Comprehensive studies of the Li⁺ effect on NaYF₄:Yb/Er nanocrystals: Morphology, structure, and upconversion luminescence

Impurity doping plays a critical role in altering the properties of target nanomaterials in terms of designed morphologies, crystal structures, and functionalities. In this work, we have performed a comprehensive investigation of the effect of Li⁺ doping on the morphology, crystal structure, and upconversion luminescence of NaYF₄:Yb/Er nanocrystals. Different Li⁺ sources, e.g., LiOA and LiOH, were used and the Li⁺ doping concentration varied from 0 to 100 mol%. The final product changes from hexagonal NaYF₄:Yb/Er to the mixture of cubic NaYF₄:Yb/Er and tetragonal LiYF₄:Yb/Er, and finally to pure tetragonal LiYF₄:Yb/Er. More importantly, at an ultra-low concentration of 0.5 mol% Li⁺ doping, as high as 34 times green and 101 times red emission enhancements are achieved.