Inversion of dynamical electron scattering for atomically-resolved structural analysis

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The method of 4D scanning transmission electron microscopy (4D-STEM) consists of scanning an electron probe across a specimen, and recording a diffraction pattern at each raster position. By iterative algorithms, atomic-scale structure and chemistry can be reconstructed. In this project, contemporary nanostructures are used to demonstrate precision enhancement of the structural characterization to the picometre-range, and to maximise chemical sensitivity so as to explore currently hidden structure-property relationships. By formulation of the scattering problem in frozen phonon approximation as a neural network, persistent model violations as to thermal diffuse scattering (TDS) are eliminated. Such networks are trained based on experiments via efficient calculation of gradients with respect to arbitrary solely-physical parameters, yielding atom positions and types as the direct measurement result. The project addresses four main objectives: 1. Ferroelectric domains in BaTiO3 are quantified, whereas O- and Ti-ions differ from their symmetry positions by a few picometres only. Since tunnel junctions with 1-2nm thick layers are promising for future data storage devices, pm-level mapping the strain-induced domains is supposed to shed light on the physics of domain formation in nanolayer systems and device failure regarding the observed non-switchability of ferroelectric domains. 2. Segregation in axial InAs/AlAsSb nanowire heterostructures is measured quantitatively at the atomic scale, so as to understand the electronic performance of high electron mobility transistors (HEMT). By exploiting TDS explicitly, we achieve enhanced chemical sensitivity in comparison to established ptychography, the replacement of fixed forward scattering models in Z-contrast STEM, and the elimination of the complex dynamical scattering at interfaces. 3. Alloyed dichalcogenide monolayers Mo{S(x)Se(1-x)] are explored chemically and structurally. Alloyed 2D crystals enable a new degree of freedom x to tune the electronic band alignment in heterobilayers. Gradients with respect to the chemical composition of group-VI sites are used to design experimental conditions in paradigmatic manner. Low-dose 4D-STEM experiments serve to map group-VI monomers, homomers and heterodimers and enable the understanding of spectra obtained with visible light. 4. The reconstruction algorithm is expanded such that chemical bonding becomes experimentally accessible. Simulations and Experiments are used to develop additional network layers and setting up their parametrization. By implementing ResNet modules, differentiability of the network as to the specimen thickness is studied. In summary, a generic reconstruction method is developed and released for scientific use in the community.