On the Nature of Trapped-Hole States in CdS Nanocrystals and the Mechanism of their Diffusion

TL;DR

This study uses DFT modeling to analyze hole trapping and diffusion mechanisms on CdS nanocrystal surfaces, revealing that holes localize on sulfur atoms and diffuse via thermally activated hopping, resulting in slow surface diffusion.

Contribution

It provides a detailed theoretical analysis of hole trapping and diffusion in CdS nanocrystals, highlighting the nature of surface trap states and the hopping diffusion mechanism.

Findings

Holes localize on sulfur non-bonding orbitals near the valence band top.

Holes form nonadiabatic small polarons that hop between sulfur sites.

Surface hole diffusion is slow due to weak electronic coupling.

Abstract

Recent transient absorption experiments on CdS nanorods suggest that photoexcited holes rapidly trap to the surface of these particles and then undergo diffusion along the rod surface. In this paper, we present a semiperiodic DFT model for the CdS nanocrystal surface, analyze it, and comment on the nature of both the hole-trap states and the mechanism by which the holes diffuse. Hole states near the top of the valence band form an energetic near continuum with the bulk, and localize to the non-bonding sp$^3$ orbitals on surface sulfur atoms. After localization, the holes form nonadiabatic small polarons that move between the sulfur orbitals on the surface of the particle in a series of uncorrelated, incoherent, thermally-activated hops at room temperature. The surface-trapped holes are deeply in the weak-electronic coupling limit and, as a result, undergo slow diffusion.