electron microscopy



Electron Energy Loss Spectroscopy (EELS)

A typical electron energy loss spectrum is shown below.
It consists of three parts:

Zero-loss peak at 0 eV:
It mainly contains electrons that still have the original beam energy E0, i.e., they have only interacted elastically or not at all with the specimen. In thin specimens, the intensity of the zero-loss beam is high, so that damage of the CCD chip can occur. Since the zero-loss beam contains not much useful information, it is often omitted during spectrum collection.

Low-loss region (< 100eV)
Here, the electrons that have induced plasmon oscillations occur. Since the plasmon generation is the most frequent inelastic interaction of electron with the sample, the intensity in this region is relatively high. Intensity and number of plasmon peaks increases with specimen thickness.

High-Loss region (> 100eV)
For the ionization of atoms, a specific minimum energy, the critical ionization energy EC or ionization threshold, must be transferred from the incident electron to the expelled inner-shell electron, which leads to ionization edges in the spectrum at energy losses that are characteristic for an element. Thus, EELS is complementary to X-ray spectroscopy, and it can be utilized for qualitative and quantitative element analysis as well. In particular, the detection of light elements is a main task of EELS.
Compared to the plasmon generation, the inner-shell ionization is a much less probable process, leading to a low intensity of the peaks. In the high-loss region, the amount of inelastically scattered electrons drastically decreases with increasing energy loss, thus small peaks are superimposed on a strongly decreasing background (s. spectrum). Because of the low intensity, the representation of the high-loss region is often strongly enhanced (here: intensity gap at about 220 eV).

Obviouisly, a lot of valuable information is present in a electron energy loss spectrum. The energy resolution (below 1 eV) is much better than in X-ray spectroscopy, and as a result more structural information can be obtained from the fine structure in EELS. The critical ionization energy EC is sensitive to the chemical situation of the element: e.g., the L edge of Cu metal and CuO are shifted in respect to each other (chemical shift). Moreover, the ionization process may take more energy than EC, and therefore there also is intensity located after the edge onset. Actually, this region, designated as ELNES (Energy-Loss Near-Edge Structure), mirrors the DOS and provides information about the bonding situation. Modulations further away from the ionization edge contain information about interatomic distances and coordination (EXELFS, Extended Energy-Loss Fine Structure).

For many questions, it is important to get this information with a high local resolution. Such mappings can be done with two different methods:
1. STEM with EELS
In STEM mode, an region of the sample is selected, and an EELS is measured on each spot of the defined grid (serial measurement).
2. EFTEM using an energy filter
An energy range is selected, and an energy-filtered image is recorded with electrons of this energy (parallel measurement).

Some EEL spectra are available in a database. Further information can be found at Gatan's EELS page.
Electron-matter interactions

Generation of X-rays

ETH Zürich | ETH chemistry department | ETH inorganic chemistry

© ETH Zürich and the authors