Phase evolution and upconversion luminescence enhancement investigation from YF3 to (α+β)-NaYF4 by doping of Cu2+ ion

A facile, but effective, method has been developed for enhancing the upconversion (UC) luminescence efficiency of YF3: Yb3+, Er3+ microcrystals through doping of Cu2+ ion, accompanied by phase transformation from YF3 to α-NaYF4 to (α+β)-NaYF4. The Cu2+ ion occupies the Y3+ ion crystal lattice site leading to positive vacancy and lower energy barrier in the reaction of forming α-NaYF4. In addition, we have also systematically investigated the phase transformation from YF3 to (α+β)-NaYF4. With the extension of the reaction time, surface energy of the [1 0 1¯0] crystal plane decreases dramatically while that of the [0 0 0 1] crystal plane increases accordingly, which may be the main reason for forming (α+β)-NaYF4. This synthesis strategy enables UC luminescence enhancement and simultaneous crystal phase transformation, which should inspire more even wider application of other UC host materials.