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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. EELS in TEM/STEM. Analyze energies of electrons transmitted through the specimen

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electron energy loss spectrometry eels
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

eels in tem stem
EELS in TEM/STEM
  • Analyze energies of electrons transmitted through the specimen
  • Also calledAnalytical 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

slide3

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

standard instrument gatan peels

Object plane

Above desk

Below desk

Image plane

Magnetic prism “lens”

Standard Instrument: Gatan PEELS

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

spectrometer collection semiangle b
Spectrometer Collection Semiangle b
  • bis 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

energy resolution
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

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

the two eels modes
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 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
        • < 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
three spectral regions
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

zero loss peak
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

low loss region plasmons
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

thickness measurements
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

inner shell ionization losses
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

energy levels and energy loss spectrum
Energy Levels and Energy-Loss Spectrum

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

chart of possible eels edges
Chart of Possible EELS Edges

from the Gatan EELS Atlas

from Ahn et al., EELS Atlas, Gatan and ASU HREM Facility, 1983

edge energy edge shape

Ec

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

l series edges and white lines

White lines

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

edge fine structure
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

elnes
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

carbon elnes
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

tetrahedral vs octahedral

Al L2,3

Crysoberyl

Rhodizite

Calculation for Al octahedrally coordinated to O

Tetrahedral vs. Octahedral

Si L2,3

from Garvie, Craven, and Brydson (1984)

from Brydson (1989)

fe l 2 3 edge in minerals
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

oxidation state
Oxidation State
  • L3/L2 ratiosa
    • Fe 3.8±0.3
    • FeO 4.6
    • Fe3O4 5.2
    • 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) 614-635

qualitative microanalysis

60 eV - EDS cannot resolve

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

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

eels quantification
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

eels quant procedure

Extracted

edge intensity

Low-loss intensity ~ IT

Fitted

background

EELS Quant Procedure
  • Collect spectrum with known collection angle bfrom 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

microanalysis example
Microanalysis Example

Courtesy J. Hunt

specimen thicknesss requirement
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

if plural scattering occurs
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

spatial resolution
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

atomic resolution eels analysis s pennycook group ornl

La M4,5 edges

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

strategy for analysis of unknown phases
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
summary
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