Ferroelectric domain walls are small-sized quasi-two-dimensional (2D) systems with great potential in developing memristor technology, non-volatile memory, and electronic components. The instantaneous polarization and swapping between conductive and resistant electrical states lead to a change in the orientation of the domain wall of electric fields.
Study: The Third Dimension of Ferroelectric Domain Walls. Image Credit: remotevfx.com/Shutterstock.com
Moreover, the domain walls in a three-dimensional (3D) material are not perfectly flat and form complex physical structures due to the formation of networks. An article published in Advanced Materials discussed the importance of 3D nanoscale structure for prominent transport properties in 3D materials. Furthermore, erbium manganese oxide (ErMnO3) was used to study the electronic conduction in charged and neutral domain walls of a 3D network.
The combination of finite element modeling and tomographic microscopy techniques helped understand the contribution of domain walls in bulk, showing the importance of curvature effects for local conduction at the nanoscale level. The results provided an understanding of the propagation of the electrical current in domain wall networks and guidelines to design technology based on domain walls.
Ferroelectric Domain Walls
Geometric corrugation effects have an essential role in determining the 2D material’s physical properties. Moreover, these effects can aid in attaining stability in transition-metal dichalcogenides and single-layer graphene, resulting in improved optical, electronic, and mechanical responses.
The ferroelectric domain walls are a newly emerged 2D system with a strong interrelation between morphology and electrical responses. The quasi-2D system has domain walls of thickness between 1 to 10 angstroms. The 2D domain walls do not have a flat structure but instead exhibit a natural bending and curvature due to point defects causing roughness of domain walls, thereby reducing the electrostatic stray fields.
Furthermore, the orientation changes due to the host material’s electric polarization modify the state of the charge, conductivity, and local carrier concentration change. The correlation between the structure of the domain wall and its electronic properties, alongside the spatial mobility of walls, endow unique functionalities to the 2D materials, which inspired the development of domain wall nanoelectronics.
The ferroelectric domain walls within the bulk separate the domains with different electric polarization (P) orientations, making it difficult to access their intrinsic electronic transport properties. To this end, the ferroelectric domain wall’s electrical conduction around the surface was analyzed using scanning probe and electron microscopy techniques.
Structural determination of 3D domain walls is essential to understanding complex nanoscale physics. Hence, different microscopic approaches were explored for the structural characterization, and the results were correlated with electrical conductance along the ferroelectric domain walls in their 3D structures.
Structural Determination of 3D Ferroelectric Domain Walls
In the present work, scanning electron microscopy (SEM), focused ion beam (FIB), and scanning probe microscopy (SPM) were employed in combination to analyze ferroelectric ErMnO3, resolve the structure of the 3D domain wall, and record the transport properties at the nanoscale level.
The tomography data obtained from FIB-SEM correlated the domain wall’s local orientation to the electronic conduction through a single experiment and facilitated accurate calculations that revealed the spreading nature of injected current within the domain wall with the 3D network.
Furthermore, curvature effects were studied through high-resolution atomic force microscopy (AFM) along with finite-element calculations. This combinational analysis quantified the variations in electronic conduction due to the deviation of the domain wall structure from flat plane geometry.
The injected current spread was calculated across the 3D domain wall network, which revealed the existence of a cut-off-length (Lc) specific to the domain wall. The Lc was determined from the relative conductance within the Lc, the charge state, and connections of one domain wall with another were critical to controlling the current path and measuring the domain wall conductivity, highlighting the importance of near-surface domain wall nanostructure.
To summarize, the ferroelectric domain wall’s non-planar nature was demonstrated as a critical parameter in maintaining the localized transport behavior. Furthermore, leveraging the variations in the curvature (temporary or permanent) permitted the regulation of domain wall resistance without the need for position altering or angle tilting of the domain wall.
Thus, variation in curvature paved a new path to domain wall engineering, supporting the development of two-terminal devices, wherein the dynamical or static curvature effects help achieve variations in conductance. This work details curvature-driven transport phenomena and suggests the fundamental criteria for designing a device, including the Lc, which determines the distance between the electrodes.
Generally, based on the conductivity of the domain wall, the Lc varies between the ranges of nanometers to micrometers. This variation in Lc suggests that the entire domain wall network rearranges to suit the electronic transport behavior in systems containing highly conducting walls. Moreover, the curvature effects-based conductivity changes allow multi-level resistance control and complex electronic responses.
Roede, E. D., Shapovalov, K., Moran, T. J., Mosberg, A. B., Yan, Z., Bourret, E., Cano, A. (2022). The third dimension of ferroelectric domain walls. Advanced Materials. https://onlinelibrary.wiley.com/doi/10.1002/adma.202202614