Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Transmission Electron Microscopy and Spectroscopy of Nanoparticles 61 terized by the quantized energy levels and the associated electronic states. Under the impact of an incident electron the electrons bounded to the atoms may be excited either to a free electron state or to a unoccupied energy level with a higher energy. The quantum transitions associated with these excitations will emit photons or X-rays and electrons such as secondary electrons Auger electrons and ionized electrons these inelastic scattering signals are the finger prints of the elements that can provide quantitative chemical and electronic structural information. Figure 3-22 shows the main inelastic processes that may be excited in high-energy electron scattering 6 43 . When a fast electron passes through a thin metal foil the most noticeable energy-loss is to plasmon oscillations in the sea of conduction electrons. For an ideal case in which the electrons can move freely in the sea the system can be treated as an electron gas. This case is best represented by aluminum in which the outer-shell electrons can be considered as free electrons. The negatively charged particles are mixed with nuclei forming a solid state plasmon gas . The resonance frequency of this plasmon is directly related to the density of electrons in the solid. The decay of plasmons results in the emission of ultraviolet light. The cathodulumines-cence . technique is based on the detection of the visible light which is emitted when an electron in a higher-energy state usually at an impurity fills a hole in a lower state that has been created by the fast electron. Figure 3-22. Schematic one-electron energy level diagram plotted against the positions of atoms showing the characteristics excitations by an incident electron in a semiconductor material. Here Ef is the Fermi level Eb the binding energy and . . and . are the conduction-band minimum valanceband maximum and vacuum level respectively A a K shell excitation .
