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. EELS in TEM/STEM. Analyze energies of electrons transmitted through the specimen - PowerPoint PPT Presentation

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Introduction to Electron DiffractionLyman - EELS
Electron Energy-Loss
Spectrometry (EELS)
Charles Lyman
Lehigh University
Bethlehem, PA
*
Analyze energies of electrons transmitted through the specimen
Also called Analytical Electron Microscopy; really AEM includes EDS, CBED, EELS, CL, Auger, etc.
Advantages:
Detectability ~ 10x better than EDS
Any solid
Rich signal includes chemical information, etc.
Difficulties:
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
*
Spectrum collected on a cooled 1024-channel diode array
Parallel-Collection EELS (PEELS)
*
Below desk
Above desk
Object plane
Image plane
b is the most important parameter for quantification
Semiangle subtended at the specimen by the entrance aperture of spectrometer
must know this angle
Image Mode
Diffraction Mode
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
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
Monochromated FEG:
*
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
Large aperture (high intensity) will degrade energy resolution
Small aperture (high energy resolution) will degrade signal intensity
Spatial Selection
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
*
Super-intense
Microanalysis
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
Measure energy energy resolution and energy spread of gun
~0.3-0.7 eV at best
Zero-loss peak
*
Most prominent in free-electron metals
Analysis
Microanalysis of Al and Mg alloys
Thickness measurements
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
IT is area under entire spectrum
Io is area under zero-loss
Subtract background first for best accuracy
Rough estimate of l:
Very thin specimens:
*
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
K-shell electron (1s)
*
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
Chart of Possible EELS Edges
from the Gatan EELS Atlas
*
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
Ec
Each element has characteristic edge energy
Sharp white lines are present when d-band unfilled
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
White lines
Sensitive to chemical bonding effects
To ~ 50 eV beyond edge
EXELFS - extended energy-loss fine structure
Analogous to EXAFS
Located beyond 50 eV for several hundred eV
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
ELNES
Note significant detail near the on-set of the edge. ELNES detail is specific to the bonding environment.
N2 in air
from the Gatan EELS Atlas
*
Lyman - EELS
Carbon ELNES
Carbon K-edges of minerals containing the carbonate anion compared with three forms of pure carbon
*
from Brydson (1989)
Al L2,3
Chemical shift
Shape change
*
Colliex et al., Phys. Rev. B 44 (1991) 11,402-11,411
Leapman et al. Phys. Rev. B 26 (1982) 614-635
from Colliex et al. (1991)
(depends on peak stripping method)
*
Aluminum extraction replica
60 eV - EDS cannot resolve
*
For each element assume:
N = number of atoms per unit area
See Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Springer, 1996
*
EELS Quant Procedure
Courtesy J. Hunt
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”
Fitted
background
Extracted
Estimate by:
t ~ 10 nm for microanalysis
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
If Plural Scattering Occurs…
For quantitation of the ionization edge we need a true single scattering distribution
Deconvolute to get this
from Williams and Carter, Transmission Electron Microscopy, Springer, 1996
*
Only electrons within 2b are collected
STEM mode
TEM mode
Lens aberrations will limit
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
*
M. Varela et al, Phys. Rev. Lett. 92 (2004) 095502
La M4,5 edges only observed in spectrum collected directly from bright spot in image: single-atom resolution
La M4,5 edges
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
EELS and HREM (structure images)
Determine the elements present (EELS)
Obtain d-values of the phases (HREM)
Only very thin specimens
Specimen thickness measurements
Bonding information from near-edge fine structure (ELNES)
Fingerprints of edge shape
Chemical shifts
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
What Can We Get from EELS?

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