Template-Mediated Synthesis of Methylammonium Lead Bromide Quantum Nanodots with Tailored Optical Properties

Abstract

This research provides critical insights into the role of block copolymers (BCP), salt dissociation/complexation, and coassembly of polymer chains with salt complexes in perovskite nanocrystal synthesis. We demonstrate the formation of hybrid nanostructures using polystyrene-block-poly(ethylene oxide) (PS-b-PEO) as a template for lead bromide (PbBr₂) dissociation and complexation in 1,3,5-trimethylbenzene (TMB) and the encapsulation of MAPbBr₃ quantum nanodots in its cosolvent with methanol. PbBr₂, typically insoluble in TMB, forms [PbBr₃]⁻ and [PbBr₄]^2– complexes in the presence of PS-b-PEO. These complexes bind with the PEO block, coassembling into crystalline intermediates such as irregular nanosheets and polygonal nanoplates studded with tiny nanodots. Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), and wide-angle X-ray diffraction (WAXD) reveal that these intermediate nanostructures are not orthorhombic PbBr₂ crystals but rather distinct crystals resulting from the coassembly of PS-b-PEO, [PbBr₃]⁻/[PbBr₄]^2– complexes and PbBr₂ nanodots. PS-b-PEO acts as a soft colloidal template, stabilizing the lead halide complexes and preventing their direct precipitation in TMB. However, our findings indicate that to effectively form encapsulated MAPbBr₃ quantum nanodots with narrow size distribution, it is crucial to remove excess PbBr₂ microcrystals, use a low concentration of PS-b-PEO, and add sufficient MABr contents. Otherwise, MAPbBr₃ quantum nanodots tend to coexist with precursor nanostructures or MA₄PbBr₆, which negatively impacts their optical properties. Moreover, high concentrations of PS-b-PEO favor the growth of polygonal nanoplates, which compete with the growth of MAPbBr₃ quantum nanodots.