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

First-principles calculations within Density Functional Theory and Many-Bodyperturbation Theory have been carried out in order to investigate the structural,electronic and optical properties of undoped and doped silicon nanostructures.We consider Si nanoclusters co-doped with B and P. We ¯nd that the electronicband gap is reduced with respect to that of the undoped crystals suggesting thepossibility of an impurity based engineering of electronic and optical propertiesof Si nanocrystals. Finally, driven by recent suggestions concerning the chanceof exploiting Ge dots for photovoltaic nanodevices, we present calculations of theelectronic and optical properties of a Ge35H36 nanocrystal, and compare the re-sults with those of the corresponding Si35H36-nc and the co-doped Si33BPH36.