Temperature Driven Transformation of CsPbBr$_3$ Nanoplatelets into Mosaic Nanotiles in Solution through Self-Assembly

Two-dimensional colloidal halide perovskite nanocrystals are promising materials for light emitting applications. In addition, they can be used as components to create a variety of materials through physical and chemical transformations. 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 the transformation of self-assembled stacks of CsPbBr$_3$ nanoplatelets in solution, capturing the different stages of the process by keeping the solutions at room temperature and monitoring the nanocrystal morphology over a period of a few months. 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 merging of nanoplatelet stacks, which first form nanobelts; ii) merging of neighboring nanobelts that form more extended nanoplates; and iii) attachment of nanobelts and nanoplates, which create objects with an atomic structure that resemble a mosaic made of broken nanotiles. We reveal that the starting nanoplatelets merge seamlessly and defect-free on an atomic scale in small and thin nanobelts. However, aged nanobelts and nanoplates, which are mainly stabilized by amine/ammonium ions, link through a bilayer of CsBr. In this case, the atomic columns of neighboring perovskite lattices shift by a half-unit-cell, forming Ruddlesden-Popper planar faults.