The case study of a 32-atoms Si nanocrystallite (NC) embedded in a SiO2 matrix, both crystalline and amorphous, or freestanding 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 by 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 DFTRPA optical spectra upon inclusion of local fields point towards the need of a proper treatment of the optical response of the interface region.
