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In transition metal dichalcogenide (TMDC) heterostructures, interlayer coupling plays a crucial role in the design and emergence of diverse properties. Here, we show the control of the interlayer coupling in Janus WSSe/WS2 heterostructures by changing the stacking order. Low-frequency Raman measurements revealed that heterostructures with the S/S interface show strong interlayer coupling compared to those with the S/Se interface. In the case of the S/S interface, photoluminescence (PL) from WS2 was quenched due to charge transfer in the type-II band alignment, while, interestingly, in the case of the S/Se interface, it was enhanced. The stacking-order dependence of the interlayer coupling and photoluminescence are attributed to the direction of interfacial electric field caused by broken out-of-plane mirror symmetry in a WSSe layer. In the case of the S/S interface, the interfacial electric field promotes the charge transfer as evidenced by strong PL quenching. On the contrary, in heterostructures with the S/Se interface, the charge transfer is blocked as the electric field prevents carrier transfer and modulates the band offset. Our results clearly show that Janus TMDCs with intrinsic fields can be effectively used to manipulate the interlayer interactions and optical properties in heterostructures.

We report etching-free and iodine-free transfer of highly aligned array of armchair-edge graphene nanoribbons (ACGNRs) and their field-effect transistor (FET) characteristics. They were prepared by on-surface polymerization on Au(788) templates. The ACGNRs were mechanically delaminated and transferred onto insulating substrates with the aid of a nano-porous support layer composed of hydrogen silsesquioxane (HSQ). The key process in the mechanical delamination is the intercalation of octanethiol self-assembled monolayers (SAMs), which penetrate the HSQ layer and intercalate between the ACGNRs and Au(788). After the transfer, the octanethiol SAMs were removed with Piranha solution, enabling the reuse of the Au single crystals. The FETs fabricated with the transferred ACGNR array showed ambipolar behavior when the channel length was as long as 60 nm. Quasi-one-dimensional conductivity was observed, which implies a good alignment of GNRs after the transfer. In contrast, short-channel ACGNR FETs (channel length ∼20 nm) suffer from a geometry-dependent short-channel effect. This effect is more severe in the FETs with ACGNRs parallel to the channel, which is an ideal geometry, than in ones perpendicular to the channel. Since the ID-VD curve is well fitted by the power-law model, the short-channel effect likely stems from the space-charge limited current effect, while the wide charge-transfer region in the GNR channel can be another possible cause for the short-channel effect. These results provide us with important insights into the designing short-channel GNR-FETs with improved performance.

Transition metal dichalcogenides (TMDs) have emerged as exciting 2D materials beyond graphene due to their promising applications in the field of electronics and optoelectronics. Hence, the ability to produce controllable and uniformly thick TMD sheets over a large area is of utmost important for large-scale applications. Here, a facile method of synthesizing large-area, layer-controlled WS2, and MoS2 sheets by sulfurization of their corresponding thin metal films is reported. A metal film, which is deposited by magnetron sputtering method, can be adjusted to produce, with great control, the desired sheet thickness down to a monolayer. Various characterization techniques, such as Raman, photoluminescence, and transmission electron microscopy, were used to evaluate the grown films. The results confirmed some of the exotic properties of TMDs such as the thickness dependent band-gap transition (indirect to direct band gap) and Raman shift. Devices made directly on the as-grown film showed modest mobility, ranging from 0.005 to 0.01 cm2 V−1s−1. Our synthesis method is simple and could also be used to synthesize other TMDs.