Enhancing red upconversion emission of Er3+ by building NaErF4@NaYbF4:2%Er3+ core-shell structure

Building core-shell structures have been 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 were successfully constructed based on high temperature co-precipitation method by epitaxial growth technology. The upconversion red emission intensity of Er³⁺ ions in different core-shell structures was effectively enhanced by regulating their structures and doping ions. The experimental structures show that the constructed core-shell nanocrystals are all hexagonal phase structure, and the size of the core-shell structure is about 40 nm. In the near infrared 980 nm laser excitation, the NaErF₄ core-shell nanocrystals showed strong single-band red emission. And the single-band red emission intensity of Er³⁺ ions was enhanced through constructing the NaErF₄@NaYbF₄:2%Er³⁺ core-shell structures. The experimental results showed that red emission intensity of Er³⁺ ions was about 1.4 times higher than that of the NaErF₄@NaYbF₄ core-shell structures by constructing the NaErF₄@NaYbF₄:2%Er³⁺ core-shell structures under 980 nm excitation, and the red/green emission intensity ratio was increased from 5.4 to 6.5. Meanwhile, when NaErF₄@NaYbF₄:2%Er³⁺ core-shell structures have recoated the NaYF₄ inert shell and introduced trace amounts of Tm³⁺ ions, the red emission intensity of Er³⁺ ions was 23.2 and 40.3 times compared with NaErF₄@NaYbF₄ core-shell structures, and the red/green emission intensity ratio reached 7.5 and 10.2, respectively. The red emssion enhancement of Er³⁺ ions was mainly caused by bidirectional energy transfer processes of high excitation energy of Yb³⁺ ions and energy trapping center of Tm³⁺ ions which effectively changed the densities of population of luminescent energy levels of Er³⁺ ions. What’s more, the coated NaYF₄ inert shell also effectively reduced the surface quenching effect of nanocrystals. The mechanism of red enhancement in different core-shell structures were discussed based on the spectral properties, the processes 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 has great application potential in the fields of colorful anti-counterfeiting, display and biological imaging.