Electronic structure, upconversion luminescence and optical temperature sensing behavior of Yb3+-Er3+/Ho3+ doped NaLaMgWO6

To develop new upconversion (UC) phosphors of high sensitivity for temperature sensing, a series of Yb3+-Er3+ and Yb3+-Ho3+ doped NaLaMgWO6 (NLMW) samples were designed via solid-state reaction method. The phase purity was examined by XRD patterns, indicating the obtained samples are all single-phase. The morphology was characterized by SEM and TEM techniques, revealing smooth particle surface. The electronic structure of NLMW was analyzed theoretically, and the optical bandgap obtained was compared with the experimental results. Upon 980 nm excitation, the characteristic transitions of Er3+ and Ho3+ were found, and their optimal concentrations were determined to be 2 and 3 mol%, respectively. By investigating the temperature-dependence of NLMW:0.1 Yb3+,0.02Er3+, it was found that the thermally-coupled 2H11/2 and 4S3/2 levels of Er3+ cause a large fluorescence intensity ratio (FIR) for 528 and 550 nm emissions, which can generate a high absolute sensitivity (0.0107 K−1 at 508 K). The corresponding explanation was given by using the dependence of UC emissions on excitation power density. For the NLMW:0.1 Yb3+,0.03Ho3+ phosphor, the non-thermally-coupled (5F4,5S2) and 5F5 levels were employed to character the temperature-sensing behavior of Ho3+. High absolute sensitivity of 8.9×10−3 K−1 was gained at 548 K. The reason for the different change of 543 and 658 nm emissions intensities with temperature was conducted by the phonon-electron coupling.