Concentration quenching of blue upconversion luminescence in Tm3+/Yb3+ co-doped Gd2(WO4)3 phosphors under 980 and 808 nm excitation

This paper reported on the upconversion (UC) spectroscopic properties of Tm3+/Yb3+ codoped Gd-2(WO4)(3) phosphors synthesized by a traditional co-precipitation method upon excitation of 980 and 808 nm commercial available fiber lasers, respectively. X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) were used to characterize the crystal structure and morphology of the samples. Intense blue emission ((1)G(4) -> H-3(6)) and near-infrared (NIR) emission (H-3(4) -> H-3(6)) as well as weak red emission ((1)G(4) -> F-3(4)) of Tm3+ were observed under 980 nm excitation. The concentration quenching processes for blue and NIR emissions were discussed. The blue emission intensity has a cubic dependence on the fiber laser working current, indicating a three-photon process involved in blue UC emission. The blue emission and its concentration quenching process for Tm3+/Yb3+ codoped Gd-2(WO4)(3) phosphors were also investigated under 808 nm excitation. Weak blue emission (D-1(2) -> F-3(4)) was detected in the UC spectra other than the weak red emissions ((1)G(4) -> F-3(4) -> F-3(2,3) -> H-3(6)). The dominant mechanism for blue emission under 808 nm excitation was attributed to the back energy transfer (BET) Tm3+ -> Yb3+ and the energy transfer (ET) Yb3+ -> Tm3+ processes. Upon 808 nm excitation, a two-photon process was deduced from the quadratic dependence of blue UC emission intensity on the fiber laser working current. (C) 2017 Elsevier B.V. All rights reserved.