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

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 4F5/2 → 4I9/2 and 4F3/2 → 4I9/2 transitions of Nd3+ and the 2F5/2 → 2F7/2 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.