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Electron Energy-Loss Spectrometry (EELS)

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Title: Electron Energy-Loss Spectrometry (EELS)


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 gt 1
  • Quantitative analysis by inner-shell ionization
    edges of elements
  • Rich signal includes chemical information, etc.
  • Difficulties
  • Need very thin specimen t lt 30 nm
  • Intensity weak for energy losses DE gt 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
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 gt1 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

Measure as width of the zero-loss peak
Zero-Loss Peak 200keV / 150pA Cold-FEG
0.37eV _at_FWHM
Field-emission distribution
Data courtesy J. Hunt
7
The Two EELS Modes
  • Image Mode
  • Energy Resolution
  • Without objective aperture, collect everything gt
    ? 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 b 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
  • lt 1 nm in FEG STEM at 0.5 nA
  • 10 nm in W electron gun at 0.5 nA

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
  • 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
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 80-120 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 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

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
N in boron nitride
N2 in air
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) 411-425
20
Tetrahedral vs. Octahedral
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) 188-200
22
Oxidation State
  • L3/L2 ratiosa
  • Fe 3.80.3
  • FeO 4.6
  • Fe3O4 5.2
  • g-Fe2O3 5.8
  • a-Fe2O3 6.5
  • Chemical shiftb
  • Fe gt FeO 1.40.2 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) 614-635

23
Qualitative Microanalysis
  • Discrimination of TiC and TiN in alloy steel
  • Aluminum extraction replica

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
EELS Quant Procedure
  • Collect spectrum with known collection angle b
    from a very thin specimen region
  • Calculate (Ib A E-r over d 20-50 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 only observed in spectrum
collected directly from bright spot in image
single-atom resolution
M. Varela et al, Phys. Rev. Lett. 92 (2004) 095502
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
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