Photoluminescence properties of surface-oxidized Ge nanocrystals: Surface localization of excitons

We have studied photoluminescence (PL) properties of surface-oxidized Ge nanocrystals with a 3.7 nm diameter. A difference between the absorption and PL excitation spectra and time-resolved PL measurements show that the site for the radiative recombination of excitons is different from that for the photogeneration of excitons. From fine structures in resonantly excited PL spectra at low temperatures, we concluded that excitons are localized on the surface of Ge nanocrystals and that the strong coupling of excitons with local vibrations of germanium oxides at the surface cause these fine structures.