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+.