In the landscape of techniques to better understand and engineer (battery) materials and their interfaces, 4D-Scanning Transmission Electron Microscopy (4D-STEM) takes a unique and powerful place. This technique is an extension or regular STEM, where 2D diffraction data is available for every imaged pixel. Because of the high energies available at the TEM, it is not only possible to collect Bragg diffraction data, but also to characterize amorphous phases through electron-based Pair Distribution Function (ePDF) analysis, unearthing subtle, but relevant effects. Additionally, structural data can be combined with co-located Energy-Dispersive X-ray spectroscopy (EDS) data. However, the large volume of data and the complexity of diffraction analysis makes manual processing of 4D-STEM data prohibitively slow.To solve this, and to leverage the possibilities of collecting spatially resolved structural and compositional data, we developed an automated workflow for analyzing 4D-STEM data. This workflow is cast as a Directional Acyclic Graph (DAG). First, the 4D data is split into spatially mapped crystalline and amorphous components. Then, non-local averaging and non-negative matrix factorization distill the amorphous data into a low number of higher-quality pair distribution functions (PDFs). Finally, these can be structurally refined using reverse Monte Carlo (RMC) fitting and molecular dynamics (MD).Here we demonstrate the characterization of the amorphous structure of undoped and doped TiO2: a simple, yet very relevant system. TiO2 finds uses in lithium-ion-based energy storage, catalysis and microelectronics. One of the reasons for its attractiveness is the possibility to tune the functional properties by means of the structure: e.g. the conductivity of the material changes by means of doping and/or phase selection through annealing (anatase vs rutile). Amorphous TiO2 is subject to similar effects. Indeed, small concentrations of impurities (e.g. precursor ligands) may have a large impact on the final structure. Similarly, subtly different structures may arise at the substrate-film or film-vacuum interface. This will affect the functional properties of the films. TiO2 was deposited by atomic layer deposition (ALD) with TiCl4 and water, using AlMe3 and ZnEt2 as precursors for respectively Al and Zn doping.Our automated approach greatly reduces the amount of time and effort necessary to interpret the data in a meaningful way. Specifically, for the TiO2, we can clearly observe how even a low level of Zn, Al and Cl impurities profoundly alter the amorphous structure. In other work submitted to this conference, we used the same approach to analyze complex, multi-element battery interfaces. Through this effort, a better understanding amorphous materials and interfaces can be gained, which can then serve as a stepping stone to study and engineer electrochemical electrode and catalyst interfaces.
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