ZnO nanorods alloyed with Mo/Er. The effect of post-deposition treatment on defect states and luminescence

Free-standing ZnO nanorods alloyed with Er/Mo were synthesized by the hydrothermal growth method. To characterize them, the number of experimental techniques was applied including X-ray diffraction (XRD), scanning emission microscopy (SEM), electron paramagnetic resonance (EPR), photo- and radioluminescence (PL, RL). EPR confirmed the existence of F⁺ centres common for ZnO-based structures in the ZnO:Er(30%) nanorods whereas in the ZnO:Mo(30%) this kind of defect was absent. Air annealing at elevated temperatures results in the reduction of F⁺ centres in all the materials studied. Moreover, Er³⁺ EPR signal also undergoes changes including broadening in the ZnO:Er. This allowed suggesting oxidation of Er ions on the ZnO nanorods surface. Red luminescence (~680 nm) appears in all studied samples regardless the dopant origin and doping level after the annealing in air. The exciton-related band at 380 nm never observed in the samples before the annealing appears upon the annealing at 350 °C in ZnO:Mo(10%) and ZnO:Er(30%). No such band was observed in the ZnO:Mo(30%) sample under the same conditions. According to SEM there are nanorods no more but microrods upon the content of Mo/Er as compared to the as-grown untreated ZnO as reported in a recent work.