Effects of surface adsorption on the photoluminescence wavelength of silicon nanocrystal

Abstract

Surface-passivated silicon nanocrystallites were prepared by pulsed laser ablation in hydrogen gas, and the photoluminescence (PL) properties were measured by varying the ambient gas to reveal the effect of surface adsorption on recombination processes. The PL peak wavelength of the as-deposited sample was about 800 nm in vacuum. This peak shifted to about 660 nm when the sample was exposed to the atmosphere for about 5 min. This peak shift was partially reversible; the 660 nm peak red-shifted to 730 nm when the chamber was evacuated again. This reversible peak shift was not observed on exposure to pure oxygen gas, but the similar shift was observed in water vapor. These results indicate that this peak shift was caused by physical adsorption of water molecules on the surface of the Si nanocrystallites. Prolonged exposure results in a peak shift to about 580 nm, and this shift was not reversible. This result indicates that the defect created by oxidation create PL centers at 580 nm which is insensitive to the adsorption of molecules. A calculation with the semi-empirical molecular orbital method suggests that the adsorption of water molecules affects the lattice relaxation of the excited state.