Enhanced upconversion luminescence and controllable phase/shape of NaYF4:Yb/Er crystals through Cu2+ ion doping

Hexagonal phase (beta-) NaYF4:Yb/Er upconversion (UC) microrods doped with Cu2+ ions were prepared via a facile solvothermal method. Simultaneous phase and shape control of NaYF4:Yb/Er crystals with enhanced upconversion luminescence (UCU was successfully realized by codoping with Cu2+ ions. With an increasing Cu2+ ion concentration in the NaYF4:Yb/Er system, the crystal structure changed from a mixture of alpha- and beta-phases to the pure beta-phase and was converted into highly uniform microrods. As for the Cu2+ ion doped beta-NaYF4:Yb/Er upconversion microrods, the maximum emission occurs with the sample added with 60 mol% Cu2+ ions and the UCL intensities of the green and red emissions were enhanced 37 and 25 times, respectively. Meanwhile, the possible mechanisms for the enhanced UCL of NaYF4:Yb/Er crystals were also discussed in detail. Moreover, the temperature dependent UCL properties of 60 mol% Cu2+ ion doped beta-NaYF4:Yb/Er were studied under 980 nm excitation, which were temperature-sensitive over a wide temperature range from 123 K to 423 K. The change in UCL intensity influenced by temperature was discussed in detail.