Synthesis and upconversion luminescence of Lu2O3:Yb3+,Tm3+ nanocrystals

Lutetium oxide nanocrystals codoped with Tm3+ and Yb3+ were synthesized by the reverse-like co-precipitation method, using ammonium hydrogen carbonate as precipitant. Effects of the Tm3+, Yb3+ molar fractions and calcination temperature on the structural and upconversion luminescent properties of the Lu2O3 nanocrystals were investigated. The XRD results show that all the prepared nanocrystals can be readily indexed to pure cubic phase of Lu2O3 and indicate good crystallinity. The experimental results show that concentration quenching occurs when the mole fraction of Tm3+ is above 0.2%. The optimal Tm3+ and Yb3+ doped molar fractions are 0.2% and 2%, respectively. The strong blue (490 nm) and the weak red (653 nm) emissions from the prepared nanocrystals were observed under 980 nm laser excitation, and attributed to the 1G4 → 3H6 and 1G4 → 3F4 transitions of Tm3+, respectively. Power-dependent study reveals that the 1G4 levels of Tm3+ can be populated by three-step energy transfer process. The upconversion emission intensities of 490 nm and 653 nm increase gradually with the increase of calcination temperature. The enhancement of the upconversion luminescence is suggested to be the consequence of reducing number of OH− groups and the enlarged nanocrystal size.