Surface-Mediated Aligned Growth of Monolayer MoS₂ and In-Plane Heterostructures with Graphene on Sapphire

Aligned growth of transition metal dichalcogenides and related two-dimensional (2D) materials is essential for the synthesis of high-quality 2D films due to effective stitching of merging grains. Here, we demonstrate the controlled growth of highly aligned molybdenum disulfide (MoS₂) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS₂ grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS₂-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS₂, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS₂ growth on sapphire was further developed to the rational synthesis of an in-plane MoS₂-graphene heterostructure connected with the predefined orientation. The in-plane epitaxy was observed by low-energy electron microscopy. Transmission electron microscopy and scanning transmission electron microscopy suggest the alignment of a zigzag edge of MoS₂ parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS₂ was obtained by the monolayer graphene compared with a conventional metal electrode. Our findings deepen the understanding of the chemical vapor deposition growth of 2D materials and also contribute to the tailored synthesis as well as applications of advanced 2D heterostructures.