Direct growth of transition metal dichalcogenides (TMDs) such as WS2 and WSe2 on graphene and hexagonal boron nitride (hBN) is of interest for the fabrication of 2D heterostructures as an alternative to exfoliation and 2D layer stacking. Commensurability between the TMD lattice and graphene or hBN is favorable for epitaxy, however, it is difficult to control nucleation and wetting of the TMD on pristine van der Waals surfaces due to the low surface energy and lack of chemical bonding. In addition, mirror twins and inversion domains can form due to rotational symmetry which introduces grain boundaries in coalesced TMD films. However, the presence of defects in hBN and graphene can dramatically alter the energy landscape providing an alternative route to control nucleation and epitaxy.
Our recent studies have focused on the epitaxial growth of WSe2, WS2 and related TMD monolayer films by gas source chemical vapor deposition (CVD) on hBN and graphene. The CVD process is carried out in a cold-wall reactor using metal hexacarbonyls and hydride chalcogen precursors in a hydrogen carrier gas. In the case of growth on epitaxial graphene on SiC, nucleation of WS2 and WSe2 occurs primarily at wrinkles and step edges resulting in different WS2 rotational orientations. In the case of WSe2 growth on hBN, however, single atom vacancies in the hBN were found to act as sites for metal atom trapping which facilitates the nucleation of WSe2 domains on the surface. In this case, the TMD nucleation density can be controlled by manipulating the density of surface atom vacancies which can be achieved through plasma irradiation and annealing in NH3. In addition, the metal atoms break the surface symmetry which leads to a preferred orientation (~95%) for WSe2 domains on hBN. Through careful control of nucleation and extended lateral growth time, fully coalesced WSe2 monolayer films on hBN were achieved which exhibit optical and transport properties superior to comparable films grown on sapphire substrates. The results demonstrate the important role of defects in nucleation and epitaxial growth of 2D heterostructures.