Electron Energy-Loss Spectrometry (EELS)

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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
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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 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
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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
from Williams and Carter, Transmission Electron
Microscopy, Springer, 1996
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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
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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 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
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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

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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, lp 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
• 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
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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

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

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
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
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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
Si L2,3
from Garvie, Craven, and Brydson (1984)
from Brydson (1989)
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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 3.80.3
• 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

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Qualitative Microanalysis

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

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
• sK the ionization cross section
• N number of atoms per unit area

See Egerton, Electron Energy-Loss Spectroscopy in
the Electron Microscope, Springer, 1996
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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
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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 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
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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
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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
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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
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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
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