Bend-Induced Ferroelectric Domain Walls in α-InSe.

The low bending stiffness of atomic membranes from van der Waals ferroelectrics such as α-InSe 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 α-InSe 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 α-InSe produces two classes of structures: arcs, which form at bending angles below ∼33°, and kinks, which form above ∼33°. 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 ∼4 Å 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 α-InSe at the highest limits of curvature and demonstrate a strategy for nanoscale ferroelectric domain patterning.