Electronic and optical properties of Si and Ge nanocrystals: an ab-initio study

First-principles calculations within density functional theory and many-body perturbation theory have been carried out in order to investigate the structural, electronic and optical properties of un- doped and doped silicon nanostructures. We consider Si nanoclus- ters co-doped with B and P. We find that the electronic band gap is reduced with respect to that of the undoped crystals, suggesting the possibility of impurity based engineering of electronic and optical properties of Si nanocrystals. Finally, motivated by recent sugges- tions concerning the chance of exploiting Ge dots for photovoltaic nanodevices, we present calculations of the electronic and optical properties of a Ge 35 H 36 nanocrystal, and compare the results with those for the corresponding Si 35 H 36 nanocrystals and the co-doped Si 33BPH36 .