Investigation of ultraviolet carrier recombination of Volmer-Weber type ZnO nanocrystals and nanorods through Varshni’s formula and Arrhenius plots

The carrier recombination in Volmer-Weber-type ZnO nanocrystals (VW ZnO NCs) and Volmer-Weber-type ZnO nanorods (VW ZnO NRs) synthesized by using a physical sputtering method and a hydrothermal method, respectively, was investigated by using temperature-dependent photoluminescence. The main mechanism of UV emission observed in both materials was attributed to the neutral donor bound to the exciton (D 0 X) and a two-electron satellite (TES). Using the energies of D 0 X and its TES, we found that the donor binding energy was higher than that of a ZnO crystal. The change in the main carrier recombination and the activation energy were estimated using Varshni’s formula and an Arrhenius plot, respectively. For the VW ZnO NCs, the change from D 0 X to free exciton (FX) was examined at 150 K, and an activation energy of 6.8 meV was obtained. For the VW ZnO NRs, no switch of the main carrier recombination was obvious at 150 K, but some deviations with different origins appeared at temperatures above 150 K on Varshni’s curve and the Arrhenius plot. The activation energy was double that of the VW ZnO NCs due to the existence of a number of donor impurities.