Enhanced red upconversion fluorescence emission of Ho<sup>3+</sup> ions in NaLuF<sub>4</sub> nanocrystals through building core-shell structure

Abstract

A series of the hexagonal-phase NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0%Ho<sup>3+</sup>/12.0% Ce<sup>3+</sup>@NaLuF<sub>4</sub>:<i>x</i>%Yb<sup>3+</sup> core-shell (CS) nanocrystals with codoping different Yb<sup>3+</sup> 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 Yb<sup>3+</sup> ion concentration increasing from 0% to 15% in NaLuF<sub>4</sub> shell, none of the crystal structures, sizes, and morphologies of the samples changes obviously because of the similarity in ionic radius between Yb<sup>3+</sup> and the ions in shell and the low doping concentration. Under 980 nm near-infrared (NIR) excitation, the NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0%Ho<sup>3+</sup>/12.0%Ce<sup>3+</sup> 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 Ho<sup>3+</sup> and Ce<sup>3+</sup> ions, which results in the enhancement of the red emission. However, the overall emission intensity of NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0%Ho<sup>3+</sup>/12.0%Ce<sup>3+</sup> nanocrystal decrease compared with that of the NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0%Ho<sup>3+</sup> nanocrystal. Thus, to further enhance the red UC emission intensity in NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0%Ho<sup>3+</sup>/12.0%Ce<sup>3+</sup> nanocrystal, the NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0% Ho<sup>3+</sup>/12.0%Ce<sup>3+</sup>@NaLuF<sub>4</sub>:<i>x</i>%Yb<sup>3+</sup> CS nanocrystal are prepared for blocking the excitation and emission energy, transmitting surface quenching center and getting more excitation energy through doping Yb<sup>3+</sup> ions in NaLuF<sub>4</sub> shell. It can be clearly seen that the red UC emission intensity of CS nanocrystal first increases and then decreases with Yb<sup>3+</sup> 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 NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup> /2.0%Ho<sup>3+</sup>/12.0%Ce<sup>3+</sup>@NaLuF<sub>4</sub>:10%Yb<sup>3+</sup> CS nanocrystal because the Ho<sup>3+</sup> ions get more energy through the following three ways: 1) Yb<sup>3+</sup> (core)-Ho<sup>3+</sup> (core); 2) Yb<sup>3+</sup> (shell)-Ho<sup>3+</sup> (core); 3) Yb<sup>3+</sup> (shell)-Yb<sup>3+</sup> (core)-Ho<sup>3+</sup> (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 NaLuF<sub>4</sub>:20.0%Yb<sup>3+</sup>/2.0%Ho<sup>3+</sup>/12.0%Ce<sup>3+</sup>@NaLuF<sub>4</sub>:Yb<sup>3+</sup> CS nanocrystals with red UC emission have great potential applications in biological field and multi-primary color.