## 3 Electron Spin-Dependent Processes in Si Nanocrystals
### Photosensitized Generation of Singlet Oxygen
The latter parameter controls the energy transfer probability, which is larger for nc-Si in as-prepared por-Si with hydrogen-terminated surface [65]. For oxidized nc-Si the increased spacing between confined excitons and adsorbed O\({}_{2}\) molecules is of the order of double the length of the Si-O bond (\(\sim\)3 A). This critically affects the efficiency of the electron exchange  
Figure 23: PL spectra (curves) of por-Si and the corresponding spectral dependences of the PL decay time (symbols) for different strength of the quenching induced by O\({}_{2}\) molecule adsorption at \(T\) = 5 K: in vacuum (curve 1, squares), intermediate level of the quenching (curve 2, circles) and strong quenching (curve 3, triangles). (From Reference [65]). interaction. Contrary to a strong coupling for hydrogen-terminated nanocrystals, the PL quenching efficiency is reduced by orders of magnitude if a thin oxide barrier is present. The degree of spatial overlap of the electronic wave functions of confined excitons and adsorbed oxygen molecules depends on the size of nc-Si. For smaller nc-Si the overlap becomes better and the probability for energy transfer is higher. On the other hand, for smaller NPs a larger number of emitted phonons is required to conserve the energy, which lowers the probability for energy exchange. The interplay between these two tendencies results in an almost spectrally constant efficiency of the energy transfer. While the involved transitions are spin forbidden in isolated nc-Si and O\({}_{2}\) molecules, they become allowed through exchange interaction [65]. To demonstrate the role of the electron spin statistics for the energy transfer rate, the magnetic-field dependence of the PL quenching efficiency was studied [65]. A magnetic field introduces the quantization axis for electron spins and it results in the lifting of the energy state degeneracy in both O\({}_{2}\) molecules and nc-Si. In general, the number of possible states participating in the electron exchange is reduced and the decreased energy transfer rate results in a weaker PL quenching. At low temperature the magnetic field results in preferential occupation of \"spin-down\" states for both O\({}_{2}\) and excitons, while to proceed with energy exchange \"spin-up states\" are required. The PL quenching was observed at temperature of 10 K in magnetic fields up to 10 T as shown in Figure 24. For the spectral region above the \({}^{1}\Sigma\) state, the high quenching level of the PL intensity prevented the observation of magnetic field effects within the experimental detection sensitivity. Besides the quenching of exciton PL, the photosensitized generation of \({}^{(1)}\)O\({}_{2}\) can be observed by measuring the infrared PL related to the \({}^{(1)}\)O\({}_{2}\) radiative de-excitation [64; 65]. The relevant time scale of the infrared PL was determined from time-resolved measurements. In the presence of adsorbed O\({}_{2}\) molecules the radiative decay of the \({}^{1}\Delta\) state of \({}^{(1)}\)O\({}_{2}\) was observed at detection energy of 0.953 eV. The initial fast decay is attributed to the defect-related PL background. The slow component with the lifetime \(\tau_{\Delta}\approx\) 0.5 ms accounts for the \({}^{1}\Delta\rightarrow\)\({}^{3}\Sigma\) transition in O\({}_{2}\). This lifetime is significantly shorter that the radiative lifetime of a free O\({}_{2}\) molecule. The shortening of \(\tau_{\Delta}\) can be explained by the interaction of the \({}^{1}\Delta\) state of \({}^{(1)}\)O\({}_{2}\) with nc-Si surface. The same interaction should also influence the radiative lifetime \(\tau_{\tau}\) of adsorbed \({}^{(1)}\)O\({}_{2}\) molecules. Firstly, the quantum yield \(\eta_{\
u}\approx\) 7\(\cdot\)10\({}^{-5}\) of the radiative decay of the \({}^{1}\Delta\) state is estimated from the comparison from the integral intensity of the exciton PL band. The radiative lifetime of \({}^{(1)}\)O\({}_{2}\)\(\tau_{\tau}\approx\) 7 s is obtained by using the simple relation \(\tau_{\tau}=\tau_{\Delta}\)/\(\eta_{\tau}\).