## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Phenomenological Model of the Photosensitization by Si Nanocrystals
The saturation of I\({}_{\text{EPR}}\) at high microwave power can be absent when \(T_{2}\) decreases due to the spin-spin relaxation. Indeed, there is no saturation of the I\({}_{\text{EPR}}\) signal on microwave power for por-Si in oxygen atmosphere (Figure 33). Since I\({}_{\text{EPR}}\) for the samples at \(P_{\text{ox}}\) = 760 Torr decreases by 40% under illumination with photon flux of about 10\({}^{20}\) cm\({}^{-2}\), the concentration of triplet O\({}_{2}\) molecules decreases by \(\Delta n\approx 0.4n_{0}\), where \(n_{0}\) is the initial O\({}_{2}\) concentration. The latter number at \(P_{\text{ox}}\) = 760 Torr is about 10\({}^{19}\) cm\({}^{-3}\) (the Avogadro number divided by the molar volume). From the comparison of \(I_{\text{EPR}}\) in the O\({}_{2}\) atmosphere in darkness and under illumination, one can estimate the \({}^{(1)}\)O\({}_{2}\) molecular concentration to be \(\Delta n\approx 4\cdot\)10\({}^{18}\) cm\({}^{-3}\). The latter value is comparable with the number of nc-Si and it is by 2 orders of magnitude larger than the Si DBs concentration. This fact demonstrates that Si DBs are only probes to monitor the \({}^{(1)}\)O\({}_{2}\) photosensitization. The observed features of I\({}_{\text{EPR}}\) for por-Si in oxygen atmosphere in darkness and under illumination were confirmed by means of the pulsed EPR technique and similar concentration of photosensitized \({}^{(1)}\)O\({}_{2}\)\(\sim\)10\({}^{18}\) cm\({}^{-3}\) was detected by using the Q-band EPR [75]. While por-Si looks rather efficient photosensitizer of the \({}^{(1)}\)O\({}_{2}\) generation in gaseous atmosphere, the photosensitization by por-Si in liquids and, especially, in aqueous solutions is required for biomedical applications [86; 87; 88; 89; 90]. Unfortunately, the fast nonradiative relaxation of \({}^{(1)}\)O\({}_{2}\) in water complicates detection of the \({}^{(1)}\)O\({}_{2}\) luminescence. In this case the \({}^{(1)}\)O\({}_{2}\) photosensitization can be indirectly probed by monitoring the intensity and lifetime of the exciton PL of por-Si. Although the exciton PL quenching for nc-Si dispersed in water is weaker, its spectral shape is similar to that measured in gaseous ambient. Note, the relative contribution of the high-energy quenching at 2-2.2 eV is stronger in water because of more efficient charging of the nc-Si surfaces followed with formation of O\({}^{-}\)\({}_{2}\) radicals [76]. The lifetime of the exciton PL of por-Si MPs dispersed in water decreases after saturation of the solution by oxygen as shown in Figure 34 [91]. The energy transfer times determined from transients of the exciton PL by using Equation (13) are plotted in inset of Figure 25:5 The spectral dependence of the energy transfer time is similar to that observed for por-Si in gaseous oxygen ambient (see Figure 26). A slightly longer energy transfer time correlates with weaker quenching of the PL intensity of por-Si in water than that in gaseous O\({}_{2}\). This fact indicates an influence of water molecules on the exciton recombination and/or energy transfer from the excitons to O\({}_{2}\) molecules. Indeed, the energy transfer is mostly efficient for nc-Si with S-H surface bonds, which are hydrophobic [77]. Therefore nc-Si dispersed in water should be partially oxidized that results in decreasing the \({}^{(1)}\)O\({}_{2}\) photosensitization efficiency [65]. The oxidation of nc-Si dispersed in water or aqueous solutions can be a limiting factor to achieve the stable and efficient \({}^{(1)}\)O\({}_{2}\) photosensitization. The oxidation process can be also stimulated during the photosensitization [81]. For biomedical applications of nc-Si in PDT it is obviously necessary to stabilize nc-Si surface against the \({}^{(1)}\)O\({}_{2}\) induced photooxidation. It can be done, for example, by using rapid thermal annealing or laser irradiation.