Enhanced upconversion emission in Yb3+ and Er3+ codoped NaGdF4 nanocrystals by introducing Li+ ions

beta-NaGdF4 : Yb3+/Er3+ nanoparticles (NPs) codoped with Li+ ions were prepared for the first time via a thermal decomposition reaction of trifluoroacetates in oleylamine. The influence of site occupancy of Li+ on the upconversion emission of beta-NaGdF4 : Yb3+/Er3+ NPs was investigated in detail. The upconversion emission intensity was significantly enhanced by introducing different concentrations of Li+ ions. In contrast to lithium-free beta-NaGdF4 : Yb3+/Er3+, the green and red UC emission intensities of the NPs codoped with 7 mol% Li+ ions were enhanced by about 47 and 23 times, respectively. The luminescence enhancement should be attributed to the distortion of the local asymmetry around Er3+ ions. The mechanisms for the enhancement of upconversion emission were discussed. In addition, it was found in our research work that beta-NaGdF4 : Yb3+/Er3+ NPs exhibited paramagnetic features at room temperature and the magnetization was slightly increased by introducing Li+ ions.