Theory of optical properties of polysilanes: Comparison with porous silicon

Abstract

We have studied the electronic structure of \ensuremath{\sigma}-bonded silicon chains containing between 2 and 66 silicon atoms. Stable atomic configurations are obtained by minimization of the total energy. The exciton spectrum is calculated by diagonalization of the full matrix of the Coulomb interaction taking into account the electron-hole exchange interaction. We predict important atomic relaxations that give polaronic effects for free molecules and explain the observed Stokes shifts. We show that the luminescence properties of silicon chains strongly depend on the interactions between the molecules and the medium in which they are embedded. The energy, the size, and the radiative lifetime of excitons are computed and their dependence on the size of the molecules is analyzed. We deduce that chains of silicon atoms are probably not at the origin of the visible luminescence of porous silicon but could explain its fast band in the blue region.