Effect of annealing on upconversion luminescence of ZnO:Er3+ nanocrystals and high thermal sensitivity

The effect of annealing on the upconversion luminescence of ZnO:Er3+ 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 degrees C, the ratio of the intensity of H-2(11/2) -> I-4(15/2) emission to that of S-4(3/2) -> I-4(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 degrees 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 H-2(11/2) and S-4(3/2) states in the two samples, originating from the local microstructure variation around Er3+ ions. In addition, a high thermal sensitivity of 0.0062/degrees C was obtained in the ZnO:Er3+ 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.