Superdomains and superdomain walls in ferroelectric thin films

In ferroelectric thin films, the complex interplay between mechanical and electrostatic boundary conditions in reduced dimensionality allows for the formation of a wide variety of domain configurations with fascinating properties. Domains are regions of uniformly oriented polarization, separated by interfaces termed domain walls (DWs). These domain walls can be as thin as a few atomic layers and exhibit physical properties and symmetry different from the parent phase—indeed, when polar, they can themselves be considered as individually switchable nanoscale ferroelectric entities. At larger length scales, domains can self-organize into superdomains—mesoscopic, spatially correlated assemblies of domains that behave as higher-order switching units. Superdomains differ from loosely arranged domain collections because their constituent domains exhibit cooperative dynamics, giving rise to collective polarization states, hierarchical organization, and emergent functional properties. Their characteristic length scales range from hundreds of nanometers to several micrometers, and their order parameter is the collective polarization. The interfaces between superdomains—superdomain walls—are distinct from conventional domain walls. They separate regions with different collective polarization arrangements and may host novel interfacial phenomena, offering opportunities for unique functionalities in devices such as non-volatile memories or for emergent effects like effective negative capacitance. Domains and superdomains can be found in different materials, in the form of bulk crystals, epitaxial, and free-standing thin films; their features, including size and density, can be tuned and/or controlled by changing, for instance, the strain state, the electrostatic boundary conditions of the system and, for thin films, the thickness, the deposition temperature, and other growth parameters, as shown in Fig. 1. In this experimentally driven review, we will explore the formation, structure, and properties of domains, superdomains, and superdomain walls in ferroelectric thin films. We will, in particular, focus on tetragonal perovskite ferroelectrics, which present the salient features we wish to address (the presence of both purely ferroelectric domains and ferroelectric/ferroelastic twins, as well as a broad range of complex polarization textures), while restraining the number of possible configurations with respect to, for example, orthorhombic or rhombohedral symmetry ferroelectrics. While this review is, thus, not intended to be exhaustive, we hope that it sparks interest in what we see as a dynamic and promising area—ferroelectric functional materials—with strong potential for future development and possible applications. It is worth mentioning that the philosophy followed for the figures is to illustrate our discussion with examples from the literature and from our own work, relatively briefly discussed in the main text. More details are found in the caption with references for the interested reader.