The pure β-NaYF4: Yb3+, Er3+ hexagonal sub-microplates were successfully prepared by the combination of coprecipitation and hydrothermal methods using sodium citrate as chelator. The size of them is about 600 nm × 400 nm (side length × thickness). The obtained sample was divided into two parts and one of them was annealed in nitrogen at 300 °C for 2 hours. The crystal structure of the β-NaYF4: Yb3+, Er3+ hexagonal sub-microplates before and after annealing treatment is hexagonal phase. Under the excitation of 980 nm diode laser, the upconversion luminescence intensity the sample after annealing is much stronger than that of the sample without annealing treatment. High temperature annealing process improved the crystallization of the sample, resulting in the decrease of the nonradiative relaxation and the enhancement of the upconversion luminescence.
High temperature annealing treatment on Co doped ZnO bulks