An effective way to improve near-infrared-II luminescence and optical thermometry performance via Gd3+ incorporation in Er3+, Yb3+: ZrO2 nanocrystals

The optical nano-thermometers with near-infrared-II (NIR-II) luminescence (1000 ∼ 1700 nm) have drawn growing interest owing to the unique merits such as deep-tissue penetration and excellent spatial resolution. Herein, we report a simple way to improve the NIR-II luminescence and temperature sensing sensitivity simultaneously in Er3+/Yb3+: ZrO2 nanocrystals with the introduction of Gd3+ ions. X-ray diffraction and transmission electron microscopy measurements validate that the addition of Gd3+ ions can change the host lattice parameter and decrease the crystalline grain size of the samples. The optical investigation shows that the remarkable enhancements in the NIR-II emissions of Yb3+ (1036 nm) and Er3+ (1527 nm) are realized via Gd3+ doping under 980 nm excitation, which can be interpreted based on the accelerated radiative transition rate and non-radiative Yb3+-Er3+ energy transfer rate. Meanwhile, the thermal sensing sensitivity of the samples shows a strong dependence on the Gd3+ concentrations, when the fluorescence intensity ratio between Yb3+: 2 F 5 ∕ 2 → 2 F 7 ∕ 2 and Er3+: 4I 13 ∕ 2 → 4I 15 ∕ 2 transitions is chosen for temperature sensing. Both the optimal NIR-II luminescence and thermal sensitivity are achieved in 1 mol%Gd3+ co-doped sample. The proposed route here may help in the development of efficient NIR-II luminescence materials for thermometry in the biological and medical fields.