Energy transfer in Tb~（3＋）,Yb~（3＋） codoped Lu_2O_3 near-infrared downconversion nanophosphors

Tb3＋ and Yb3＋ codoped Lu2O3 nanophosphors were synthesized by the reverse-strike co-precipitation method. The obtained Lu2O3：Tb3＋,Yb3＋ nanophosphors were characterized by X-ray diffraction （XRD） and photoluminescence （PL） spectra. The XRD results showed that all the prepared nanophosphors could be readily indexed to pure cubic phase of Lu2O3 and indicated good crystallinity. The Tb3＋→Yb3＋ energy transfer mechanisms in the UV-blue region in Lu2O3 nanophosphors were investigated. The experimental results showed that the strong visible emission around 543 nm from Tb3＋ （5D4→7F5） and near-infrared （NIR） emission around 973 nm from Yb3＋ （2F5/2→2F7/2） of Lu2O3：Tb3＋,Yb3＋ nanophosphors were observed under ultraviolet light excitation, respectively. Tb3＋ could be effectively excited up to its 4f75d1 state and relaxed down to the 5D4 level, from which the energy was transferred cooperatively to two neighboring Yb3＋. The Yb3＋ concentration dependent luminescent properties and lifetimes of both the visible and NIR emissions were also studied. The lifetime of the visible emission decreased with the increase of Yb3＋ concentration, verifying the efficient energy transfer from the Tb3＋ to the Yb3＋. Cooperative energy transfer （CET） from Tb3＋ to Yb3＋ was discussed as a possible mechanism for the near-infrared emission. When doped concentrations were 1 mol.% Tb3＋ and 2 mol.% Yb3＋, the intensity of NIR emission was the strongest.