  The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO2
matrix, both crystalline and amorphous, or free-standing with different
conditions of passivation and strain is analyzed through ab-initio approaches.
The Si32/SiO2 heterojunction shows a type I band offset highlighting a
separation between the NC plus the interface and the matrix around. The
consequence of this separation is the possibility to correctly reproduce the
low energy electronic and optical properties of the composed system simply
studying the suspended NC plus interface oxygens with the appropriate strain.
Moreover, through the definition of an optical absorption threshold we found
that, beside the quantum confinement trend, the amorphization introduces an
additional redshift that increases with increasing NC size: i.e. the gap tends
faster to the bulk limit. Finally, the important changes in the calculated
DFT-RPA optical spectra upon inclusion of local fields point towards the need
of a proper treatment of the optical response of the interface region.
