A primary energetic beam experiences energy loss when it interacts with materials. Under high energy electron irradiation, e.g. in TEM and SEM, the outer-shell (valence) electrons can undergo single-electron excitations:
i) The valence electrons can transit to the condition band across the band gap (so-called interband transition for insulators and semiconductors).
ii) The conduction electrons transit to a higher energy state (for metal).
iii) Emission of secondary electrons.
Figure 4467 shows the schematic illustration of an energy-loss spectrum and the formation of the main energy-loss peaks related to the energy levels of electrons surrounding atom A and atom B in materials.
Figure 4467. Schematic illustration of an energy-loss spectrum and the formation of three main energy-loss peaks.
In this above cases, the high energy electrons normally lose several to tens of electron volts and are scattered at small angles (e.g. 1 ~ 5 mrad for incident electrons of 100 ~ 200 keV). In the interaction of incident electrons with solids, valence electrons are delocalized and produce collective excitations called plasmons. The energy region of the EEL spectrum (EELS) up to the energy loss of ∼50 eV is dominated by the collective excitations of valence electrons (plasmon) and by interband transitions.
Except for the single-electron excitation, inelastic scattering of outer-shell electrons can involve many atoms in the specimen. Based on quantum theory, this excitation can be described in terms of the creation of a pseudoparticle (plasmon) at energy of Ep = ħωp ( ħ is Planck’s constant and ωp is the plasmon frequency). For most solids, Ep is in the energy range of 5–30 eV.
For instance, during the irradiation of Si by the primary electrons, the electrons suffer energy losses due to the excitation of the valence-band electrons towards the conduction band. This inelastic process induces electron–hole (e–h) pair formation. These carriers (holes and electrons) generated inside the interaction volume undergo several processes e.g. escape from the surface, diffuse away from the generation region, undergo recombination, and become partially trapped.
The EELS of the low-energy loss region less than 50 eV is particularly called valence electron energy loss spectroscopy (VEELS) and mainly reflects the excitation of valence band electrons, such as interband transition (single excitation) and plasmon excitation (collective excitation). VEELS can be applied to analyze, for instance, local electronic and optical properties of materials  in nanoscale with STEM [2–7]. The STEM-VEELS method provides various advantages over conventional optical spectroscopy, for instance, it can measure a wider energy (wavelength) range and local electronic and optical properties from a small volume in a nano-region. The difficulty of STEM-VEELS application is that it is hard to fit a zero-loss tail, to extract ELF (energy loss function) attributable to the large zero-loss tail and to interpret the collective excitation such as plasmon, etc.
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