Intense upconversion luminescence and energy-transfer mechanism of Ho3+/Yb3+ co-doped SrLu2O4 phosphor

A series of novel SrLu2O4: x Ho3+, y Yb3+ phosphors (x=0.005-0.05, y=0.1-0.6) were synthesized by a simple solid-state reaction method. The phase purity, morphology, and upconversion luminescence were measured by X-ray diffraction (XRD), scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. The doping concentrations and sintering temperature were optimized to be x=0.01, y=0.5 and T=1400 degrees C to obtain the strongest emission intensity. Under 980 nm laser diode excitation, the SrLu2O4: Ho3+, Yb3+ phosphors exhibit intense green upconversion (UC) emission band centered at 541 nm (F-5(4),S-5(2) -> I-5(8)) and weak red emission peaked at 673 nm (F-5(5) -> I-5(8)). Under different pump-power excitation, the UC luminescence can be finely tuned from yellow-green to green light region to some extent. Based on energy level diagram, the energy-transfer mechanisms are investigated in detail according to the analysis of pump-power dependence and luminescence decay curves. The energy-transfer mechanisms for green and red UC emissions can be determined to be two-photon absorption processes. Compared with commercial NaYF4: Er3+, Yb3+ and common Y2O3: Ho3+, Yb3+ phosphors, the SrLu1.49Ho0.01Yb0.5O4 sample shows good color monochromaticity and relatively high UC luminescence intensity. The results imply that SrLu2O4: Ho3+, Yb3+ can be a good candidate for green UC material in display fields.