Fluorescence enhancement and inverse Boltzmann distribution in Li+/Er3+ co-doped Y2O3 nanocrystals

Er3+/Li+ co-doped Y2O3 nanocrystals were synthesized by the sol-gel method. Fluorescence intensity has been enhanced about 270 times under 980 nm laser excitation by combined laser annealing (90 times) and co-doped with Li+ ions (3 times). The doping of Li+ ions increased the asymmetry around Er3+, while laser annealing reduced surface defects. Fluorescence characteristics with the temperature of laser annealed Er3+/Li+ co-doped Y2O3 nanocrystals were studied. An interesting phenomenon that the non-thermal coupling of I563/I799 transformed into the thermal coupling as the temperature rises is found. The possible mechanisms were discussed, and fluorescence thermometry based on I563/I799 showed higher temperature sensitivity than other reports. The effective fluorescence enhancement method and the new fluorescence thermometry energy levels can promote the application of rare earth fluorescent materials.