Atomic reconstruction and moir\'e patterns in transition metal dichalcogenide van der Waals heterostructures

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding which enables one to create van der Waals heterostructures (vdWH). Recent work has shown that control of the twist angle between layers can have a dramatic effect on vdWH properties. For TMD vdWH, twist angle has been treated solely through the use of rigid-lattice moir\'e patterns. No atomic reconstruction, i.e. any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles less than 1{\deg} undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moir\'e pattern as has been assumed in prior work. Using conductive atomic force microscopy (CAFM), we show that the stacking orientation of the two TMD crystals has a profound impact on the atomic reconstruction, consistent with recent theoretical work on graphene/graphene and MoS2/MoS2 structures and experimental work on graphene bilayers and hBN/graphene vdWH. Transmission electron microscopy (TEM) provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moir\'e pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra from reconstructed vdWH. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provides fundamental insight into the behavior of this important class of heterostructures.