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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 Si 18 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.

In order to study the electronic properties of porous silicon (por-Si) we have investigated silicon nanoclusters with atomic configurations satisfying the general features of the radial and angular distribution functions of the por-Si structure modelled using the diffusion-limited aggregation model. The electronic structure calculations were performed within the intermediate neglect of differential overlap approximation of the Hartree-Fock-Roothaan scheme, while the restricted configuration interaction was employed in calculating excited states and transition rates. Both the electronic structure and optical transition strength are found to depend strongly on the cluster size and local environment. Importantly, the inclusion of many-electron effects is crucial in determining the transition rates. The low reactivity of some nanoclusters, together with their intermediate size and well-determined electronic properties, make them prospective initial blocks for the manufacturing of optoelectronic materials with desired characteristics.

We have implemented a modified diffusion-limited aggregation model to simulate the porous silicon structure obtained by electrochemical dissolution. The resulting fractal structures were fully equilibrated using the molecular dynamics method. An analysis of the relaxed structure shows it to be quite stable with the presence of one-, two- and three-coordinated atoms as well as the four-coordinated atoms found in bulk silicon. It is suggested that the different substructures or nanocrystals might be responsible for the observed photoluminescence