Stark sublevels of Er3+–Yb3+ codoped Gd2(WO4)3 phosphor for enhancing the sensitivity of a luminescent thermometer
We report a strategy for enhancing the sensitivity of an optical thermometer by utilizing the Stark sublevels. Under 980 nm excitation, the upconversion emission originating from the Stark levels of Er3+ (4S3/2(2)/4S3/2(1) and 4F9/2/4F9/2(1)) is observed in a monoclinic phase Gd2(WO4)3:Er3+/Yb3+ phosphor, which is synthesized through the co-precipitation method. The temperature sensing behavior is studied over the range of 296–620 K based on thermal coupling levels 2H11/2/4S3/2(2), 2H11/2/4S3/2, 2H11/2/4S3/2(1), 4S3/2(2)/4S3/2(1) and 4F9/2(2)/4F9/2(1) using fluorescence intensity ratio (FIR) technology. The sensitivity based on Stark sublevels 2H11/2/4S3/2(2) or 2H11/2/4S3/2(1) is almost twice more than that based on the traditional 2H11/2/4S3/2 levels. The obtained maximum sensitivity of the optical thermometer is 16.5 × 10−3 K−1 at 395 K (2H11/2/4S3/2(2)). These results suggest that the use of Stark levels is a promising approach for enhancing the sensitivity of optical thermometers.