Citric-assisted sol-gel based Er 3+ /Yb 3+ -codoped Na 0.5 Gd 0.5 MoO 4 : A novel highly-efficient infrared-to-visible upconversion material for optical temperature sensors and optical heaters

The Er3+/Yb3+-codoped Na0.5Gd0.5MoO4 phosphors were prepared via a facile sol-gel method. Under 980 nm excitation, the characteristic emissions of Er3+ ions were observed in all the samples and the upconversion (UC) emission intensity increased gradually with the increment of Yb3+ ion concentration, achieving its optimum value when the doping concentration was 0.11 mol. Meanwhile, the UC mechanism for both green and red emissions in Er3+/Yb3+-codoped Na0.5Gd0.5MoO4 phosphors was a two-photon process. Furthermore, the fluorescence intensity ratio of two green UC emissions from the thermally-coupled energy levels (2H11/2, 4S3/2) of Er3+ ions was studied as a function of temperature in the range of 298–778 K. The maximum sensitivity was determined to be around 0.00856 K−1 at 590 K, which was independent of the UC emission intensity. The heating effect which was induced by the laser excitation was also investigated. It was found that the temperature of sample varied from 265.143 to 328.094 K with elevating the pump power from 334 to 1032 mW. These results demonstrated that the Er3+/Yb3+-codoped Na0.5Gd0.5MoO4 phosphors may have potential applications in optical temperature sensors and optical heaters.