Engineering Atomic-Scale Disorder for Enhanced Optical Thermometry in NIR Upconversion Materials

Abstract

Near-infrared (NIR) photon upconverting (UC) crystals have recently attracted considerable attention as promising materials for contactless nanothermometry. In such systems, temperature sensing is generally based on the fluorescence intensity ratio (FIR) of the emitted photoluminescence, and therefore improving FIR remains a major challenge for achieving high-resolution temperature detection. In this work, we present a crystal-engineering strategy to enhance the temperature sensitivity of Y2WO6-based UC crystals by tuning the atomic-scale disorder within the host lattice. A series of Li+-doped Y2WO6 crystals co-doped with Yb3+ and Er3+ were synthesized, all crystallizing in the monoclinic P2/c structure. Controlled incorporation of defects in the lattice generated structural disorder, which was identified through the presence of compressive microstrain. This structural modification was accompanied by a noticeable enhancement in upconversion photoluminescence intensity. To gain deeper insight into the local structural environment, pair distribution function (PDF) analysis was performed using high-energy synchrotron X- 0.55946 Å), allowing real-space examination of the diffraction data. In addition, X-ray absorption fine structure (XAFS) measurements were carried out to probe the local atomic arrangement. These complementary analyses revealed that the composition denoted as WO-3, which exhibited the highest absolute and relative temperature sensitivities, also possessed the greatest degree of local lattice disorder. The findings conformed the hypothesis of relaxation of the parity-forbidden selection rules, through local symmetry distortion enhances the optical transitions responsible for upconversion emission. Overall, this study highlights controlled lattice disorder as an effective strategy for improving the temperature sensitivity of NIR-UC materials for nanothermometry applications.