The Electron Microscopy Center

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Electron Energy Loss Spectroscopy (EELS)

Description

When a high energy electron passes through matter and scatters ineleastically, a change in energy can be observed in the electron afterwards. If a beam of electrons is passed through a sample and a percentage of the electrons scatter inelastically, then an entire range of energies and scattering angles will be oberved in the electron beam afterwards. It turns out the spectrum of these electrons is nearly equivalent to x-ray absorption spectra. Hence, XANES and EXAFS experiments may be carried out in the TEM. In the case of electrons these experiments are called ELNES and EXELFS respectively. However additional information is obtainable from EELS experiments, this includes spectral information localized to regions less than two angstrom and the momentum dependance of the spectra. This information can be used to construct a dielectric function for the material as a function of energy and momentum for example. Localized bonding and localized electronic structure information can also be obtained at interfaces. These measurements can be directly related to the DOS using software on our computational analysis workstation. Thus a full structural analysis may be performed.In addition GATAN Corp has developed energy loss imaging capabilities which we have available. This can greatly improve contrast in acquiring CBD patterns and allows imaging of specific elements in a sample. The EELS spectrometer is particularly sensitive to the detection of light elements allowing it to compliment the EDS spectrometer which looses sensitivity for light elements. This is very useful for the simple detection and quantification of light elements.

Specifications

Energy range available from 0 to 2000 eV.
Best energy resolution obtainable on our machine is .65 eV.

Instrumentation

  • JEOL 2010F STEM equipped with GATAN GIF 2000 EELS spectrometer

Advantages

Sensitive to light elements. The only technique where near edge structure can be recorded with a resolution of 2 angstrom.

Limitations

EELS work requires very thin specimens. This depends on the material, but around 500 angstrom is usually good. The thicker a specimen is, the more multiple-scattering effects will become present in the spectrum. There are ways to remove the effects reliably up to about 1000 angstrom, but this requires more scope and processing time. It is better to have a thin sample to begin with. There are some elements which do not have major edges within the 2000 eV energy range available. Also the technique is complex and to apply correctly requires a good deal of background studying, including STEM operation.

Examples

Typical EELS spectra taken from CeO2 showing oxygen k edge and two different oxidative states determined by the differing ratios of the Ce M5,4 peaks taken at two locations.

Spectrum acquired by Alan Dozier. Project funded via grants of Uschi Graham and Burt Davis.

EFTEM image taken of a fly ash particle showing distribution of elements

Image acquired by Alan Dozier. Project funded by Uschi Graham grant.