Flexible Polymer-Assisted Mesoscale Self-Assembly of Colloidal CsPbBr3 Perovskite Nanocrystals into Higher Order Superstructures with Strong Inter-Nanocrystal Electronic Coupling

Surface-passivating ligands, although ubiquitous to colloidal nanocrystal (NC) syntheses, play a role in assembling NCs into higher order structures and hierarchical superstructures, which has not been demonstrated yet for colloidal CsPbX3 (X = Cl, Br, and I) NCs. In this work, we report that functional poly(ethylene glycols) (PEG(6)-Y, Y = -COOH and -NH2) represent unique surface-passivating ligands enabling the synthesis of near-uniform CsPbBr3 NCs with diameters of 3.0 nm. The synthesized NCs are assembled into individual pearl necklaces, bundled pearl necklaces, lamellar, and nanorice superstructures, in situ. It is believed a variety of forces, including van der Waals attractions between hydrophilic PEG tails in a nonpolar solvent and dipole dipole attraction between NCs, drive mesoscale assembly to form superstructures. Furthermore, postsynthetic ligand treatment strengthens the argument for polymer-assisted mesoscale assembly as pearl necklace assemblies can be successfully converted into either lamellar or nanorice structures. We observe an similar to 240 meV bathochromic shift in the lowest energy absorption peak of CsPbBr3 NCs when they are present in the lamellar and nanorice assemblies, representing strong inter-NC electronic coupling. Moreover, pearl necklace structures are spontaneously assembled into micrometer length scale twisted ribbon hierarchical superstructures during storage of colloidal CsPbBr3 NCs. The results show that the self-assembled superstructures of CsPbBr3 NCs are now feasible to prepare via template-free synthesis, as self-assembled structures emerge in the bulk solvent, a process that mimics biological systems except for the use of nonbiological surface ligands (PEG(6)-Y). Taken together, emergent optoelectronic properties and higher order superstructures of CsPbBr3 NCs should aid their potential use in solid-state devices and simplify scalable manufacturing.