Department of Materials Science and EngineeringFollowing the rise in popularity of various two-dimensional (2D) materials, e.g., graphene (Gr), hexagonal boron nitride (h-BN), phosphorene, and transition metal dichalcogenides (TMDs), the stacked structure of these layered 2D materials (called van der Waals heterostructures) has played an important role in overcoming the limitations of individual 2D materials and expanding the range of attainable properties for use in various fields.
In this thesis, we introduced the source contact geometry between a source and a substrate and optimized the direct growth method to obtain high-quality and uniform WS2/Gr heterostructures with strong interlayer coupling. The obtained, epitaxially grown WS2/Gr heterostructure possesses increased domain sizes and an enhanced coverage of monolayer WS2 without compromising the quality of the WS2 or significantly damaging the underlying graphene. Particularly, the symmetrical and narrow photoluminescence (PL) peak reveals the superior crystallinity of the grown WS2. This result is in contrast to those obtained by different conventional methods, such as the exfoliation method and CVD growth with conventional opened geometries.
While they host many exciting potential applications, some of these 2D materials are subject to environmental instability issues induced by interaction between material and gas molecules in air, which poses a barrier to further application and manufacture. To overcome this, it is necessary to understand the origin of material instability and degradation processes in air, as well as developing strategies to extend air-stabilty. The long-term investigations on air stability indicate that the resulting WS2/Gr heterostructures exhibit outstanding stability, in contrast to the general short-term degradation of 2H phase TMD (2H-TMDs) flakes grown on conventional substrates. This results not only describe the superior crystallinity of as-grown WS2 on graphene, but electron microscopy images, X-ray photoelectron spectroscopy (XPS) results, and Photoluminescence (PL) spectra collected over months suggest that graphene serves a new role by obstructing the aging propagation.
This result shows how important interlayer interactions are in the stacking configuration of the vdW heterostructure. As an additional study, we show that the characteristics of the subsequent layer in vertical van der Waals (vdW) heterostructure are considerably affected by the structural defects of the template layer, comparing two types of directly synthesized WS2 flakes on CVD grapheneon the pristine basal plane of graphene (B-WS2) and on graphene defects (D-WS2). Both of WS2 flakes show the same crystal structure without atomic displacement and lattice distortion. However, they strongly influenced by interlayer interactions between the stacked layers, affecting the physical and electrical properties including deformability, thermal stability, and junction property. Combined experimental and theoretical studies have shown that the difference in the properties of D-WS2 flakes could be originated from the covalent bonds formed via W atomic bridges with hybridized orbitals at defect sites of graphene. These results suggest that it is importance to understand the interlayer interactions in 2D vdW heterostructures.clos
