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

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.