2 mol% Tm3+ doped NaYF4 phosphors with 0-98 mol% Yb3+ codoping were synthesized by sal-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 D-1(2) -> F-3(4) and (1)G(4) -> H-3(6) 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 F-3(2) -> H-3(6) and F-3(3) -> H-3(6) transitions of Tm3+ was strongly related to the Yb3+ codoping concentration and temperature, implying a potential application of Tm3+ -> Yb3+:NaYE4 phosphors for optical temperature sensing. (C) 2013 Elsevier B.V. All rights reserved.