Yb3+/Tm3+ and Yb3+/Ho3+ doped NaY9(SiO4)6O2 phosphors: upconversion luminescence processes, temperature-dependent emission spectra and optical temperature-sensing properties

Abstract The improvement of sensitivity is an important issue for optical temperature-sensing materials. In this work, a series of oxide-based upconversion phosphors were designed by a conventional solid state reaction method. The XRD patterns revealed that the as-prepared samples are single-phase. The optimal Tm3+/Ho3+ doping concentrations were determined, and the luminescence quenching features were studied by using the energy level diagram and decay curves. The temperature-dependent emission spectra were measured to evaluate the optical temperature-sensing properties. It was found that the fluorescence intensity ratio (FIR) for emissions from thermally-coupled levels of Tm3+ follows Boltzmann distribution, and the FIR for emissions from non-thermally-coupled levels of Ho3+ can be fitted by a linear relation. Both the absolute and relative sensitivities were calculated and compared for different strategies in the present system. High relative and absolute sensitivities were obtained by employing I697/I789 of Tm3+ and I660/I548 of Ho3+, respectively. Besides, the population channel on the excited states of Ho3+ was clarified by analyzing the emission spectra excited by different ways under various temperatures.