Effect of Tm3+ concentration on upconversion fluorescence in Y2(MoO4)3:Tm3+,Yb3+ phosphors

Tm3+/Yb3+ co-doped Y2(MoO4)3 phosphors with various Tm3+ concentrations were prepared via a traditional high temperature solid state reaction method using NH4HF2 as a flux. The microstructure analysis by means of X-ray diffraction revealed that the as-synthesized samples presented a pure phase Y2(MoO4)3. Under 980 nm excitation, intense blue, near infrared (NIR) and weak red upconversion emissions which centered at 487, 800 and 649 nm were observed, and these emissions were corresponding to the 1G4→3H6, 3H4→3H6 and 1G4→3F4 transitions of Tm3+, respectively. The upconversion luminescence mechanisms of Y2(MoO4)3:Tm3+,Yb3+ phosphors were analyzed based upon the power dependent upconversion emissions. The results show that the populations of 1G4 and 3H4 levels come from three- and two-photon energy transfer upconversion processes, respectively. The possible upconversion mechanisms for the blue, red and NIR emissions were discussed based on the energy level diagrams of Tm3+ and Yb3+. In addition, with the increase of Tm3+ concentration, all of the emission bands obviously increase firstly and then decrease, which can be attributed to the concentration quenching. Meanwhile, the intensity ratio of the blue emission and the NIR emission decreases with the increase of Tm3+ concentration.