Well-aligned ZnO nanorods grown on silicon substrates buffered with ZnO film were postannealed at 200-1000 degrees C in various atmospheres. The optical properties of ZnO nanorods were much improved by annealing at high temperatures in both atmospheres of N-2 and O-2. This improvement was ascribed to the decrease in structure defects as compared to as-grown ZnO nanorods. However, for the ZnO nanorods annealed in H-2/N-2, a stronger UV emission occurs at 600 degrees C; above that the UV emission rapidly disappeared, which was attributed to the collapse of the ZnO nanorods due to H-2 etching during a higher-temperature annealing in H-2/N-2. The crystallinity and optical properties of the ZnO nanorods can be controlled by postannealing treatment. (c) 2006 American Vacuum Society.