  In recent years many experiments have demonstrated the possibility to achieve
efficient photoluminescence from Si/SiO2 nanocrystals. While it is widely known
that only a minor portions of the nanocrystals in the samples contribute to the
observed photoluminescence, the high complexity of the Si/SiO2 interface and
the dramatic sensitivity to the fabrication conditions make the identification
of the most active structures at the experimental level not a trivial task.
Focusing on this aspect we have addressed the problem theoretically, by
calculating the radiative recombination rates for different classes of
Si-nanocrystals in the diameter range of 0.2-1.5 nm, in order to identify the
best conditions for optical emission. We show that the recombination rates of
hydrogenated nanocrystals follow the quantum confinement feature in which the
nanocrystal diameter is the principal quantity in determining the system
response. Interestingly, a completely different behavior emerges from the
OH-terminated or SiO2-embedded nanocrystals, where the number of oxygens at the
interface seems intimately connected to the recombination rates, resulting the
most important quantity for the characterization of the optical yield in such
systems. Besides, additional conditions for the achievement of high rates are
constituted by a high crystallinity of the nanocrystals and by high confinement
energies (small diameters).
