Electron Energy Loss Spectroscopy (EELS)

Suggested Reading: Used electron spectrometer

Used TEM

Review of theory for elastic + inelastic scattering

Examples for graphite and gold

Problem:

How can we gain information on unoccupied electronic states of the system

IPES—good, but low count rates, few labs do this.

Auger spectroscopy—common, high count ratesXPS-- not bad either.

Can we use such instrumentation and excitation methods?

Yes! (Well, sort of…)

Electron Energy Loss—basic process

Losses in XPS spectra

Electron Energy Loss Spectroscopy—electron excitation*TEM+EELS analyzer*Electron energy analyzer

Near Edge X-ray Absorption Fine Structure (NEXAFS)—synchrotron based photon absorption measurement.

Core

hv

e-

Evac

E field

EFermi

CB

VBΔE

Transit of an electron through a solid (ionization of core electron shown) induces a strong, local electric field (E)

This electric field can induce excitations of higher lying electrons:(a)From ground to higher vibrational states (HREELS)(b)From filled to empty electronic states (EELS) –considered here.

EK = hv-EB- Φanal. - ΔE

Promotion of an electron from VB to CB

EELS effects observed in XPS, Auger, etc. as well as from reflected/transmitted electrons

C(1s)XPS from ~ 3ML graphene /Co3O4/Co (Zhou, et al., JPCM 24 (2012) 72201

Loss spectra from backscattered electrons, few layer graphene/SiCUsing high resolution electron spectrometer

Can also use TEM with electron energy loss analyzer for this purpose(Can get EELS spectra from VERY localized area)

EELS in TEM mode

United States Patent 7067805

From uspatentsonline.com

Double pass Cylindrical Mirror Analyzer (CMA) with co-axial gun (STAIB instruments)

--Coaxial gun, reflection geometry--Large angular acceptance.--Field free region between analyzer and sample

EELS can be used to monitor evolution of surface vs. bulk plasmons in deposited nanoparticles. (Plasmon collective excitation of electron gas in, e.g., metals) note, surface plasmon typically distinct in energy from bulk)

Used double pass CMA

EP

60

One can get some idea of energy band dispersion by collecting at well defined angles (CMA not good for this), but note effects on resolution of varying the primary energy:

ΔE/E = const. for electrons from excitation source.

Also better surface sensitivity

Langer, et al.

Energy and Emission Angle Effects

1.Surface excitations (surface plasmons) enhanced at higher emission angles.

2.Excitation cross section for specific loss energy EL increases as ΔE = Ep – EL increases.

Bulk plasmon

Surface Plasmon

Note: Surface plasmon intensity increases with emission angle of reflected electron

REELS of Si

Note relative intensities of surface and bulk plasmons at 500 vs 2000 eV primary energy—due to both cross section and imfp effects

Can get sampe phenomena during X-ray absorption

From Grant Bunker, NEXAFS overview

One can monitor photo yield, or electron yield

Absorption vs. wavelength

Ask to borrow your Mom and Dad’s synchrotron!

Near Edge and pre-edge region

Excitation of Core level to unoccupied bound state

EXAFS region, excitation into continuum

NEXAFS (XANES) Core unoccupied bound states, like EELS but different, it is a direct absorption measurement

O 1s (or N, or C

e-