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

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 MAPbBr(3) 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 MAPbBr(3) 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, MAPbBr(3) quantum nanodots tend to coexist with precursor nanostructures or MA(4)PbBr(6), 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 MAPbBr(3) quantum nanodots.