Electron Energy Loss Spectroscopy (EELS)

• The most versatile technique which involves analysis of the energy distribution of the in-elastically scattered electrons in the transmitted beam

• The energy resolution of present-day spectrometers is as high as 1 meV.

Every primary electron has one of three possibilities in

terms of its interactions with

atoms of the specimen.

• Elastic Scattering• In-elastic Scattering

• For most of the electrons, the change in energy is not random but is directly related to which electron, from which atom, from which orbital shell the inelastic collision took place.

• This specific loss of energy is known as Electron Energy Loss Spectroscopy or EELS

Electron Energy Loss Spectroscopy(EELS) Magnetic Spectrometer• Discriminates the energy loss

electrons on the basis of their absolute energy.

• The signal from the electron energy loss spectrometer can be used to generate an EELS spectrum

• The spectrometer can be used to produce a compositional map

Electron Energy Loss Spectroscopy(EELS)The EELS spectrum • Three regions – Each region arises due to a different

group of electron/sample interactions.

• Region 1 (0 to 10 eV) is the zero-loss region.

• Region 2 (10 to 60 eV) is the low-loss region.

• Region 3 (>60 eV), the core-loss region

EELS Spectra

• The Zero-Loss Peak:1). It is the main feature in EELS spectra of thin

specimens.2). Originates from electrons that have lost NO

energy3). Width of the zero-loss peak: energy spread

of the electron source.4). Less analytical information about the

sample5).Used to calibrate the Energy scale

• What is phonon? -- Phonons are lattice vibrations, which are equal to heating the specimen. – This effect may lead to a damage of the sample.

EELS SpectraLow-Loss area: Plasmon• It reflects excitation of plasmons

and interband transitions.

• What is plasmons: Plasmons are longitudinal oscillations of free electrons, which decay either in photons or phonons.– It is caused by weakly bonded. – It depends on local density of the weakly

bonded electrons.– The typical lifetime of plasmons is about

10-15 s.

EELS Spectra• Interband transition: the

transition between the conduction and valence bands (electrons and holes)

• Intraband transitions: the transitions between the quantized levels within the conduction or valence band. It known also as the itersubband transition.

EELS SpectraLow-Loss area: Plasmon• In the EELS spectra, plasmon losses always occur,

except the ultra-thin specimens. • Used to estimate the thickness of the sample.– However, when the specimen is quite thick, multiple

plasmon losses will make the straightforward analysis impossible.Thin Thick

EELS Spectra• High-loss Region1). The most important region of the EELS

spectrum for microanalysis. 2).The signal in the core-loss region is very weak

relative to that in the zero-loss and low-loss regions. Therefore, the core-loss region of the spectrum is often amplified 50 to 100 times.

3).The peaks, or edges, arise because of interactions between the incident electrons and the inner-shell electrons of atoms in the specimen.

4). When an incident electron ionizes an atom, it produced a specific amount of energy. The amount of energy lost in ionizing the target atoms is the electron energy loss.

Why is it ionization (absorption) edge?

1). Electrons should lose at least the energy equal to the critical ionization energy EC

for ionization.2). Intensity of the absorption edge

decreases with increasing energy loss 3). The edge has a high background. The

background is originated from: multiple inelastic electron

scattering

4). The most important thing is: from the edge the critical ionization energy Ec can be identified. We then can identify the elements in our sample from the energy.

Two more things about the ionization (absorption) edge

1). ELNES, “electron loss near-edge structure”: features in the spectrum with energy loss E = Ec to Ec + 50 eV. – It contains information on local density of empty states,

oxidation state.

2). EXELFS, “extended energy-loss fine structure”: features in the spectrum with energy loss E > Ec + 50 eV.– It contains information on local coordination of the

respective atom.

Example• Diamond, graphite and

fullerene all consist of only carbon.

• All of these specimens have absorption peaks around 284 eV in EELS corresponding to the existence of carbon atoms.

• From the fine structure of the absorption peak, the difference in bonding state and local electronic state can be detected.

Qualitative EELS• The most frequent use of EELS is the

qualitative determination of the composition of precipitates containing light elements

• Metallurgical analysis typically necessitates

differentiating among carbides, nitrides, and carbo-nitrides.

The quantification of EELS spectrum

• Divided into three steps – The first requires the removal of the background

from beneath the ionization edges– The second the integration of area within the edge – The third the determination of composition from

the integrated intensities.

Limitations

• A severe limitation of EELS is specimen thickness.

– For aluminum, the usable thickness for EELS analysis is of the order of 10 to 20 nm.

– For heavier materials, such as steels, the limiting thickness is even less.

ComparisonEELS

• High detection efficiency for low Z elements

• Elemental and chemical information

• Excellent energy resolution (0.3-2 eV) results in few peak overlaps; fine structures of the ionization edges can be analyzed (ELNES, EXELFS)

• Very efficient and high sensitivity to most elements => fast and efficient mapping technique

• Fast technique; complex processing required

EDXS• High detection efficiency for high

Z elements• Elemental information only

• Low energy resolution (> 100 eV) causes frequent overlaps

• Inefficient signal collection and detection => X-ray mapping is time-consuming

• Slow technique; only simple data processing required