Maximizing Sensitizer and Activator Concentrations in Upconversion Nanostructures via Passivation‐Regulated Energy‐Transfer Kinetics

ABSTRACT Upconversion luminescence (UCL) is fundamentally limited by the low dopant concentrations required to avoid concentration quenching, making it difficult to simultaneously maximize light absorption and emission efficiency in lanthanide nanostructures. Here we address this bottleneck by engineering a region‐selective core–shell–shell architecture, β‐NaErF 4 @NaYbF 4 @NaYF 4 , in which activator‐rich and sensitizer‐rich domains are spatially separated yet optically coupled. This design enables both 100% Er 3+ and 100% Yb 3+ doping in their respective domains without the conventional concentration quenching penalty, yielding a 116‐fold enhancement in UCL brightness and a 3.3‐fold increase in upconversion quantum yield relative to the canonical NaYF 4 : Yb,Er@NaYF 4 nanostructure. Systematic doping‐concentration‐dependent studies show that increasing Yb 3+ concentration markedly improves upconversion efficiency, whereas increasing Er 3+ concentration mainly enhances brightness through a combined contribution from absorption and emission. Mechanistic investigations further indicate that the high‐brightness regime cannot be explained by spatial separation alone. Instead, the passivation shell suppresses surface‐loss channels and prolongs sensitizer excited‐state survival, establishing a dynamic population equilibrium in which energy backflow counterbalances the detrimental impact of backward energy transfer at high dopant densities. These results provide a kinetic framework for designing high‐density upconversion systems and offer a general strategy for developing ultra‐bright lanthanide photonic nanostructures.