Metal halide perovskites are highly promising materials for optoelectronic applications; however, realizing their full potential requires a detailed understanding of structure–property relationships as well as strategies to improve their intrinsic instability [1], [2]. At the nanoscale, the shape and crystallography of individual perovskite nanocrystals (NCs) can strongly influence their optical properties, thus making transmission electron microscopy–based approaches indispensable despite the pronounced electron-beam sensitivity of this class of materials. We developed a workflow that enables the structural characterization of beam-sensitive perovskite NCs, providing access to both 3D shape- and structure-dependent information at the single-particle level. Moving beyond the single-particle regime, we address the topology of the perovskite NC assemblies, emerging materials for collective and mesoscale functionalities [3], [4], using a recently developed secondary-electron electron-beam-induced current technique. To determine structural organization in depth and quantify packing fractions, we apply advanced segmentation algorithms to electron tomography reconstructions.

While such structural control is essential for optimizing functional performance, the practical implementation of perovskite NCs is currently limited by their poor phase stability. To distinguish intrinsic degradation from electron-beam-induced effects, we first evaluate the critical electron dose for reliable microscopy. Then we use four-dimensional electron microscopy to track the black-to-yellow phase transition in CsPbI₃ at the nanoscale. Phase mapping based on the diffraction data, combined with direct-space intensity analysis, reveals the predominant role of water and a non-local, vapor-mediated mass-transfer mechanism. The same transition is further investigated at the film level using environmental scanning electron microscopy, based on the strong contrast difference between the black and yellow phases. Kinetic curves extracted from these data agree with X-ray diffraction measurements, while simultaneously uncovering PbI₂ formation confined to the porous regions of the films.

Together, this work provides an integrated framework for low-dose transmission electron microscopy, enabling reliable 2D and 3D investigations as well as in situ studies.