Imaging Moire Superlattice of Transition Metal Dichalcogenides using Torsional Force Microscopy

Stacking atomically thin Van der Waals (VdW) layers at a small twist angle forms moire superlattices that can dramatically transform the system’s electronic properties. Direct imaging of moire superlattices provides important information about their structures. Various scanning probe microscopy techniques have been applied for this purpose, but oftentimes, the applicability is specific to particular moire material systems. Recently, Torsional Force Microscopy (TFM), which is sensitive to tip-sample dynamic friction, has been demonstrated to image graphene based moire superlattices. In this work, we apply the TFM technique to image the moire superlattice of transition metal dichalcogenides (TMD) of WS₂/WSe₂. We study how the scanning parameters affect the TMF signal contrasts from the moire lattice, based on which we will discuss the underlying mechanism of moire contrast from friction. We will also compare TFM with piezoelectric force microscopy (PFM) in their capability to image TMD moire systems.