Role of interfaces and structural defects in ferroelectric thin films

Over the last decades, ferroelectric materials have become essential components in a range of applications, from sensors and actuators to non-volatile memories. This was made possible by unprecedented progress in the synthesis and characterization of ferroelectric materials, allowing for precise control of important properties, such as the spontaneous electrical polarization or the electro-mechanical coupling. An essential step in this development was the advancement in oxide thin-film engineering, which enabled the study of ferroelectric materials beyond bulk crystals. However, under the spatial confinement of a thin-film geometry, the functional properties of ferroelectric materials can differ greatly from their bulk-crystal counterpart. It is therefore imperative to study the influence that structural defects and interfaces have on the functionality of thin-film ferroelectrics. In this thesis, we use scanning transmission electron microscopy to study the crystal structure and electronic properties of a variety of ferroelectric materials. In lead titanate, we map the ferroelectric polarization at the atomic scale under competitive and cooperative interface configurations, which, together with complementary in-situ second harmonic generation microscopy, allowed the stabilization of a robust, single-domain configuration in a ferroelectric capacitor. We then developed a focus-ion beam workflow that allows us to prepare ferroelectric thin-films in a capacitor-like geometry for in-situ electrical biasing experiments in the transmission electron microscope. Applying this technique to lead zirconate titanate thin films, we investigate the response of the ferroelectric polarization to an applied electric field in the presence of structural defects and domain boundaries. Combining atomically resolved high-angle annular dark-field and medium resolution differential phase contrast, we observed a reduction of the average domain-wall mobility in highly defective regions, with inhomogeneous domain wall motion at the local scale. Finally, we employed electron energy loss spectroscopy to study the electronic structure and chemical composition of our materials. In lead zirconate titanate, we find the accumulation of oxygen vacancies stabilizing energetically unfavorable domain configurations in the presence of the substrate interface. In hexagonal yttrium manganite, we quantify the Mn valence state to correlate it with changes in the structural distortion of the material, observed at its interfaces. We further combine this analysis with the in-situ heating capabilities of our microscope to investigate the changes in the electronic structure of yttrium manganite during its ferroelectric phase transition. The results presented in this work demonstrate the utility of investigating the properties of ferroelectric materials down to the atomic scale. Such characterizations will enable the design of energy-efficient oxide electronics of the future. Furthermore, our work also highlights the synergies with complementary characterization techniques, such as piezo-response force microscopy, in-situ harmonic generation and theoretical modelling, which, together with transmission electron microscopy, allow for a more complete understanding of the material properties across all length scales