## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Phenomenological Model of the Photosensitization by Si Nanocrystals
The first one is a growth of the specific surface area of nc-Si with porosity increasing that promotes the O\({}_{2}\) adsorption and then the total rate of \({}^{(1)}\)O\({}_{2}\) photosensitization. The second reason is related to a decrease of the nonradiative recombination with increasing the porosity that results in an increase of the exciton concentration. In fact, the PL quantum yield increases in por-Si with high porosity that indicates the second reason impact [83]. Besides the characteristic energy of about 1.63 eV (the \({}^{1}\Sigma\rightarrow{}^{3}\Sigma\) transition energy in O\({}_{2}\) molecule), the spectrally selective quenching of the exciton PL intensity in por-Si at room temperature demonstrate an additional maximum at photon energy of 2-2.2 eV, which is related to the formation of superoxide radicals, that is, O\({}^{-}\)\({}_{2}\)[76]. The PL quenching at 2-2.2 eV can be also explained by the charging of nc-Si due to the O\({}^{-}\)\({}_{2}\) radical formation on the nc-Si surface [83] because the positively charged Si nanocrystals exhibit an enhanced rate of the nonradiative Auger recombination [61; 80]. The infrared PL of nc-Si in por-Si MPs at room temperature was investigated to get information on the photosensitized generation of \({}^{(1)}\)O\({}_{2}\)[70]. Since the radiative transitions in \({}^{(1)}\)O\({}_{2}\) molecules are characterized by extremely low probability, the experimental observation of the \({}^{(1)}\)O\({}_{2}\) phosphorescence at 0.98 eV required very sensitive detection system and was possible only for por-Si with rather efficient generation of \({}^{(1)}\)O\({}_{2}\). Figure 30 shows typical infrared PL spectra of por-Si powder obtained by mechanical milling of por-Si films [73]. While the PL spectrum of por-Si powder in vacuum shows a featureless emission band related to defects states in nc-Si [61], the PL line of the \({}^{(1)}\)O\({}_{2}\) phosphorescence at 0.98 eV appears just after the admission of O\({}_{2}\) to por-Si powder. For several minutes after the O\({}_{2}\) admission the \({}^{(1)}\)O\({}_{2}\) phosphorescence intensity decreases, while the intensity of the defect-related band increases. The relative intensity of the \({}^{(1)}\)O\({}_{2}\) phosphorescence intensity was found to decrease in por-Si subjected to mechanical grinding, which resulted in increasing the defect-related PL [73]. Figure 29: Energy transfer time (circles) and energy transfer efficiency (triangles) _vs_ porosity of por-Si. \(E_{\rm exc}=3.7\) eV, \(ln\
u_{\rm PL}=1.6\) eV, \(P_{\rm ox}=1\) bar, \(T=300\) K. (Reprinted from Reference [83] by permission from John Wiley and Sons © 2007). Dependences of the intensity of \({}^{(1)}\)O\({}_{2}\) luminescence at 0.98 eV and defect-related PL at 0.94-0.95 eV for por-Si photoexcited at room temperature indicate formation of new defect states on the nc-Si surfaces due to the photosensitized generation of \({}^{(1)}\)O\({}_{2}\)[73]. Most probable reason of the defect formation is the photo-induced oxidation of nc-Si by \({}^{(1)}\)O\({}_{2}\), which is accompanied by irreversible quenching of the exciton PL [81]. The latter process induces decreasing the \({}^{(1)}\)O\({}_{2}\) photosensitization efficiency and then the \({}^{(1)}\)O\({}_{2}\) luminescence intensity drops down (see Figure 31). Therefore por-Si acts both as the photosensitizer and quencher of \({}^{(1)}\)O\({}_{2}\). The lifetime \(\tau_{\Delta}\approx 15\) ms was estimated from the comparison of \({}^{(1)}\)O\({}_{2}\) luminescence and exciton PL of nc-Si at room temperature at \(P_{\rm ox}=1\) bar [73]. On the one hand, the estimated lifetime is significantly shorter than the radiative lifetime of \({}^{(1)}\)O\({}_{2}\)[67]. On the other hand, \(\tau_{\Delta}\approx 15\) ms is larger than the \({}^{(1)}\)O\({}_{2}\) lifetime in water (3 \(\upmu\)s) or biosystems (\(<\)1 \(\upmu\)s) [69]. This fact implies that nc-Si can be considered as a potential photosensitizer of \({}^{(1)}\)O\({}_{2}\) in liquids, including aqueous media [70,71].