Chapter/Index: Introduction | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Appendix
| 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). Figure 4467a 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 4467a. 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 [1] 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. Figure 4467b shows an energy-level diagram of a solid with core-level excitation and electron emission processes at solid surface. This energy-level diagram provides a detailed view of the electronic structure of a solid, highlighting the core levels (K- and L-shells), the valence band, and critical energy levels such as the Fermi level (EF) and the vacuum level (Evac). The diagram illustrates the primary processes of electron excitation, where external energy sources, such as X-rays or incident electrons, can eject electrons from inner core levels (K and L). This excitation can lead to various secondary emission processes, including the emission of secondary electrons, photons (light and X-rays), and Auger electrons. The Fermi level (EF) is shown as the energy level at which electrons are in equilibrium at absolute zero temperature, representing the dividing line between occupied and unoccupied electronic states. The vacuum level (Evac) indicates the energy threshold an electron must exceed to escape the solid into the vacuum. The valence band, represented as a shaded region, contains delocalized states where electrons are free to move throughout the material, playing a crucial role in its conductive properties.
In details, the diagram in Figure 4467b describes:
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