2D metal-halide perovskites are interesting materials due to their controllable optoelectronic properties, such as anisotropic charge diffusion, and composition-dependent tuneable band gap. The 2D perovskite crystals we investigated are organic–inorganic hybrids composed of semiconducting metal–halide octahedral layers sandwiched between two layers of mostly insulating organic cations [1]. This architecture gives rise to highly anisotropic charge transport, with carriers preferentially moving in the in-plane direction, strongly weakening the transport between the two octahedra layers [2]. We exploit this intrinsic anisotropy by growing lateral heterostructures in the PEA₂PbBr₄–PEA₂PbI₄ system, where the higher-band-gap PEA₂PbBr₄ core is surrounded by the lower-band-gap PEA₂PbI₄. This configuration enables directional charge or energy flow in the in-plane direction from the core toward the surrounding material. Going beyond conventional optical characterization, we employ Scanning Transmission Electron Microscopy, including both compositional analysis using Energy-Dispersive X-ray Spectroscopy (EDX) and scanning electron diffraction (4D-STEM). 4D-STEM is particularly insightful as it enables probing of the crystallography at high lateral resolution (probe size ~few nm) with very limited electron dose (dose < 1 e^-/Ả²), preventing damage to the sample [3]. Combining powder X-Ray diffraction with nanoscale diffraction provides a complementary set of information with both ensemble and single-particle insight.

We observe a high degree of crystallinity in all particles, which generally comprise a single crystal or a few large domains that grow along the ab-plane (the one containing the inorganic sheets) and are observed from the [001] direction in STEM. Two different growth modes are distinguished using STEM imaging and STEM-EDX, leading to a majority fraction of the particles growing as a bromide micro-platelet with iodide extensions on two opposing facets, and a minority fraction with the iodide phase fully framing the bromide micro-platelet on all sides. Both architectures are emissive, and with 4D-STEM we not only identified the crystallographic relationships between core and surrounding phase, gaining insight into the growth mode, but also studied the crystallographic relationship between core and frame, which includes lattice distortion, and the degree of alloying, while taking into account the in-plane rotation of the octahedral lattice. The interfaces between halide phases indicate a coherent lattice direction, suggesting a good potential for seamless electron transport.

Overall, this study demonstrates an approach for understanding local crystallographic properties in a complex, beam-sensitive system, revealing crystal correlations at the nanoscale, therefor paving the way to engineer new 2D perovskite materials and devices.