Optical Properties of Silicon Nanocrystals: a First Principles Study

Ab initio quasiparticle gaps, self-energy corrections, exciton Coulomb energies, and optical gaps of Si nanocrystals are calculated using the higher-order finite difference pseudopotential method. The calculations are performed in real space on hydrogen-passivated Si clusters with diameters up to 30 Å (> 1000 atoms). The size-dependent self-energy correction is enhanced substantially compared to bulk, and quantum confinement and reduced electronic screening result in appreciable excitonic Coulomb energies. Calculated optical gaps are in very good agreement with absorption data from Si nanocrystals.