Density-functional and many body perturbation theory calculations have
been carried out in order to study the structural, electronic, and optical
properties both in ground and excited state configuration, of silicon
nanocrystals in different conditions of surface passivation and doping.
Starting from hydrogenated clusters, we have considered different Si/O
bonding geometries at the interface. We provide strong evidences that
not only the quantum confinement effect but also the chemistry at the
interface has to be taken into account in order to understand the physical
properties of these systems. In particular we show that only the
presence of a surface Si-O-Si bridge bond induce an excitonic peak in
the emission-related spectra, redshifted with respect to the absorption
onset, able to provide an explanation for both the observed Stokes shift
and the near-visible PL experimentally observed in Si-nc. For the silicon
nanocrystals embedded in a SiO2 matrix, the electronic and optical
properties are discussed in detail. The strong interplay between the
nanocrystal and the surrounding host environment and the active role
of the interface region between them is pointed out, in very good agreement
with the experimental results. Finally, concerning doping, we will
show that, thanks to electronic transitions between donor and acceptor
states present at the band edges and considering also the effect of
quantum confinement it is possible to engineer the absorption and emission
spectra of Si nanocrystals. For each considered system optical gain
calculations have been carried out giving some insights on the system
characteristics necessary to optimize the gain performance of Si-nc.
