Effect of annealing temperature on the morphology and optical properties of ZnO tetrapods grown by a thermal evaporation technique

The effect of the thermal annealing temperature was investigated on ZnO tetrapods grown by a thermal evaporation method. The ZnO tetrapods were synthesized by thermal evaporation of Zn powder in air. The annealing was done in an oxygen gas environment at temperatures ranging from 400 to 1000 °C for 1 h. As the annealing temperature increased from 400 °C to 800 °C, the morphology of the tetrapod remained unchanged; however, the size of the tetrapods increased. With a further increase in the annealing temperature from 800 °C to 1000 °C, the ZnO tetrapod changed drastically to nanoneedles. As-grown and annealed samples had an identical crystal structure, which was a wurtzite structure. A strong and sharp ultraviolet emission at 380 nm was observed for the 600 °C -annealed sample indicating the high crystalline quality. The ultraviolet emission intensity decreased abruptly for the samples annealed at 800 °C and 1000 °C, which exhibits the degradation in crystallinity.