EELS

Electron energy loss spectroscopy (EELS) for chemical and compositional analysis.

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Overview: 

TEM specimen beam interactions.Electron energy loss spectroscopy (EELS) is a family of techniques that measure the change in kinetic energy of electrons after they interact with a specimen. This technique is used to determine the atomic structure and chemical properties of a specimen, including: the type and quantity of atoms present, chemical state of atoms and the collective interactions of atoms with their neighbors. Some of these techniques include: spectroscopy, energy-filtered transmission electron microscopy (EFTEM), and DualEELS™.

Atom-scale view of electron energy loss in TEMAs electrons pass through a specimen, they interact with atoms of the solid. Many of the electrons pass through the thin sample without losing energy. A fraction will undergo inelastic scattering and lose energy as they interact with the specimen. This leaves the sample in an excited state. The material can de-excite by giving up energy typically in the form of visible photons, x-rays or Auger electrons.

As the incident electron interacts with the sample, it changes both its energy and momentum. You can detect this scattered incident electron in the spectrometer as it gives rise to the electron energy loss signal. The sample electron (or collective excitation) carries away this additional energy and momentum.

Core-loss excitations occur when tightly bound core electrons are promoted to a higher energy state by the incident electron. The core electron can only be promoted to an energy that is an empty state in the material. These empty sates can be bound states in the material above the Fermi level (so called anti-bonding orbitals in the molecular orbital picture). The states can also be free electron states above the vacuum level. It is the sudden turn-on of the scattering at the Fermi energy and the probing of empty states which makes the EELS signal sensitive to both the atom type and its electronic state.Correlation between EELS and specimen features

You can visualize the initial spectral features in the core-loss excitations when you align the Fermi level with the zero-loss peak (ZLP) of the spectrum. The edges can now be seen as the point where the electrons lose enough energy to promote the core level atomic electrons to the Fermi level. This analogy fails to reproduce the scattering above the Fermi level, but is helpful to visualize the core level edge sudden increase in intensity.What an EELS spectrum looks like

A typical energy loss spectrum includes several regions. The first peak, the most intense for a very thin specimen, occurs at 0 eV loss (equal to the primary beam energy) and is therefore called the zero-loss peak. It represents electrons that did not undergo inelastic scattering, but may have been scattered elastically or with an energy loss too small to measure. The width of the zero-loss peak mainly reflects the energy distribution of the electron source. It is typically 0.2 – 2.0 eV but may be as narrow as 10 meV or lower in a monochromated electron source.

For more information on the EELS family of techniques, please visit EELS.info, an educational site.

Research Spotlight

TEM team & collaborators from left to right: Dayne Swearer, Rowan Leary, Emilie Ringe, and Sadegh Yazdi.

The Ringe Group was established in 2014 in the department of Materials Science and NanoEngineering (MSNE) at Rice University, Houston...

Resources:

 

Applications

EELS.info

Atomic level EELS mapping using high energy edges in DualEELS™ mode
High-speed EELS composition analysis, in DualEELS mode, of metal alloy ohmic contacts for the fabrication of III-V MOSFET devices
EELS: A tool for investigating biological materials
Fast simultaneous acquisition of low- and core-loss regions in the EELS spectrum from catalyst particles containing the heavy metals Au and Pd using the GIF Quantum® system
Fast STEM spectrum imaging using simultaneous EELS and EDS in Gatan Microscopy Suite® software
Review of recent advances in spectrum imaging and its extension to reciprocal space
The use of MLLS fitting approach to resolve overlapping edges in the EELS spectrum at the atomic level

Posters

Fast STEM EELS spectrum imaging analysis of Pd-Au based catalysts
The high efficiency of the latest generation EELS spectrometers allow highly detailed EELS spectra from heavy elements to be acquired in a matter of milliseconds resulting in composition maps with outstanding information content.

A quantitative investigation of biological materials using EELS
EELS has proved to be a valuable tool to obtain compositional information from biological samples. In addition to the composition, EELS also gives insight into the chemistry unveiling the nature of the chemical bonds and different oxidation states.

High-speed composition analysis of high-z metal alloys in DualEELS mode
Demonstrating that high-speed atomic EELS composition maps with high contrast and high signal-to-noise ratio can be acquired routinely from high-energy edges.

Fast atomic level EELS mapping analysis using high-energy edges in DualEELS mode
Demonstrating that atomic EELS mapping using high-energy edges is very effective. The high signal-to-background ratio of high-energy edges leads to simplified data extraction.

Atomic resolved EELS analysis across interfaces in III-V MOSFET high-k dielectric gate stacks
Demonstrating that EELS SI can reveal the elemental distribution at the gate of high-k MOSFET devices at atomic column level.

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