Bend-Induced Ferroelectric Domain Walls in ?-In2Se3

The low bending stiffness of atomic membranes from van der Waals ferroelectrics such as alpha-In2Se3 allow access to a regime of strong coupling between electrical polarization and mechanical deformation at extremely high strain gradients and nanoscale curvatures. Here, we investigate the atomic structure and polarization at bends in multilayer alpha-In2Se3 at high curvatures down to 0.3 nm utilizing atomic-resolution scanning transmission electron microscopy, density functional theory, and piezoelectric force microscopy. We find that bent alpha-In2Se3 produces two classes of structures: arcs, which form at bending angles below similar to 33 degrees, and kinks, which form above similar to 33 degrees. While arcs preserve the original polarization of the material, kinks contain ferroelectric domain walls that reverse the out-of-plane polarization. We show that these kinks stabilize ferroelectric domains that can be extremely small, down to 2 atoms or similar to 4 angstrom wide at their narrowest point. Using DFT modeling and the theory of geometrically necessary disclinations, we derive conditions for the formation of kink-induced ferroelectric domain boundaries. Finally, we demonstrate direct control over the ferroelectric polarization using templated substrates to induce patterned micro-and nanoscale ferroelectric domains with alternating polarization. Our results describe the electromechanical coupling of alpha-In2Se3 at the highest limits of curvature and demonstrate a strategy for nanoscale ferroelectric domain patterning.