Presentation on EELS spectroscopy.

1. ELECTRON ENERGY LOSS
SPECTROSCOPY

2. Introduction
 It is an analytic technique based on the inelastic
scattering of fast moving electrons into a thin
specimen.
 It provides structural and chemical information about
the specimen using TEM (Tunneling electron
microscope.
 It provides information with high spatial resolution.

3. Introduction
 Each type of interaction between the electron beam
and thin specimen produces a characteristic change in
energy and angular distribution of scattered electrons.
 Electron energy-loss spectroscopy offers unique
possibilities for advanced materials analysis.

4. Every primary
electron has one of
three possibilities in
terms of its
interactions with
atoms of the
specimen.

6. Electron Energy Loss 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

7. EEL Spectrum
The zero loss peak
 It is the main feature in EELS spectra of thin
specimens.
 Originates from electrons that have lost NO
energy
 Width of the zero-loss peak: energy spread
of the electron source.
 Less analytical information about the
sample
 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.

8. EEL Spectrum
Low Loss Area
 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.

9. EEL Spectrum
Low Loss Area
 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

10. EEL Spectrum
Low Loss Area
 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

11. EEL Spectra
High Loss Region
 The most important region of the EELS
spectrum for microanalysis.
 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.
 The peaks, or edges, arise because of
interactions between the incident electrons
and the inner-shell electrons of atoms in the
specimen.
 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.

12. EEL Spectroscopy
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

13. Examples
 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.

14. Conclusion
 Within last decade, TEM based EELS has gone a lot of
development .
 EELS very efficient and high sensitivity to most
elements.
 EELS imaging and spectroscopy offers many new
opportunities to study fundamental questions of
material science atomic resolution.
 The energy resolution of present-day spectrometers is
as high as 1 meV.