and emission spectra were taken on a Hitachi F-4600 fluorescence spectrophotometer with xenon lamp as the light source, the slit width of 5 nm, the excitation wave- length of 360 nm, and the scan speed of 1200 nm/min. The afterglow decay curves were recorded by a PR- 305 Long Rays Fluorescence Tester (Zheda Sensing In- struments Co., Ltd.) with excitation illumination of 1000 lx and excitation time of 15 min, and were tested 10 s after switching off the excitation source.