Effect of heat-treatment temperature on upconversion luminescence of β-NaYF₄:Yb³⁺, Er³⁺ nano/microparticles

β-NaYF₄:20%Yb³⁺, 2%Er³⁺ nano/microparticles were successfully synthesized by solvothermal and thermal decomposition method. According to the results obtained with TG-DTA, the as-synthesized samples were heat-treated at different temperatures in air. The microstructure, morphology and upconversion luminescence property of annealed samples were characterized by XRD, SEM, FT-IR and photoluminescence (PL) spectra. The results show that the total upconversion luminescence intensity of nano/microparticles firstly increases and then decreases with a rise of temperature. Comparing with the as-prepared samples, appropriate heat treatment temperature (580°C) can improve upconversion luminescence property of nanoparticles. The results in present research indicate that the improvement of crystallization, the decrease in the concentration of defects and the decomposition of the organic molecules on the surface of the particles induce the enhancement of upconversion luminescence. However, the intensity of upconversion luminescence decreases sharply under higher temperature treatment (≤580°C), which may be due to the phase transition (β→α) and the existence of Na₂CO₃. The differences in luminescence property between nanoparticles and microparticles are mainly attribute to different agglomeration degree of particles during thermal treatment.