Improved stability of nanocrystalline porous silicon after coating with a polymer

The effect of light soaking (LS) on nanocrystalline porous silicon (nc-Si) is found to depend on the ambient conditions. Light soaking in air decreases the photoluminescence (PL) intensity and increases the number of dangling bond density measured by electron spin resonance (ESR) monotonically as the exposure time is increased. In vacuum, however, short light exposures increase the PL intensity and decrease the ESR signal, but longer exposures have the opposite effect. Thus PL intensity has a maximum as a function of exposure time. The temperature dependence of LS in vacuum is measured at 300, 325, and 350K. We found that the maximum PL intensity occurs at smaller exposure times as the temperature is increased. These results are explained in terms of two kinds of photostructural changes having opposite LS effects on the PL. Coating nc-Si with a thin layer of polystyrene stabilizes PL against light soaking as well as water vapor, without affecting the PL intensity significantly. Modified bonding configurations at the nc-Si/polystyrene interface, as observed by Fourier transform infrared, seem responsible for the improved stability.