Enhanced red upconversion fluorescence emission of Ho3+ ions in NaLuF4 nanocrystals through building core-shell structure

Abstract

A series of the hexagonal-phase NaLuF₄:20.0%Yb³⁺/2.0%Ho³⁺/12.0% Ce³⁺@NaLuF₄:<i>x</i>%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:<i>x</i>%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.