In this work we investigate, by first-principles calculations, the structural, electronic and optical properties of: (1) oxygenated silicon-based nanoclusters of different sizes in regime of multiple oxidation at the surface, and (2) hydrogenated Si nanoclusters (H-Si-nc) in their ground and excited state configurations. Structural relaxations have been fully taken into account in all cases through total energy pseudopotential calculations within density functional theory.
In the first case we have varied systematically the number of Si=O bonds at the cluster surface and found a nonlinear reduction of the energy gap with the Si=O bond number. A saturation limit is reached, which allows us to provide a consistent interpretation of the photoluminescence (PL) redshift observed in oxidized porous silicon samples. Our results help also to explain some very recent findings on the single silicon quantum dot photoluminescence bandwidth.
In the second case, after a preliminary study of the clusters stability, the properties of the ground and excited states have been compared varying the cluster dimensions from 1 to 29 Si atoms. Ab-initio calculations of the Stokes shift as a function of the cluster dimension will be presented. A structural model linked to the four level scheme recently invoked to explain the experimental outcomes relative to the observed optical gain in Si-nc embedded in a SiO2 matrix will be also suggested.
We present ab initio results for the structural, electronic and optical properties of silicon nanostructures confined by silicon dioxide. We investigate the role of the dimension, symmetry and bonding situations at the interfaces. In particular we consider Si/SiO2 superlattices and Si nanocrystals embedded in SiO2 matrix. In the case of Si/SiO2 superlattices the presence of oxygen defects at the interface and the dimensionality are the key points in order to explain the experimental outcomes concerning photoluminescence. For Si nanocrystals embedded in SiO2 we show, in agreement with experimental results, the close interplay between chemical and structural effects on the electronic and optical properties.
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