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.