Unraveling the Excited-State Dynamics of Er in LiErF-Based Upconversion Nanoparticles.

Lanthanide (Ln)-enriched upconversion nanoparticles (UCNPs) with high dopant concentrations have garnered significant attention due to their unique optical properties. However, their practical applications are hindered by the deleterious concentration quenching effect. Herein, through kinetic modeling of Er excited-state dynamics employing energy diffusion theories, we demonstrate that concentration quenching in LiErF UCNPs predominantly originates from long-range energy migration through the I level toward surface and lattice defects, rather than the conventionally attributed cross-relaxation mechanism. Such migration-mediated energy dissipation can be effectively suppressed by the synergistic engineering strategies combining surface passivation, spatial confinement via a sandwiched LiYF@LiErF@LiYF core-shell-shell architecture to restrict Er migration, and incorporation of Tm as energy trapping centers, boosting upconversion quantum yield from <0.01% to 2.29% (980 nm@70 W cm). The established mechanistic framework and material design principles provide critical insights for engineering heavily doped UCNPs, particularly advancing their application potential in single-particle spectroscopy and optoelectronic nanodevices.