Unlocking Near‐Unity Photoluminescence Quantum Yield for Cs 3 TbCl 6 Perovskite Nanocrystals Via Fluoroquinolone Surface Engineering

Rare‐earth‐based lead‐free halide perovskite nanocrystals (NCs) are promising for optoelectronics due to their narrow‐band emission and physicochemical stability. However, the intrinsic 4f‐4f forbidden transitions of rare‐earth ions result in limited absorption cross‐sections. Furthermore, inevitable surface defects at the nanoscale trigger non‐radiative recombination, limiting luminescence efficiency. Herein, a surface engineering strategy utilizing fluoroquinolone (FQ) molecules is proposed to construct a hybrid system. Specifically, by introducing norfloxacin (NFLX) as a surface ligand, NFLX‐modified Cs 3 TbCl 6 nanocrystals (NCs) have been developed that achieve a near‐unity photoluminescence quantum yield (PLQY) of 99.67%. Experiments and theoretical analyses reveal that the organic ligands effectively passivate surface defects, thereby suppressing non‐radiative transitions. Simultaneously, the ligand acts as an efficient antenna that amplifies excitation light capture and establishes energy transfer pathways to NCs. Notably, it is demonstrated that beyond energy level matching, the electronic effects, steric configurations, and absorption capacities of ligand are crucial factors for efficient antenna effect. Finally, the effectiveness of this strategy is validated through applications in extreme‐environment‐resilient fluorescent anti‐counterfeiting fibers and high‐quality white light‐emitting diodes (WLEDs). This work overcomes the bottleneck limiting the luminescence efficiency of rare‐earth halide NCs, paving a new technical pathway for the design of high‐efficiency surface‐coordinated hybrid rare‐earth luminescent NCs.