Effect of Yb3+ concentration on upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ co-doped YNbO4 nanoparticles prepared via molten salt route

Yb3+/Er3+ co-doped YNbO4 nanoparticles (NPs) with an average size of 35 nm were prepared via modified molten salt method for the first time. The phase purity, crystal structure, morphologies, and upconversion luminescence (UCL) properties as well as quantum yield and temperature sensing behavior of the as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and upconversion luminescence (UCL) spectra, respectively. The XRD Rietveld refinements based on the XRD data were employed to reveal the phase purity and structure of the as-prepared samples. It was confirmed that the optimal doping concentration of Yb3+ ions in YNbO4:Yb3+, Er3+ NPs is around 10 mol%, which has a maximal quantum yield of 0.1%. The temperature sensing behavior of the as-prepared YNbO4:Yb3+, Er3+ NPs was studied based on the fluorescent intensity ratio (FIR) technique from two thermal coupled 2H11/2 and 4S3/2 levels. It was found that the temperature sensitivity was sensitive to the doping concentration of Yb3+ ions. In addition, the dependence of UCL colors on temperature was observed and the corresponding mechanism was proposed. Therefore, the as-prepared Yb3+/Er3+ co-doped YNbO4 NPs have double function of optical thermometer and safety sign for the high temperature environment.