Two-dimensional (2D) transition metal dichalcogenides (TMDs) are technologically consequential materials owing to their attractive properties such as indirect to direct band gap transition upon thinning to a monolayer [1] . Integration of multiple 2D TMDs into heterostructures in various geometries (vertical and in-plane) has accelerated the development of these materials into targeted applications in optoelectronics [2] . However, the performance of these heterostructures is highly dependent

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... the interfaces formed between constituent TMDs [3] . For example, defects and strain at the interface, originating from the lattice mismatch between the two 2D materials, can significantly affect the interface structure and consequently the heterostructure performance [4] . At the same time, this provides an opportunity to control the interface characteristics by careful selection of TMDs and synthesis methods to tune the properties of resulting heterostructures. Most of the studies on heterostructures so far focused on those formed by isotropic TMDs such as MoS 2 , WS 2 , WSe 2 and MoSe 2 . Whereas, reports on heterostructures formed between an isotropic and an anisotropic TMDs are sparse. In this study, ReS 2 was chosen to form an interface with MoS 2 because those two materials have different crystal structures. ReS 2 has an anisotropic structure, as opposed to isotropic MoS 2 [5] . Such anisotropy in the system can introduce modulations in the atomic and interfacial structure that is derived from strain and lattice mismatch. Moreover, MoS 2 -ReS 2 heterostructures are expected to exhibit type I band alignment, strong interlayer interaction and demonstrate excellent photoresponse properties [6] . Therefore, it is imperative to synthesize MoS 2 -ReS 2 heterostructures with well-defined interfaces and to understand their atomic structure. Here, we used a two-step CVD process to synthesize vertical and in-plane MoS 2 -ReS 2 heterostructures wherein, MoS 2 is grown on c-plane sapphire during the first step and ReS 2 is subsequently grown on MoS 2 /sapphire in the second step. The as grown heterostructures are transferred to Cu quantifoil holey carbon grids for studying their microstructure. We employed scanning/transmission electron microscopy (S/TEM) techniques to investigate the interface atomic structure, defects, epitaxy and strain between the MoS 2 and ReS 2 layers. Figure 1a shows a low magnification TEM image of MoS 2 -ReS 2 vertical heterostructure. Selected area diffraction pattern (SADP) obtained from the area outlined by the dashed