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

This paper reported on the upconversion (UC) spectroscopic properties of Tm3+/Yb3+ codoped Gd2(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 (1G4 → 3H6) and near-infrared (NIR) emission (3H4 → 3H6) as well as weak red emission (1G4 → 3F4) 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 Gd2(WO4)3 phosphors were also investigated under 808 nm excitation. Weak blue emission (1D2 → 3F4) was detected in the UC spectra other than the weak red emissions (1G4 → 3F4, 3F2,3 → 3H6). 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.