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Lyman - EELS
Electron Energy-Loss
Spectrometry (EELS)Charles LymanLehigh UniversityBethlehem, PA
Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School
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EELS in TEM/STEM
Analyze energies of electrons transmitted through the specimen
Also called Analytical Electron Microscopy; really AEM includes EDS, CBED, EELS, CL, Auger, etc.
Advantages: » Spatial resolution in STEM ~ d, the electron
beam size» Detectability ~ 10x better than EDS » Any solid» Qualitative analysis of any element of Z > 1» Quantitative analysis by inner-shell ionization
edges of elements» Rich signal includes chemical information, etc.
Difficulties: » Need very thin specimen: t < 30 nm» Intensity weak for energy losses E > 300 eV» L- and M- edges not very obvious for some
elements from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Parallel-Collection EELS (PEELS)
Gatan PEELS» Under TEM viewing screen» Entrance aperture selects
electrons» Magnetic prism disperses
electrons by energy» Spectrum collected on a
cooled 1024-channel diode array
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Standard Instrument: Gatan PEELS
Below desk
Above desk
Object plane
Image plane
Magnetic prism “lens”
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Spectrometer Collection Semiangle
is the most important parameter for quantification» Semiangle subtended at the specimen by the entrance aperture of spectrometer » must know this angle» must keep constant for spectral comparisons
Image Mode is controlled by objective aperture
Diffraction Mode is controlled by EELS
entrance aperture€
≈deff
2
2θB
b
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Energy Resolution
Energy resolution is limited by the probe-energy distribution and spectrometer resolution
Probe energy resolution (depends on gun current)» W: 2-3 eV
» LaB6: >1 eV
» Warm FEG: 0.55-0.9 eV» Cold W FEG: 0.25-0.5 eV» Monochromated FEG:
– 0.01 eV demonstrated– 0.1-0.3 eV typical use– Approximately Gaussian zero-loss peak
0.37eV@FWHM
Zero-Loss Peak200keV / 150pA
Cold-FEG
Field-emissiondistribution
Data courtesy J. Hunt
Measure as width of the zero-loss peak
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The Two EELS Modes
Image Mode » Energy Resolution
– Without objective aperture, collect everything => ~ 100 mrad– Energy resolution is controlled by spectrometer entrance aperture (energy resolution is not
compromised)
» Spatial Selection– Position analysis area on optic axis, lift screen – Area selected is effective aperture size demagnified back to the specimen plane– Spatial resolution poor (10-30 nm)
Diffraction Mode» Energy Resolution
– Control with spectrometer entrance aperture– Large aperture (high intensity) will degrade energy resolution– Small aperture (high energy resolution) will degrade signal intensity
» Spatial Selection– Select area with STEM beam– Area selected is function of beam size and beam spreading
< 1 nm in FEG STEM at 0.5 nA ~ 10 nm in W electron gun at 0.5 nA
– Best for high spatial resolution microanalysis
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Three Spectral Regions
Zero-loss peak» No useful info, except FWHM» Super-intense
Low-loss region» 0-50 eV loss» Plasmons» Inter/intra band transition
Inner-shell ionizations» 30 eV loss and higher» Microanalysis» Very low intensity
Usually set energy range to 1000 eV loss
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Zero-Loss Peak
Elastically scattered electrons
Collected from either 000 or hkl
Measure energy energy resolution and energy spread of gun
» ~0.3-0.7 eV at best
Very intense » can overload and damage
photodiode array
Zero-loss peak
from Ahn et al., EELS Atlas, Gatan and ASU HREM Facility, 1983
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Low-Loss Region: Plasmons
Collective oscillations of weakly bound electrons
» Most prominent in free-electron metals
Analysis» Energy loss sensitive to changes in
free-electron density» Microanalysis of Al and Mg alloys
Thickness measurements» Plasmon mean-free-path,p ~100 nm
» Multiple peaks for thick specimens
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Thickness Measurements
Log ratio method
is total mean free path for all scattering
» IT is area under entire spectrum
» Io is area under zero-loss» Subtract background first for
best accuracy
€
t
λ= ln
IT
Io
⎛
⎝ ⎜
⎞
⎠ ⎟
Rough estimate of nmso for 100-keV electons is 80-120 nm various materials
Very thin specimens: t = p(Ip/Io)
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Inner-Shell Ionization Losses
Inner-shell electron ejected by beam electron
» We measure energy loss in beam electron after event
Ionization event occurs before emission of either x-ray or Auger electron emitted
» Get EELS signal regardless
Can observe “edges” for all inner-shell electrons
» K-shell electron (1s)» L-shell electron (2s or L1)
(2p or L2 , L3)
from Spence, in High Resolution Electron Microscopy, Buseck et al. (eds.),Oxford, 1987
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Energy Levels and Energy-Loss Spectrum
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Chart of Possible EELS Edges
from the Gatan EELS Atlas
from Ahn et al., EELS Atlas, Gatan and ASU HREM Facility, 1983
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Edge Energy - Edge Shape
K-edge» Ideal triangular “saw tooth”
sitting on background
Intensity decreases beyond edge
» Less chance of ionization above Ec since cross section decreases with increasing E Ec
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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L-Series Edges and White Lines
Each element has characteristic edge energy
Sharp white lines are present when d-band unfilled
White lines
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Edge Fine Structure
ELNES - electron loss near edge structure
» Sensitive to chemical bonding effects
» To ~ 50 eV beyond edge
EXELFS - extended energy-loss fine structure
» Analogous to EXAFS» Sensitive to atomic nearest
neighbors» Located beyond 50 eV for several
hundred eV
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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ELNES
Note significant detail near the on-set of the edge. ELNES detail is specific to the bonding environment.
N2 in air N in boron nitride
from the Gatan EELS Atlas
from Ahn et al., TEELS Atlas, Gatan and ASU HREM Facility, 1983
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Carbon ELNES
Carbon K-edges of minerals containing the carbonate anion compared with three forms of pure carbon
from Garvie, Craven, and Brydson, American Mineralogist, 79, (1984) 411-425
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Tetrahedral vs. Octahedral
from Garvie, Craven, and Brydson (1984) from Brydson (1989)
Si L2,3
Crysoberyl
Rhodizite
Calculation for Al octahedrally coordinated to O
Al L2,3
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Fe L2,3 Edge in Minerals
Chemical shift Shape change
Almandine
Hedenbergite
Hercynite
Fe “orthoclase”
Brownmillerite
andradite
Van Aken and Liebscher, Phys Chem Minerals 29 (2002) 188-200
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Oxidation State
L3/L2 ratiosa
» Fe 3.8±0.3» FeO 4.6
» Fe3O4 5.2
» -Fe2O3 5.8
» -Fe2O3 6.5
Chemical shiftb
» Fe –> FeO 1.4±0.2 eV
a. Colliex et al., Phys. Rev. B 44 (1991) 11,402-11,411b. Leapman et al. Phys. Rev. B 26 (1982) 614-635
from Colliex et al. (1991)
(depends on peak stripping method)
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Qualitative Microanalysis
Discrimination of TiC and TiN in alloy steel Aluminum extraction replica
60 eV - EDS cannot resolve
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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EELS Quantification
Single scattering in a very thin specimen assumed For each element assume:
PK = the probability for ionization
K = the ionization cross section
N = number of atoms per unit area
€
IK = PK IT
PK = Nσ K expt
λ K
⎛
⎝ ⎜
⎞
⎠ ⎟
IK ≈ Nσ K IT (very thin specimen, t ≈ 0)
N =IK
σ K IT
for a single element when IT is known
Not collecting all the electrons so we must use IK (β,Δ) and σ K (β,Δ)
where σ K (β,Δ) = partial ionization cross - section
See Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Springer, 1996
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Courtesy J. Hunt
Fittedbackground
Extractededge intensity
Low-loss intensity
~ IT
€
€
NA =IA β ,Δ( )
σ A β,Δ( )IT
or NA
NB
=IK
A β,Δ( )IK
B β,Δ( )
σ KB β,Δ( )
σ KA β,Δ( )
€
IA β,Δ( )
€
IB β,Δ( )
EELS Quant Procedure
Collect spectrum with known collection angle from a very thin specimen region
Calculate (Ib = A E-r over = 20-50 eV) and remove background under edge
Integrate edge intensity for a certain energy window
Determine sensitivitiy factor called the “partial ionization cross section”
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Microanalysis Example
Courtesy J. Hunt
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Specimen Thicknesss Requirement
Microanalysis requires a very thin specimen
» Estimate by:
» Estimate thickness using:
» Assuming p ~ 100 nm:
€
Ip
Io
≤1
10
t ≈ p(Ip/Io) for very thin only
t ~ 10 nm for microanalysis
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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If Plural Scattering Occurs…
For quantitation of the ionization edge we need a true single scattering distribution
Deconvolute to get this
Plural scattering removed by a deconvolution procedure
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Spatial Resolution
EELS not affected by beam spreading like XEDS
» Only electrons within 2 are collected
STEM mode» Beam size governs spatial
resolution TEM mode
» Selection apertures govern spatial resolution
» Lens aberrations will limit
Delocalization» Ionization by a “nearby” fast
electron» Small effect: 2-5 nm
EELS ionization loss spectra have been
obtained from single columns of atoms from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
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Atomic Resolution EELS Analysis (S. Pennycook Group, ORNL)
Atomic-resolution Z-contrast STEM image of CaTiO3 doped with La
M. Varela et al, Phys. Rev. Lett. 92 (2004) 095502
La M4,5 edges
La M4,5 edges only observed in spectrum collected directly from bright spot in image: single-atom resolution
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Strategy for Analysis of Unknown Phases
Start with light microscopy, SEM, powder x-ray diffraction (XRD), the library
» Straightforward interpretation (usually helps TEM analysis)» Less expensive» Far more time may be needed to prepare a suitable thin specimen
Use at least two analysis methods» EDS and CBED (powerful when used together)
– Determine the elements present (EDS)– Determine the phases present (CBED)– All electron transparent specimens– Keep the ICDD PDF handy to identify d-values
» EELS and HREM (structure images) – Determine the elements present (EELS)– Obtain d-values of the phases (HREM)– Only very thin specimens
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Summary
Microanalysis by ionization-loss edges» Light element analysis complements XES
Specimen thickness measurements» Complements XES when absorption correction needed
Bonding information from near-edge fine structure (ELNES)» Fingerprints of edge shape
– Reveal metal oxides, sulfides, carbides, nitrides, etc.» Chemical shifts
– L3/L2 ratio can reveal a change in oxidation state
» Use known standards for comparison, e.g., Fe, FeO, Fe2O3, Fe304
Interatomic distances from extended energy-loss fine structure (EXELFS)
» Information similar to EXAFS, but from nano-sized region rather than the bulk
What Can We Get from EELS?
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