Influence of Ce3+ and Gd3+ co-doping on the structure and upconversion emission in hexagonal Ho3+ doped NaYbF4 phosphors

Hexagonal Ho3+ doped NaYbF4 phosphors are synthesized via a hydrothermal method. The influence of Gd3+ and Ce3+ content on the phase structure and upconversion (UC) emission of NaYbF4 phosphors is investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM) and UC spectra. The results of XRD and TEM indicate that the solubility of Ce3+ in hexagonal NaYbF4 is low due to the large difference of ionic radius between Ce3+ and Yb3+. With help of Gd3+ co-doping (15 mol%), pure hexagonal NaYbF4 phosphors with high doping concentration of Ce3+ (15 mol%) and small crystal size are obtained. When excited by a 980 nm laser diode, Ho3+ doped hexagonal NaYb0.85Gd0.15F4 phosphors exhibit strong green UC emission at 540 nm and weak red one at 646 nm. UC luminescence tuning from green emission to red emission is observed in hexagonal Ho3+ doped NaYb0.85Gd0.15F4 phosphors by co-doping with Ce3+ ions. The UC luminescence tuning phenomenon is attributed to two resonant energy transfer processes of 5S2/5F4(Ho3+)+2F5/2(Ce3+)→5F5(Ho3+)+5F7/2(Ce3+) and 5I6(Ho3+)+2F5/2(Ce3+)→5I7(Ho3+)+5F7/2(Ce3+) between Ho3+ and Ce3+, which suppress the green emission at 540 nm, while promote the red one at 646 nm.