Atomically Resolved Observation of Continuous Interfaces between an As-Grown MoS2 Monolayer and a WS2/MoS2 Heterobilayer on SiO2

van der Waals (vdW) heterostructures synthesized through the chemical vapor deposition (CVD) method allow the creation and tuning of intriguing electronic and optical properties of two-dimensional (2D) materials. Especially, local structures in the heterostructures, such as interfaces, edges, and point defects, are critical for their wide range of potential applications. However, up to now atomic-scale measurements of local structures in as-grown 2D heterostructures on insulating substrates are still rare. Here we report our scanning tunneling microscopy and spectroscopy (STM/STS) study of as-grown MoS2 monolayer and WS2/MoS2 heterobilayer on SiO2. The heterobilayer appears smoother than the MoS2 monolayer, with a root-mean-square roughness of 0.230 nm in the former and 0.329 nm in the latter. For the first time, to our knowledge, we directly observed a novel type of continuous interface between the MoS2 monolayer and the top layer of the heterobilayer with atomic resolution. Our STS results and density functional theory calculations revealed the band gaps of the heterobilayer and the MoS2 monolayer. The finding of the continuous interfaces and the systematic characterizations could have significant impacts on the optimization and design of new 2D heterostructures.