Temperature-dependence of efficient up-conversion luminescence in NaY(WO4)2 nanophosphor doped with Er3+ for cryogenic temperature sensor

The fluorescence-based temperature sensor using temperature-dependent fluorescence properties is a great candidate to replace traditional temperature-measurement detector like thermocouple, because the latter is inaccessible to conquer some special circumstances such as a corrosive environment. In this study, the nano-sized Er3+ doped NaY(WO4)2 phosphors were synthesized by solvothermal method. The origins of three emission peaks located at 527 nm, 549 nm and 665 nm were identified. The 2H11/2 → 4I15/2 (green emission 1) and 4S3/2 → 4I15/2 (green emission 2) transitions of the Er3+ ion possessed a temperature dependent behavior over the range of 300-30 K and it was proposed for temperature sensing (optical thermometry) using the fluorescence intensity ratio (FIR) method. The FIR value of nano-sized phosphors decreased with the decrease of temperature, and the experimental data were well fitted with a theoretical function. A large energy difference (1048.5) was obtained from the fitting curve, which indicate that the sensitivity of nano-sized NaY(WO4)2: Er is higher than that of micro-sized one prepared by high temperature solid state reaction. Particularly, at temperatures below 150 K, the sensitivity of nano-sized UC phosphors was enhanced obviously. The results suggested that this nano-sized phosphor could be a splendid option for next generation luminescence-based temperature sensing devices in cryogenic region.