Active Shell Engineering for Efficient Cascade Triplet Energy Transfer in Lanthanide Heterostructures

ABSTRACT Lanthanide‐doped nanoparticles (LnNPs) exhibit unique optical properties but suffer from severe surface quenching and weak absorption that fundamentally limit their performance. Here, we demonstrate a breakthrough cascade triplet energy transfer (TET) mechanism in precisely engineered NaYbF 4 @Ca 0.8 F 2 :Nd 0.2 @9‐anthracenecarboxylic acid (ACA) heterostructures. This core/active shell/organic molecule configuration combines both molecular sensitization with surface passivation, transforming conventional inert barriers into functional energy conduits. We explore in detail the synthetic conditions required to grow not just optimally active shells but also how best to assemble the organic ligands on the surface of core‐shell LnNPs. Systematic shell thickness optimization (0.8–4.6 nm) reveals an optimal shell thickness of ∼2.0 nm. When coupled with an appropriate ligand exchange strategy, we achieve a remarkable 1200‐fold emission enhancement compared to bare cores. Comprehensive spectroscopic investigations confirm near‐unity TET efficiency and reveal the cascade TET mechanism utilizing Nd 3+ ions as energy intermediates to maximize the Yb 3+ emission. Thus, our mechanism and heterostructure design present one of the most promising synthetic strategies to overcome the existing limitations of traditional LnNPs, establishing new paradigms for high‐performance heterostructures with broad applications in bioimaging, photon conversion, and optoelectronic devices.