Simultaneous morphology control and upconversion fluorescence enhancement of NaYF4:Yb,Er crystals through alkali ions doping

Abstract We report a facile oleic acid-assisted hydrothermal route to synthesize NaYF 4 :Yb/Er crystals doped by alkali ions (Li + or K + ions), and the effects of alkali ion doping on the crystal structure and upconversion (UC) fluorescence emission of the NaYF 4 :Yb,Er crystals are investigated in detail. With the increasing dosage of Li + ions in the NaYF 4 :Yb/Er crystals, their morphologies are changed from rod to disk and finally to polyhedron. However, the morphologies of NaYF 4 :Yb/Er crystals doped with K + ions almost keep the rod-like shape. In addition, the phase transition of Li x Na 1− x YF 4 crystals initiates when x is equal to or larger than 0.5, but the phase of K y Na 1− y YF 4 remains unchanged even if y reaches 0.85. Furthermore, the UC emission intensity of NaYF 4 :Yb/Er crystals reduces with the doping of Li + ions. On the contrary, the UC emission intensity of NaYF 4 :Yb/Er crystals increases as K + ions are introduced. It is interesting that the green and red UC emissions of the K 0 . 7 Na 0.3 YF 4 :Yb,Er crystal are high 8 and 7 times, respectively.