Mechanical Manipulation of Moiré Ferroelectric Domain Structures in Twisted Bilayer WSe₂

Two-dimensional (2D) materials with vertical ferroelectric and piezoelectric properties are highly promising for future ultrathin electronic devices. In this viewpoint, moiré superlattices in van der Waals heterostructure with alternating vertical-polarization have attracted considerable attention in the communities of 2D materials and electronic devices. Bilayer of transition metal dichalcogenide (TMD, MX₂ structure where M: transition metal and X: chalcogen) stacked with a small angle gives birth to a micro-to-nano scale of moiré superlattices with two types of ferroelectric domains, so-called MX and XM domains. This new type of ferroelectric domains changes its polar structures by interlayer lateral sliding, which is a novel polarization switching mechanism distinct from conventional ferroelectrics.<br/><br/>Here, we present an ongoing study about the mechanical manipulation of ferroelectric domain structures (particularly, nodes where domain walls meet) performed by atomic force microscopy (AFM). Normal load of electrically-grounded AFM tip and its subsequent motion can guide to the top layer slips relative to the bottom layer, resulting in ripple-motion-like rearrangement in ferroelectric domains. Interestingly, this mechanical manipulation enabled the movement of the nodes or even break-up of them in the region where four domains meet (hereinafter referred to as “4-domain region”). However, such manipuation of nodes was not possible in the region where six domains meet (6-domain region). Theoretically, the 6-domain region has higher energy barrier for the node slip, leading to huge energy penalty compared to that of 4- domain’s. In accordance to both calculation and experimental results, we suggest the existence of these topologically protected and unprotected nodes in moiré bilayer heterostructures. Understanding the planar-nodes in moiré ferroelectric domains can be a major step to manipulate the switches made of a bistate topological dislocations. This work on dislocations opens up a new venue for the nanoscale control of ferroelectric domains and resultant topological structures in twisted TMD bilayer via non-electrical ways.