Metal halide perovskites (MHP) are promising semiconductors for the next generation of optoelectronic devices. Due to their tunable bandgap, they fulfil the requirement for various applications, including solar cells, light-emitting diodes, lasers and photodetectors. Unfortunately, MHPs still lack of long-term stability which hampers their commercial application. Degradation occurs due to their instability under environmental triggers, such as moisture and oxygen. Thus, the combination of in situ and ex situ TEM techniques can provide valuable insights about the underlying degradation mechanisms to extend the working lifetime of perovskites. Another challenge for TEM investigations of MHPs is their high electron beam sensitivity. Therefore, we develop low-dose protocols using 4D STEM. Hence, we are able to investigate the MHP without degradation due to Pb-cluster formation or amorphization and quantitatively interpret the STEM images using statistical parameter estimation theory.

Via low-dose 4D STEM measurements we investigated FAPbBr₃ nanocrystals (NCs), which are of great interest for green light-emitting diodes. To retrieve phase contrast imaging from 4D STEM datasets, we utilize the capacity of machine learning. In the phase contrast reconstructions, the different atomic columns in the perovskite structure could be detected, and it was also possible to clearly distinguish the light FA cations and the Br atomic columns from the image intensity. Furthermore, we quantified the observed elongation of the projections of the Br atomic columns, suggesting an alternation in the position of the Br atoms perpendicular to the Pb−Br−Pb bonds. Together with molecular dynamics simulations, these results remarkably reveal local distortions in an on-average cubic structure [1,2].

We also investigated CsPbI₃, whose photoactive phases spontaneously convert into a non-photoactive yellow orthorhombic δ-phase under ambient conditions. This transformation results in a significant increase in bandgap and a loss of photoactive functionality. We studied the impact of Zn²⁺ and Cd²⁺ dopants on the phase stability of CsPbI₃ NCs, emphasizing the formation of Ruddlesden–Popper (RP) planar defects. Via TEM we followed the temporal evolution of the phase transformation, where black-phase NCs agglomerate and form elongated microtubes with a yellow-phase crystal structure. Our observations demonstrate that doped samples are significantly more stable, while the dopants are key factors in the formation of the RP-like defects with specific atomic arrangements [3].

Additionally, we investigate II–VI semiconductor nanoplatelets (NPLs), in particular CdSe NPLs. It has been shown that via ligand exchange a high circular dichroism (CD) can be achieved [4]. The chiral morphology of helical NPLs was analyzed via electron tomography. Furthermore, exchanging the native acetate ligands with tartrate ligands results in a flat morphology and an orthorhombic distortion of the lattice, which we investigated via local nano-beam 4D STEM diffraction patterns.