Imaging moiré superlattices in twisted bilayers of 2D materials*

Two-dimensional materials stacked into van der Waals (vdW) heterostructures form moiré superlattices as a result of a relative rotational misalignment between layers as well as a lattice mismatch between two dissimilar materials. The resulting physics of the vdW heterostructures can be dominated by the large scale periodicity of the moiré superlattice which is generally substantially larger than the atomic scale lattice. In this work we outline a method to visualize the real-space moiré superlattices in a variety of twisted vdW heterostructures utilizing piezoresponse force microscopy which locally measures an electromechanical sample surface deformation. Its origin is suggested to arise due to the flexoelectric effect wherein electric polarization is induced through strain gradients present within moiré superlattices.