tem-sci mat
DESCRIPTION
temTRANSCRIPT
7212019 TEM-Sci Mat
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Transmission Electron Microscopy -TEM-
Scanning Electron Microscopy ndash SEM -
The first electron microscope was built 1932 by the German physicist Ernst Ruska who
was awarded the Nobel Prize in 1986 for its inventionHe knew that electrons possess a wave aspect so he believed he could treat them as light
waves Ruska was aware that magnetic fields affect electron trajectories possibly focusing
them as optical lenses do to light After confirming these principles he set out to design the
electron microscope which he knew would be much more powerful than an ordinaryoptical microscope since electron waves were shorter than ordinary light waves Electrons
would therefore allow for greater magnification and to visualize much smaller structures
The first practical electron microscope was built by in 1938 and had 10 nm resolution
Although modern electron microscopes can magnify an object 2 million times they are still
based upon Ruskas prototype
The electron microscope is now an integral part of many laboratories Researchers use it to
examine biological materials (such as microorganisms and cells) a variety of large
molecules medical biopsy samples metals and crystalline structures and the
characteristics of surfaces
7212019 TEM-Sci Mat
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Aim of the lecture
Electron Microscopy is a very large and specialistfield
Just a few information on
Electron Microscopy
bullWhat is it possible to dobullHow do instruments work
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HISTORY OF THE TRANSMISSION ELECTRON MICROSCOPE
(TEM)
bull1897 J J Thompson Discovers the electron
bull1924 Louis de Broglie identifies the wavelength for electrons as λ=hmv
History of TEM
bull1926 H Busch Magnetic or electric fields act as lenses for electronsbull1929 E Ruska PhD thesis on magnetic lenses
bull1931 Knoll amp Ruska 1st electron microscope (EM) built
bull1931 Davisson amp Calbrick Properties of electrostatic lenses
bull1934 Driest amp Muller Surpass resolution of the Light Microscope
bull1938 von Borries amp Ruska First practical EM (Siemens) - 10 nm resolution
bull1940 RCA Commercial EM with 24 nm resolution
bull 2000 new developments cryomicroscopes primary energies up to 1 MeV
7212019 TEM-Sci Mat
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Scheme of TEM
000251
Resolution
(nm)
Wavelength
(nm)
Electrons at 200kV
~02
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TEM lens system
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Application of magneticLenses TransmissionElectron Microscope(Ruska and Knoll 1931)1945 - 1nm resolution
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vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
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β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
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Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
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Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
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Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
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CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
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Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
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Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
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Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
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Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
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Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
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Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
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Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
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Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
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Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
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Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
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Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
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Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
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Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
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Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
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Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
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Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
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Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
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Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
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Diffraction contrast
Suppose only two beams are on
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Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
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Amplitude contrast - 1
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Amplitude contrast - 2
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Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
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Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
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Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
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(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
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To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
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beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
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STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
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Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
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EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
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EELS analysis
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TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
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Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
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TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
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TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
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SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
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SEM
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Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
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Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
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Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
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Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
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Effect of the working distance
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Depth of fieldD
D
WD
WD
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La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
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Profonditagrave di campo
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Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
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SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
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BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
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Imaging with BS and SE
Sh d ff t
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Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
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channeling BSE
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BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
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Aim of the lecture
Electron Microscopy is a very large and specialistfield
Just a few information on
Electron Microscopy
bullWhat is it possible to dobullHow do instruments work
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 366
HISTORY OF THE TRANSMISSION ELECTRON MICROSCOPE
(TEM)
bull1897 J J Thompson Discovers the electron
bull1924 Louis de Broglie identifies the wavelength for electrons as λ=hmv
History of TEM
bull1926 H Busch Magnetic or electric fields act as lenses for electronsbull1929 E Ruska PhD thesis on magnetic lenses
bull1931 Knoll amp Ruska 1st electron microscope (EM) built
bull1931 Davisson amp Calbrick Properties of electrostatic lenses
bull1934 Driest amp Muller Surpass resolution of the Light Microscope
bull1938 von Borries amp Ruska First practical EM (Siemens) - 10 nm resolution
bull1940 RCA Commercial EM with 24 nm resolution
bull 2000 new developments cryomicroscopes primary energies up to 1 MeV
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 466
Scheme of TEM
000251
Resolution
(nm)
Wavelength
(nm)
Electrons at 200kV
~02
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 566
TEM lens system
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 666
Application of magneticLenses TransmissionElectron Microscope(Ruska and Knoll 1931)1945 - 1nm resolution
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 766
vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
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HISTORY OF THE TRANSMISSION ELECTRON MICROSCOPE
(TEM)
bull1897 J J Thompson Discovers the electron
bull1924 Louis de Broglie identifies the wavelength for electrons as λ=hmv
History of TEM
bull1926 H Busch Magnetic or electric fields act as lenses for electronsbull1929 E Ruska PhD thesis on magnetic lenses
bull1931 Knoll amp Ruska 1st electron microscope (EM) built
bull1931 Davisson amp Calbrick Properties of electrostatic lenses
bull1934 Driest amp Muller Surpass resolution of the Light Microscope
bull1938 von Borries amp Ruska First practical EM (Siemens) - 10 nm resolution
bull1940 RCA Commercial EM with 24 nm resolution
bull 2000 new developments cryomicroscopes primary energies up to 1 MeV
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 466
Scheme of TEM
000251
Resolution
(nm)
Wavelength
(nm)
Electrons at 200kV
~02
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 566
TEM lens system
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 666
Application of magneticLenses TransmissionElectron Microscope(Ruska and Knoll 1931)1945 - 1nm resolution
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 766
vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 466
Scheme of TEM
000251
Resolution
(nm)
Wavelength
(nm)
Electrons at 200kV
~02
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 566
TEM lens system
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 666
Application of magneticLenses TransmissionElectron Microscope(Ruska and Knoll 1931)1945 - 1nm resolution
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 766
vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 566
TEM lens system
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 666
Application of magneticLenses TransmissionElectron Microscope(Ruska and Knoll 1931)1945 - 1nm resolution
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 766
vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 666
Application of magneticLenses TransmissionElectron Microscope(Ruska and Knoll 1931)1945 - 1nm resolution
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 766
vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 766
vm
h
0
=λ
2
02
1
vmeV =2
1
2
0
02
12
+
=
cm
eV eV m
h
λ
minus=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
Attention
relativistic
electrons
Accelerating voltage
(kV)
Nonrelativistic λλλλ
(nm)
Relativistic λλλλ
(nm)
Velocity
(times108 ms)
100 000386 000370 1644
200 000273 000251 2086
400 000193 000164 2484
1000 000122 000087 2823
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 866
β
λ 610=thr
000251~550
Electrons at 200kVGreen light
Wavelength (nm)
Resolution
β= semi-collection angle of magnifying lens
λ= electron wavelength
4
1
4
3
670 sC r λ = Best attained resolution ~007 nm
Nature (2006)
e reso u on o e ransm ss on e ec ron
microscope is strongly reduced by lens aberration
(mainly spherical aberration C s )
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 966
Emitters
tun sten Field emitters single oriented
hexaboride
etched to a fine tip
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1066
Thermoionic emitters
Heating current
Emitter
Wehnelt
A virtual probe of size d can beassumed to be present at the
first cross-over
kT c e AT d
i J
Φminus
== 2
2
0
4
π
Brightnessdensity per unit solid angle
E
x
Anode
02 eV
Ω= π β 2
0
4
d
ic
4A Area Emitting
2
0d π
=
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1166
Schottky and Field emission guns
+
++
Φminus
Φ+Φ=
E e
E J
514x10862
6x1026micro
micro
Emission occurs by tunnel effect
E=electric field
Φ=work function
micro=Fermi level
bullHigh brilliancebullLittle cross over bullLittle integrated current
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1266
CoherenceCoherence a prerequisite for interference is a superposition of wavesystems whose phase difference remains constant in time Twobeams are coherent if when combined they produce an interference
pattern
Two beams of light from self luminous sources are incoherentIn practice an emitting source has finite extent and each point of the
system of Fresnel fringes at the edge The superposition of these fringesystems is fairly good for the first maxima and minima but farther awayfrom the edge shadow the overlap of the fringe patterns becomessufficiently random to make the fringes disappear
The smaller is the source the larger is coherence
Using a beam with more than one single wave vector k (polychromatic beam)
reduces the coherence
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1366
Magnetic lenses
( )
int
int
infin+
infinminus
infin+
infinminus
=
=
dx x BV
dx x BV f
)(8
8
1 2
η θ
η
lens with focal length f but with a rotation θ
V acceleration voltage of the electronsη charge to mass ratio of the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1466
Magnetic lenses bell shaped field(((( ))))2
0
1 a z
B B z
++++====
z
Br B
z r
partpartpartpart
partpartpartpartminusminusminusminus====2
Newtonrsquos law
3)
2ϕ ampampamp mr F r m r +=
( ) ϕ ϕ rF mr
dt
d =amp
2
z F z m =ampamp
2ϕ ϕ ampamp mr r eB z +minus=
= z Br
e
dt
d 2
2ϕ ampr eB r =
1)
2)
C Br e
mr z += 22
2ϕ ampfrom 2) with C=0 for per trajectories
z Bme
2====ϕ ϕϕ ϕ amp
Br is small for paraxial trajectories eq 3) gives vz=constwhile the coordinate r oscillates with frequency ω=radic(1+k2)
r Bm
e B
m
emr B
m
er eBr m z z z z
222
422minus=
+minus=ampamp
ar y =a z x =
0
2202
8 U m
aeBk ==== y
x
k
dx
yd 22
2
2
2
)1( ++++minusminusminusminus====
from 1)
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1566
Aberrations
Spherical aberration3α δ s s C =
Defocus
α δ f f =
Scherzer in a lens system with
Chromatic aberration
∆+
∆=
I
I
E
E C C C 2δ
radial symmetry the spherical
aberration can never be completelycorrected
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1666
Astigmatism
different gradients of the fielddifferent focalization in the two directions
It can be corrected
Other aberrations exist likethreefold astigmatismComabut can be corrected or are negligible
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1766
Deflection coils
At least two series of coilsare necessary to decouplethe shift of the beam from
its tilt
Position remains in p
while different tilts arepossible
Position is shiftedwithout changing theincidence angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1866
Revelators
Scintillator emits photons when hit by high-energy electrons The emitted photons are
collected by a lightguide and transported to aphotomultiplier for detection
hos hor screen the electron excites hos hors that
emit the characteristic green light
CCD conversion of charge into tension
Initially a small capacity is charged with respect a referencelevel The load is eventually discharged Each load
corresponds to a pixel The discharge current is proportionalto the number of electrons contained in the package
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 1966
Trajectories of 10KeV electrons in matter
GaAs bulk
httpwwwgelusherbrookecacasinodownload2html
Energy released in the matrix
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2066
Trajectories of 100KeV electron in a thinspecimen
GaAs thin film
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2166
Interaction electronic beam ndash sample electron diffraction
backscattering
scattering
Electrons can be focused by electromagnetic lenses
The diffracted beams can be recombined to form an image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2266
Electron diffraction - 1
Diffraction occurs when the Ewaldrsquos sphere cuts a point of the reciprocal lattice
λ θ =sin2d Braggrsquos Law
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2366
Electron diffraction - 2
λ 1
1 d
L
R=
Ld
λ =
Recorded spots correspond mainly to one plane in reciprocal space
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2466
Scattering
Fast electrons are scattered by the protons in
the nuclei as well as by the electrons of the
atoms
X-rays are scattered only by the electrons of
the atoms
Diffracted intensity is concentrated in the forward direction
Coherence is lost with growing scattering angle
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2566
Electrons (200 kV)
λ = 251 pm
Comparison between high energy electron diffraction
and X-ray diffraction
X-ray (Cu K α)
λ = 154 pm
r E
= 325983223109 m
r E
= 983223 m
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2666
Objective lens
The magneticpre-field of the
objective lens can
Objective is the most important lens in a TEM it has a very high field (up to 2 T)
The Specimen is completely immersed in its field so that pre-field and post field canbe distinguished
The post field isused to create
image or diffraction
obtain a parallelillumination on thespecimen
beams
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2766
Diffraction mode
Different directions correspond todifferent points in the back focal
plane
f
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2866
Imaging mode
Different point correspond to differentpoints
All diffraction from the same point inthe sample converge to the same
image in the image plane
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 2966
Contrast enhancement by single diffraction mode
f
Bright field Dark field
Objectiveaperture
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3066
Darkbright field images
Dark field Bright field
05 05
Using 200 Using 022
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3166
Diffraction contrast
Suppose only two beams are on
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3266
Imaging
Lattice Fringe Image High Resolution Image
Phase contrastAmplitude contrast
Bright Field Image Dark Field Image
Perfect imaging would require the interference of all difffraction
channels Contrast may however be more important
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3366
Amplitude contrast - 1
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3466
Amplitude contrast - 2
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3566
Phase contrast in electron microscopy
Fringes indicate two Dim periodicity
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3666
Phase contrast in electron microscopy
t φ What happens if we consider all beamsimpinging on the same point Interference
~
t Φ
~
Φ
FFT
2
2
sumsumsumsumΨΨΨΨ==== BEAMS
g i φ φφ φ
g a vector of the reciprocal lattice
g ΨΨΨΨ s the component beam scattered by a vector g
But notice that g ΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨsumsumsumsum
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffractedbeam For an amorphous material all Fourier components are possible but in a crystalonly beams with the lattice periodicity are allowed these are the diffracted beams
NOTE the diffraction pattern is just the Fourier transform of the exit wave
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3766
Effect of the sample potential V
Phase shift
Amplitude variation
θ θθ θ φ φφ φ i
t e====
t e
micro micromicro micro minusminusminusminus====
V ie V i
t σ φ σ +asympasymp 1
Example of exit wave function (simulation)
Real part Modulus phase
O ti l Ph C t t i
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3866
(useful for biological specimen which absorb little
radiation but have different diffraction index withrespect to surrounding medium thus inducing a
phase shift)
Optical Phase Contrast microscope
Image for regular brightfield
objectives Notice the air bubbles
at three locations some cells arevisible at the left side
Same image with phase contrast objectives
White dots inside each cell are the nuclei
Phase contrast in electron microscopy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 3966
To build an ideal phase microscope we must dephase (by π2) alldiffracted beams while leaving the transmitted unchanged
Phase adjustment device
Phase contrast in electron microscopy
The device is ~150 983221 wide and 30 983221 thick Theunscattered electron beam passes through a drift
tube A and is phase-shifted by the electrostatic
potential on tubesupport B Scattered electrons
passing through space D are protected from thevoltage by grounded tube C
An alternative use of the electron microscope is to concentrate the electron
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4066
beam onto a small area and scan it over the sample Initially it was developedto gain local chemical information Actually structural information can be
gained too since the beam spot can be as small as 13 Aring
STEM
Diffraction mode
While scanning the beam over the differentpart of the sample we integrate overdifferent diffraction patterns If thetransmitted beam is included the method is
called STEM-BF otherwise STEM-DFThe dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4166
STEM probeIt depends on
aperture Cs defocus
2
2)()(2max
)0( int minusminus=∆minus
K
r r k ik i
probe k d eer r P probe
r
r
rrrr rrr χ
=
It is the sum of the waves at different angles each with its own phase factor
If there is no aberration the larger is the convergence the smaller is the probe
The presence of aberrations limits the maximum value of the convergence angle up to 14 mrad
The different wavevectors contributing to the incoming wave
blur up the diffraction pattern causing superposition of the spots
Interference effects are unwanted and smallest at the largest angles
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4266
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4366
Chemical analysis energy dispersive spectrometry
SEM TEM
The electron beam can be focused to obtain a spot less than 500 nm
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4466
EELSUsed to collect spectra and imagesUsable in both
scanning and temmode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the localenvironment of the species If the relevant inelastic event corresponds
to a localized interatomic transition the information is local too
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4566
EELS analysis
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4666
TomographyBy observing atdifferent angle itit is possible toreconstruct the
3D image
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4766
Drawback of TEM necessity of sample preparation
Ion milling
Milling
Hand milling
Powders
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4866
TEM DEVELOPMENTS
bull An additional class of these instruments is the electron cryomicroscope
which includes a specimen stage capable of maintaining the specimen atliquid nitrogen or liquid helium temperatures This allows imaging specimens prepared in vitreous ice the preferred preparation technique for imagingindividual molecules or macromolecular assemblies
bull
determined by analysing its X-ray spectrum or the energy-loss spectrum ofthe transmitted electrons
bull Modern research TEMs may include aberration correctors to reduce theamount of distortion in the image allowing information on features on thescale of 01 nm to be obtained (resolutions down to 008 nm have beendemonstrated so far) Monochromators may also be used which reduce theenergy spread of the incident electron beam to less than 015 eV
Other Electron Microscopes
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 4966
TEM transmissionelectron microscope
SEM scanning electronmicroscope
Other Electron Microscopes
transmission electronmicroscope
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5066
SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
s equ va en o e rs par o e u
there is are important differencesa) The sample is outside the objective lensb) The signal recorded corresponds to reflected
or to secondary electrons
c) No necessity for difficult sample preparationd) Max resolution 2 nm
The distance of the sample iscalled working distance WD
WD
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5166
SEM
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5266
Range vs accelerating voltage
+
minus+=
3269541221660
1)(
R
z
R
z
R
z
R z g
( ) 7510520 beam E R ρ = Range of electrons in a material
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5366
Accelerating voltage
Higher voltage -gt smaller probeBut larger generation pear
Higher voltage -gt more backscattering
Low energy-gt surface effects
Low energy (for bulk) -gt lower charging
High energy (for thin sample) -gt less charging
Effect of apertures
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5466
Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
St th f C d L
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5566
Strength of Condenser Lens
The effect is similar to that of changing the aperture
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5666
Effect of the working distance
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5766
Depth of fieldD
D
WD
WD
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5866
La divergenza del fascio provoca unallargamento del suo diametro sopra esotto il punto di fuoco ottimale In prima
approssimazione a una distanza D2 dalpunto di fuoco il diametro del fascioaumenta di ∆r asympαD2
Ersquo possibile intervenire sulla profonditagrave dicampo aumentando la distanza di lavoro ediminuendo il diametro dellrsquoapertura finale
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 5966
Profonditagrave di campo
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6066
Minore ersquo lrsquoapertura della lente obiettivo e maggiore ersquo ladistanza di lavoro WD maggiore ersquo la profonditagrave di fuoco
SECONDARY l t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6166
SECONDARY electrons
A large number of electrons of lowenergy lt30-50eV produced by the
passage of the primary beamelectrons
narrow area of radius λSE (fewnm) = the SE creation meanfree path
The Everhart-Thornley detectorsare suited to detect these electronby means of a gate with a positive
bias
BACKSCATTERING
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6266
BACKSCATTERING
Nt E E
E E
E
Z e2
0
0
22
0
24
216
+
+=
πε η
Backscattering coefficient=fraction of the incidentbeam making backscattering
Characterised by a quite large energy
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6366
Imaging with BS and SE
Sh d ff t
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6466
Shadow effect
detector
b reci rocit it is almostequivalent to light most
of time intuitiveinterpretation
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6566
channeling BSE
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction
7212019 TEM-Sci Mat
httpslidepdfcomreaderfulltem-sci-mat 6666
BSDE Backscattering electron diffraction