In-depth insight into the Yb3+ effect in NaErF4-based host sensitization upconversion: a double-edged sword

Abstract

NaErF₄ is the most extensively studied host for self-sensitized upconversion (UC), and Yb³⁺ is the most commonly used energy absorber. It has been reported that the red luminescence of Er³⁺ can be enhanced by introducing Yb³⁺ into the NaErF₄ host lattice, where Yb³⁺ ions serve as trapping centers to confine the excitation energy. Also, it has been pointed out that the Yb³⁺ doping in the shell of NaErF₄-hosted core–shell nanocrystals can further improve the red emission intensity. Conversely, it can be argued that the Yb³⁺ doping in the shell always results in the luminescence quenching of the NaErF₄ core. These imply that the impact of Yb³⁺ on NaErF₄-based host-sensitized UC is rather complicated and must be probed deeply. In this study, we thoroughly discussed the effect of Yb³⁺ located in the core/shell on the NaErF₄-based host sensitization UC and afforded the related mechanism interpretations. In the NaErF₄ core nanocrystals, the green-dominated UCL presented an enhancement on increasing the concentration of the Yb³⁺ dopant owing to the promoted energy harvesting for luminescence. Furthermore, the emission properties of NaErF₄:10%Yb shelled with diverse inert layers were also investigated, and the intensity difference of these core-inert shell nanoparticles could be explained by the lattice mismatch and shell thickness. In NaErF₄:10%Yb@NaYF₄:Yb with variable Yb³⁺ doping in the shell, the red-dominated UCL was generally weakened with more Yb³⁺ localized in the shell, which was ascribed to the competition of energy pooling and energy dissipation of Yb³⁺ in the outer layer. Therefore, Yb³⁺ ions wield a two-sided influence (termed a “double-edged sword”) on the UC emissions of the Er³⁺ host. Additionally, we demonstrated the application potential of such UCNPs in water sensing and high-level anti-counterfeiting. This work offers an in-depth insight into the UC mechanism of Yb³⁺-doped NaErF₄ nanocrystals and inspires the engineering of novel luminescent materials with distinguished properties.