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

Abstract

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 (MoS) on c-plane sapphire with two distinct orientations, which are highly controlled by tuning sulfur concentration. We found that the size of the aligned MoS grains is smaller and their photoluminescence is weaker as compared with those of the randomly oriented grains, signifying enhanced MoS-substrate interaction in the aligned grains. This interaction induces strain in the aligned MoS, which can be recognized from their high susceptibility to air oxidation. The surface-mediated MoS growth on sapphire was further developed to the rational synthesis of an in-plane MoS-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 MoS parallel to a zigzag edge of the neighboring graphene. Moreover, better electrical contact to MoS 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.

Van der Waals epitaxy of continuous thin films of 2D materials using atomic layer deposition in low temperature and low vacuum conditions

“…The failure to obtain epitaxial growth might be ascribed to the relatively low deposition temperatures compared to the earlier reports of vdW epitaxy on sapphire (see Section S13 for a literature survey). It has been suggested that epitaxy of TMDCs on sapphire is enabled by chemical reduction of the sapphire surface in a chalcogen environment at high temperatures, 50 or by formation of a chalcogen-terminated surface layer on the sapphire. 51 Both are be unlikely to occur under our mild ALD conditions.…”

Epitaxial Growth of Rectangle Shape MoS2 with Highly Aligned Orientation on Twofold Symmetry a‐Plane Sapphire

“…[ 28 ] In the past several years, researchers have paid great efforts to pursue the orientation‐controlled growth of MoS 2 (or WS 2 ) on several kinds of single‐crystal substrates, such as hexagonal boron nitride (h‐BN), graphene and c‐plane sapphire. [ 29–38 ] Among them, the threefold symmetry c‐plane sapphire has great potential for the orientation‐controlled growth of the MoS 2 when suitably control the growth parameters. [ 33–38 ] All above research works demonstrate the fact that the orientation of MoS 2 grain is closely related to the lattice direction and symmetry of substrate, and that would be limited if the lattice symmetry of substrate is lowered.…”

“…[ 29–38 ] Among them, the threefold symmetry c‐plane sapphire has great potential for the orientation‐controlled growth of the MoS 2 when suitably control the growth parameters. [ 33–38 ] All above research works demonstrate the fact that the orientation of MoS 2 grain is closely related to the lattice direction and symmetry of substrate, and that would be limited if the lattice symmetry of substrate is lowered. This is reasonable because the reduction of symmetry of substrate can decrease the degree of freedom of materials growth.…”

“…In addition, the PL peaks become broader after the transfer process, which is attributed to the increase of charge density due to the unintentional doping induced by the transfer process. [ 35 ] Notably, the alignment growth on a‐plane sapphire can be extended to the synthesis of other 2D TMDs, for example, we can also grow the well‐aligned rectangle WS 2 grains (Figure S6, Supporting Information).…”

“…The few deviation (≈14%) of MoS 2 grains from the dominant crystallographic orientation is reasonable, because each grain goes through different environment during the growth process, resulting in the original difference of lattice orientation in the early stages of nucleation. [ 35 ] Thus, it would be possible to achieve the complete alignment of all MoS 2 grains on the entire a‐plane sapphire substrate in the future, by carefully optimizing the substrate treatment and growth conditions.…”