Temperature dependence of radiative lifetimes, optical and electronic properties of silicon nanocrystals capped with various organic ligands.

Abstract

Despite the known temperature effects on the optical and photoluminescence properties of silicon nanocrystals (Si NCs), most of the density functional theory calculations thus far have been carried out at zero temperature, i.e., fixed atomic positions. We present a study of the effect of finite temperature on the radiative lifetimes and bandgaps of Si NCs capped with six different organic ligands, CH3, C2H5, C2H4Cl, C2H4OH, C2H4SH, and C2H4NH2. In addition, we show the differences in electronic and optical properties, as well as the wavefunctions (WFs) around the bandgap, of the capped Si NCs at zero temperature. We show that the NCs capped with alkyl and C2H4Cl ligands have larger HOMO-LUMO and optical absorption gaps compared to the C2H4NH2, C2H4OH, and C2H4SH capped NCs. We demonstrate that this big difference in both gaps comes from the increased contribution to the states at the top of the valence band from the NH2, OH, and SH groups of the C2H4NH2, C2H4OH, and C2H4SH ligands, respectively. Additionally, we assigned the rather weak dependence of the radiative lifetimes of C2H4NH2 capped NCs on the NC size to the slightly changing symmetry of the highly localized HOMO WF at the NH2 group. Furthermore, we demonstrate that the temperature effect on the radiative lifetimes and bandgaps is larger in alkyl and C2H4Cl capped Si NCs. We indicate that the decrease in radiative lifetime of the CH3 capped NCs with increasing temperature comes from the changing symmetry of the LUMO WF and the increased dipolar overlap between the HOMO and LUMO WFs. Finally, we show that there is a constant decrease in the bandgaps of the Si NCs with increasing size, with the bandgap change of CH3 capped NCs being larger compared to the bandgap change of the C2H4NH2 capped NCs.