Enhancing Ultraviolet Upconversion in SrYbF5 Nanoparticles through a Spatial-Confinement Strategy toward Information Encryption and Anticounterfeiting

Abstract

Ultraviolet (UV) upconversion has attracted much attention in recent years due to its applications in biomedicine, nanophotonics, and other fields. However, it remains a challenge to achieve efficient UV upconversion in lanthanide nanocrystals due to the insufficient content of the sensitizer in regular sensitizer–activator coupled systems. Here, we report a spatial-confinement strategy to enhance the UV upconversion greatly by increasing the content of sensitizer Yb³⁺ in a core–shell–shell structure with the activator Tm³⁺ being confined spatially inside the cubic SrYbF₅ lattice. A set of alkaline-earth metal cation substitutions suggests that a stable lattice is necessary for high-efficiency UV upconversion. In addition, the multiphoton upconversion of Tb³⁺ and Eu³⁺ is also obtained through Gd³⁺-mediated energy migration in the SrGdF₅ sublattice. By taking advantage of different luminescence features of Eu³⁺/Tb³⁺ and Tm³⁺, the information encryption and decoding are realized through a time-gating technique. Our results provide an efficient approach to enhance multiphoton UV upconversion of lanthanides and further promote their frontier applications in anticounterfeiting and information security.