The influence of oxygen on optical absorption spectra, radiative lifetimes, and localization of states is analyzed within the quasiparticle formalism based on the GW approximation for defect-free Si nanocrystallites. Passivation with hydroxyls and oxide coverage have significant influence on the spatial distribution of the highest-occupied and lowest-unoccupied molecular orbitals as well as on the radiative lifetimes. Oxidation blueshifts the onset of absorption, whereas oxide coverage and passivation with hydroxyls tend to redshift it. The indirect-gap nature of the optical transitions in Si bulk is transferred to the Si nanocrystallites independently of the oxygen contents.

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