Two-dimensional (2D) colloidal perovskite CsPbBr3 nanoplatelets (NPLs) have recently attracted attention as promising blue-emitting materials because of their large exciton binding energy and precisely controlled thickness. However, synthesizing NPLs that are homogeneous and highly fluorescent while exhibiting long-term stability has been challenging, which has limited the successful integration of such materials in optoelectronic devices (e.g., light-emitting devices). Herein, we introduce a strategy reiying on surface ligand-engineering, using PbBr2-complexed polymers as coating ligands that impart morphological and colloidal stability while enhancing the photoluminescence quantum yield of perovskite NPLs. The polymers are salt-rich compounds that present several ammonium (or imidazolium) bromide anchoring groups and alkyl chains with different lengths as solubilizing blocks. The presence of quaternary ammonium salts facilitates the dissolution of PbBr2 in organic solutions, forming polysalt-Pb_x.Br_y~(2x-y) complexes. These complexes strongly interact with the NPLs and eliminate surface defects, compared to the as-grown materials, and improve their structural and morphological stability. This strategy has yielded high-quality blue-emitting NPLs that maintain color purity (with narrow profiles at ~460 nm) with photoluminescence quantum yield (PLQY) up to 80. Additionally, significantly enhanced long-term stability of the nanoplatelets under several challenging conditions, including UV irradiation and highly diluted conditions, has been achieved.
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