Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light emitting applications. Recent studies focused on nanoplatelets that are able to self-assemble and transform on solid substrates. Yet, the mechanism behind the process and the atomic arrangement of their assemblies remain unclear. Here, we present in detail the transformation of self-assembled stacks of CsPbBr₃ nanoplatelets in solution over a period of a few months by using ex-situ transmission electron microscopy and surface analysis. We demonstrate that the transformation mechanism can be understood as oriented attachment, proceeding through the following steps: i) desorption of the ligands from the particles surfaces, causing the seamless atomic merging of nanoplatelet stacks into nanobelts; ii) merging of neighboring nanobelts that form more extended nanoplates; and iii) attachment of nanobelts and nanoplates, forming objects with an atomic structure that resembles a mosaic made of broken nanotiles. We reveal that aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr, with the atomic columns of neighboring perovskite lattices shifted by a half-unit-cell, forming Ruddlesden-Popper planar faults. We also show, via in-situ monitoring of the nanocrystal photoluminescence combined with transmission electron microscopy analysis, that the transformation is temperature-driven and that it can take place within tens of minutes in solution and in spin-coated films. Understanding this process gives crucial information for the design and fabrication of perovskite materials, where control over the type and density of defects is desired, stimulating the development of perovskite nanocrystal structures with tailored electronic properties.

A detailed investigation into the synthesis of green-emitting powders of Cs₄PbBr₆ and CsPbBr₃ materials by antisolvent precipitation from CsBr–PbBr₂ precursor solutions in dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) is reported. Various solvated lead bromide and polybromide species (PbBr₂, [PbBr₃]⁻, [PbBr₄]^2–, and possibly [PbBr₅]^3– or [PbBr₆]^4–) are detected in the precursor solutions by optical absorbance and emission spectroscopies. The solvodynamic size of the species in solution is strongly solvent-dependent: ∼1 nm species were detected in DMSO, while significantly larger species were observed in DMF by dynamic light scattering. The solvodynamic size of the lead bromide species plays a critical role in determining the Cs-Pb-Br composition of the precipitated powders: smaller species favor the precipitation of Cs₄PbBr₆, while larger species template the formation of CsPbBr₃ under identical experimental conditions. The powders have been characterized by ¹³³Cs and ²⁰⁷Pb solid-state nuclear magnetic resonance, and ¹³³Cs sensitivity toward the different Cs environments within Cs₄PbBr₆ is demonstrated. Finally, the possible origins of green emission in Cs₄PbBr₆ samples are discussed. It is proposed that a two-dimensional Cs₂PbBr₄ inclusion may be responsible for green emission at ∼520 nm in addition to the widely acknowledged CsPbBr₃ impurity, although we found no conclusive experimental evidence supporting such claims.