Recrystallization-Driven Assembly of Cs3Cd2Cl7 Nanoplatelets into Hybrid Organic-Inorganic Ruddlesden-Popper Stacks

The long-term aging of metal-halide perovskite nanocrystals is usually associated with uncontrolled degradation or transformation into unwanted phases. In contrast with this expected behavior, we report the spontaneous evolution of A3Cd2Cl7 (A = Cs, Rb) Ruddlesden-Popper nanoplatelets into crystalline multilayered stacks during prolonged storage in nonpolar solvents. This thermodynamically driven recrystallization is initiated by ligand interdigitation, which promotes face-to-face self-assembly, and is followed by Ostwald ripening within the stacks into a hybrid layered phase. These early assemblies are crucial in imposing the geometric constraints that make platelets converge toward the same morphology. This drives coordinated lateral growth and size-focusing over several months, yielding large, morphologically homogeneous nanosheet stacks. Multilayer X-ray diffraction reveals that these stacks consist of atomically precise inorganic slabs alternating with oleylammonium cation bilayers, corresponding to a stoichiometry of (oleylammonium)2Cs4Cd4Cl14, thereby identifying the assemblies as a new hybrid organic-inorganic Ruddlesden-Popper phase. Their persistence beyond one year without further evolution and their resistance to sonication demonstrate interplatelet interactions far more durable than typical colloidal assembly interactions. Nevertheless, stresses generated during solvent evaporation can induce lateral sliding, twisting and the collapse of stacks, generating characteristic card-deck patterns on transmission electron microscopy grids. Similar early assembly and lateral growth were observed for Cs2PbI2Cl2 nanoplatelets, suggesting that this pathway extends beyond Cd-based systems to other 2D-layered metal halides. These findings establish ligand interdigitation and aging-assisted ripening as a route to hybrid organic-inorganic phases and provide mechanistic insight into the self-organization of colloidal 2D nanocrystals into ordered superstructures inaccessible through direct synthetic routes.