Effect of Annealing on Upconversion Luminescence of ZnO:Er³⁺ Nanocrystals and High Thermal Sensitivity

The effect of annealing on the upconversion luminescence of ZnO:Er 3+ nanocrystals was investigated in detail. The green and the red upconverted emissions under infrared 978-nm light excitation were remarkably enhanced with an increase of annealing temperature. Moreover, for the sample annealed at 500 °C, the ratio of the intensity of 2 H 11/2 → 4 I 15/2 emission to that of 4 S 3/2 → 4 I 15/2 emission increased from less than to more than unity with an increase of the excitation density. However, the same case did not occur to the sample annealed at 700 °C, where the ratio was independent of excitation density except when the excitation density was higher than 42 700 W/cm 2 . This distinction was attributed mainly to the difference in energy gap between the 2 H 11/2 and 4 S 3/2 states in the two samples, originating from the local microstructure variation around Er 3+ ions. In addition, a high thermal sensitivity of 0.0062/°C was obtained in the ZnO:Er 3+ nanocrystals based on the temperature-dependent fluorescence intensity ratio (FIR) of the green upconverted emission, which would make this material a promising candidate for the nanoscaled thermal sensor of high accuracy and resolution.