Electron ptychographic phase imaging of all-inorganic halide perovskites (AIHPs) using 4D STEM

Abstract

Both inorganic and organic-inorganic 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 and the extreme sensitivity of HPs towards electron-beam irradiation prevents access to intrinsic information. Imaging is severely limited, especially at high magnifications, and the key factor for TEM studies is the electron dose. However, technological advances and the development of direct electron cameras (DECs) that can accurately determine the position of an incoming scattered electron have opened up a whole new field of electron microscopy: 4D STEM. A four-dimensional dataset represents two dimensions in real space, the position of the electron probe, and two dimensions in reciprocal space, the detector plane. For each probe position, a convergent electron beam diffraction (CBED) pattern is recorded. From the recorded CBEDs it is not only possible to extract the signal for any STEM detector geometry, but also the phase problem can be solved. Utilizing the coherent interference in the overlap region between direct and scattered beams, the phase and amplitude of the electron wave can be reconstructed. This thesis aims to show the potential of 4D STEM, in particular phase reconstructions via focused probe ptychography. Two non-iterative reconstructions based on weak-phase- and phase-object approximation are presented. Single-side-band (SSB) and Wigner distribution deconvolution (WDD) ptychography are introduced as low-dose, dose-efficient techniques to image the atomic structure of beam-sensitive HPs. Atomically resolved, almost aberration-free phase images of three all-inorganic halide perovskites, CsPbBr3, CsPbIBr2, and CsPbI3 are presented with a resolution down to the aperture-constrained diffraction limit.