1. Electron Energy-Loss Spectrometry (EELS)
Charles Lyman
Lehigh University
Bethlehem, PA
Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School

2. 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 DE > 300 eV
L- and M- edges not very obvious for some elements
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

3. 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

4. Standard Instrument: Gatan PEELS
Below desk
Above desk
Object plane
Image plane
Magnetic prism “lens”
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

5. Spectrometer Collection Semiangle b
b 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
b is controlled by objective aperture
Diffraction Mode
b is controlled by EELS entrance aperture
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

6. 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: eV
Cold W FEG: eV
Monochromated FEG:
0.01 eV demonstrated
eV typical use
Approximately Gaussian zero-loss peak
Measure as width of the zero-loss peak
Zero-Loss Peak
200keV / 150pA
Cold-FEG
0.37eV
@FWHM
Field-emission
distribution
Data courtesy J. Hunt

7. The Two EELS Modes Image Mode Diffraction 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
Control b with spectrometer entrance aperture
Large aperture (high intensity) will degrade energy resolution
Small aperture (high energy resolution) will degrade signal intensity
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

8. 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

9. Zero-Loss Peak Zero-loss peak Elastically scattered electrons
Collected from either 000 or hkl
Measure energy energy resolution and energy spread of gun
~ 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

10. 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, lp ~100 nm
Multiple peaks for thick specimens
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

11. Thickness Measurements
Log ratio method
l 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
Rough estimate of l:
l ~ 0.8Eo nm
so for 100-keV electons
l is nm various materials
Very thin specimens:
t = lp(Ip/Io)
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

12. 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

13. Energy Levels and Energy-Loss Spectrum
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

14. Chart of Possible EELS Edges
from the Gatan EELS Atlas
from Ahn et al., EELS Atlas, Gatan and ASU HREM Facility, 1983

15. Edge Energy - Edge ShapeEc
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
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

16. L-Series Edges and White Lines
Each element has characteristic edge energy
Sharp white lines are present when d-band unfilled
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

17. 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

18. ELNES
N2 in air
N in boron nitride
from the Gatan EELS Atlas
Note significant detail near the on-set of the edge. ELNES detail is specific to the bonding environment.
from Ahn et al., TEELS Atlas, Gatan and ASU HREM Facility, 1983

19. 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)

20. Tetrahedral vs. Octahedral
Crysoberyl
Rhodizite
Calculation for Al octahedrally coordinated to O
Al L2,3
Si L2,3
from Garvie, Craven, and Brydson (1984)
from Brydson (1989)

21. 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)

22. Oxidation State L3/L2 ratiosa Chemical shiftb Fe 3.8±0.3 FeO 4.6
g-Fe2O3 5.8
a-Fe2O3 6.5
Chemical shiftb
Fe –> FeO 1.4±0.2 eV
(depends on peak stripping method)
from Colliex et al. (1991)
Colliex et al., Phys. Rev. B 44 (1991) 11,402-11,411
Leapman et al. Phys. Rev. B 26 (1982)

23. 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

24. EELS Quantification Single scattering in a very thin specimen assumed
For each element assume:
PK = the probability for ionization
sK = the ionization cross section
N = number of atoms per unit area
See Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Springer, 1996

25. Low-loss intensity ~ IT
EELS Quant Procedure
Fitted
background
Extracted
edge intensity
Low-loss intensity ~ IT
Collect spectrum with known collection angle b from a very thin specimen region
Calculate (Ib = A E-r over d = eV) and remove background under edge
Integrate edge intensity for a certain energy window D
Determine sensitivitiy factor called the “partial ionization cross section”
Courtesy J. Hunt

26. Microanalysis Example
Courtesy J. Hunt

27. Specimen Thicknesss Requirement
Microanalysis requires a very thin specimen
Estimate by:
Estimate thickness using:
Assuming lp ~ 100 nm:
t ≈ lp(Ip/Io) for very thin only
t ~ 10 nm for microanalysis
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

28. If Plural Scattering Occurs…
Deconvolute to get this
For quantitation of the ionization edge we need a true single scattering distribution
Plural scattering removed by a deconvolution procedure
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

29. Spatial Resolution EELS not affected by beam spreading like XEDS
Only electrons within 2b 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

30. Atomic Resolution EELS Analysis (S. Pennycook Group, ORNL)
Atomic-resolution Z-contrast STEM image of CaTiO3 doped with La
La M4,5 edges
La M4,5 edges only observed in spectrum collected directly from bright spot in image: single-atom resolution
M. Varela et al, Phys. Rev. Lett. 92 (2004)

31. 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

32. Summary What Can We Get from EELS?
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