Preparation and temperature sensing properties of Tm<sup>3+</sup>, Yb<sup>3+</sup> co-doped Bi<sub>2</sub>WO<sub>6</sub> upconversion luminescent materials

Tm³⁺ and Yb³⁺, with different concentrations, co-doped Bi₂WO₆ up-conversion luminescence materials are prepared by high temperature solid state method. The microstructure, upconversion emission spectra, and optical temperature sensing properties of the synthesized powders are characterized and analyzed. The X-ray diffraction results show that the doping of Tm³⁺ and Yb³⁺ ions has little effect on the orthorhombic structure of Bi₂WO₆ matrix material. Under the 980 nm excitation, the maximum emission intensity of Tm³⁺ ions is obtained when the doping concentration of Tm³⁺ and Yb³⁺ are 1% and 6%, respectively. The intensities of four emission peaks of Tm³⁺ in 1%Tm³⁺, 6%Yb³⁺:Bi₂WO₆ sample increase with the excitation pump power increasing from 199 to 400 mW. With the excitation power of 199–400 mW, the sample light intensity<i> I</i> and the excitation power <i>P</i>^<i>n</i> show a linear relationship. The relationship between the excitation pump power and the emission intensity of Tm³⁺ in this range is investigated. The four emission peaks of Tm³⁺ at 478, 650, 685 and 705 nm correspond to the <i>n</i> values of 1.01, 1.34, 1.77 and 1.75, respectively, indicating that the above emission peaks are derived from two-photon absorption. Under 980 nm excitation (power 379 mW), when the temperature increases from 298 to 573 K, the thermal coupling energy levels of Tm³⁺ in 1%Tm³⁺, 6%Yb³⁺:Bi₂WO₆ samples produce 705 and 685 nm emission whose intensities are increased by 28.4 times and 31.6 times, respectively. The relationship between the fluorescence intensity ratio of the thermal coupling energy levels (³F₃, ³F₂) of Tm³⁺ in the sample and the temperature is fitted. The maximum absolute temperature sensitivity of the sample is 0.00254 K^–1 at 298 K, and the maximum relative temperature sensitivity is 0.00144 K^–1. Under the same conditions, the relationship between the fluorescence intensity ratio of 705 and 650 nm produced by the non-thermal coupling energy level pair (³F₃, ¹G₄) and the temperature is fitted, and the maximum absolute temperature sensitivity is calculated to be 0.167 K^–1 at 573 K. The maximum relative temperature sensitivity is 0.0378 K^–1 at 298 K, which is 26 times higher than the relative maximum temperature sensitivity <i>S</i>_r of the thermal coupling level (³F₃, ³F₂).