Silicon nanocrystals have been synthesized in an SiO2 matrix using Si ion implantation. Si implantations have been performed at 35 keV with doses from I x 10(16) cm(-2) to 7 x 10(16) cm(-2), corresponding to peak excess concentrations of 3 at.% to 17 at.%. The samples have been annealed in vacuum at various temperatures to obtain Si nanoclusters of different sizes. Photoluminescence (PL) experiments show a distinct shift of the spectra towards lower energies and a decrease in intensity with increasing a annealing temperature between 700 degrees C and 900 degrees C for doses higher than 5 x 10(16) cm(-2). We also observe for these samples that the peak position appears at lower energy for higher doses, Additional annealing in H-2/N-2 results in a strong increase of the PL intensity which is attributed to the passivation of defects. Annealing the samples again in vacuum leads to a reproduction of the former PL spectra showing the reversibility of the passivation, These experiments are in qualitative agreement with the quantum confinement model and in contradiction to recent observations made on Ge nanocrystals in a SiO2 matrix (K.S. Min et al., Appl. Phys. Lett., submitted).