  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.
