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⁺ co-doped Bi₂WO₆ up-conversion luminescence materials with different concentrations were prepared by high temperature solid state method. The microstructure, upconversion emission spectra and optical temperature sensing properties of the synthesized powders were characterized and analyzed. XRD results show that the doping of Tm³⁺ and Yb³⁺ ions has little effect on the orthorhombic structure of Bi₂WO₆ matrix materials. Under 980nm excitation, the maximum emission intensity of Tm³⁺ was obtained when the doping concentration of Tm³⁺ and Yb³⁺ was 1% and 6%, respectively. The intensity of four emission peaks of Tm³⁺ in 1% Tm³⁺, 6% Yb³⁺: Bi₂WO₆ samples increased with the increase of excitation pump power from 199mW to 400mW. Under the excitation power of 199mW-400mW, the sample light intensity I and the excitation power Pⁿ showed a linear relationship. The relationship between the excitation pump power and the emission intensity of Tm³⁺ in this range is calculated. The four emission peaks of Tm³⁺ at 478nm, 650nm, 685nm and 705nm correspond to the n 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 980nm excitation (power 379mW), when the temperature increases from 298K to 573K, the thermal coupling energy levels of Tm³⁺ in 1% Tm³⁺, 6% Yb³⁺: Bi₂WO₆ samples produce 685nm and 705nm emission intensity increased by 31.6 times and 28.4 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 was fitted. The maximum absolute temperature sensitivity of the sample was 0.00254K^-1 at 298K, and the maximum relative temperature sensitivity was 0.00144K^-1. Under the same conditions, the relationship between the fluorescence intensity ratio of 705nm and 650nm 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 0.167K^-1 at 573K. The maximum relative temperature sensitivity is 0.0378K^-1 at 298K, which is 26 times higher than the relative maximum temperature sensitivity S_r of the thermal coupling level (³F₃, ³F₂).