High-sensitivity optical thermometer operating in cryogenic and high temperature ranges based on luminescence of divalent and trivalent thulium ions in CaAl4O7:Tm2+/3+

Blue-emitting calcium dialuminate (CaAl₄O₇) phosphor doped with thulium ions was prepared by the microwave-assisted combustion synthesis route for the first time, including fabrication of the phosphor-in-glass (P-i-G) structure. The samples crystallize in the monoclinic structure as validated through the powder X-ray diffraction studies. A wide bandgap of 4.60 eV was found in the CaAl₄O₇:Tm³⁺ phosphor. Upon excitation with 359 nm, the Ca_(1-x)Al₄O₇:0.03Tm³⁺ sample generated a bright blue emission around 459 nm that can be ascribed to the ¹D₂→³F₄ transition of Tm³⁺. The performed temperature-dependent measurement (-180 to 300 ^oC) revealed the presence of Tm²⁺ ions, as a broad-band emission centered around 500 nm, observed in the cryogenic temperatures. The observed unusual enhancement in the signal intensity of Tm³⁺ with temperature may be beneficial for general lighting technology and LED devices. Thanks to diverse thermal quenching rates and thermalization processes of Tm²⁺ and Tm³⁺ emissions we were able to develop a highly sensitive, multi-parameter (ratiometric) luminescent thermometer operating from cryogenic to high temperature range. The highest relative sensitivity (nearly 2% K^-1) was achieved for the 448/500 nm luminescence intensity ratio, associated with Stark component of Tm³⁺ 4f-4f emission and d-f emission of Tm²⁺.