Local-fields and disorder effects in free-standing and embedded Si nanocrystallites

The case study of a 32-atoms Si nanocrystallite (NC) embeddedin a SiO2 matrix, both crystalline and amorphous, or freestandingwith different conditions of passivation and strain isanalyzed through ab-initio approaches. The Si32/SiO2 heterojunctionshows a type I band offset highlighting a separationbetween the NC plus the interface and the matrix around. Theconsequence of this separation is the possibility to correctlyreproduce the low energy electronic and optical properties ofthe composed system simply by studying the suspended NCplus interface oxygens with the appropriate strain. Moreover,through the definition of an optical absorption threshold wefound that, beside the quantum confinement trend, theamorphization introduces an additional redshift that increaseswith increasing NC size, i.e. the gap tends faster to the bulklimit. Finally, the important changes in the calculated DFTRPAoptical spectra upon inclusion of local fields point towardsthe need of a proper treatment of the optical response of theinterface region.