Energy Migration Upconversion in Ce(III)-Doped Heterogeneous Core−Shell−Shell Nanoparticles

One major challenge in upconversion research is to develop new materials and structures to expand the emission spectrum. Herein, a heterogeneous core−shell−shell nanostructure of NaYbF₄:Gd/Tm@NaGdF₄@CaF₂:Ce is developed to realize efficient photon upconversion in Ce³⁺ ions through a Gd-mediated energy migration process. The design takes advantage of CaF₂ host that reduces the 4f–5d excitation frequency of Ce³⁺ to match the emission line of Gd³⁺. Meanwhile, CaF₂ is isostructural with NaGdF₄ and can form a continuous crystalline lattice with the core layer. As a result, effective Yb³⁺ → Tm³⁺ → Gd³⁺ → Ce³⁺ energy transfer can be established in a single nanoparticle. This effect enables efficient ultraviolet emission of Ce³⁺ following near infrared excitation into the core layer. The Ce³⁺ upconversion emission achieved in the core−shell−shell nanoparticles features broad bandwidth and long lifetime, which offers exciting opportunities of realizing tunable lasing emissions in the ultraviolet spectral region.