Red upconversion emission of Er<sup>3+</sup> enhanced by building NaErF<sub>4</sub>@ NaYbF<sub>4</sub>:2%Er<sup>3+</sup> core-shell structure

Building core-shell structures are widely used to enhance and regulate the luminescence properties of rare-earth-doped micro/nano materials. In this work, a variety of different NaErF₄ core-shell and core-shell-shell nanocrystals are successfully constructed based on high temperature co-precipitation method by epitaxial growth technology. The upconversion red emission intensities of Er³⁺ ions in different core-shell structures are effectively enhanced by regulating their structures and doping ions. The experimental structures show that the constructed core-shell nanocrystals each have a hexagonal phase structure, and core-shell structure of about 40 nm. In the near infrared 980 nm laser excitation, the NaErF₄ core-shell nanocrystal shows a strong single-band red emission. And the single-band red emission intensity of Er³⁺ ions is enhanced through constructing the NaErF₄@NaYbF₄:2%Er³⁺ core-shell structure. The experimental results show that red emission intensity of Er³⁺ ions is about 1.4 times higher than that of the NaErF₄@NaYbF₄ core-shell structure by constructing the NaErF₄@NaYbF₄:2%Er³⁺ core-shell structures under 980 nm excitation, and its red/green emission intensity ratio increases from 5.4 to 6.5. Meanwhile, when NaErF₄@NaYbF₄:2%Er³⁺ core-shell structure recoats the NaYF₄ inert shell and is added with a small quantity of Tm³⁺ ions, their red emission intensities of Er³⁺ ions are 23.2 times and 40.3 times that of NaErF₄@NaYbF₄ core-shell structures, and their red/green emission intensity ratios reach 7.5 and 10.2, respectively. The red emission enhancement of Er³⁺ ions is mainly caused by bidirectional energy transfer process of high excitation energy of Yb³⁺ ions and energy trapping center of Tm³⁺ ions which effectively change the density of population of luminescent energy levels of Er³⁺ ions. Furthermore, the coated NaYF₄ inert shell also effectively weakens the surface quenching effect of nanocrystals. The mechanisms of red enhancement in different core-shell structures are discussed based on the spectral properties, the process of interion energy transfer, and luminescence kinetics. The constructed NaErF₄@NaYbF₄:2%Er³⁺@NaYF₄ core-shell structures with high-efficiency red emission in this work have great potential applications in the fields of colorful anti-counterfeiting, display and biological imaging.