High Luminescence in small Si/SiO2 Nanocrystals: a theoretical study

In recent years, many experiments have demonstrated the possibility to achieve efficient photoluminescencefrom SiSiO2 nanocrystals. While it is widely known that only a minor portion of the nanocrystals in thesamples contributes to the observed photoluminescence, the high complexity of the SiSiO2 interface and thedramatic sensitivity to the fabrication conditions make the identification of the most active structures at theexperimental level not a trivial task. Focusing on this aspect, we have addressed the problem theoretically, bycalculating the radiative recombination rates for different classes of Si nanocrystals in the diameter range of0.2–1.5 nm, in order to identify the best conditions for optical emission. We show that the recombination ratesof hydrogenated nanocrystals follow the quantum confinement feature in which the nanocrystal diameter is theprincipal quantity in determining the system response. Interestingly, a completely different behavior emergesfrom the OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the interface seemsintimately connected to the recombination rates, resulting the most important quantity for the characterizationof the optical yield in such systems. Besides, additional conditions for the achievement of high rates areconstituted by a high crystallinity of the nanocrystals and by high confinement energies and mall diameters