Transmission electron energy loss spectroscopy (EELS) consists of measuring energy loss dispersion of inelastically scattered high-energy electrons transmitted through a thin film. The high-energy electrons, which interact with the electrons in the solid, lose a certain amount of energy and transfer momentum to the solid. Because of the energy and momentum conservation rules, energy loss and the corresponding momentum-transfer (q) of the probed electron represents the energy and the momentum of the electronic excitations in solids.
Although some optical techniques, such as soft X-ray absorption and optical reflectance measurements, provide comparative information about solids with higher energy resolution, EELS enjoys several unique advantages over optical spectroscopies. First of all, unlike optical reflectance measurements which are sensitive to the surface condition of the sample, the transmitted EELS represents the bulk properties of the material. Secondly, EELS spectra can be measured with q along specific controllable directions and thus, can be used to study the dispersion of plasmons, excitons, and other excitations [8.1–8.5]. Such experiments offer both dynamics as well as symmetry information about the electronic excitations in solids. In addition, the capability to probe the electronic structure at finite momentum-transfer also allows one to investigate the excited monopole or quadrupole transitions, which cannot be directly observed by conventional optical techniques limited by the dipole selection rule.
Because of the significant energy spread of conventional TEM electron sources (e.g. LaB6, W-hairpin filaments with ΔE ∼ 1–2 eV), EELS measurements to investigate the electronic structure of solids have been generally limited to dedicated electron energy loss spectrometers with energy resolutions ∼0.1 eV.