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 NaYbF 4 :Gd/Tm@NaGdF 4 @CaF 2 :Ce is developed to realize efficient photon upconversion in Ce 3+ ions through a Gd‐mediated energy migration process. The design takes advantage of CaF 2 host that reduces the 4f–5d excitation frequency of Ce 3+ to match the emission line of Gd 3+ . Meanwhile, CaF 2 is isostructural with NaGdF 4 and can form a continuous crystalline lattice with the core layer. As a result, effective Yb 3+ → Tm 3+ → Gd 3+ → Ce 3+ energy transfer can be established in a single nanoparticle. This effect enables efficient ultraviolet emission of Ce 3+ following near infrared excitation into the core layer. The Ce 3+ 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.