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1PASI - Electron Microscopy - Chile

Lyman - EELS

Electron Energy-Loss

Spectrometry (EELS)Charles LymanLehigh UniversityBethlehem, PA

Based on presentations developed for Lehigh University semester courses and for the Lehigh Microscopy School


Lyman - EELS

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 E > 300 eV» L- and M- edges not very obvious for some

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


Lyman - EELS

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


Lyman - EELS

Standard Instrument: Gatan PEELS

Below desk

Above desk

Object plane

Image plane

Magnetic prism “lens”



Lyman - EELS

Spectrometer Collection Semiangle

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 is controlled by objective aperture

Diffraction Mode is controlled by EELS

entrance aperture€

≈deff

2

2θB

b



Lyman - EELS

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

0.37eV@FWHM

Zero-Loss Peak200keV / 150pA

Cold-FEG

Field-emissiondistribution

Data courtesy J. Hunt

Measure as width of the zero-loss peak


Lyman - EELS

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 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


Lyman - EELS

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



Lyman - EELS

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


Lyman - EELS

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,p ~100 nm

» Multiple peaks for thick specimens



Lyman - EELS

Thickness Measurements

Log ratio method

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

€

t

λ= ln

IT

Io

⎛

⎝ ⎜

⎞

⎠ ⎟

Rough estimate of nmso for 100-keV electons is 80-120 nm various materials

Very thin specimens: t = p(Ip/Io)



Lyman - EELS

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


Lyman - EELS

Energy Levels and Energy-Loss Spectrum



Lyman - EELS

Chart of Possible EELS Edges

from the Gatan EELS Atlas

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


Lyman - EELS

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 Ec



Lyman - EELS

L-Series Edges and White Lines

Each element has characteristic edge energy

Sharp white lines are present when d-band unfilled

White lines



Lyman - EELS

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



Lyman - EELS

ELNES

Note significant detail near the on-set of the edge. ELNES detail is specific to the bonding environment.

N2 in air N in boron nitride

from the Gatan EELS Atlas

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


Lyman - EELS

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


Lyman - EELS

Tetrahedral vs. Octahedral

from Garvie, Craven, and Brydson (1984) from Brydson (1989)

Si L2,3

Crysoberyl

Rhodizite

Calculation for Al octahedrally coordinated to O

Al L2,3


Lyman - EELS

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


Lyman - EELS

Oxidation State

L3/L2 ratiosa

» Fe 3.8±0.3» FeO 4.6

» Fe3O4 5.2

» -Fe2O3 5.8

» -Fe2O3 6.5

Chemical shiftb

» Fe –> FeO 1.4±0.2 eV

a. Colliex et al., Phys. Rev. B 44 (1991) 11,402-11,411b. Leapman et al. Phys. Rev. B 26 (1982) 614-635

from Colliex et al. (1991)

(depends on peak stripping method)


Lyman - EELS

Qualitative Microanalysis

Discrimination of TiC and TiN in alloy steel Aluminum extraction replica

60 eV - EDS cannot resolve



Lyman - EELS

EELS Quantification

Single scattering in a very thin specimen assumed For each element assume:

PK = the probability for ionization

K = the ionization cross section

N = number of atoms per unit area

€

IK = PK IT

PK = Nσ K expt

λ K

⎛

⎝ ⎜

⎞

⎠ ⎟

IK ≈ Nσ K IT (very thin specimen, t ≈ 0)

N =IK

σ K IT

for a single element when IT is known

Not collecting all the electrons so we must use IK (β,Δ) and σ K (β,Δ)

where σ K (β,Δ) = partial ionization cross - section

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


Lyman - EELS

Courtesy J. Hunt

Fittedbackground

Extractededge intensity

Low-loss intensity

~ IT

€

€

NA =IA β ,Δ( )

σ A β,Δ( )IT

or NA

NB

=IK

A β,Δ( )IK

B β,Δ( )

σ KB β,Δ( )

σ KA β,Δ( )

€

IA β,Δ( )

€

IB β,Δ( )

EELS Quant Procedure

Collect spectrum with known collection angle from a very thin specimen region

Calculate (Ib = A E-r over = 20-50 eV) and remove background under edge

Integrate edge intensity for a certain energy window

Determine sensitivitiy factor called the “partial ionization cross section”


Lyman - EELS

Microanalysis Example

Courtesy J. Hunt


Lyman - EELS

Specimen Thicknesss Requirement

Microanalysis requires a very thin specimen

» Estimate by:

» Estimate thickness using:

» Assuming p ~ 100 nm:

€

Ip

Io

≤1

10

t ≈ p(Ip/Io) for very thin only

t ~ 10 nm for microanalysis



Lyman - EELS

If Plural Scattering Occurs…

For quantitation of the ionization edge we need a true single scattering distribution

Deconvolute to get this

Plural scattering removed by a deconvolution procedure



Lyman - EELS

Spatial Resolution

EELS not affected by beam spreading like XEDS

» Only electrons within 2 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


Lyman - EELS

Atomic Resolution EELS Analysis (S. Pennycook Group, ORNL)

Atomic-resolution Z-contrast STEM image of CaTiO3 doped with La

M. Varela et al, Phys. Rev. Lett. 92 (2004) 095502

La M4,5 edges

La M4,5 edges only observed in spectrum collected directly from bright spot in image: single-atom resolution


Lyman - EELS

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


Lyman - EELS

Summary

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

What Can We Get from EELS?

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