Surface-Trapped Hole Diffusion in CdS and CdSe: The Superexchange Mechanism

In contrast to the conventional view that surface-trapped holes in CdS and CdSe nanoparticles are static and immobile, recent experimental and theoretical work has shown that these holes are dynamic and undergo diffusion along the surfaces of these particles. We present semiperiodic density functional theory calculations for nanocrystal surfaces of these materials and employ Green’s function methods to assess the mechanisms of trapped-hole diffusion. Bulk electronic states can serve as intermediate virtual states that link the donor and acceptor hole traps on the surface. The associated superexchange tunneling mechanism dominates over the conventional direct tunneling pathway. The methodology presented in this work is nonperturbative in nature and should be applicable to a wide range of other nanoscale problems in which underlying phenomena are strongly influenced by coupling to a dense manifold of electronic states.