Electron Energy Loss Spectrometry (EELS)Electron Energy Loss Spectrometry (EELS)

Inelastic scattering causes loss of the energy of electrons Electron-electron interactions Loss in Energy + Change in Momentum Energy loss electrons leads to higher chromatic aberration Thin specimen required EELS spectrometer has a very high energy resolution

[(FEG ~ 0.3 eV), (XEDS → resolution ~ 100eV)]Note: beam energy can be 400 kV

Can be used in forming energy filtered images + diffraction patterns

Electron Energy Loss Spectrometry (EELS)Electron Energy Loss Spectrometry (EELS)

Can be used in forming energy filtered images + diffraction patterns Not just elemental information (as in XEDS) but

Chemical information like bonding Material parameters like dielectric constant

Beam Interaction with the specimen

Spatial and Angulardistribution of electrons

CoherencyEnergy distribution

Changes

Spectrometers

Omega Filter (LEO- formerly Zeiss)

Magnetic Prism (Gatan)

EELS

Inter of intra band transitions

Plasmon excitation

Phonon excitation

Inner shell ionization

Part of the zero loss peak. Not resolved in an EELS spectrum Causes specimen to heat up. (~ 0.2 eV, 5-15 mrad)

(5-25 eV, 5-10 mrad)

(~5-25 eV, < 0.1 mrad)

(~30-1000 eV, 1-5 mrad)

Bulk plasmon

Surface plasmon

λp ~ 100nm

• Transverse waves• Half the energy of bulk plasmons

Signature of the structure

• Longitudinal waves

Low loss region (< 50 eV)

High loss region (> 50 eV)

Elemental information

COLLECTIVE OSCILLATIONS

PLASMONS PHONONS

Collective oscillations of free electrons Most common inelastic interaction Damped out in < 1015 s Localized to < 10 nm Predominant in metals (high free electron density) Angles → < 0.1 mrad

Collective oscillations of atoms Can be generated by other inelastic

processes. (Auger / X-ray energy) Will heat up the specimen Small energy loss < 0.1 eV Phonon scattered electrons to large angle (5 – 15 mrads) Diffuse background

Plasmon excitation

Longitudinal wave like oscillations of weakly bound electrons Rapidly damped (lifetime ~1015 sec, localized to < 10 nm) Dominate in materials with free electrons (n) (Li, Na, Mg, Al )

But occur in all materials to some extent or other

EP = f(n) Microanalytical information

Carry contrast formation, limit image resolution through chromatic aberration Can be removed by energy filtering

122

02 2Plasmons P

h h neE

m

EPlasmon → Energy lost by the electron beam when it generates a plasmon

h → Planck’s constant m → Mass of electron n → Free electron density (sometimes plasmon peaks are observed from materials without free electrons)

P → Frequency of the plasmon generated

Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

Plasmon PeaksAl specimen

Thin sample Thick sample

Inter- and Intra Band Transitions

Excitations in outermost orbital → delocalized, interatomic bonding→ reflects the solid state character of the sample

Change in orbital state of the core electron Interactions with molecular orbital → can be used for finger printing (by storing

low loss spectra of known specimens in a database) Secondary electron emission (< 20eV)

(hence in same energy-loss regime as band transitions) Weakly bound outer-shell electrons control the reaction of a material to an

external field → controls the dielectric response (can measure with the signalfrom the < ~ 10eV region)

Incident High-kV Beam

Direct Beam(Energy loss electrons)

SPECIMEN

Secondary Electrons(E < 20eV)

The electrons causing SE emission appear in the loss regime

Low loss spectrum from Al and Al compounds→ The differences in the spectra are due to differences in bonding

Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

Energy-loss (eV) →

Spe

ctra

ver

tica

lly

disp

lace

d fo

r ea

sy v

isib

ilit

y →

Inter- and Intra Band Transitions

Inner shell ionization

The other side of the coin of XEDS. Small cross section (ΔE ↑ cross section ↓) (Ionization edge intensity much

smaller than the plasmon peak)

K , L (L1 , L2 , L3) , M (M1 , M2 , M3 , M4;5).

Combination of ionization loss with plasmon loss can occur.

There are intensity variations superimposed on the ionization edge

ELNES (Energy Loss Near Edge Structure)

(starting from about 30 eV of the edge and extending ~100s of eV)

EXELFS (Extended Energy Loss Fine Structure)

(~50 eV after ELNES)

ELNES and EXELFS arise due to the ionization process imparting more than

critical energy (Ec) for ionization

Li → 50 eV to ionize K shell electron (Z ↑ Eionization ↑)

U → 99 KeV to ionize K shell electron → Use L, M edges for heavy elements

Idealized Ionization edgeOnly found for isolated Hydrogen atoms

Ec → minimum energy required to ionize a atom

Ec

Energy Loss

Inte

nsit

yDecreasing probability of ionization

↓ as E↑

Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

Idealized edgeEdge superimposed on plural scattering

ELNES

Bonding Effects

Diffraction Effects from atoms surrounding the ionized atom

EXELFS

Thick Specimen → Combination of ionization and plasmon losses

Ionization loss

Plasmon loss

Detection of the beam electron that ionized the atom is independent of the atom emitting Auger electron or a X-ray

(hence EELS is not affected by the fluorescence yield limitation that limits X-ray analysis by EDX)

EELS Ionization Edge vs Characteristic X-Rays in EDX

Can lead to

The EELS spectrum

Gain Change 100

Zero loss peak

ELNES EXELFS

Inner shell ionization edges

Inte

nsit

y

Energy Loss (eV)0 500

0 40 80v

Io

Plasmon peak

250 300 350 400 450

Energy-loss (eV)

ELNES

EXELFS

Zero loss peak

Forward scattered (cone of few mrad) 000 spot of DP Bragg diffracted peak (~20mrad) rarely enters the spectrometer) Includes energy loss of ~0.3 eV Includes phonon loses EELS does not resolve phonon loses

FWHM defines the energy resolution E resolution kV resolution

Low loss region ~ 50 eV

Ionization Edge

Bonding effects

~50 eV after ELNES Diffraction effects from the atoms surrounding the ionized atom

The EELS spectrum

ELNES- Energy Loss Near Edge StructureEXELFS- EXtended Energy Loss Fine Structure

Coordination, Bonding effects, Density of states, Radial distribution function

Information available in the low loss region of the spectrum

0 40 80v

Io

Plasmon peak

Low loss region ~ 50 eV

Free electron density (plasmon peak)

Composition of the specimen (In some binary free electron systems the plasmon peak shift reflects the composition of the specimen)

Dielectric constant of the specimen

The plasmon peak

Valley before the plasmon peakParts to the low loss region

1

1a

1b

Intraband transition characteristic of

Polystyrene

Band gap differences manifesting itself in the low loss region of the EELS

spectrum

Low loss region before Plasmon peak1a

Transmission Electron Microscopy, David B. Williams & C. Barry Carter, Plenum Press, New York, 1996.

Al → free electron metal Fe → Transition metal

NiO – ZrO2 interfaceNiO

ZrO2

6 nm interface

1b

Transmission Electron Microscopy, David B. Williams & C. Barry Carter, Plenum Press, New York, 1996.

Plasmon peak

Bonding of the ionized atom

Coordination of the ionized atom

The density of states of the solid

The radial distribution function

Information available in ELNES and EXELFS

250 300 350 400 450Energy-loss (eV)

ELNES

EXELFS

Arise as more than critical energy for ionization (Ec) imparted to the core electron

The excess energy can be thought of as a wave emanating from the ionized atom If the excess energy is ~ few eV → the electron undergoes plural elastic scattering

from the surrounding atoms → ELNES → Bonding between atoms Excess energy is greater → interaction can be approximated to a single scattering

event→ EXELFS → Local atomic arragement

2

Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

250 300 350 400 450Energy-loss (eV)

ELNES

EXELFS

For convenience

Characteristics of the three principal sources operating at 100kV

Units Tungsten LaB6 Field emission

Current density A/m2 5 104 106 1010

Brightness A/m2/Sr 109 5 1010 1013

Energy spread eV 3 1.5 0.3

Vacuum Pa 102 104 108

Lifetime hr 100 500 >1000

Thin specimen is better (plasmon peak intensity < 1/10 th zero loss peak) Use high E0 (scattering cross-section , but benefit is in of plural scattering + edge signal to noise ratio ) Energy resolution limited by electron source

Chemical analysis (structural and elemental) using EELS

Spatial Resolution

TEM

STEM Limited by size of probe (~1nm)

Limited by selecting aperture at spectrometer entrance (its effective size at the plane of the specimen)

Energy Resolution 1 eV (incident energy 200-400 eV!)

• Somewhat better than XEDS• FEG < 1nm resolution

• Using FEG DSTEM Krivanek et al. [Microsc. Microanal. Microst. 2, 257 (1991).] detected single atom of Th on C film

Difficult to do quick semi-quantitative analysis (as possible with XEDS)

EELS spectrum (high loss region) from a particle of BN over a ‘holey’ C film

Equal amounts of B and N but intensities very different

• Variation of ionization cross section with E• Varying nature of the plural scattering background• C and N edges sit on tails of preceding edges

Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

Stainless steel specimen

EELS spectra

Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

TiN

TiC

Ti L2,3

No visible Grain Boundary

2.761 Å

Fourier filtered image

Dislocation structures at the Grain boundary

counts

eV

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

counts

1850 1900 1950 2000 2050 2100 2150 2200 2250eV

counts

eV

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

counts

1850 1900 1950 2000 2050 2100 2150 2200 2250eV

Si peak at 1839 eV Sr L2,3 peaks

Grain Boundary

Grain

eV1900 2000

EELS

2100 2200

~8º TILT BOUNDARY IN THE SrTiO3 POLYCRYSTAL

GB23x4

MX23x4

Di st 5 nm

VG microscope

Chapter 40 in Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

EELS microanalysis detection of a single atom of Th on C film!

EELS microanalysis

Chapter 40 in Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

Si L edge ELNES changes across a Si-SiO2 interface due to change in Si bonding atomic level images with spectra localized to individual atomic columns

Differences from C K edge from graphite and diamond

Cu L edge from Cu and CuO

ELNES

http://eels.kuicr.kyoto-u.ac.jp/eels.en.html

Diamond, graphite and fullerene are the matters which consists of only carbon, so that, all of these specimens have absorption peaks around 284 eV in EELS corresponding to the existence of carbon

atom. From the fine structure of the absorption peak, the difference in bonding state and local electronic state can be detected. The sharp peak at absorption edge corresponds to the excitation of

carbon K-shell electron (1s electron) to empty anti-bonding pi-orbital. It is not observed for diamond, because of no pi-electron in it.

Excitation of: Carbon K shell e (1s) → antibonding orbital

Carbon K shell

Energy FilteringEnergy Filtering

Imaging

Diffraction

Filter out the inelastically scattered electrons

http://eels.kuicr.kyoto-u.ac.jp/eels.en.html

Energy-loss (eV) →

v

280 290 300 310 320

Graphite

Diamond

Differences between C edge between graphite and diamond

Change in Cu L edge as Cu metal is oxidized

Energy-loss (eV) →

v

920 940 960 980

CuO

Cu

L3

L2

[1] [1]

[1] Chapter 40 in Transmission Electron Microscopy by David B. Williams and C. Barry Carter, Plenum Press, New York, 1996.

ELNES