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We have investigated the different mechanisms of photoluminescence (PL) of silicon nanocrystals due to the quantum confinement effect (QCE) and interface states. Si nanocrystals were formed by pulsed-laser deposition in inert argon and reactive oxygen gas. The collisions between the ejected species greatly influence the morphology of the Si nanocrystals and cause a transition from a film structure to a porous cauliflower-like structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The oxygen content of the Si nanocrystals increases with increasing O2 ambient pressure, and nearly SiO2 stoichiometry is obtained when the O2 pressure is higher than 100 mTorr. Broad PL spectra are observed from Si nanocrystals. The peak position and intensity of the PL band at 1.8–2.1 eV vary with ambient gas pressure, while intensity changes and blueshifts are observed after oxidation and annealing. The PL band at 2.55 eV shows vibronic structures with periodic spacing of 9769 meV, while no peak shift is found before and after oxidation and annealing. Raman and transmission electron microscope measurements show consistent results in crystal size while more accurate atomic force microscope measurements reveal a smaller crystal size. X-ray diffraction reveals a polycrystal structure in the Si nanocrystals and the crystallinity improves after annealing. Combined with the PL spectra of Si nanocrystals obtained by crumbling electrochemically etched porous Si layer, the results clearly demonstrate that the PL band at 1.8–2.1 eV is due to the QCE in the Si nanocrystal core, while the PL band at 2.55 eV is related to localized surface states at the SiOx /Si interface.