Negative thermal quenching of photoluminescence in annealed ZnO–Al2O3 core–shell nanorods

Abstract

ZnO–Al₂O₃ core–shell nanorods (NRs) have been fabricated through the vapor phase condensation method and atomic layer deposition. It is found that the nanorod comprises a wurtzite single crystalline ZnO core with the main axes along the [0001] direction and an amorphous Al₂O₃ shell. The temperature-dependent photoluminescence (PL) properties of the as-grown and annealed ZnO/Al₂O₃ NRs are investigated systematically. The PL of the as-grown ZnO/Al₂O₃ NRs demonstrates a normal thermal quenching feature. However, the salient behavior of negative thermal quenching (NTQ), i.e., the increase in PL intensity with an increase in temperature, is clearly observed in the annealed ZnO/Al₂O₃ NRs. A multi-level model is adopted to account for this behavior and the thermal activation energy of the NTQ process is estimated to be ∼69 meV. Moreover, we suggest that the activation energy is related to the Al donor defect in ZnO resulting from the inter-diffusion between the ZnO core and the Al₂O₃ shell during the annealing process.