Calculations of the electronic structure of silicon quantum dots: oxidation-induced redshifts in the energy gap
We study oxidation-induced redshifts in the energy gap for spherical Si30, Si42, Si87, Si99, Si167 and Si191 dots (of 1?2 nm in diameter) passivated with hydrogen by self-consistent calculations using the extended H?ckel-type nonorthogonal tight-binding method for three different oxygen configurations (double-bonded, backbonded and inserted). While the nature of the lowest unoccupied molecular orbital (LUMO) state does not depend significantly on the dot size, the highest occupied molecular orbital (HOMO) state is associated closely with oxygen in the Si167 or smaller Si dots, and has a much larger Si contribution in the largest Si191 dot, so that the HOMO energy in the Si167 or smaller Si dots depends significantly on the oxygen configuration, while that in the Si191 dot does not. We find that the HOMO?LUMO energy gaps calculated for these Si dots double-bonded to oxygen are all dipole allowed, but gradually decrease from 2.2 eV to about 1.7 eV with increasing dot size, while the inserted oxygen configuration does not cause a significant energy-gap redshift even in the smallest Si dot. Finally, it is found that the energy gaps calculated for the Si dots backbonded to oxygen coincide well with luminescence redshifts observed in porous Si.