Monodisperse, shape-selective synthesis of YF₃:Yb³⁺/Er³⁺ nano/microcrystals and strong upconversion luminescence of hollow microcrystals

Synthesizing upconversion materials with 3-dimensional structures is of fundamental importance in understanding the relationship between their optical properties and their structures. In this paper, we report a simple but useful synthetic method to produce highly monodisperse 3-dimensional YF₃:Yb³⁺/Er³⁺ microcrystals showing variable upconversion efficiencies. By tuning the pH, precursor salt, and solvent, the shapes of the YF₃:Yb³⁺/Er³⁺ crystals could be tuned from hollow hexagon to hollow dumbbell, tetragonal bipyramid, and truncated tetragonal bipyramid structures. The upconversion efficiency showed significant dependence on the structure of YF₃:Yb³⁺/Er³⁺; hollow structures revealed a large enhancement in the luminescence intensity. The upconversion efficiency of the structures decreased in the order hollow hexagon > hollow dumbbell > tetragonal bipyramid > truncated tetragonal bipyramid.