Upconversion luminescence of Ba9Y2Si6O24:Yb3+-Ln3+ (Ln = Er, Ho, and Tm) phosphors for temperature sensing

To develop new phosphor materials for optically thermal sensing, the Yb3+-Er3+, Yb3+-Ho3+ and Yb3+-Tm3+ doped Ba9Y2Si6O24 samples were prepared by the solid-state reaction method, and the orange-yellow, green and blue emissions for upconversion luminescence have been obtained, respectively. The phase composition of the samples was characterized by X-ray diffraction, indicating the pre-prepared phosphors are all single-phase. For the Ba9Y1.52Si6O24:0.4Yb3+,0.08Er3+ sample, it has been found that the intensity ratio of the 2H11/2-4I15/2 to 4S3/2-4I15/2 emissions of Er3+ shows a continuous increase with increasing temperature upon 980 nm excitation owing to the well-known thermally coupled levels of Er3+. It was also found that decay lifetime on 4S3/2 level of Er3+ changes with temperature. For the Ba9Y1.52Si6O24:0.4Yb3+,0.08Ho3+ phosphor, the green (5F4,5S2)-5I8 emission of Ho3+ exhibits a faster decay rate in intensity relative to the red 5F5-5I8 one when the measure temperature increases. The corresponding reason has been discussed by the energy level diagram and decay lifetime. For the Ba9Y1.58Si6O24:0.4Yb3+,0.02Tm3+ sample, it was found that the change of the emission intensity ratio of the Tm3+ 3H4-3H6 to (3F2,3F3)-3H6 transitions with the temperature follows the Boltzmann distribution. The sensitivities of the above phosphors were calculated, and the high sensitivity values indicate the present phosphors could be a promising candidate for optical temperature sensors.