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

KGaA, Weinheim 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.