Size-dependent oxygen-related electronic states in silicon nanocrystals

Silicon nanocrystals embedded in ${\mathrm{SiO}}_2$ were isolated with a selective etching procedure, and the isolated nanocrystals’ excitonic emission energy was studied during controlled oxidation. Nanocrystals having initial diameters, $d_0$ , of $\sim 2.9–3.4\mathrm{nm}$ showed a photoluminescence $\left(\mathrm{PL}\right)$ blueshift upon oxidatively induced size reduction, as expected from models of quantum confinement. Oxidation of smaller $\mathrm{Si}$ nanocrystals $\left(d_0\sim 2.5–2.8\mathrm{nm}\right)$ also initially resulted in a $\mathrm{PL}$ blueshift, but a redshift in the $\mathrm{PL}$ was then observed after growth of $\sim 0.3$ monolayers of native oxide. This decrease in excitonic emission energy during oxidation is consistent with the theoretically predicted formation of an oxygen-related excitonic recombination state.