Visible photoluminescence from oxidized Si nanometer-sized spheres: Exciton confinement on a spherical shell

We report strong visible photoluminescence (PL) at room temperature from oxidized Si nanometer-sized spheres with a spherical crystalline Si (c-Si) core and an amorphous ${\mathrm{SiO}}_2$ (a-${\mathrm{SiO}}_2$) surface layer. The peak energy of the broad PL spectrum is about 1.65 eV, which is independent of the core diameter. We propose a model in which excitons are confined on a spherical shell, an interfacial layer between the c-Si core and the a-${\mathrm{SiO}}_2$ surface layer, and in which the exciton confinement enhances the oscillator strength and the PL intensity.