Silicon nanocrystals (Si-ncs) have been produced by implantation of Si+ in excess into SiO2 followed by both annealing and passivation using argon or nitrogen. Nitrogen increases the photoluminescence (PL) emission and shifts the spectra toward the blue. The measured Si-nc diameter is 4.3 and 3.8 nm after annealing performed under Ar and N2, respectively. A significant quantity of nitrogen atoms has been detected in all samples by resonant nuclear reaction analysis (RNRA). The nitrogen concentration is significantly higher when the annealing and passivation are performed in a nitrogen environment, in agreement with a larger Si–N vibration signal on the Raman spectra. The depth profiles of nitrogen are very similar to those of Si-nc, suggesting that the N2 molecules may diffuse in the SiO2 during the annealing and then are trapped in proximity to the Si-nc. In addition to Si+, the implantation of N2+ to concentrations of 3 and 6 at. % produced a decrease in the PL intensity (accentuated at the higher concentration) and an increase in the Raman signal associated to Si–N vibrations. These results suggest that a relatively low nitrogen atomic fraction enhances the PL emission, since a large nitrogen concentration impedes the formation of Si-nc thus significantly decreasing the PL intensity.