Density functional theory study on a 1.4 nm silicon nanocrystal coated with carbon

Abstract

Charge carrier transport associated with silicon nanocrystals (Si NCs) can be improved by removing hydrocarbon chains that are routinely attached to the NC surface by means of hydrosilylation. Thermal annealing for the hydrocarbon-chain removal may lead to carbon-coated Si NCs. But the optical behavior of carbon-coated Si NCs has not been clearly understood. By comparing a carbon-coated Si NC with those fully passivated by hydrogen (H) or coated with silicon oxide (SiO₂) in the framework of density functional theory, we find that carbon coating causes both the excitation energy and emission energy of the Si NC to significantly decrease. The carbon-coated Si NC exhibits a smaller Stokes shift than the fully H-passivated and SiO₂-coated Si NCs. The radiative recombination rate of the carbon-coated Si NC is two orders of magnitude lower than those of the fully H-passivated and SiO₂-coated Si NCs. The thermal removal of hydrocarbon chains at the NC surface is not recommended for Si-NC-based light-emitting devices because carbon-coated Si NCs with rather low light emission efficiency may be produced. In contrast, the carbon coating of Si NCs may be beneficial for Si-NC-based solar cells.