Unraveling the Excited-State Dynamics of Er³⁺ in LiErF₄-Based Upconversion Nanoparticles

Lanthanide (Ln³⁺)-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 Er³⁺ excited-state dynamics employing energy diffusion theories, we demonstrate that concentration quenching in LiErF₄ UCNPs predominantly originates from long-range energy migration through the ⁴I_13/2 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 LiYF₄@LiErF₄@LiYF₄ core-shell-shell architecture to restrict Er³⁺ migration, and incorporation of Tm³⁺ as energy trapping centers, boosting upconversion quantum yield from <0.01% to 2.29% (980 nm@70 W cm^-2). 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.