Nd³⁺/Yb³⁺ co-doped Y₂Mo₃O₁₂ phosphor for high-sensitivity near-infrared contactless thermometry via fluorescence...

Near-infrared (NIR) luminescent thermometers based on rare-earth co-doped phosphors offer compelling advantages for non-invasive temperature sensing in biomedical and industrial environments. In this work, a Y₂Mo₃O₁₂ phosphor co-doped with 1 mol% Nd³⁺ and 10 mol% Yb³⁺ (denoted YMO: 1% Nd/10% Yb) was prepared via the sol gel method and comprehensively characterized as a NIR optical thermometer. Phase identification and Rietveld refinement of X-ray diffraction (XRD) data confirmed single-phase formation with the expected orthorhombic crystal structure, while scanning electron microscopy (SEM) revealed the surface morphology and grain distribution of the sintered material. Diffuse reflectance spectroscopy and Kubelka-Munk analysis yielded an optical band gap of 2.77 eV, consistent with the transparency of the Y₂Mo₃O₁₂ host in the visible-to-NIR spectral window and its suitability for photoluminescence applications. Temperature-dependent NIR emission spectra were collected under 325 nm laser excitation across the physiologically and industrially relevant range of 298-543 K. The spectra exhibit well-resolved emission bands centered at approximately 802, 874, 974, and 1058 nm, attributed to the ⁴F_5/2 → ⁴I_9/2 and ⁴F_3/2 → ⁴I_9/2 transitions of Nd³⁺ and the ²F_5/2 → ²F_7/2 transition of Yb³⁺. All emission intensities display systematic, reversible temperature dependence, confirming the thermal stability and reproducibility of the luminescent response. Thermometric performance was evaluated using the fluorescence intensity ratio (FIR) technique based on four emission band pairs: I ₈₀₂/I ₈₇₄, I ₈₀₂/I ₁₀₅₈, I ₈₀₂/I ₉₇₄, and I ₈₇₄/I ₉₇₄. The Nd³⁺ thermally coupled level (TCL)-based FIRs (I ₈₀₂/I ₈₇₄ and I ₈₀₂/I ₁₀₅₈) follow the Boltzmann thermalization model, whereas the Nd³⁺/Yb³⁺ cross-ion FIRs (I ₈₀₂/I ₉₇₄ and I ₈₇₄/I ₉₇₄) exhibit energy-transfer-controlled temperature dependence and were fitted using polynomial functions. Among the pairs examined, the I ₈₀₂/I ₉₇₄ ratio delivers superior thermometric performance, achieving a maximum relative sensitivity of S _r = 2.57% K^-1 at 298 K, a value that compares favorably with state-of-the-art rare-earth NIR thermometers reported in the literature. The reversibility and repeatability of the FIR response further support the potential of YMO: Nd/Yb as a robust, high-sensitivity platform for contactless optical thermometry.