Lecture 4 - Spectroscopy

Analytical Electron Microscopy (AEM), Energy Dispersive Spectroscopy (EDS),

Electron Energy Loss Spectroscopy (EELS),,

EDS-EELS Spectrum Imaging,

Energy Filtered TEM (EFTEM)

What we are mostly interested in measuring by EELS in the TEM is inelastic electron scattering

Elastic scattering

Phonon scattering (few meV)Quasi-elasticThermal diffuse scattering

Plasmon excitation (10-30 eV)Collective excitation ofconduction electronsValence electron excitation

Inner-shell ionizationCore lossesAbsorption edgesMost useful for:CompositionBonding

Gatan parallel-collection electron energy-loss spectrometer (PEELS)Attaches to base of camera/viewing chamber of TEMAdditional ports for scintillator and PMT for on-axis STEM detector

Gatan electron energy-loss spectrometer (EELS)Old-style serial-collection; newer parallel-collection (PEELS)Latest PEELS (Enfina) uses a CCD detector instead of a photodiode array

Gatan parallel-collection electron energy-loss spectrometer (PEELS)curved pole piece entrance/exit faces; double-focusing 90°

magnetic prism

Information from energy-loss spectrum

Nomenclature for inner-shell ionization edges

L3 and L2 white lines for 3d transition metalsTransitions from 2p to unfilled 3d statesSimilar M5 and M4 white lines for Lanthanides (unfilled 4f states)

Anatomy of an electron energy-loss spectrum(TiC, 100kV, = 4.7 mrad, = 2.7 mrad, t/ = 0.52)from Disko in Disko, Ahn, Fultz, Transmission EELS in Materials Science, TMS, Warrendale PA, 1992

I0 zero loss or elastic peaklow-loss region <40eV, dominated by bulk plasmon at 23.5 eVCarbon K edge, 285 eV, 1s shell electron excitedTitanium L23 edge, 455 eV, 2p shell

Low-loss regionPlasmons and thickness determination

Plasmons are collective excitations of valence electronsLifetime 10-15 s, localized to <10 nmEp = hp/2 = h/2 (ne2/0m)0.5 h is Planck’s constant, n is free-electron density,e and m are electron charge and mass, 0 is permittivity of free spaceCharacteristic scattering angles are small <1 mrad

Thickness (t) determination:t/ = ln (IT/I0) is inelastic scattering mean free path (average distance between scattering events)and is inversely proportional to scattering cross sectionIT is total intensity, I0 is zero-loss intensity

(nm) = 106 F (E0/Em) / ln (2 E0/Em)E0 is in kV, in mrad, F is a relativistic correction factor ~1 for E0 < 300 kV,Em is the average energy loss in eVEm = 7.6 Z0.36 where Z is average atomic numberF = {1 + (E0/1022)} / {1 + (E0/511)2}Java script at Nestor’s web site http://tpm.amc.anl.gov/NJZTools/NJZTools.html

Quantitative microanalysis with core-loss EELS(TiC spectrum from Disko)Isolation of core-loss intensities that scale with atomic concentrationsAtomic fractions or atoms/area with use of atomic scattering cross sections

Least squares fit of form AE-r to model background ~50 eV beforeeach edge

Extrapolated to higher energy losses

Integrated counts above extrapolatedbackground give shaded core-edgeintensities in energy windows width

IC() = NC C(E0,) I()NC carbon atoms/areaIL() = total spectrum intensity up to an energy loss C = partial ionization cross section at incident beam energy E0 up to amaximum scattering angle (collection semiangle)

No need for I0() if use element ratios:NC/NTi = {IC() / ITi()} {Ti(E0,) / C (E0,)}

Quantitative microanalysis with core-loss EELSSelection and measurement of acquisition parameters

The sample must be thin !Typically t/0.3 to 0.5

The collection angle should be set to an appropriate value wrt the characteristic scattering angle = E/2E0

(E0 should be relativistic)

Typically ~ few times (few mrad)If is too large:S/B decreases (just extra background)Include diffracted beams But should be larger than the incident beam convergence semi-angle Joy proposed a correction to reduce ().Reduction factor R = [ln{1+(/)2} ] / [ln{1+(/)2} ]

Background fitting

Background comes from tails of (multiple) plasmons and core edges at lowerenergy losses (especially outer-shells)Inverse power law, IB = AE-r

Least squares fit to ln(I) versus ln(E)A and r valid over a limited energy ranger is typically between 2 and 5, and decreases for increases in t, , and E

For E < ~200 eV, AE-r commonly fails to give a good fit and extrapolationWorking with 3dTM borides in the early 80’s we developed the log-polybackground fitting for B. It is the most useful and reliable alternative to AE-r.Polynomials do not extrapolate sensibly - do not use them.Usually a quadratic, sometimes a third order polynomial, will cope with the small curvature in ln(I) versus ln(E)

Mike Kundmann wrote a log-poly function for Gatan’s EL/P software.JK Weiss included it in ESVision as nth order power law fit (select n).

Cross sectionsCalculated (Egerton, Rez), parameterized (Joy)Measured from standards, similar to k-factors in EDS

Egerton’s SIGMAK and SIGMAL used in EL/P(Fortran) code listed in Egerton’s bookHydrogenic model but works wellA white-line correction is also selectable inEL/P, but best to define beyond WLs

EL/P v3 also has Rez’s Hartree-Slater models(includes M edges)

Plural scatteringSpectral components near core edge for “real” spectrum

1 detector noise and spuriousscattering in the spectrometer

2 single scattering tails of valenceOr lower-energy core excitations

3 plural inelastic scattering involving(2) Combined with one or more“plasmon” excitations

4 single scattering core edge intensity

5 plural inelastic scattering involvingCore excitation combined with one orMore “plasmon” excitations

If component 1 is small, AE-r inverse power law background fitting still works

Plural scattering - effect of increasing thicknessBN at 100 kV (Leapman in Disko et al)S/B for boron decreases by a factor of 15

Energy-loss near-edge structure (ELNES) indicative of empty(unfilled) density of states (DOS)

Additional examples of ELNES

Radial distribution functions by EXELFS analysisExtended energy-loss fine structure (cf EXAFS extended x-ray absorption fine structure)EXELFS good for low-Z major constituents at high spatial resolution(EXAFS advantageous for higher-Z and low concentrations)

Spectrum images and linesA complete spectrum acquired and stored for each pixel in an image terminology from Jeanguillaume and Colliexspectral images/profiles also used

Acquisition of spectra one at a time is still useful in many investigations, e.g. phaseIdentification, in-situ changes in composition or bonding

For composition gradients, repetitively re-positioning a small probe manually to measure spectra is inaccurate, inefficient and time consuming

Modern integrated acquisition systems are available to automate the set-up, acquisition, and processing of spectral series

Post processing with user interaction is usual, but can be done on-the-fly (e.g., to create elemental maps)

Gatan - EELS only but have tried simultaneous EDSEmispec Vision (Cynapse), TIA on FEI Tecnai - multiple simultaneous spectroscopies, including EELS and EDS (any manufacturer) - less comprehensive processing for EELS

Spectrum imaging in STEM - Philips CM200FEG with Emispec VisionSimultaneous EDS and EELS (with GIF)Co-Cr-Pt-B developmental media

DF STEM B K Cr L23 Co L23

Cr K Co KPt M0-20 keV

1 nA in 1.6 nm probe 1 s dwell/pixel 64 x 64 pixels All elements accessible

with combined EDS and EELS

Clear intergranular boron segregation

Log-polynomial background fitting insufficient for reliable boron intensities

Compositions from ratios of maps with k factors

Typical EDX spectrum from a B-poor region in Sample 4965B

Elemental Composition

Edge Intensity k-factor Weight% Atomic%

Cr Ka 29 1.308* 11.68% 17.93%

Co Ka 106 1.493* 48.60% 65.82%

Pt La 25 5.279* 39.72% 16.25%

Calculated k-factor for Pt-La may be suspect.

No k-factor for Pt Ma available.

Typical EDX spectrum from a B-rich region in Sample 4965B Similar Pt, higher Cr, and lower Co compared to B-poor region

Elemental Composition

Edge Intensity k-factor Weight% Atomic%

Cr Ka 33 1.308* 19.33% 29.75%

Co Ka 58 1.493* 39.22% 53.25%

Pt La 17 5.279* 41.45% 17.00%

B-rich PEELS (from single pixel in Emspec Vision, transferred to Gatan EL/P)Low S/B for B, C is largest peak (overcoat + contamination?), O-K in front of Cr L23, low Co L23

B

C

OCr

Co

Green curve is x16

B-rich PEELS background fit, regular AE-r (log-poly same)Shape of B edge as expected Quant: B:Co = 0.37 +/- 0.06

Purple curve is x8

Normalized CompositionCo Cr Pt B44 25 14 17 at%

Spectrum Lines of Soft Magnetic Multilayers

50 nm

Energy (keV)

Co

un

ts

151050

400

300

200

100

0Ta

Ta

Mn

Mn

MnFe

Fe

Ir

Ir

IrIr

Ir

Ir

Position (nm)

Co

un

ts

302520151050

2500

2000

1500

1000

500

0

Ir Lb

Ta La

Fe Kb

Mn Ka

Nine Layer FeTaN/IrMn

Energy (keV)

Co

un

ts

151050

400

300

200

100

0

Fe

Fe

Fe

Mn

Mn

Mn

Ta

Ta

Ir

Ir

Ir

Ir

Ir

Overlapping EDS Peaks Makes Quantification Difficult

Combining EDS and EELS Allows for Fe, Ta, N, Ir, and Mn Quantification

-0.10

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

-10.00 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00

distance nm

atom

frac

tion

Ta

Ir

N

Fe

Mn

Two types of imaging filter for EFTEMIn-column omega (or variants), used by Leo (Zeiss) and JEOLPost-column Gatan imaging filter (GIF)

Gatan imaging filter

Basic EFTEM operationImages or diffraction patternsGIF magnification ~19, so MSC chip 10242 24um pixels equivalent to ~1mm on TEM screen

Select pass band of energies (energy-losses) with slits.Lower slit edge is fixed, upper slit edge is movable to adjust slit width.

Instead of displacing the slit or changing the magnet current to select different energy losses, the accelerating voltage is increased by the energy loss desired. The initial nominal accelerating voltage is reduced by 3kV to avoid exceeding the manufacturers specifications.

Electrons are always the same energy after passing through sample.No chromatic shifts or changes in magnification.Do not have to change the excitation of the imaging lenses or imaging filter multipoles.The probe-forming (condenser) lenses must track with the accelerating voltage to keep illumination “constant;” in practice there is a lot of hysteresis.

OKCrL23 CoL23

1 2 3

Co Jump ratioCo Jump ratio Co Map

Co Pre-edge 1 Co Pre-edge 2 Co Post-edge

100 nm

Co80Cr16Ta4 Generation of Co map and jump ratio imagesMap: subtraction from post-edge image AE-r extrapolation of pre-edge 1 and 2Jump ratio: post-edge image divided by pre-edge image

OK

CrL23 CoL23

1 2 3 4

O pre-edge 1 O pre-edge 2 O post-edge Cr post-edge

100 nm

Cr mapCr jump ratioCr jump ratio

Generation of Cr map and jump ratio imagesMap: 4-window (DM custom script) to account for O edge from surface oxide(AE-r fit: Two O pre-edge images define exponent r, O post-edge to define A)Jump ratio: Cr post-edge image divided by O post-edge image

Quantitative compositions from EFTEM elemental map ratiosCompensates for diffraction contrast, and variations in thickness and illuminationUse k-factors or calculated cross sections to convert to concentration ratios

=Cr map

Co map

Cr/Co map ratio

100 nm

2.0 nm

1.3 nm

1.5 nm

N signal O signal low loss

100 nm

Si3N4-SiCw composite sintered with Y2O3 and Al2O3 Composition differences between intergranular films and pockets~30 at.% N in intergranular films (<5% N in triple-point pockets)S/N for oxygen suggests fractional monolayer detectability

100 nm

Fe M jump ratio Ti L jump ratio

MA 12YWT (Fe-12%Cr-3%W-0.4%Ti-0.25%Y2O3) ferritic steel crept at 800°CEFTEM Fe-M and Ti-L jump ratio images reveal nano-clusterst/ = 0.22 (31nm), cluster concentration = 2.5 x 1023 m-3 (c.f. APT 1 x 1024 m-3)

More difficult to perform and interpret than EDS Plasmons - not elementally specific but large signal Core losses - integrated intensities yield compositions - not all elements have

edges that can be used in practice ELNES - information on chemistry - bonding and valence EXELFS - radial distribution functions for low-Z major constituents EFTEM - quantitative elemental mapping at 1 nm resolution

“EELS in the Electron Microscope,” R F Egerton, Plenum 1986, 1995 “Transmission EELS in Materials Science,” M M Disko et al eds, TMS 1992

(second edition in preparation, Cambridge University Press) Gatan EELS software (EL/P for Mac now obsolete) “EELS Atlas,” C C Ahn and O L Krivanek, Gatan Inc and ASU 1983

Electron energy-loss spectroscopy (EELS) andenergy-filtered transmission electron microscopy

(EFTEM)Summary and resources