Ab initio study on oxidized silicon clusters and silicon nanocrystals embedded in SiO2: Beyond the quantum confinement effect

Density-functional theory calculations have been carried out in order to study the structural, electronic, and optical properties of oxidized silicon clusters and silicon nanocrystals embedded in SiO2. For the isolated clusters, different Si/O bonding geometries and various levels of oxidation have been investigated, checking also the dependence of the results on the structure size. We provide strong evidences that not only the quantum confinement effect but also the chemistry at the interface has to be taken into account in order to understand the physical properties of these systems. In particular we show how the multiple presence of silanonelike Sidouble bondO bonds can be a reliable model for explaining the photoluminescence redshift observed in oxidized porous silicon samples and it can be used as possible explanation also for the unexpected large photoluminescence bandwidth in single oxidized Si quantum dots. For the silicon nanocrystals embedded in a SiO2 matrix, the electronic and optical properties are discussed in detail. The strong interplay between the nanocrystal and the surrounding host environment and the active role of the interface region between them is pointed out, in very good agreement with the experimental results.