Ab initio absorption spectra and optical gaps in nanocrystalline silicon

The optical absorption spectra of Si-n H-m nanoclusters up to similar to 25 0 atoms are computed using a linear response theory within the rime-depende nt local density approximation (TDLDA). The TDLDA formalism allows the elec tronic screening and correlation effects, which determine exciton binding e nergies, to be naturally incorporated within an ab initio framework. We fin d the calculated excitation energies and optical absorption gaps to be in g ood agreement with experiment in the limit of both small and large clusters . The TDLDA absorption spectra exhibit substantial blueshifts with respect to the spectra obtained within the time-independent local density approxima tion.