Multifunctional Optical Thermometry Based on the Rare-Earth-Ions-Doped Up-/Down-Conversion Ba2TiGe2O8:Ln (Ln = Eu3+/ Er3+/ Ho3+/ Yb3+) Phosphors

The fabrication of a multifunctional sensor together with a widening temperature-sensing range is an essential challenge in optical thermometers especially for trivalent lanthanide-doped materials. Herein, we design a wide range, highly sensitive, and multifunctional thermometer by exploiting the emission spectrum of Eu³⁺ ions, and further detailed discussion has been made on the new temperature-sensing mechanism. The sensor can be operated between 358 and 548 K with a maximum relative sensitivity ( S_r) of 0.93% K^-1 at 358 K, which is higher than that of most temperature-sensing materials. A paramount superiority is that the calibration parameter can be directly calculated from the single Eu³⁺ emission spectrum, avoiding the demand of other calibrations, which realizes the coexistence of a simple structure and high precision. Furthermore, other up-conversion thermometers based on Er³⁺/Ho³⁺/Yb³⁺ co-doped Ba₂TiGe₂O₈ (BTG) phosphors as well as the down-conversion thermometer based on Eu³⁺-doped Ba₂TiGe₂O₈ (BTG:Eu³⁺) phosphor have been synthesized for comparison, and the results show that the novel thermometer (BTG:Eu³⁺) has a much higher sensitivity than that of the traditional thermometers (BTG:Er³⁺/Ho³⁺/Yb³⁺). In addition, the versatility of the phosphor (BTG:Eu³⁺) is simultaneously reflected in its applications to red phosphor for white-light emitting diodes (W-LEDs) and plant growth lamps. All of the results suggest that BTG:Eu³⁺ could be a good candidate with its highly sensitive S_r value for optical thermometry and as a safety sign in high-temperature environments.