  Several models of oxygenated and hydrogenated surfaces of Si quantum wells
and Si nanocrystals of variable shapes have been constructed in order to assess
curvature effects on energy gaps due to the three Si-suboxides. Si-suboxides in
partially oxydized models of nanocrystals of spherical shapes (or quantum dots)
are shown to reduce energy gaps compared to the hydrogenated nanocrystals,
consistent with previous results in the literature. This trend is shown to be
reversed when planar interfaces are formed in Si-NC inside SiO2 thin films, as
in Si quantum wells. At planar interfaces (or surfaces) the electronic charge
density is shown to become extended and to distribute among the Si-suboxides,
thus generating along these planar interfaces extended, or delocalized, states.
This delocalization of the electronic states then increase the energy gap
compared to equivalent hydrogenated interfaces (or surfaces). Determination of
geometric effects of curvature on the band gaps are based on density functional
theory (DFT) using the real-space numerical approach implemented in the
``Pseudopotential Algorithm for Real-Space Electronic Structure'' (PARSEC)
program. A method for measuring interface effects due to Si-suboxides in Si-NC
in SiO2 is also suggested: by comparing photoluminescence (PL) between as-grown
and annealed Si-NC samples. The DFT calculations suggest that blueshift of more
than 0.2 eV of the PL should be observed in as-grown samples having
Si-suboxides at their planar interfaces, in comparison to annealed samples
above 950 degree Celcius that have fewer or no Si-suboxides once annealed,
providing that the bulk of Si-NC remains unaltered.
