Electron Ptychographic Phase Imaging of Beam-sensitive All-inorganic Halide Perovskites Using Four-dimensional Scanning Transmission Electron Microscopy

Halide perovskites (HPs) are promising candidates for optoelectronic devices, such as solar cells or light-emitting diodes. Despite recent progress in performance optimization and low-cost manufacturing, their commercialization remains hindered due to structural instabilities. While essential to the development of the technology, the relation between the microscopic properties of HPs and the relevant degradation mechanisms is still not well understood. The sensitivity of HPs toward electron-beam irradiation poses significant challenges for transmission electron microscopy (TEM) investigations of structure and degradation mechanisms at the atomic scale. However, technological advances and the development of direct electron cameras (DECs) have opened up a completely new field of electron microscopy: four-dimensional scanning TEM (4D-STEM). From a 4D-STEM dataset, it is possible to extract not only the intensity signal for any STEM detector geometry but also the phase information of the specimen. This work aims to show the potential of 4D-STEM, in particular, electron exit-wave phase reconstructions via focused probe ptychography as a low-dose and dose-efficient technique to image the atomic structure of beam-sensitive HPs. The damage mechanism under conventional irradiation is described and atomically resolved almost aberration-free phase images of three all-inorganic HPs, CsPbBr3, CsPbIBr2, and CsPbI3, are presented with a resolution down to the aperture-constrained diffraction limit.