Mechanically Generated Ferroelectric Domains in van der Waals α-In₂Se₃

Mechanical deformations, such as strain or strain gradient, offer a facile way to modulate the polarization of ferroelectric materials. As the sizes of the electromechanical systems approach nanoscale, strain gradient becomes important due to its inverse scaling with sample size. So, it is imperative to understand how extreme strain gradient, such as bending to a nanoscale radius of curvature, tunes the electrical polarization of ferroelectrics. Conventional perovskite ferroelectric systems are fundamentally limited in their ability to undergo nanoscale mechanical deformations before fracture. In this regard, van der Waals ferroelectric materials, such as α-In₂Se₃, provide a unique opportunity to understand the interplay between electrical polarization and mechanical deformation at strain gradients orders of magnitude higher than previously explored, due to their ability to deform to nanoscale curvatures. The key questions here are how bending multilayer α-In₂Se₃ to an extremely high and previously unexplored strain gradient regime modulates the ferroelectric domains.<br/>In this study, we investigate the modulation of out-of-plane polarization in highly localized bends in multilayer α-In₂Se₃. Bending of the material is achieved by buckle delamination created by the residual stress from mechanical exfoliation process. We study the atomic structure, energetics of the domains, and electrical polarization using scanning transmission electron microscopy (STEM), density functional theory (DFT), and piezoelectric force microscopy (PFM). Finally, we fabricate patterned trenches and transfer α-In₂Se₃ flakes to controllably tune the material.<br/>We show that bending multilayer α-In₂Se₃ to a nanoscale curvature leads to two different types of deformation which depends on the bending angle. Bending angle below 35° results in smooth arcs where the polarization of the material is uniformly maintained throughout the arc. However, bending above 35° results in highly localized kink formation. This kink formation is associated with a structural transformation and creation of a domain wall that results in polarization switching. Using DFT, we show the energetic favorability of kink formation at high bending angle. We also explain the origin of threshold bending angle from the geometric favorability of one disclination formation per layer. PFM measurement shows that the newly formed domain at the kink propagates inside the material and is arrested by material defects, such as crack, edge, or another kink. Finally, we demonstrate control over this bend-induced domain wall formation by transferring α-In₂Se₃ onto patterned trenches to artificially bend the material over threshold angle and subsequently create alternating domain structures.<br/>Altogether, these results unveil the interplay between ferroelectricity and mechanical deformation at extreme curvatures between 7×10⁶ m^-1 and 3×10⁹ m^-1, establishing opportunities of electrical polarization manipulation and domain wall engineering. These findings lay the groundwork for flexible and reconfigurable nanoelectronics based on van der Waals ferroelectric materials.