The electronic structure and luminescence properties of porous silicon and silicon nanoclusters

We present the results of ab initio Hartree-Fock and configuration interaction calculations performed on selected small silicon clusters with a view to study their luminescent properties. We found that the covalent character of bonding is much reduced at the surface of the silicon clusters. The charge density of the clusters considered tends to be polarized towards their bulk-like core. This reflects the ‘self-passivation’ character of the chemical bonding in clusters. The optical transitions in the Si18 cluster are found to have essentially many-electron character. The energies of three lowest optical transitions are found to be in the near-IR region and the corresponding oscillator strengths are 0.0001–0.004. Our results suggest that there is a strong dependence of the optical properties of silicon nanostructures not only on their size but also on their shape, which strongly affects the charge density and the character of correlation between the excited electrons.