Mechanosynthesis of polymer-stabilized lead bromide perovskites: Insight into the formation and phase conversion of nanoparticles

The application of polymers to replace oleylamine (OLA) and oleic acid (OA) as ligands for perovskite nanocrystals is an effective strategy to improve their stability and durability especially for the solution-based processing. Herein, we report a mechanosynthesis of lead bromide perovskite nanoparticles (NPs) stabilized by partially hydrolyzed poly(methyl methacrylate) (h-PMMA) and high- molecular-weight highly-branched poly(ethylenimine) (PEI-25K). The as-synthesized NP solutions exhibited green emission centered at 516 nm, possessing a narrow full-width at half-maximum of 17 nm and as high photoluminescence quantum yield (PL QY) as 85%, while showing excellent durability and resistance to polar solvents, e.g., methanol. The colloids of polymer-stabilized NPs were directly processable to form stable and strongly-emitting thin films and solids, making them attractive as gain media. Furthermore, the roles of h-PMMA and PEI-25K in the grinding process were studied in depth. The h-PMMA can form micelles in the grinding solvent of dichloromethane to act as size-regulating templates for the growth of NPs. The PEI-25K with large amounts of amino groups induced significant enrichment of PbBr₂ in the reaction mixture, which in turn caused the formation of CsPb₂Br₅-mPbBr₂ and CsPbBr₃-Cs₄PbBr₆-nCsBr NPs. The presence of CsPbBr₃-Cs₄PbBr₆-nCsBr NPs was responsible for the high PL QY, as the Cs₄PbBr₆ phase with a wide energy bandgap can passivate the surface defects of the CsPbBr₃ phase. This work describes a direct and facile mechanosynthesis of polymer-coordinated perovskite NPs and promotes in-depth understanding of the formation and phase conversion for perovskite NPs in the grinding process.