Implantation of Si+ ions into thermal oxides grown on silicon has been used to synthesise a two phase structure consisting of Si nanocrystals in a SiO2 matrix. Various processing conditions have been used in order to modify the size and population distributions of the Si inclusions. Photoluminescence spectra have been recorded from samples annealed in nitrogen, forming gas and oxygen. Both red and blue shifts of the luminescence peaks have been observed. It is concluded that the photoluminescence is a consequence of the effects of quantum confinement but is also dependent on the presence of irradiation-induced defects or Si/SiO2 interface states.