Bi3+ Effects on Down-/up-conversion Luminescence, Temperature Sensing and Optical Transition Properties in Bi3+/Er3+ Co-doped YNbO4  Phosphors

A series of Bi 3+ single-doped and Bi 3+ /Er 3+ co-doped YNbO 4 phosphors with various concentrations of Bi 3+ ions were prepared by a conventional high temperature solid-state reaction method. The results of XRD and Rietveld refinement confirmed that monoclinic phase YNbO 4 samples were achieved. The down-/up-conversion luminescence of Er 3+ ions were investigated under the excitation of ultraviolet light (327 nm) and near infrared light (980 nm). Under 327 nm excitation, broad visible emission band from Bi 3+ ions and characteristic green emission peaks from Er 3+ ions were simultaneously observed, while only strong green emissions from Er 3+ ions were detected upon excitation of 980 nm. Remarkable emission enhancement was observed in down-/up-conversion luminescence processes by introducing Bi 3+ ions into Er 3+ -doped YNbO 4 phosphors. By analyzing the laser working current dependent up-conversion luminescence spectra, two-photon processes were confirmed to be responsible for both the green and the red up-conversion emissions of Er 3+ ion. The temperature sensing property of Er 3+ was studied by using the temperature dependent up-conversion luminescence spectra and it was found that the temperature sensitivity was sensitive to the doping concentration of Bi 3+ ions. By comparing the experimental values of the radiative transition rate ratio of the two green emission levels of Er 3+ ions and the theoretical values calculated by Judd-Ofelt (J-O) theory, it was concluded that energy level splitting had significant influences on the temperature sensing property of Er 3+ ions.