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

Abstract

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.