Enhancing Ultraviolet\nUpconversion in SrYbF<sub>5</sub> Nanoparticles through a Spatial-Confinement\nStrategy toward Information\nEncryption and Anticounterfeiting

Ultraviolet (UV) upconversion has\nattracted much attention\nin recent\nyears due to its applications in biomedicine, nanophotonics, and other\nfields. However, it remains a challenge to achieve efficient UV upconversion\nin lanthanide nanocrystals due to the insufficient content of the\nsensitizer in regular sensitizer–activator coupled systems.\nHere, we report a spatial-confinement strategy to enhance the UV upconversion\ngreatly by increasing the content of sensitizer Yb<sup>3+</sup> in\na core–shell–shell structure with the activator Tm<sup>3+</sup> being confined spatially inside the cubic SrYbF<sub>5</sub> lattice. A set of alkaline-earth metal cation substitutions suggests\nthat a stable lattice is necessary for high-efficiency UV upconversion.\nIn addition, the multiphoton upconversion of Tb<sup>3+</sup> and Eu<sup>3+</sup> is also obtained through Gd<sup>3+</sup>-mediated energy\nmigration in the SrGdF<sub>5</sub> sublattice. By taking advantage\nof different luminescence features of Eu<sup>3+</sup>/Tb<sup>3+</sup> and Tm<sup>3+</sup>, the information encryption and decoding are\nrealized through a time-gating technique. Our results provide an efficient\napproach to enhance multiphoton UV upconversion of lanthanides and\nfurther promote their frontier applications in anticounterfeiting\nand information security.