Oxygen hole centers

BA BE BNFL BOHC BPP BWR Bottom ash Binding energy British Nuclear Fuels Ltd. Boron-oxygen hole center Balti Power Plant Boiling water reactor... 

Another type of paramagnetic defects that are attributed to the main intrinsic defects of silica glass, are the so-called non-bridging oxygen hole centers - the (=Si-0)3Si-0 radicals. The EPR spectrum of these radicals, stabilized on the silica surface is presented in Figure 7.16a (conclusions about their existence were made earlier based on the indirect data). The temperature dependence of the intensity of the EPR signal does not obey the Curie law, and the EPR signal can be detected only at T< 100 K. 

In the latter case, the vacancy is believed to have a stabilizing effect on the oxygen hole center at the surface (40). The EPR parameters of these bridge and terminal oxygen hole centers were found to be very close to those of the same bulk defects and O" adsorbed species, respectively (Table II). The reactivity of the hole centers at the surface was similar to that of O" adsorbed species (39, 40). 

Figure 23.5. Mechanism of photoluminescence from nc-Si/Si02 nanocomposites Left pictorial illustration, right energy levels involved in (1) the photogeneration of electron-hole pairs in the silicon nanocrystals, the band gap and transition probabilities of which increase with decreasing crystallite size, which is followed by an energy transfer to localized radiative centers, such as (2) a non-bridging oxygen hole center (NBOHC) in the Si02 layer or (2 ) a metallic impurity.

The experimental results presented here clearly show that the "red" PL from nc-Si/Si02 nanocomposites prepared by ultra-pure processing originates from the non-bridging oxygen hole centers in the passivating Si02 matrix. The... 

The mechanism of the photoluminescence from nc-Si/Si02 nanocomposites includes a quantum confinement controlled photogeneration of electron-hole pairs within the silicon nanocrystals, followed by an energy transfer to nonbridging oxygen hole centers within the passivating Si02 matrix. The relatively inefficient and slow PL from such radiative centers is of little interest. A variety of other PL that have been reported in the literature and attributed to radiative recombination within the Si nanocrystals are most probably due to impurities. Several examples are shown to illustrate that extreme care has to be taken in order to avoid such artefacts. 

Aral, K., Imai, H., Isoya, J., Hosono, H., Abe, Y., and Imagawa, H., Evidence for pair generation of an E center and a nonbridging oxygen-hole center in Y-ray-irradiated finorine-doped low-OH synthetic silica glass, Phys. Rev. B 45,10818 (1992). 

Vaccaro, L., Cannas, M., Boizot, B., and Parlato, A., Radiation induced generation of non-bridging oxygen hole center in silica Intrinsic and extrinsic processes, J. Non-Cryst. Solids 353, 586 (2007). 

Although vitreous silica usually remains colorless following irradiation to very high doses, doped silicas can become colored through the formation of defects associated with impurities. Purple samples, for example, are formed if the glass contains a small amount of aluminum, due to the formation of aluminum-oxygen hole centers (AlOHC). Other impurities, such as germanium or titanium, can also produce colored vitreous silica by formation of defect centers. 

Apart from other pubhshed experiments [4.57] related to EURECA, mission radiation and recording of EPR spectra have been done at room temperature (see spectrum 3 in Fig. 4.66). Spectra 1 and 2 result from samples irradiated at 77 K, recorded at 170 K and irradiated at 77 K, annealed at 440K, and recorded at 170K, respectively (published in [4.57]). The main, quite complicated signal around consists basically of a variety of the so-called oxygen hole centers (OHCs) close to aluminum or silicon, aluminum has a natural abundance of 100% Al with I = 5/2, yielding a hyperfine coupling. In accordance with other publications it has been shown that all... 

It has been suggested that the hole center O- generated from the lattice oxygen... 

BaTiOs crystallizes in the perovskite structure. This structure may be described as a barium-oxygen face-centered cubic lattice, with barium ions occupying the corners of the unit cell, oxide ions occupying the face-centers, and titanium ions occupying the centers of the unit cells, (a) If titanium is described as occupying holes in the Ba-O lattice, what type of hole does it occupy (b) What fraction of the holes of this type does it occupy (c) Suggest a reason why it occupies those holes of this type but not the other holes of the same type ... 

Other groups are assuming that the trapped hole is an oxygen radical centered at the surface of the titanium dioxide particle, having an energy state lower than the valence band edge of the semiconductor (reaction (7.8)) [lid]. 

---