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.