Electron Energy Loss Spectroscopy
Electron Energy Loss Spectroscopy (EELS) is a spectroscopic technique that can be combined with TEM or STEM to gather chemical and elemental data during electron microscopy analysis. EELS has a wide range of applications in the research and development of new materials and technologies.
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What is EELS analysis used for?
EELS is often used in tandem with electron microscopy techniques, such as transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) to help gather additional data. It can be used to investigate various qualities of thin film samples, including detecting the atoms present, their quantities, the nature of their chemical bonding, oxidation states, and even their allotropic forms. This makes EELS useful across a range of industries for analyzing chemical fingerprints and creating spatial maps of new materials as they are developed.
EELS also provides information on the electronic properties of materials and can therefore be used to characterize semiconductors and other electronically active materials that form the backbone of a wide range of current and future technologies. EELS can operate on a sub-microscopic scale and therefore finds applications in the development of new nanotechnology.
How does electron energy loss spectroscopy work?
In EELS, a fine beam of electrons with known kinetic energy is fired toward a sample. The electrons will interact with the atoms in the sample in a variety of ways, including causing the electrons inside the atoms to shift to different energy levels or become removed entirely. This process causes the incident electrons to lose some kinetic energy and become scattered. The scattered electrons are then detected by a spectrometer, which measures the remaining energy that they carry.
The amount of energy lost can then be calculated, which will be specific to the event it caused and the type of atom it interacted with. This will tell the user about the nature of the interaction and therefore which atoms are present, where they are located, and in what quantities they exist. It also provides information about how they are bonded and what other atoms are nearby. By repeating this process, EELS makes it possible to create a map of the different elements in the sample.
Suitable samples and sample preparation
Samples for EELS are most often in the form of extremely thin films. Therefore, they must be deposited as a thin film, or suitably thinned from a bulk sample. Repeated thinning steps with methods like FIB may be required to achieve the desired thickness. Damage and contamination to the surface should be minimized to reduce their impact on the final results.
EELS vs. EDX
Energy-dispersive X-ray spectroscopy (EDX) is another spectroscopic elemental analysis technique that is used in a similar manner to EELS, usually in connection with either SEM or TEM. Generally, EDX works best when used to identify relatively heavy elements that appear later on the periodic table. It is also easy to use and generally more time and cost-effective when compared to EELS, making it a more common ‘workhorse’ technique.
However, while EDX is excellent for analyzing simple elemental compositions in a sample, EELS can also provide additional information about the structure, chemical bonding, electronic properties, and surface features of a sample. It can also readily record data for lighter elements that may not appear in EDX analysis.
In some cases, EELS can even be used to differentiate between different forms of the same element, such as carbon, which gives it an edge in both organic and inorganic analysis. As with most cases, often the two techniques can be seen as complementary to one another and are frequently used in tandem.
Suitable sample matrices
- Thin films
- Carbon-based materials
Ideal uses of EELS
- Elemental analysis
- Surface mapping
- Monitoring different allotropes of carbon
- Detecting impurities
- Determining bond characteristics
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Frequently asked questions
EELS is most often used to complement TEM or STEM analysis with elemental composition information on nanoscale samples.
EELS generally requires an electron-transparent sample, the preparation of which may be tricky. When compared with EDX, elemental analysis with EELS is also less widely available and usually more expensive.
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