Trends for the crystallinity, optical and electrical properties of post-thermal annealed ZnO nanorods

Hexagonal ZnO nanorods have been prepared by a simple low-temperature hydrothermal method using ZnCl2 and aqueous ammonia as reactants. The crystallinity, photoluminescence (atomic defects) and charge transport properties of ZnO can be tuned by post-thermal annealing (in the range of 200–500 °C under argon atmosphere). It was found that (1) the crystallinity of the nanorods first increases and then decreases with the annealing temperature, and the optimal annealing temperature was found to be about 300 °C; (2) nanorods with the highest crystallinity exhibited the highest photoluminescence intensity and the highest band-edge to defect-related emission intensity ratio; (3) the nanorods became conducting rather than semiconducting upon annealing treatment, and nanorods with the highest crystallinity and photoluminescence intensity revealed the highest current densities. These trends provide insights for tuning ZnO opto-electronic properties for various applications.