introduction to eels in the aem - argonne national...
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Introduction to EELS in the AEMIntroduction to EELS in the AEM
Nestor J. ZaluzecNestor J. Zaluzec
zaluzec@[email protected]
zaluzec@[email protected]
What are the Limits - Today?What are the Limits - Today?
S. Pennycook etal from the TEAM Project Report
5 Å Spectroscopicidentification atsubnanometerresolution.
8% collectionefficiency
820 850 880
Inte
nsity
Energy (eV)
La M4/5
La in CaTiO3grown by MBE
P.E. Batson IBM Yorktown, Jan 2004
AEM - High Spatial Resolution Analysis
Electronic Structure changes at the Si/SiO2 Interface
Brief Review of Energy Loss ProcessesBrief Review of Energy Loss Processes
Instrumentation: Detector SystemsInstrumentation: Detector Systems
Instrumentation: AEM Systems Instrumentation: AEM Systems
Data Analysis and Quantification: Data Analysis and Quantification:
Advanced TopicsAdvanced Topics
Brief Review of Energy Loss ProcessesBrief Review of Energy Loss Processes
Electron Excitation of Inner Shell & Continuum Processes Electron Excitation of Inner Shell & Continuum Processes Spectral Shapes Spectral Shapes Notation of Edges Notation of Edges
Electron Scattering Angular DistributionsElectron Scattering Angular Distributions
The Emission Process:The Emission Process:
1-Excitation1-Excitation2-Relaxation2-Relaxation3-Emission3-Emission
Electron Distribution Relaxation
Ejected Inner ShellElectron
Incident Electron
Inelastically ScatteredPrimary Electron
X-ray PhotonEmission
Internal Conversion and Auger Electron
Emission
Electron Excitation of Inner Shell Processes
Eo
Eo-δE δE
Electron Energy Loss SpectroscopyElectron Energy Loss Spectroscopy
Measure the changes in the energy distribution of an electron beam transmittedMeasure the changes in the energy distribution of an electron beam transmittedthrough a through a thinthin specimen. specimen.
Each type of interaction between the electron beam and the specimen produces aEach type of interaction between the electron beam and the specimen produces acharacteristiccharacteristic change in the energy and angular distribution of scattered change in the energy and angular distribution of scatteredelectrons.electrons.
The energy loss process is the The energy loss process is the primaryprimary interaction event. All other sources of interaction event. All other sources ofanalytical information ( i.e. X-rays, Auger electrons, etc.) are analytical information ( i.e. X-rays, Auger electrons, etc.) are secondarysecondary products productsof the initial inelastic event. Thus, EELS has the highest potential yield ofof the initial inelastic event. Thus, EELS has the highest potential yield ofinformation/inelastic eventinformation/inelastic event
Geometrical Collection Efficiency in XEDS
!" = "4#=
14#
AR2
Geometrical Collection Efficiency in EELS
!$ = ln 1 + $
%E
2
ln 2%E
%E = &E2'oTo
; 'o = 11-$2
; To = 12
mov 2 = 255.530 1- 11+VO(kV)
511.060
2 k e V
Accelerating Voltage Vo
(kV) To =12 mov2 (keV).
Energy Loss
&E (eV)
Accelerating Voltage (kV) Scattering Angle %E (mr)
50 43.5 10 120 0.057100 76.8 100 120 0.569120 87.9 200 120 1.14150 102.8 500 120 2.84200 123.5 1000 120 5.69250 140.3 500 100 3.26300 153.8 500 200 2.02350 165.5 500 300 1.62400 175.1 500 400 1.431000 226.3 500 1000 1.10
!
2 "
e -
Coll imator
Aper tu re
XEDS
Character i s t i cX-ray Emission
Angular Dependance(Isotropic Distr ibution)
Character i s t i cElectron Energy Loss Angular Dependance
(Aniostropic Distr ibution)
d-band
s-band
1 2 3
AES
XPS/UPS
XEDS/XRF
EELS/XAS
DO
S
Incident Electrons Incident Photons
L shell M shell2 4 531
}
Schematic Diagram Illustrating Sources ofSchematic Diagram Illustrating Sources ofInelastic Scattering SignalsInelastic Scattering Signals
Experimental XEDS, XPS, and EELS data from the Copper L shell. Note theExperimental XEDS, XPS, and EELS data from the Copper L shell. Note thedifferences in energy resolution, and spectral featuresdifferences in energy resolution, and spectral features.
Comparision Comparision Light Element Spectroscopy ResolutionLight Element Spectroscopy ResolutionXEDS XEDS vs vs EELSEELS
Comparision Comparision of WL XEDS Detector and EELS spectraof WL XEDS Detector and EELS spectrataken from the same taken from the same NiO NiO specimenspecimen
Note the enhanced spectral information in the EELS data. Vertical scale isNote the enhanced spectral information in the EELS data. Vertical scale isarbitrary andarbitrary and chozen chozen for clarity of presentation. for clarity of presentation.
HeliumHeliumIn PdIn Pd
D. D. Taverna etalTaverna etalPRL 100, 035301 (2008)PRL 100, 035301 (2008)
Electron Scattering
Scattering from an isolated atom in free space
0 10 20
Scattering Angle [mR]
I( )
/Io!
Elastic
Plasmon
PhononInner-Shell
Unscattered
R = Average Interatomic Spacing
Scattering from an collection of an amorphous collection of atoms - neighboring atoms give rise to interference
AmorphousSilica (SiO2)
0 10 20
Scattering Angle [mR]
I( )
/Io!
Elastic
Plasmon
PhononInner-Shell
Unscattered
0 10 20
Scattering Angle [mR]
I( )
/Io!
Elastic
Plasmon
PhononInner-Shell
Unscattered
Unfiltered Elastic
Plasmon
Elastic Plasmon
0
1 105
2 105
3 105
4 105
5 105
6 105
7 105
-12 -8 -4 0 4 8 12
Amorphous Carbon
Expt - 50eV Theory - Normalized
Rel
ativ
e In
tens
ity
Angle (mR)
0
5000
1 104
1.5 104
2 104
2.5 104
3 104
3.5 104
4 104
270 280 290 300 310 320 330
0.110.35.50
Rel
ativ
e In
tens
ity
Energy Loss (eV)
θE ~1.15 mR
Intensity of Edges is Directly Related to Mean Scattering AngleExample: Amorphous Carbon
Intensity of Edges is Directly Related to Mean Scattering AngleExample: Boron - Nitride
θE ~1.62 mRθE ~0.777 mR
0 10 20Scattering Angle [mR]
I( )
/Io!
Elastic
Plasmon
Phonon
Unscattered
Inner-Shell
0 10 20Scattering Angle [mR]
I( )
/Io!
Elastic
Plasmon
Phonon
Unscattered
Inner-Shell
Unfiltered Elastic
1st Plasmon2nd Plasmon
0 eV
15 eV30 eV
Instrumentation: Detector Systems
Energy Loss Spectrometers Basic Principles Electrostatic/Electromagnetic Serial/Parallel Detector Systems Spectral Artifacts
Multichannel Analyzers
CommericalCommericalSpectrometersSpectrometers
VsVsImaging FiltersImaging Filters
Imaging Filters Imaging Filters vs vs SpectrometersSpectrometers
Spectral ImagingSpectral ImagingSlice-by-Slice Slice-by-Slice vs vs Point-by-PointPoint-by-Point
Unfiltered Elastic
PlasmonUnfiltered + Obj. Aperture
Unfiltered Elastic
1st Plasmon2nd Plasmon
Silicon 111 ZAP- Gamma Corrected
Spectroscopy Spectroscopy vs vs Filtered ImagingFiltered Imaging
W. W. Grogger etal Grogger etal MM 2003MM 2003 9, s.3 , 729, s.3 , 72
Data Analysis and Quantification:Data Analysis and Quantification:
Spectral Processing Spectral Processing Thin Film Quantification Methods Thin Film Quantification Methods Specimen Thickness Effects Specimen Thickness Effects
MM2323CC6 6 in Steel :in Steel :Spectral OverlapSpectral Overlap
SpectralSpectralOverLapOverLap
Problems alsoProblems alsoexist in EELSexist in EELS
Two Related Methods are sometimes used:Two Related Methods are sometimes used:
Second Difference Filtering ( Second Difference Filtering (Shuman etal MAS,1983Shuman etal MAS,1983))
Record 3 Energy Loss Spectra which are displaced in energy by dERecord 3 Energy Loss Spectra which are displaced in energy by dEMathematically combine in computer to form the Second DifferenceMathematically combine in computer to form the Second Difference(SD) Spectrum(SD) Spectrum
SD(E) = ISD(E) = I11(E-dE) - 2 I(E-dE) - 2 I22 (E) + I (E) + I33 (E+dE) (E+dE)
Spectrum has the appearance of a derivitive, removes channel toSpectrum has the appearance of a derivitive, removes channel tochannel gain variation in parallel EELS and slowly varyingchannel gain variation in parallel EELS and slowly varyingbackgrounds. Sharp features are enhanced in visibility.backgrounds. Sharp features are enhanced in visibility.
Digital FilteringDigital Filtering
This is related to a simple numerical differentiation of a singleThis is related to a simple numerical differentiation of a singlespectrumspectrum
IIkk = P = Pkk* I* Ioo
IIkk = Number of electron having excited a kth inner shell= Number of electron having excited a kth inner shell
PPkk = Probability of excitation of the kth shell = Probability of excitation of the kth shell
IIoo = Incident electron current= Incident electron current
PPkk = N = N σσkk
N N = Number of atoms of the element analyzed= Number of atoms of the element analyzedσσkk = Ionization cross-section for the kth shell= Ionization cross-section for the kth shell
Alternatively;Alternatively;
Consider the ratio of Intensities of any two Edges in the same spectrum IConsider the ratio of Intensities of any two Edges in the same spectrum IA A and I and IBB
Invoke the Ratio Method and obtain the exact equation:Invoke the Ratio Method and obtain the exact equation:
Note the similiarity of this equation with that of Thin Film XEDSNote the similiarity of this equation with that of Thin Film XEDS
But in the real world the assumptions used in theabove simple arguments are never realized:
• Measure all scattered electrons (!="=180 0)• Integration over all energy Losses
Because we must measure over a finite energywindow (#E) we modify the expression to:
NA= IA(#E)
!$A(#E)*I 0
we also measure over a finite angular window (!)and therefore, we modify the expression to:
NA= !IA(#E,!)$A(#E,!)*I0
and the ratio equation becomes:
NANB = kAB
IA(#E,!)I B(#E,!)
with
kAB = $B(#E,!)$A(#E,!)
!
"
Specimen
IncidentBeam
Convergence
InelasticScattering
Angle
e -
Hexagonal Boron- Nitride
θE ~1.62 mRθE ~0.777 mR
Problems in EELS QuantificationProblems in EELS QuantificationCross-section CalculationsCross-section Calculations
Problems In EELS QuantificationProblems In EELS Quantification Collection Angle Errors Collection Angle Errors
0 10 20Scattering Angle [mR]
I( )
/Io!
Elastic
Plasmon
Phonon
Unscattered
Inner-Shell
Effects of Specimen ThicknessEffects of Specimen Thickness
• Multiple ScatteringMultiple Scattering
•• Low Loss Low Loss
•• Core Loss Core Loss -Visiblity-Visiblity
•• Quantification EffectsQuantification Effects
•• To measure the thickness ofTo measure the thickness ofcompare the intensity of thecompare the intensity of thezero loss (Izero loss (I00) to the total) to the totalintegrated intensity in theintegrated intensity in thespectrum (Ispectrum (ITT).).
•• This ratio is directlyThis ratio is directlyproportional to the localproportional to the localthickness of the specimen.thickness of the specimen.
t = t = λ λ * * ln ln (I(Io/o/// IITT))λ =λ = mean free path mean free path
0
1 105
2 105
3 105
4 105
5 105
-100 0 100 200 300 400
Inten
sit
y
Energy Loss (eV)
I0
IT
450 500 550 600 650 700 750
Experimental EELS Edge/Background Ratioas a Function of Specimen Thickness
at Constant Accelerating Voltage
EELS
Inte
nsity
Nor
mal
ized
to P
re-E
dge
Bgn
d
Energy Loss (eV)
t/ λ = 0.23
t/ λ = 0.62
t/ λ = 1.21
O K Edge in NiO
VariationVariationin EELSin EELSEdge P/BEdge P/B
withwithThicknessThickness
VariationVariationin EELSin EELSEdge P/BEdge P/B
withwithkVkV
450 500 550 600 650 700 750
EELS
Inte
nsity
Nor
mal
ized
to P
re-E
dge
Bac
kgro
und
Energy Loss (eV)
Experimental EELS Edge/Background Ratioas a Function of Accelerating Voltage
at Constant Specimen Thickness
O K Edge in NiO
300 kV
200 kV100 kV
Specimen Thickness Effects on EELSSpecimen Thickness Effects on EELS
Deconvolution Deconvolution of Multiple Scatteringof Multiple Scatteringusing using Leapman/Swyt Leapman/Swyt MethodMethod
Orientation & Thickness Effects on EELS SignalsOrientation & Thickness Effects on EELS Signals
Specimen ContaminationSpecimen ContaminationThe The Microscopists Microscopists BaneBane
15 sec15 sec
30 sec 30 sec
60 sec 60 sec
120 sec 120 sec
300 sec 300 sec
Spectral ImagingSpectral ImagingSlice-by-Slice Slice-by-Slice vs vs Point-by-PointPoint-by-Point
Boron Nitride on Holey Carbon
Elastic Plasmon
Energy Loss0 200 400
170 eV 195 eV
265 e V 290 eV
BN Data
170 eV 195 eV
Filtered Elemental Imaging
Net Boron K Image
265 e V 290 eV
BN Data
Filtered Elemental Imaging
Net Carbon K Image
EFTEM elementalmaps of mousepancreaticIslet cells
Nitrogen PhosphorusSulfur
Mut
Mut
Cnt
Cnt
Leapman et al. (2003)Leapman et al. (2003)
1µm
Shows sulfur-richinsulin granules in β cells
Model of axon andoligodendrocyte showinganatomy of degeneration
0
2
4
6
8
10
12
600 650 700 750 800
CCD
Coun
ts/1
000
Energy Loss (eV)
EELS of ferritin molecule extracted from spectrum-image
Fe L2,3 edge
100 nm
FerritinFe
STEM-ADF
Ferritinlocalizedin invaginationsofoligodendrocytes
Mapping Ferritin in Brain:Misregulation of Iron Metabolism inIRP Knockout Mice
Pre Fe L2,3 Post Fe L2,3
STEM-EELS
(P. Zhang et al., J. Struct. Biol. 2005)
Hofer & Hofer & Warbichler Warbichler - Graz- Graz TUTU
Hofer & Hofer & Warbichler Warbichler - Graz- Graz TUTU
Steps in Quantitative (Elemental) Analysis
Select the operating mode: Is it appropriate? CTEM, STEM..... Image Coupled, Diffraction Coupled
Obtain a typical spectrum Optimize the experimental conditions
Accelerating Voltage (maximum consistent with your specimen) Chooze the best Edges to analyze (K, L, M,....) Optimize β for the weakest edges (θE= δE/2Eo) Optimize α to minimize problems (α < β/2 ) Optimize the Acquisition Mode
Normal - High Concentrations Difference - Low Concentrations
Optimize the Data Acquisition Select Energy Resolution and Range DQE & Statistics
Process the Data to Extract Intensities Normal Difference Reference Spectra
Check the relative specimen thickness (t/λ< 1) Calculate the compositions
Absolute # of atoms Relative # of atoms Standards/Standardless?
ReCheck for Artifacts/Problems Diffraction Orientation/Channeling Unidentified Edges Spectral Overlaps Radiation Damage
Core Loss Electron Core Loss Electron SpectrosopySpectrosopy
Linear Linear dichroismdichroism::
The linear polarization of theincident electrons parallel to thedirection of momentum transfer.This polarization can be usedsense the anisotropy of thevalence states involved in thecore excitation process. It candetect the number or changes inthe number of the valence holesin different directions of theatomic volume.
In many cases, the anisotropy ofthe charge in the atomic volumeis caused by crystal-fieldinteraction and is due to ananisotropy in the bonding.
Linear Linear Dichroism Dichroism andand
Momentum Resolved EELS (MREELS) in Core Loss SpectroscopyMomentum Resolved EELS (MREELS) in Core Loss Spectroscopyderives its information from transitions from initial to final statesderives its information from transitions from initial to final states
K shell p-> s L shell p->s and p->dK shell p-> s L shell p->s and p->d
**
A Conventional EELS experiment in the TEM
Angular Resolution typically Angular Resolution typically 2-102-10 mRmR
Beam ConvergenceBeam Convergence2-10+2-10+ mR mR
0
0.2
0.4
0.6
0.8
1
1.2
270 280 290 300 310 320 330
.11
.35
.50
Rel
ativ
e In
tens
ity
Energy Loss (eV)
In some systems/conditions the difference isIn some systems/conditions the difference is subtlesubtleBut But ……. this also depends upon what your looking. this also depends upon what your looking for.for.
Variation of Amorphous Carbon NES with Collection Angle (ββ)
For Anisotropic Materials there can be large variations in the relative intensities of spectral featuresas a function of Angle (Momentum)
Variation of Graphite NES with Collection Angle (ββ)
0
1 104
2 104
3 104
4 104
5 104
6 104
7 104
260 280 300 320 340 360
A
0
2000
4000
6000
8000
1 104
1.2 104
260 280 300 320 340 360
A
B
0
1 104
2 104
3 104
4 104
5 104
260 280 300 320 340 360
A
0
5000
1 104
1.5 104
2 104
2.5 104
3 104
3.5 104
260 280 300 320 340 360
A
0
5000
1 104
1.5 104
2 104
2.5 104
260 280 300 320 340 360 0
200
400
600
800
1000
260 280 300 320 340 360A
0 0 mRmR 0.35 mr0.35 mr 0.7mr0.7mr
1.05 mR1.05 mR 1.73 mR1.73 mR 7 mR7 mR
θE ~1.15 mRα1/2 ~ 0.025 mR β1/2 ~ 0.1 mR
Variation of CK (π* σ*) in Graphite with Scattering Angle
Advanced TopicsAdvanced Topics
Low Loss SpectroscopyLow Loss SpectroscopyPlasmon Plasmon Losses StudiesLosses StudiesDielectric PropertiesDielectric Properties
Core Loss Spectroscopy Core Loss SpectroscopyNear Edge StructureNear Edge StructureExtended Fine StructureExtended Fine Structure
Radiation Damage Radiation Damage
StructureStructure in in EEL Spectra EEL Spectra is a manifestation of: is a manifestation of:Bonding Bonding (Chemistry/Physics(Chemistry/Physics) ) and/and/or Crystallographyor Crystallography ((Structure)Structure)
Low Loss RegimeLow Loss Regime
Optical, Dielectric,Optical, Dielectric,Electronic, MagneticElectronic, Magnetic
PropertiesProperties
Core Loss RegimeCore Loss Regime
Composition, Bonding,Composition, Bonding,Electronic, Magnetic ,Electronic, Magnetic ,Structural PropertiesStructural Properties
EELS Measurements of Valence Electron DensitiesEELS Measurements of Valence Electron Densities
Information in Core-Loss Profiles
ELNES
Copper L-shell Core Loss Spectroscopyin
Metallic, Oxide and High TcSuperconductor Phases
Magnetism in EELS is detectableusing the L shell transitions
p—>d
p —> s
• d electrons are dominant in determining magnetic properties• the L3/L2 "white lines" are the principle signals used to measure magnetism
x=0.2
x=0.3
x=0.4
x=0.5
x=0.6
x=1.0 Fe x Ge 1-x
Energy Loss (eV)640 680 720 760
AmorphousAmorphousFeFexxGeGe1-x1-x Magnetic Magnetic
MaterialsMaterials
700 705 710 715 720 725 730
FeL-RT-Norm
FeL-LN-Norm
Fe L
She
ll N
orm
aliz
ed In
tens
ity
Energy Loss (eV)
Summed Spectra 4 x 300 sec ~ 200KCnts at L3 Summed Spectra 4 x 300 sec ~ 200KCnts at L3 PkPkNormalized to LShell IntegralNormalized to LShell Integral
FeFe22OO3 3 (hematite)(hematite)
700 705 710 715 720 725 730
FeL-RT-Norm
FeL-RT0ffaxis-Norm
Fe L
She
ll No
rmal
ized
Inte
nsity
Energy Loss (eV)
Normalized to Normalized to LShell LShell IntegralIntegral
Momentum Resolved On-Axis vs Off-Axis
RutileRutile
Dc = 0Native State
Dc = 5Fully Amorphized State
Typical Specimenused in the EELS work
Anatase
Dc = 0
Anatase
Dc = 5
OKOKTiLTiL
Rutile Comparison Dc = 0 / 5