Doping concentration-independent optical thermometric properties in Stark sublevels-based Er³⁺-activated BaGd₂O₄ luminescent thermometers

We reported a facile strategy to modify the sensor sensitivities of Er³⁺-doped BaGd₂O₄ phosphors by simultaneously modulating the dopant concentration and utilizing the proper Stark sublevels. Under the excitation of 377 nm, visible emissions originating from the Stark sublevels of Er³⁺ ions (²H_11/2(1), ²H_11/2(2), ⁴S_3/2(1), ⁴S_3/2(2), ⁴F_9/2(1) and ⁴F_9/2(2)) were detected in all the prepared samples. The optimal doping concentration for the Er³⁺ ions in the BaGd₂O₄ host lattice was 2 mol% and the electric dipole-dipole interaction can contribute to the concentration quenching mechanism. By utilizing the fluorescence intensity ratio technique, the optical thermometric properties of the obtained phosphors in the temperature range of 288–483 K were studied based on the thermally coupled levels of ²H_11/2(1)/⁴S_3/2(1), ²H_11/2(2)/⁴S_3/2(1), ²H_11/2(1)/⁴S_3/2(2), ²H_11/2(2)/⁴S_3/2(2) and ²H_11/2/⁴S_3/2. The maximum relative sensor sensitivities of the resultant compounds based on the Stark levels of ²H_11/2(1)/⁴S_3/2(2) were much higher than that based on the ²H_11/2/⁴S_3/2 ordinary thermally coupled levels. Furthermore, the relative sensor sensitivities of the synthesized phosphors were insensitive to the doping concentration and its maximum value was up to 0.0175 K⁻¹ at 288 K. These characteristics demonstrated that the Er³⁺-doped BaGd₂O₄ material system was suitable for optical thermometers.