Using ultrasonic vibration of as-made porous silicon in water, the suspension of Si nanocrystals is prepared for exploring light-emitting properties of Si nanocrystals. By analyzing the photoluminescence (PL) property of the as-made suspension, we conclude that the observed red PL band arises from radiative recombination of carriers in the interface layer full of Si-H-x (x = 1, 2) and Si=O bonds. To effectively passivate the Si nanocrystal surface, we mixed the suspension with peroxide and then heated it. Spectral examinations show that the PL and PL excitation spectra all shift under different excitation wavelengths and at different monitoring wavelengths, respectively. This result illuminates clear evidence for the quantum confine effect of the Si nanocrystals. We suggest that the surfaces of the Si nanocrystals are covered by SiO2 after treated by peroxide, which leads to the currently observed blue PL phenomenon. (C) 2007 Elsevier B.V. All rights reserved.