| One of the most challenging issues in semiconductor physics is to engineer band structures for a particular device. Contemporary photovoltaic (PV) and photoelectrochemical (PEC) devices rely on defect energy levels and nano-scaling to customize their band structures. As the length scale of a material becomes comparable to the exciton Bohr radius the free particle behavior of charge carriers transition to bound states where energy levels are quantized. In this thesis, hybrid density functional theory has been used to study the electronic properties of silicon nanocrystals (SiNCs) having 75, 150 and 300 silicon atoms. The atomic coordinates were defined by two geometries (diamond and wurtzite) of bulk phase silicon. The global minimum energy structures for both geometries at each size were found for particular variation on magnetic moments, dopant, dopant position, and surface passivation with hydrogen. We report our results on bond lengths, binding energies, formation energies, HOMO-LUMO gaps, and density of states. We also report results on electronic occupations derived from Mulliken population analysis. Our results show that the SiNCs have tunable HOMO-LUMO gaps with respect to size and that the inclusion of noble metals produces inter-gap defect levels. In addition, we have found that hydrogen passivation affected the doping behavior significantly. Contrary to the general expectation, hydrogen passivation contributed to the energy levels near the highest occupied orbital. Overall, our results suggest the SiNCs can be used to construct optimal photovoltaic applications or used individually as photocatalysts. |
