Investigation of near-infrared-to-ultraviolet upconversion luminescence of Tm3+ doped NaYF4 phosphors by Yb3+ codoping

2 mol% Tm3+ doped NaYF4 phosphors with 0–98 mol% Yb3+ codoping were synthesized by sol–gel method. The phase transition from the mixture of hexagonal and cubic phases to single cubic phase of Tm3+–Yb3+:NaYF4 phosphors was investigated with increasing of Yb3+ concentration. Near-infrared, red, blue, violet and ultraviolet upconversion emissions of Tm3+ were observed from the Tm3+–Yb3+:NaYF4 phosphors under 976 nm laser diode excitation, with the strongest near-infrared to ultraviolet emissions at 20 mol% Yb3+ codoping. The violet and blue emissions for the 1D2 → 3F4 and 1G4 → 3H6 transitions of Tm3+ can be tuned by varying Yb3+ codoping concentration, which was elucidated using steady-state equations. The intensity ratio of red emissions for the 3F2 → 3H6 and 3F3→3H6 transitions of Tm3+ was strongly related to the Yb3+ codoping concentration and temperature, implying a potential application of Tm3+–Yb3+:NaYF4 phosphors for optical temperature sensing.