Yb3+-Concentration dependent upconversion luminescence and temperature sensing behavior in Yb3+/Er3+ codoped Gd2MoO6 nanocrystals prepared by a facile citric-assisted sol-gel method

Yb3+/Er3+ codoped Gd2MoO6 upconversion nanocrystals were synthesized via the traditional citric-assisted sol-gel method. X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy and upconversion emission spectroscopy were employed to characterize the final products. Under the irradiation of 980 nm light, the synthesized nanocrystals emitted visible green and red upconversion emissions arising from the intra-4f transitions of Er3+ ions. The upconversion emission intensities were found to be dependent on the dopant concentration and the emission colour gradually changed from green to yellow, and finally to red on elevating the Yb3+ doping concentration. The involved upconversion luminescence mechanism was systematically studied and the steady-state rate expressions were applied to analyze the influence of energy back transfer on the multicolour upconversion emissions. Moreover, the temperature dependent emission intensity ratio of the (H-2(11/2), S-4(3/2)) thermally coupled levels of the Er3+ ion in the range of 303-703 K was recorded to study the optical thermometric properties of the resultant nanocrystals. The maximum sensor sensitivity of the Yb3+/Er3+ codoped Gd2MoO6 nanocrystals was determined to be as high as 0.0053 K-1 at 350 K. In addition, the temperature sensing performance of the synthesized compounds was found to be greatly dependent on the Yb3+ doping concentration. These results reveal that the Yb3+/Er3+ codoped Gd2MoO6 nanocrystals with high sensor sensitivity and wide operation range are suitable for non-contact optical thermometry.