Acta Physica Sinica, Volume. 68, Issue 17, 174204-1(2019)
A series of the hexagonal-phase NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0% Ce3+@NaLuF4:x%Yb3+ core-shell (CS) nanocrystals with codoping different Yb3+ ions in the shell is successfully built by a sequential growth process. The crystal structures and morphologies of samples are characterized by X-ray diffractometer and transmission electron microscope. With the Yb3+ ion concentration increasing from 0% to 15% in NaLuF4 shell, none of the crystal structures, sizes, and morphologies of the samples changes obviously because of the similarity in ionic radius between Yb3+ and the ions in shell and the low doping concentration. Under 980 nm near-infrared (NIR) excitation, the NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ core nanocrystal produce green and red UC emission. And the red UC emission intensity is higher than green emission intensity. This is because two effective cross-relaxation processes happen between Ho3+ and Ce3+ ions, which results in the enhancement of the red emission. However, the overall emission intensity of NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ nanocrystal decrease compared with that of the NaLuF4:20.0%Yb3+/2.0%Ho3+ nanocrystal. Thus, to further enhance the red UC emission intensity in NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+ nanocrystal, the NaLuF4:20.0%Yb3+/2.0% Ho3+/12.0%Ce3+@NaLuF4:x%Yb3+ CS nanocrystal are prepared for blocking the excitation and emission energy, transmitting surface quenching center and getting more excitation energy through doping Yb3+ ions in NaLuF4 shell. It can be clearly seen that the red UC emission intensity of CS nanocrystal first increases and then decreases with Yb3+ ion concentration increasing. Meanwhile, the corresponding red-to-green ratio increases from 4.9 to 5.6. The highest red UC emission intensity is observed in each of the NaLuF4:20.0%Yb3+ /2.0%Ho3+/12.0%Ce3+@NaLuF4:10%Yb3+ CS nanocrystal because the Ho3+ ions get more energy through the following three ways: 1) Yb3+ (core)-Ho3+ (core); 2) Yb3+ (shell)-Ho3+ (core); 3) Yb3+ (shell)-Yb3+ (core)-Ho3+ (core). Thus, building CS nanocrystals is one of the most effective approaches in order to improve the UC efficiency by suppressing the non-radiative decay of activators in the core and getting more excitation energy through different energy transfer ways. These NaLuF4:20.0%Yb3+/2.0%Ho3+/12.0%Ce3+@NaLuF4:Yb3+ CS nanocrystals with red UC emission have great potential applications in biological field and multi-primary color.
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Xue-Wen Yan, Zhao-Jin Wang, Bo-Yang Wang, Ze-Yu Sun, Chen-Xue Zhang, Qing-Yan Han, Jian-Xia Qi, Jun Dong, Wei Gao.
Received: Mar. 28, 2019
Accepted: --
Published Online: Sep. 16, 2020
The Author Email: Gao Wei (gaowei@xupt.edu.cn)