A Small Step for Epitaxy, a Large Step Towards Twist Angle Control in 2D Heterostructures

Two-dimensional (2D) materials have received a lot of interest over the past decade. Especially van der Waals (vdW) 2D materials, such as transition metal dichalcogenides (TMDCs), and their heterostructures exhibit semiconducting properties that make them highly suitable for novel device applications. Controllable mixing and matching of the 2D materials with different properties and a precise control of the in-plane twist angle in these heterostructures are essential to achieve superior properties and need to be established through large-scale device fabrication. To gain fundamental insight into the control of these twist angles, 2D heterostructures of tungsten disulfide (WS2) and graphene grown by bottom-up synthesis via metal-organic chemical vapor deposition (MOCVD) are investigated using a scanning transmission electron microscope (STEM). Specifically, the combination of conventional high-resolution imaging with scanning nano beam diffraction (SNBD) using advanced 4D STEM techniques is used to analyze moiré structures. The latter technique is used to reveal the epitaxial alignment within the WS2/Gr heterostructure, showing a direct influence of the underlying graphene layers on the moiré formation in the subsequent WS2 layers. In particular, the importance of grain boundaries within the underlying WS2 and Gr layers for the formation of moiré patterns with rotation angles below 2 is discussed.