The surface-state-induced Stark effect in ZnO nanocrystals

Surface states in ZnO nanoparticles play an important role in controlling their electrical transport properties, and these properties can be modified greatly by exposure to UV light. In order to investigate the origin of these effects, we investigate the change in the optical absorption of films of ZnO nanoparticles on exposure to UV light. The modulation spectrum changes from the first derivative to the second derivative of the absorption spectrum as the films are annealed. UV illumination changes the surface states of the nanocrystals, leading to a change in the electric field within the films, which we study using electrostatic force microscopy. The modulation of the optical absorption is found to be consistent with a Stark effect caused by the change in the electric field.