Bismuth(III)-Doped NaYbF:Tm Fluorides with Highly Efficient Upconversion Emission under Low Irradiance.

Concentration quenching of upconversion (UC) luminescence (UCL) is a common phenomenon in rare-earth-doped materials that seriously restricts the concentration of the activator and sensitizer and withholds their UC emissions and quantum yields. In particular, it remains a tremendous challenge to develop one novel strategy based on the introduction of trivalent bismuth (Bi) ions to exceed the typical thulium (Tm) ion concentration and reach high-efficiency UC under low illumination. In this work, the Tm accommodation capacity can be increased from 2.0 to 8.0 mol % in NaYbF:Tm materials with the assistance of Bi ions, which maintains strong UC emissions with large absolute UC quantum yields under low illumination. Specifically, the total upconversion quantum yield (UCQY) of the as-obtained Na(TmYbBi)F (8Tm60Yb32Bi) sample can reach as high as 1.45% upon continuous-wave (CW) laser excitation at 40 W cm. Strikingly, the total UCQY still remains at a high level (0.41%) even though the CW power density decreases to 1.5 W cm. Moreover, the intrinsic mechanism of the breakthrough in the threshold of concentration quenching of UCL by Bi ions was also fully explored. These advances in enhancing UC emissions and UCQYs under a low pump power density offer exciting opportunities for important photonic applications.