Abstract
Ferroelectric domain wall (DW) based nano-electronics is an emerging new field of research. It is only recently with advancements in electron and atomic force microscopy instrumentation that the complex nature of these 2D entities can be probed. In this Research Update, the advances in aberration corrected scanning transmission electron microscopy applied to ferroelectric topological defects are summarized. We discuss sub-atomic imaging and diffraction techniques used to observe changes in polarization, chemical composition, charge density, and strain at DWs and vortices. We further highlight the current achievements in mapping the 3D nature of ferroelectric polar skyrmions and in situ biasing. This Review will focus on both the fundamental physics of DW and polar vortex formation and their dynamics. Finally, we discuss how electron spectroscopy can be used to relate the quantified structural distortions of polar topological entities to changes in their oxidation state and band structure.
Highlights
In the past 20 years, there has been a significant shift in interest within the ferroelectric community from domains to domain walls (DWs) as the active element for device applications.1 At almost the exact same time, commercially available aberration corrected scanning transmission electron microscopy (STEM) was achieved.2–4 This significant advancement in STEM enabled researchers to start analyzing the sub-atomic shifts at ferroelectric domain walls and polar interfaces,5 probing the fundamental physics at the spatial resolution of the DW itself
On the other hand, are freer to adjust their domain structure to minimize their free energy during the lift-out and thinning of a lamella. This factor should be kept in mind when choosing the focused ion beam (FIB) lamella orientation, shape, thickness, and whether to include extra thin “windows.” Altering the FIB preparation techniques can result in drastically different strain throughout the lamella and DW pattern confinement effects
For ABO3 perovskite ferroelectrics, high angle annular dark field (HAADF) can identify the heavier “A” and “B” site atoms, while annular bright field (ABF) can be used to see the displacements or rotations of oxygen octahedra (Fig. 4).90–96. These complimentary aspects of high resolution (HR) STEM are crucial as the positions of all atoms in the unit cell must be measured to determine the displacements with respect to the paraelectric phase and the net polarity
Summary
In the past 20 years, there has been a significant shift in interest within the ferroelectric community from domains to domain walls (DWs) as the active element for device applications. At almost the exact same time, commercially available aberration corrected scanning transmission electron microscopy (STEM) was achieved. This significant advancement in STEM enabled researchers to start analyzing the sub-atomic shifts at ferroelectric domain walls and polar interfaces, probing the fundamental physics at the spatial resolution of the DW itself. While the crystal symmetry is the overriding energy consideration for DW orientation in bulk crystals, DW or vortex orientation in nanoscale samples such as TEM lamellae can vary significantly These factors are important as the DW contrast is often the most accessible clue for understanding the overall domain structure and polarity. A careful understanding of TEM contrast mechanisms and the applicability of different polarization mapping techniques is crucial for DW studies In this Review, we discuss how aberration corrected STEM techniques can be used to obtain reliable maps of the polarization, electric fields, and charge distribution within ferroelectric DWs and other polar topologies such as vortices, and how to avoid the most common sources of error and artifacts attributed to these techniques. We highlight some of the most recent advances in STEM characterization methods for ferroelectrics such as visualizing electric charge density at sub-angstrom resolution and the benefits of coupling polarization characterization with electron energy loss spectroscopy (EELS) band structure analysis.
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