In-Situ Switching of van der Waals Ferroelectrics with In-Plane Electric Biasing

Two-dimensional (2D) van der Waals (vdW) ferroelectrics offer the enticing opportunity of both stabilizing ferroelectricity down to atomic thickness while seamlessly integrating with current complementary metal-oxide-semiconductor (CMOS) technologies [1-3]. Here, we perform <i>in-situ</i> in-plane biasing scanning transmission electron microscopy (STEM) imaging to investigate the switching dynamics in vdW (anti)ferroelectrics. By visualizing the metastable intermediate states during switching processes at atomic scale, we reveal the pivotal role of stacking-polarization coupling in governing the switching pathways of SnSe.<br/><br/>Our in-situ biasing experiments utilize a micro-electromechanical system (MEMS)-based holder to apply an in-plane biasing to SnSe. A SnSe flake is transferred to a MEMS chip and subsequently thinned by focused ion beam (FIB). We estimate the applied electric field (ranging from 0 to 50 kV cm^-1) by measuring the distance between Pt electrodes and the voltage supplied by a constant voltage source. High angle annular dark-field (HAADF) STEM images reveal the polarization order and interlayer stacking order. Upon applying in-plane electric field to pristine AFE-order SnSe, both AFE-to-FE polarization order transition and AB-to-AC stacking order transition were observed through a 180° switching pathway. In addition, 90° switching can also introduce stacking order transition and concurrently switch armchair to zigzag direction. To quantify the in-plane strain, out-of-plane strain and atomic displacement, we performed strain mapping derived from the atomic-scale images for understanding the switching mechanisms.<br/><br/>In summary, by combining <i>in-situ</i> in-plane biasing method and atomic position analysis, we reveal the intrinsic coupling between stacking and polarization order in 2D vdW ferroelectrics and highlight the strain-mediated switching pathways of AFE-to-FE order transition. Additionally, this experimental methodology is adaptable to any in-plane (A)FE materials and <i>in-situ</i> heating technology, underscoring the <i>in-situ</i> in-plane biasing method for understanding fundamental mechanisms of functional materials.<br/><br/><b>References:</b><br/>1. Wang C, <i>et al</i>., <i>Nat. Mater.</i>, <b>22</b>, 542, 2023.<br/>2. Shi C, <i>et al</i>., <i>Nat. Commun.</i>, <b>14</b>, 7168, 2023.<br/>3. Xu B, <i>et al</i>., <i>Npj Comput. Mater.</i>, <b>8</b>, 47, 2022.