Deep Tomographic Piezoresponse Force Microscopy—A New Approach to Unveil Novel Topological Structures in Uniaxial Ferroelectrics

With the development of scanning probe microscopy and related techniques (piezoresponse force microscopy, conductive atomic force microscopy), we now have access to nanoscale images of ferroelectric domain structure and in-situ electrical, mechanical or magnetic properties with unprecedented resolution. While representing an incredible leap forward in our understanding of these systems, the information available is quite literally superficial: the entire subsurface structure is traditionally inaccessible by these techniques. Recently, researchers have been interested in experimentally revealing the third dimension of ferroelectric domain structures [1,2] to understand the complex networks that exist below the surface.<br/><br/>Here, we develop tomographic piezoresponse force microscopy to reveal and reconstruct the subsurface ferroelectric domain structure in two well-known uniaxial ferroelectrics: lead germanate and triglycine sulfate. Pioneered by Huey et al. [3], tomographic atomic force microscopy methods involve scanning the surface of a sample with an atomic force microscopy tip in contact with a sample, while employing a significant compressive force from tip to sample. This “machines” the surface of the material, meaning subsequent images are collected at different depths into the material. If carried out in conjunction with standard piezoresponse force microscopy imaging of the ferroelectric domain structure, the 3D domain structure can be recreated. We take this tomographic method to new depths, milling in excess of 5 microns into the surface of the ferroelectric, where previously 100’s of nanometres had been typical [3].<br/><br/>In the case of lead germanate, we find evidence for the theoretically predicted domain wall saddle point features. Models suggest that this is the consequence of the mutual bifurcation of head-to-head domains, and accompanying rotation of polarisation away from the singular polarisation axis. This prevents the build-up of bound charge at head-to-head or tail-to-tail sections of domain walls, and gives the polarisation network a topological character[4]. In triglycine sulphate, we find charged sections of domain walls, which contradicts the notion that domain walls in this material are uncharged, fully penetrating the crystal along the polar axis. Tomographic atomic force microscopy adds a new dimension to scanning probe studies on ferroelectrics, and can reveal topological features which may have been previously unexpected, especially in uniaxial ferroelectrics.<br/><br/>[1] Roede, E. D., <i>et al.</i>, The Third Dimension of Ferroelectric Domain Walls, <i>Advanced Materials </i><b>34</b>, 2202614 (2022).<br/>[2] Kirbus, Benjamin, et al. "Real-time 3D imaging of nanoscale ferroelectric domain wall dynamics in lithium niobate single crystals under electric stimuli: implications for domain-wall-based nanoelectronic devices." <i>ACS Appl. Nano Mater., </i><b> 2.9,</b> 5787-5794. (2019).<br/>[3] Steffes, J. J., Ristau, R. A., Ramesh, R., & Huey, B. D., Thickness scaling of ferroelectricity in BiFeO3 by tomographic atomic force microscopy. <i>Proc. Natl Acad. Sci.</i>, <b>116,</b> 2413-2418, (2019).<br/>[4] Tikhonov, Yurii, et al. "Polarisation Topology at the Nominally Charged Domain Walls in Uniaxial Ferroelectrics." <i>Adv. Mater., </i>2203028, (2022).