transmission electron microscopy -tem- - unige.it
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Transmission Electron Microscopy
-TEM-
The first electron microscope was built 1932 by the German physicist Ernst Ruska, who
was awarded the Nobel Prize in 1986 for its invention. He knew that electrons possess a
wave aspect, so he believed he could treat them in a fashion similar to light waves. Ruska
was also aware that magnetic fields could affect electron trajectories, possibly focusing
them as optical lenses do to light. After confirming these principles through research, he set
out to design the electron microscope. Ruska had deduced that an electron microscope
would be much more powerful than an ordinary optical microscope since electron waves
were shorter than ordinary light waves and electrons would allow for greater magnification
and thus to visualize much smaller structures. The first crude electron microscope was
capable of magnifying objects 400 times. 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 Ruska's prototype and his correlation
between wavelength and magnification. 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 various surfaces.
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Aim of the lecture
Electron Microscopy is a very large and specialist field
Just a few information on•What is it possible to do •How do instruments work
Electron Microscopy
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HISTORY OF THE TRANSMISSION ELECTRON MICROSCOPE
(TEM)
•1897 J. J. Thompson Discovers the electron
•1924 Louis de Broglie identifies the wavelength for electrons as λ=h/mv
•1926 H. Busch Magnetic or electric fields act as lenses for electrons
•1929 E. Ruska Ph.D thesis on magnetic lenses
•1931 Knoll & Ruska 1st electron microscope (EM) built
•1931 Davisson & Calbrick Properties of electrostatic lenses
•1934 Driest & Muller Surpass resolution of the Light Microscope
•1938 von Borries & Ruska First practical EM (Siemens) - 10 nm resolution
•1940 RCA Commercial EM with 2.4 nm resolution
• 2000 new developments, cryomicroscopes, primary energies up to 1 MeV
History of TEM
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Scheme of TEM
0.00251
Resolution
(nm)
Wavelength
(nm)
Electrons at 200kV
~0.2
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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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2.8230.000870.001221000
2.4840.001640.00193400
2.086 0.00251 0.00273 200
1.6440.003700.00386100
Velocity
(×108 m/s)
Relativistic λλλλ(nm)
Nonrelativistic λλλλ(nm)
Accelerating voltage
(kV)
vm
h
0
=λ
2
02
1vmeV =
2
1
2
0
02
12
+
=
cm
eVeVm
hλ
−=
2
2
0 1c
vmm
Basis of the transmission electron microscopy
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βλ
61.0=thr
0.00251~550
Electrons at 200kVGreen light
Wavelength (nm)
4
1
4
3
67.0 sCr λ=
Resolution
Best attained resolution ~0.07 nm
Nature (2006)
The resolution of the transmission electron
microscope is strongly reduced by lens aberration
(mainly spherical aberration Cs )
β = semi-collection angle of magnifying lens
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Emitters
tungsten Lanthanum hexaboride
Field emitters: single oriented crystal of tungstenetched to a fine tip
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Thermoionic emitters
KTeATJΦ
−= 2
A virtual probe of size d can beassumed to be present at the
first cross-over
=J
π2
0
4
d
iJ c= Brightness or Brillance:
density per unit solid angle
Heating current
E
x
Emitter
Wehnelt
Anode
0.2 eV
Ω=
πβ
2
0
4
d
ic
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Schottky and Field emission guns
Φ−
Φ+Φ= E
eE
J
5.14x108.62
6x102.6µ
µ
Emission occurs by tunnel effect
E=electric field
Φ=work function
µ=Fermi level
+
++
•High brilliance •Little cross over•Little integrated current
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CoherenceCoherence: A prerequisite for interference is a superposition of wave systems whose phase difference remains constant in time. Two beams are coherent if, when combined, they produce an interference pattern.
Two beams of light from self luminous sources are incoherent.In practice an emitting source has finite extent and each point of the
source can be considered to generate light. Each source gives rise to a system of Fresnel fringes at the edge. The superposition of these fringe systems is fairly good for the first maxima and minima but farther away from the edge shadow the overlap of the fringe patterns becomes sufficiently 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 reduces the coherence
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B
Magnetic lenses
( )
∫
∫
∞+
∞−
∞+
∞−
=
=
dxxBV
dxxBVf
)(8
8
1 2
ηθ
η
It is a lens with focal length fbut with a rotation θ
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Magnetic lenses: bell shaped field(((( ))))2
0
/1 az
BBz
++++====
z
BrB
zr
∂∂∂∂∂∂∂∂
−−−−====2
Newton’s law
3)
2ϕ&&& mrFrm r +=
( ) ϕϕ rFmrdt
d=&
2
zFzm =&&
2ϕϕ && mrreB z +−=
= zBre
dt
d 2
2
ϕ&reB r=
1)
2)
CBre
mr z += 22
2ϕ&
zBm
e
2====ϕϕϕϕ&
Br is small for paraxial trajectories, eq. 3) gives vz=const, while the coordinate r oscillates with frequency ω=√(1+k2)
rBm
eB
m
emrB
m
ereBrm zzzz
222
422−=
+−=&&
ary /=azx /=
*0
2202
8 Um
aeBk ==== y
x
k
dx
yd22
2
2
2
)1( ++++−−−−====
from 2):
from 1):
with C=0 for per trajectories
in meridian planes
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Aberrations
Spherical aberration
Chromatic aberration
∆+
∆=
I
I
E
ECCC 2δ
3αδ ss C=
Defocus
αδ ff =
Scherzer: in a lens system withradial symmetry the spherical
aberration can never be completelycorrected
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Astigmatism
different gradients of the field:different focalization in the two directions
It can be corrected
Other aberrations exist like threefold astigmatism Coma…but can be corrected or are negligible
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Deflection coils
At least two series of coils are necessary to decouple the shift of the beam from its tilt
Position remains in pwhile different tilts are possible
Position is shifted without changing the incidence 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 a photomultiplier for detection.
phosphor screen: the electron excites phosphors that emit the characteristic green light
CCD conversion of charge into tension. Initially, a small capacity is charged with respect a reference
level. The load is eventually discharged. Each load corresponds to a pixel. The discharge current is proportional
to the number of electrons contained in the package.
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Trajectories of 10KeV electrons in matter
http://www.gel.usherbrooke.ca/casino/download2.html
Energy released in the matrix
GaAs bulk
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Trajectories of 100KeV electron in a thin
specimen
GaAs thin film
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Interaction electronic beam – sample: electron diffraction
backscattering
forward
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
λθ =sin2dBragg’s Law
Diffraction occurs when the Ewald sphere cuts a point of the reciprocal lattice
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λ1
1 d
L
R=
R
Ld
λ=
Electron diffraction - 2
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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X-ray (Cu Kα)
λ = 154 pm
rE = 3.25·109 m
Electrons (200 kV)
λ = 2.51 pm
rE = 2.0·1011 m
Comparison between high energy electron diffraction
and X-ray diffraction
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Objective lens
The magnetic pre-field of the
objective lens can also be used to obtain a parallel
illumination on the specimen
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 can be distinguished
The post field is used to create
image or diffraction from diffracted
beams
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Diffraction mode
f
Different directions correspond to different points in the back focal
plane
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Imaging mode
Different point correspond to different points.
All diffraction from the same point in the sample converge to the same
image in the image plane
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Contrast enhancement by single diffraction mode
Bright field Dark field
Objective aperture
f
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Dark/bright field images
Dark field Bright field
0.5 m0.5 m
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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Fringes indicate two Dim. periodicity
Phase contrast in electron microscopy
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Phase contrast in electron microscopy
tφ
iφ
What happens if we consider all beams impinging on the same point ? Interference !!!
~
tΦ
~
iΦ
FFT
2
2
∑∑∑∑ ΨΨΨΨ====BEAMS
giφφφφ
g a vector of the reciprocal lattice
gΨΨΨΨ s the component beam scattered by a vector g
But notice that gΨΨΨΨ is the Fourier component of the exit wavefunction1
2====ΨΨΨΨ∑∑∑∑
BEAMS
g
Indeed each electron has a certain probability to go in the transmitted or diffracted beam. For an amorphous material all Fourier components are possible but in a crystal
only 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
θθθθφφφφ it e====
tt e
µµµµφφφφ −−−−====
Example of exit wave function (simulation)
Real part Modulus phase
Vie Vi
t σφ σ +≈≈ 1
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(useful for biological specimen which absorb little
radiation but have different diffraction index with
respect to surrounding medium, thus inducing a
phase shift)
Image for regular brightfield
objectives. Notice the air bubbles
at three locations, some cells are
visible at the left side
Same image with phase contrast objectives.
White dots inside each cell are the nuclei.
Optical Phase Contrast microscope
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To build an ideal phase microscope we must dephase (by π/2) all diffracted beams while leaving the transmitted unchanged
Phase adjustment device
The device is ~150 µ wide and 30 µ thick. The
unscattered electron beam passes through a
drift tube A and is phase-shifted by the
electrostatic potential on tube/support B.
Scattered electrons passing through space D are
protected from the voltage by grounded tube C.
Phase contrast in electron microscopy
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Diffraction mode
While scanning the beam over the different part of the sample we integrate over different diffraction patterns. If the transmitted beam is included the method is called STEM-BF otherwise STEM-DF
An alternative use of the electron microscope is to concentrate the electron beam onto a small area and scan it over the sample. Initially it was developedto gain local chemical information. Actually structural information can begained, too, since the beam spot can be as small as 1.3 Å.
STEM
The dark field image corresponds
to less coherent electrons and
allows therefore for a more
accurate reconstruction
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STEM probe
It depends on aperture , Cs , defocus
2
0
2)()(2max
),0,( ∫=
−−=∆−K
K
rrkiki
probe kdeerrP 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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Selected area electron diffraction - SAED
The diffracting
area is ~0.5 µµµµm
in diameter
Single crystal Polycrystalline Amorphous
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Convergent beam electron diffraction - CBED
The
diffracting
area is <1 nm
in diameter
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Chemical analysis: energy dispersive spectrometry
The electron beam can be focused to obtain a spot less than 500 nm
SEM TEM
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EELSUsed to collect spectra and images
Usable in both scanning and tem
mode
Each edge is characteristic for each material
The fine structures of the edge reveal the characteristics of the local environment 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 at different angle it it is possible to reconstruct 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
• An additional class of these instruments is the electron cryomicroscope, which includes a specimen stage capable of maintaining the specimen at liquid nitrogen or liquid helium temperatures. This allows imaging specimensprepared in vitreous ice, the preferred preparation technique for imagingindividual molecules or macromolecular assemblies.
• In analytical TEMs the elemental composition of the specimen can bedetermined by analysing its X-ray spectrum or the energy-loss spectrum of the transmitted electrons.
• Modern research TEMs may include aberration correctors, to reduce the amount of distortion in the image, allowing information on features on the scale of 0.1 nm to be obtained (resolutions down to 0.08 nm have beendemonstrated, so far). Monochromators may also be used which reduce the energy spread of the incident electron beam to less than 0.15 eV.
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TEM : transmission electron microscope
SEM : scanning electron microscope
STEM: scanningtransmission electron
microscope
Other Electron Microscopes
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SEM
The signal arises from secondary electronsejected by the sample and captured by the fieldof the detector
It is equivalent to the first part of the TEM butthere is are important differences:a) The sample is outside the objective lens. b) 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
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SEM
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Range vs accelerating voltage
+
−+=32
69.54.1221.66.01
)(R
z
R
z
R
z
Rzg
( ) 75.1/052.0 beamER ρ= Range of electrons in a material
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Accelerating voltage
Higher voltage -> smaller probeBut larger generation pear
Higher voltage -> more backscattering
Low energy -> surface effects
Low energy (for bulk) -> lower charging
High energy (for thin sample) -> less charging
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Effect of apertures
Larger aperture means in the case of SEM worse resolution(larger probe) but higher current
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Strength of Condenser Lens
The effect is similar to that of changing the aperture
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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 un allargamento del suo diametro sopra e sotto il punto di fuoco ottimale. In prima approssimazione, a una distanza D/2 dal punto di fuoco il diametro del fascio aumenta di ∆r ≈αD/2.
E’ possibile intervenire sulla profondità di campo aumentando la distanza di lavoro e diminuendo il diametro dell’apertura finale
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Minore e’ l’apertura della lente obiettivo e maggiore e’ la distanza di lavoro WD, maggiore e’ la profondità di fuoco.
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SECONDARY electrons
A large number of electrons of low energy <30-50eV produced by the
passage of the primary beam electrons.
they are mainly created in a narrow area of radius λSE (few nm) = the SE creation mean
free path
The Everhart-Thornley detectors are suited to detect these electron by means of a gate with a positive
bias.
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BACKSCATTERING
NtEE
EE
E
Ze2
0
0
22
0
24
216
++
=πε
η
Backscattering coefficient=fraction of the incident beam making backscattering
Characterised by a quite large energy
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Imaging with BS and SE
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Shadow effect
detector
by reciprocity it is almost equivalent to light : most
of time intuitive interpretation
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channeling BSE
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BSDE Backscattering electron diffraction
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reciprocity principle
inverting the direction of the electron the
image formation is equivalent
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LEEM
The low energy electron microscope allows tostudy surfaces with 1 nm resolution. The electrons are decelerated to few tens of eV in front of the sample and reaccelerated again after being back reflected into the microscope. Contrast is achieved by selecting differentdiffraction channels corresponding to substrateand adsorbate or using the different reflectivityof different atomic species.