## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Photosensitized Generation of Singlet Oxygen  
Since there is a similarity between the electronic structure of excitons in nc-Si and that of many organic molecules as dyes, the energy transfer through spin-split states of the latter can be also probable for the former. A bright example of the spin-dependent energy transfer from photoexcited dye molecules to molecular oxygen in its triplet state followed with its transformation into the singlet oxygen, that is, \({}^{(1)}\)O\({}_{2}\) where (1) in the superscript indicates the electron spin degeneracy, which is used in the photodynamic therapy (PDT) [63]. It was found that nc-Si in por-Si layer and powder can act as a photosensitizer of the \({}^{(1)}\)O\({}_{2}\) generation [64]. The observed photosensitization was explained by the spin-dependent energy transfer from excitons in nc-Si with sizes of 1-10 nm to O\({}_{2}\) molecules adsorbed on the nc-Si surface [65]. According to the quantum mechanics the relaxation of \({}^{(1)}\)O\({}_{2}\) back to the ground (triplet) state is \"spin-forbidden\" and thus a \({}^{(1)}\)O\({}_{2}\) molecule is characterized by rather long lifetime [66], which is favorable for transferring the excitation energy to oxidation reactions. The latter can be used for the detection and application of \({}^{(1)}\)O\({}_{2}\)[67]. The latter is known to be an electronically excited state of molecular oxygen [67] and it influences oxidative biochemical processes in living systems [68]. The \({}^{1}\)O\({}_{2}\)-mediated reaction, which is employed in PDT of cancer, consists of a drug, that is, photosensitizer of \({}^{(1)}\)O\({}_{2}\), administration followed with resonant photoexcitation of the drug [63]. The efficient \({}^{1}\)O\({}_{2}\) photosensitization by nc-Si is promoted by its huge specific surface area up to 10\({}^{3}\) m\({}^{2}\)/g [62] and by long radiative lifetimes (10\({}^{-5}\)-10\({}^{-3}\) s) of the triplet excitons confined in nc-Si [61]. The \({}^{(1)}\)O\({}_{2}\) generation by photoexcited nc-Si can be quantified by measuring the quenching of the exciton photoluminescence (PL) of nc-Si [64] and by analyzing the radiative decay of \({}^{(1)}\)O\({}_{2}\) molecules at 0.98 eV (1270 nm) as it is usually used for molecular sensitizer [68; 69]. In fact, the \({}^{(1)}\)O\({}_{2}\) photosensitization bypor-Si was successfully monitored by means of the PL spectroscopy of \({}^{(1)}\)O\({}_{2}\) emission at 0.98 eV [70]. While the \({}^{(1)}\)O\({}_{2}\) luminescence indicates a high efficiency of the photosensitization process [71], a strong deactivation (quenching) of \({}^{(1)}\)O\({}_{2}\) molecules in nc-Si assemblies is revealed from a comparison of the photosensitizing efficiency of the standard molecular sensitizer as porphyrin molecules and nc-Si [72]. It can be explained by considering a process of the nonradiative collisional quenching of \({}^{(1)}\)O\({}_{2}\) molecules on the nc-Si surfaces in por-Si layer and powder. The \({}^{(1)}\)O\({}_{2}\) quenching acts as a limiting factor to achieve high concentration of \({}^{(1)}\)O\({}_{2}\) due to the photosensitized generation in por-Si MPs [73]. A method of the electron paramagnetic resonance (EPR) is well known to detect \({}^{(1)}\)O\({}_{2}\) in gaseous ambient [67]. The conventional X-band EPR spectroscopy of photoexcited por-Si in O\({}_{2}\) atmosphere allowed us to estimate the \({}^{(1)}\)O\({}_{2}\) concentration photosensitized by nc-Si to be of the order of 10\({}^{19}\) cm\({}^{-3}\) under excitation with photon flux about 10\({}^{20}\) cm\({}^{-2}\)[74]. Similar results were obtained by means of the Q-band EPR spectroscopy, which demonstrated a conversion of about 30% of O\({}_{2}\) molecules in the singlet state upon excitation of por-Si powder in oxygen atmosphere [75]. The \({}^{(1)}\)O\({}_{2}\) photosensitization was shown to be also possible for por-Si dispersed in different oxygen-saturated organic and inorganic liquids [70] as well in water [76].