Calculations of the electronic structure of silicon quantum dots: oxidation-induced redshifts in the energy gap

We stud), oxidation-induced redshifts in the energy gap for spherical Si-30, Si-42 Si-87. Si-99 Si-167 and Si-191 dots (of 1-2 nm in diameter) passivated with hydrogen by self-consistent calculations using the extended Huckel-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 Si-167 or smaller Si dots, and has a much larger Si contribution in the largest Si-191 dot, so that the HOMO energy in the Si-167 or smaller Si dots depends significantly on the oxygen configuration, while that in the Si-191 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.