Size-Dependent Surface Effects on the Photoluminescence in ZnO Nanorods

One-dimensional semiconductor nanostructures are expected to show significant surface effects, which results in remarkable modification of the optical properties. In this work, an experimental study of surface effects on the photoluminescence (PL) of ZnO nanorods with different sizes is reported. A thin shell layer of Al(2)O(3) is used to passivate the nanorods' surface, which allows one to compare the PL spectra before and after dielectric coating. It is found that strong surface exciton recombination is present in ZnO nanorods with average diameter as large as similar to 500 nm. Coating the nanorods by Al(2)O(3) significantly reduces the surface state-related emissions, indicating that surface passivation rather than surface band-bending mechanism dominates. We also provide evidence that the long controversial 3.31 eV emission in ZnO is not related to surface states but a free-to-bound transition involving an unknown acceptor level of similar to 125 meV. In the visible spectral region, an orange emission around 2.1 eV together with the normal green emission is observed in the thick nanorods. The little change in intensity after Al(2)O(3) coating allows us to conclude that the visible emissions are unlikely from the surface. Based on a DAP-like transition model, we are able to interpret the blue shift of the orange emission with increasing temperature and attribute the emission to zinc vacancy defects.