Dominant red upconversion emission in LiErF₄-based nanoparticles by reducing spontaneous emission of Yb³⁺

In this study, a series of LiErF4:Tm@LiYF4:Yb,Er,Ce@LiYF4 core/shell/shell (C/S/S) UC nanoparticles (UCNPs) were prepared by a co-precipitation method. In this design of C/S/S structure, Er3+ is used as an energy collector in the first shell, which can reduce the energy loss by absorbing spontaneous emission from Yb3+. Then, by adding appropriate concentrations of Ce3+ into the same shell, the cross relaxation effect between Er3+ and Ce3+ is exploited to improve the efficiency of Er3+ in collecting the energy of spontaneous emission from Yb3+. During the upconversion (UC) process, Yb3+ plays a role as an energy transfer bridge, transferring the energy back to Er3+ in the core, thus enhancing the red UC emission intensity. Meanwhile, Tm3+ acts as an energy trapping center that contributes to the red emission. The results show that the spontaneous emission loss of Yb3+ has a certain effect on the UC fluorescence intensity, which will weaken the fluorescence to some extent, however is not the cause of the substantial decay of fluorescence. Therefore, this study provides clues for researchers to identify main factors affecting the luminescence efficiency, which will help develop more efficient UCNPs.