Philip Kim1
Harvard University1
Controlling the interlayer twist angle in an artificial two-dimensional (2D) van der Waals (vdW) interface offers an experimental route to create a moire superlattice. In the small twist angle homojunction, vdW interlayer interaction can cause atomic-scale structural reconfiguration at the interface, creating the arrays of moire domain structures. The boundaries of these domain structures are structural solitons, forming 2D dislocation line arrays. The crossing points of these dislocation lines are topological objects, which cannot be removed by local deformation. Particularly, in the crystal symmetry engineered twisted bilayer polar crystals, we find unconventional domain anti-ferroelectricity can arise to exhibit the ferroelectric/antiferroelectric switching mechanism. We performed an in-situ transmission electron microscopy (TEM) investigation on dual gated twisted bilayer transition metal dichalcogenides (TMD) devices that enable real-time observation of polar domain dynamics in a 2D system. In combination with the theoretical investigation, we find the polarizability of the twisted bilayer TMD sensitively depends on the moire length and the domain shapes. As the size of domain approaches to the sample size, spontaneous polarization in the engineered TMD interface shows analogy as well as interesting distinction compared with the conventional ferroelectrics, exhibiting the electrically switchable bipolar structure in the engineered TMD. However, as the domain size becomes smaller than the size of the samples, the clear demonstration of alternating polar domains appears with an antiferroelectric configuration. Unlike atomic-scale antiferroelectric, these antiferroelectric domain structures cannot be transformed into a single ferroelectric domain due to the topological protection of the domain topology. We will discuss the implication of engineering (anti)-ferroelectric domain structures for the ferroelectric device applications based on 2D twisted homojunction.