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

Abstract

Blue-emitting calcium dialuminate (CaAl₄O₇) phosphor doped with thulium ions was prepared via a microwave-assisted combustion synthesis route for the first time, and a phosphor-in-glass (P-i-G) structure was fabricated. The samples crystallized in the monoclinic structure were validated through the powder X-ray diffraction studies. A wide bandgap of 4.60 eV was observed in the CaAl₄O₇:Tm³⁺ phosphor. Upon excitation at 359 nm, the Ca_(1−x)Al₄O₇:0.03Tm³⁺ sample generated a bright blue emission around 459 nm that was ascribed to the ¹D₂ → ³F₄ transition of Tm³⁺. The temperature-dependent measurements (−180 to 300 °C) revealed the presence of Tm²⁺ ions as a broad-band emission centered around 500 nm at cryogenic temperatures. The unusual enhancement in the signal intensity of Tm³⁺ with varying temperature could be beneficial for general lighting technology and LED devices. Owing to the 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 that operated in the cryogenic to high temperature range. The highest relative sensitivity (nearly 2% K⁻¹) was achieved for the 448/500 nm luminescence intensity ratio, which was associated with the Stark components of 4f–4f emission of Tm³⁺ and d–f emission of Tm²⁺.