We report a new solution route for the preparation of SiO2-capped silicon nanocrystals (Si NCs). The Si NCs terminated with SiO2 are fully characterized by transmission electron microscopy, X-ray diffraction, UV-vis absorption, photoluminescence decay and Fourier transform infrared spectra. The photoluminescence spectra reveal that the Si NCs solution emits green luminescence at 535 and 578 nm excited at 490 nm. The origin of the green luminescence of Si NCs is studied. Our theoretical calculations reveal that the green emission is due to the surface related localized states of self-trapped excitons rather than the purely quantum-confined states, which agrees well with the experimental results. A self-trapped exciton model is proposed to take into account the stepwise localization of electron and hole at the Si-SiO2 interface. From the localization energies the effective Bohr radii of the localized electrons and holes are estimated to be about 1.71 and 1.57 nm, respectively. (C) 2009 Elsevier B.V. All rights reserved.