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 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

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 Below desk Above desk Object plane Image plane Magnetic prism “lens” from Williams and Carter, Transmission Electron Microscopy, Springer, 1996

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

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 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

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

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 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 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 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 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

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

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 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) 614-635

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

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

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 = 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 Courtesy J. Hunt

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… 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 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) 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) 095502

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 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