Electron energy-loss spectroscopy (EELS)
Inelastic scattering by the TEM specimen results in energy losses of the electron beam that are measured in the electron energy-loss spectrum. At low energy losses (e.g. < 50 eV), energy loss is mainly due to interactions with oscillations of weakly-bound electrons (“plasmons”) in the specimen. Intensity in this region of the specimen can, for instance, give information on sample optical properties. At higher energy losses, inner-shell ionization events produce intensity edges in the energy-loss spectrum that are atomically characteristic, and can therefore be used for elemental analysis. Compared to EDX, the better collection efficiency of the EEL spectrometer can allow shorter spectral acquisition times for compositional line-scans or mapping in STEM mode. However, compositional quantification is more complicated and less accurate than for EDX, especially if there is overlap of edges from different elements (i.e. it is not a “push button” technique). Nevertheless, EELS is often more chemically sensitive than EDX, and further provides better identification/quantification of low atomic number elements such as carbon or oxygen. Additionally, measurement of the fine structure of the ionization edge spectrum can be used to analyze atomic bonding states or valency for many elements. Specimen thickness, relative to the inelastic scattering mean-free path, is also be easily determined from the EEL spectrum.