Electron irradiation defects produced

Both spin- Vi and spin-1 centres have been observed following particle irradiation. Electron-irradiation of 6H SiC revealed new Ti-related triplets through ODMR and cross relaxation [11]. A spin-Vi defect was also reported [11]. Similarly, electron-irradiation of the 3C polytype produced both an S = Vz centre, possibly the N-donor, and triplets [14]. Neutron-irradiation of both 4H and 6H polytypes produced luminescence-quenching donor resonances (see TABLE 1) [1]. 

Color centers can be produced in the alkali metal azide by ultraviolet light and ionizing radiation at low temperatures. The phenomenon has been of interest for some time since the defects produced are involved in the process of photochemical decomposition (cf. Chapter 7). In earlier studies [54a, b, c] purely speculative identifications of optical absorption bands with F, V, and aggregate F centers were made by analogy with the alkali halides. The most prominent visible absorption band in each case was attributed to the F center—a defect involving an electron trapped at an azide (N3) vacancy. In the case of NaNa, spin resonance [55] and recent point ion calculations [56] clearly point to the existence of a F center. However, in the case of KN3, spin-resonance studies [54a] point to the existence of molecular centers of type N2 (on low-temperature irradiation) and NJ (on room-temperature irradiation). Infrared absorptions [57] and Raman scattering [58] have been observed in the irradiated alkali azides, which can be correlated with modes associated with these defects. 

A final example demonstrates the improved textural properties of USS products compared to the citric acid route. Smaller SrFeOs-s nanoparticles (20-30 nm against 70-100 nm determined by electron microscopy) were produced by USS that exhibited a larger fraction of surface defect from interpretation of the XPS data [102]. As a result of the defects and particle structure, the band gap of the material shifted toward longer wavelength making the USS-SrFeOs-s sample a more efficient photocatalyst upon visible light irradiation. 

Any material which can form a color center contains two types of precursors as shown in Figure 2a. The hole center precursor is an atom, ion, molecule, impurity, or other defect which contains two paired electrons, one of which can be ejected by irradiation, leaving behind a hole center (Fig. 2b). The electron center precursor is an atom, ion, etc, which can produce an electron center by trapping the electron ejected from the hole center precursor. A hole and an electron center are thus formed simultaneously. Either or both can be the color center. Almost all materials have hole center precursors. If there is no electron center precursor, however, the displaced electron returns to its original place and the material remains unchanged. 

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