  We compare, through first-principles pseudopotential calculations, the
structural, electronic and optical properties of different size silicon
nanoclusters embedded in a SiO2 crystalline or amorphous matrix, with that of
free-standing, hydrogenated and hydroxided silicon nanoclusters of
corresponding size and shape. We find that the largest effect on the
opto-electronic behavior is due to the amorphization of the embedded
nanocluster. In that, the amorphization reduces the fundamental gap while
increasing the absorption strength in the visible range. Increasing the
nanocluster size does not change substantially this picture but only leads to
the reduction of the absorption threshold, following the quantum confinement
rule. Finally, through the calculation of the optical absorption spectra both
in a indipendent-particle and many-body approach, we show that the effect of
local fields is crucial for describing properly the optical behavior of the
crystalline case while it is of minor importance for amorphous systems.
