Lawrence Berkeley National Laboratory

Van der Waals epitaxy enables the integration of 2D transition metal dichalcogenides with other layered materials to form heterostructures with atomically sharp interfaces. However, the ability to fully utilize and understand these materials using surface science techniques such as angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM) requires low defect, large area, epitaxial coverage with ultra-clean interfaces. We have developed a chemical vapor deposition van der Waals epitaxy growth process where the metal and chalcogen sources are separated such that growth times can be extended significantly to yield high coverage while minimizing surface contamination. We demonstrate the growth of high quality 2D WS2 over large areas on graphene. The as-grown vertical heterostructures are exceptionally clean as demonstrated by ARPES, STM and spatially resolved photoluminescence mapping. With these correlated techniques we are able to relate defect density to electronic band structure and, ultimately, optical properties. We find that our synthetic approach provides ultra-clean, low defect density (~1012 cm-2), ~10 μm large WS2 monolayer crystals, with an electronic band structure and valence band effective masses that perfectly match the theoretical prediction for pristine WS2.