From Bulk to Colloidal Nanocrystals: Top‐Down Synthesis of Highly Luminescent Cs 2 ZrCl 6 :Te 4+ Double Perovskite Nanocrystals for X‐Ray Imaging

Lead‐free halide double perovskites (e.g., Cs 2 ZrCl 6 , CZC) have attracted enormous attention in view of their nontoxic composition and high stability compared to lead‐based analogues. However, despite the near‐unity photoluminescence quantum yield (PLQY) of bulk CZC, colloidal nanocrystals (NCs) typically exhibit much weaker emission, a common limitation of double perovskite NCs arising from surface defects. Thus, the realization of highly luminescent, colloidally stable CZC NCs remains challenging. Herein, we introduce a general top‐down approach for the synthesis of brightly emissive colloidal Cs 2 ZrCl 6 : Te 4+ (CZCT) NCs by direct ultrasonication of bulk powders with the assistance of ligands. This ligand‐assisted ultrasonication strategy is not limited to CZCT but is potentially extendable to other structurally related lead‐free double perovskite materials (such as Cs 2 AgInCl 6 :Bi 3+ ). The obtained CZCT NCs exhibit larger lateral size (over 100 nm) and PL quantum yield up to 65%, significantly outperforming the NCs prepared by the conventional hot‐injection approach (24%). High‐resolution atomic‐level analysis indicates that the doping of Te 4+ increases the lattice strain, and Te atoms prefer to segregate inside CZCT NCs. The NC colloidal dispersion enabled solution‐processed thin films that showed a sensitive response to x‐ray irradiation, with a light yield of 42 600 ± 2300 photons/MeV and high spatial resolution imaging of 12.2 lp/mm.