Halide perovskite dynamics at work: Large cations at 2D-on-3D interfaces are mobile

Ultra-thin hydrophobic capping layers of two-dimensional (2D) onto three-dimensional (3D) metal halide perovskites (HaPs) are an attractive strategy for preventing ambient-induced degradation and minimizing interfacial non-idealities of 3D HaPs. However, it is not obvious in how far the unusual 3D HaP lattice dynamics affect 2D-on-3D HaP composites' stability, especially at their interface, an issue important for devices made with such composites. Using low electron-fluence, four-dimensional scanning transmission electron microscopy and nanobeam electron diffraction, we show formation of the 2D (n = 1) phase on top of 3D perovskite, using focused-ion beam-prepared cross-sections, under conditions that minimize radiation damage. The 2D-on-3D HaP composites were prepared by controlled gas-phase surface cation exchange of 3D MAPbI3 films to form A2PbI4, where A = (fluoro- )phenyl-ethyl-ammonium, (F)PEA. We provide direct evidence for 2D phase formation also inside the 3D matrix, likely via A cation grain boundary diffusion, and, over time, of quasi-2D phases near the surface. These results show that the 2D/3D heterointerface is dynamic; they imply that not only small, but also large A cations, (F)PEA+, migrate. Structural rearrangements, leading to quasi-2D phase formation can be followed with the electron beam, which provides hitherto unknown atomistic insights into such interfaces, needed to assess their (in)stability. Apart from directly illustrating effects of HaP lattice dynamics, our results help understanding extensive (semi)empirical data on engineering 2D-on-3D composites and provide guidance for enhancing stability of such systems. Critically, our direct observation of electron beam-induced loss of long-range periodicity defines conditions for damage-free atomicresolution studies of HaP samples, also in device-relevant configurations.