Enhanced Red UC emission of Er3+ ions by Constructing Multi-heterojunction Core-shell Nanoparticles

Abstract

The construction of core-shell structures with multiple heterojunctions facilitates ion energy transfer and minimizes surface quenching effects, which are essential for enhancing and regulating the upconversion emission of materials. In this study, we effectively regulated the spatial distribution of Yb³⁺ and Er³⁺ ions in multilayered heterogeneous core-shell structures to significantly enhance the red upconversion emission of Er³⁺ ions. Specifically, the red emission intensity of the NaErF₄@NaYbF₄: 2%Er³⁺@NaYF₄ core-shell nanoparticles was enhanced nearly 24.4-fold compared to that of NaErF₄ nanoparticles. This enhancement results from a bidirectional energy transfer process. Moreover, when we swapped the positions of the NaErF₄ core and the NaYbF₄: 2%Er³⁺ shell and introduced NaYF₄ inert isolation layer and Yb³⁺ ions, the red emission intensity of the NaYbF₄:2%Er³⁺@NaYF₄:20%Yb³⁺@NaErF₄@ NaYF₄ multilayer core-shell structure increased significantly by nearly 254.3-fold compared to NaErF₄ nanoparticles. The remarkable enhancement of the red emission of Er³⁺ ions is primarily attributed to a well-organized bidirectional energy transfer process between the heavily doped core and shell Er³⁺-Yb³⁺ ion pairs. The energy transfer behavior of these multi-heterojunction core-shell nanoparticles was studied based on their spectral characteristics. The multilayer heterogeneous core-shell nanoparticles exhibited colorful emissions under different excitation conditions, highlighting their potential for biomedical applications and colorful anti-counterfeiting.