Enhancing multiphoton upconversion emissions through confined energy migration in lanthanide-doped Cs2NaYF6 nanoplatelets

Abstract

Lanthanide (Ln³⁺)-doped upconversion (UC) nanocrystals have drawn tremendous attention because of their intriguing optical properties. Currently, it is highly desired but remains challenging to achieve efficient multiphoton UC emissions. Herein, we report the controlled synthesis of a new class of UC nanocrystals based on Cs₂NaYF₆:Yb/Tm nanoplatelets (NPs), which can effectively convert the 980 nm light to five-photon and four-photon UC emissions of Tm³⁺ without the fabrication of a complicated core/multishell structure required in traditional nanocrystals. Particularly, the as-prepared Cs₂NaYF₆:Yb/Tm NPs exhibit a maximal UV-to-NIR emission intensity ratio of 1.2, which is the highest among Tm³⁺-doped core-only UC nanocrystals. We reveal that the enhanced multiphoton UC emissions may benefit from the confined energy migration of Ln³⁺ dopants in the unique two-dimensional-like structure of Cs₂NaYF₆ NPs. As such, intense red and green UC emissions of Eu³⁺ and Tb³⁺ can further be generated via the cascade sensitization of Tm³⁺ and Gd³⁺ in Cs₂NaYF₆:Yb/Tm/Gd/Eu and Cs₂NaYF₆:Yb/Tm/Gd/Tb NPs, respectively. These results validate the superiority of Cs₂NaYF₆ for the future design of efficient UC nanocrystals towards versatile applications.