Enhancement Mechanism of Temperature Sensitivity in Y ₂ WO ₆ -Based NIR-Upconverting Crystals through Atomic-Level Disorder

Near-infrared (NIR) photon-upconverting (UC) crystals are emerging as a useful class of material for contactless nanothermometry. Since the fluorescence intensity ratio (FIR) of the photoluminescent UC crystals is used as the basis of temperature sensing, enhancement of the FIR has remained the key challenge for the utilization of such materials in high-resolution nanothermometry. Herein, a rational strategy of crystal engineering was adopted to demonstrate that temperature sensitivity of Y2WO6-based UC crystals can be enhanced and optimized by tuning the atomic-level disorder of the host lattice, for the first time. A deliberate incorporation of crystal defects was undertaken systematically by synthesizing a series of Li+-doped Y2WO6-based UC crystals, codoped with Yb3+ and Er3+, crystallizing in the monoclinic P2/c space group. A structural disorder was sensed through the detection of compressive microstrain inside the lattice with observation of the enhancement of UC photoluminescence (PL) intensity. The excitation wavelength was 980 nm and the dominant emissions were Er3+: 2H11/2/4S3/2 → 4I15/2 (525/545 nm) and Er3+: 4F9/2 → 4I15/2 (658 nm). Further, the samples were subjected to the pair distribution function (PDF) analysis by the high-energy (λ = 0.55946 Å) synchrotron X-ray diffraction, using real space analysis of the diffraction patterns and also X-ray absorption fine structure (XAFS) study. Such atomic-level studies concluded that the composition (WO-3) that showed the highest absolute and relative temperature sensitivity contained the highest level of local disorder, which was associated with perturbation of local crystal field symmetry. The findings conformed to the hypothesis of the relaxation of the parity-forbidden selection rule through local symmetry distortion. The reported approach is expected to frame a general enhancement strategy of temperature sensitivity for NIR-UC crystal-based nanothermometry.