The electrical contact is one of the main issues preventing semiconducting 2D materials to fulfill their potential in electronic and optoelectronic devices. To overcome this problem, a new approach is developed here that uses chemical vapor deposition grown multilayer graphene (MLG) sheets as flexible electrodes for WS 2 field-effect transistors. The gate-tunable Fermi level, van der Waals interaction with the WS 2 , and the high electrical conductivity of MLG significantly improve the overall performance of the devices. The carrier mobility of single-layer WS 2 increases about a tenfold (50 cm 2 V −1 s −1 at room temperature) by replacing conventional Ti/Au metal electrodes (5 cm 2 V −1 s −1 ) with the MLG electrodes. Further, by replacing the conventional SiO 2 substrate with a thin (1 µm) parylene-C flexible film as insulator, flexible WS 2 photodetectors that are able to sustain multiple bending stress tests without significant performance degradation are realized. The flexible photodetectors exhibited extraordinarily high gate-tunable photoresponsivities, reaching values of 4500 A W −1 , and with very short (<2 ms) response time. The work of the heterostacked structure combining WS 2 , graphene, and the very thin polymer film will find applications in various flexible electronics, such as wearable high-performance optoelectronics devices.

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.

Recently, research on transition metal dichalcogenides (TMDCs) has been accelerated by the development of large-scale synthesis based on chemical vapor deposition (CVD). However, in most cases, CVD-grown TMDC sheets are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs) which deteriorate the physical properties of the TMDC. Here, we demonstrate the epitaxial growth of monolayer tungsten disulfide (WS 2 ) on sapphire by introducing a high concentration of hydrogen during the CVD process. As opposed to the randomly oriented grains obtained in conventional growth, the presence of H 2 resulted in the formation of triangular WS 2 grains with the welldefined orientation determined by the underlying sapphire substrate. Photoluminescence of the aligned WS 2 grains was significantly suppressed compared to that of the randomly oriented grains, indicating a hydrogen-induced strong coupling between WS 2 and the sapphire surface that has been confirmed by density functional theory calculations. Scanning transmission electron microscope observations revealed that the epitaxially grown WS 2 has less structural defects and impurities. Furthermore, sparsely distributed unique dislocations were observed between merging aligned grains, indicating an effective stitching of the merged grains. This contrasts with the GBs that are observed between randomly oriented grains, which include a series of 8-, 7-, and alternating 7/5membered rings along the GB. The GB structures were also found to have a strong impact on the chemical stability and carrier transport of merged WS 2 grains. Our work offers a novel method to grow high-quality TMDC sheets with much less structural defects, contributing to the future development of TMDC-based electronic and photonic applications.

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We demonstrate the synthesis of unique heterostructures consisting of SnS and WS (or SnS and MoS) by two-step chemical vapor deposition (CVD). After the first CVD growth of triangular WS (MoS) grains, the second CVD step was performed to grow square SnS grains on the same substrate. We found that these SnS grains can be grown at very low temperature with the substrate temperature of 200 °C. Most of the SnS grains nucleated from the side edges of WS (MoS) grains, resulting in the formation of partly stacked heterostructures with a large overlapping area. The SnS grains showed doped p-type transfer character with a hole mobility of 15 cm V s, while the WS and MoS grains displayed n-type character with a high on/off ratio of >10. The SnS-WS and SnS-MoS heterostructures exhibited clear rectifying behavior, signifying the formation of p-n junctions at their interfaces. This heterostructure growth combined with the low temperature SnS growth will provide a promising means to exploit two-dimensional heterostructures by avoiding possible damage to the first material.

Influence of sapphire substrate on the epitaxial growth of WS2 was investigated in terms of the optical and electrical properties.

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