Ab initio calculation of the optical gap in small silicon nanoparticles

摘要

We have calculated the optical gap of small silicon nanocrystals, passivated by hydrogen, with diameters up to 12 A, using the high level Configuration Interaction Singiets (CIS) method based on both Hantree-Fock (HF) and density functional theory (DET). The hybrid nonlocal exchange-correlation functional of Becke and Lee, Yang and Parr, including partially exact exchange (B3LYP) was used throughout the calculations. Our results, which can be rather safely extrapolated for larger nanoparticles with diameters up to about 16 A, clearly show that for nanocrystals of this size, quantum confinement by itself cannot explain the observed visible photoluminescence since the calculated optical gap is definitely larger than 3.0 eV. Thus, additional factors, such as the presence of oxygen in the surface of the samples must play a significant role. This it in agreement with other theoretical calculations, and our own preliminary work for oxygen covered nanocrystals.