Monolayers of transition metal dichalcogenides (TMDCs) have attracted a great interest for post‐silicon electronics and photonics due to their high carrier mobility, tunable bandgap, and atom‐thick 2D structure. With the analogy to conventional silicon electronics, establishing a method to convert TMDC to p‐ and n‐type semiconductors is essential for various device applications, such as complementary metal‐oxide‐semiconductor (CMOS) circuits and photovoltaics. Here, a successful control of the electrical polarity of monolayer WSe2 is demonstrated by chemical doping. Two different molecules, 4‐nitrobenzenediazonium tetrafluoroborate and diethylenetriamine, are utilized to convert ambipolar WSe2 field‐effect transistors (FETs) to p‐ and n‐type, respectively. Moreover, the chemically doped WSe2 show increased effective carrier mobilities of 82 and 25 cm2 V−1s−1 for holes and electrons, respectively, which are much higher than those of the pristine WSe2. The doping effects are studied by photoluminescence, Raman, X‐ray photoelectron spectroscopy, and density functional theory. Chemically tuned WSe2 FETs are integrated into CMOS inverters, exhibiting extremely low power consumption (≈0.17 nW). Furthermore, a p‐n junction within single WSe2 grain is realized via spatially controlled chemical doping. The chemical doping method for controlling the transport properties of WSe2 will contribute to the development of TMDC‐based advanced electronics.

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.

Organic–inorganic hybrid perovskites have attracted increased interest owing to their exceptional optoelectronic properties and promising applications. Monolayers of transition metal dichalcogenides (TMDCs), such as tungsten disulfide (WS2), are also intriguing because of their unique optoelectronic properties and their atomically thin and flexible structures. Therefore, the combination of these different types of materials is very attractive in terms of fundamental science of interface interaction, as well as for the realization of ultrathin optoelectronic devices with high performance. Here, we demonstrate the controlled synthesis of two-dimensional (2D) perovskite/WS2 heterostructures by an all vapor-phase growth approach. This involves the chemical vapor deposition (CVD) growth of monolayer WS2, followed by the vapor-phase selective deposition of 2D PbI2 onto the WS2 with the successive conversion of PbI2 to organic–inorganic perovskite (CH3NH3PbI3). Moreover, the selective growth of the perovskite on prepatterned WS2 enables the direct synthesis of patterned heterostructures, avoiding any damage to the perovskite. The photodetectors utilizing the perovskite/WS2 heterostructure show increased responsivities compared with isolated thin perovskite obtained by conventional solution methods. The integration of 2D perovskite with TMDCs opens a new avenue to fabricate advanced devices by combining their unique properties and overcoming current processing difficulties of perovskites.

Metallic two-dimensional (2D) transition metal dichalcogenides (TMDCs) are attracting great attention because of their interesting low-temperature properties such as superconductivity, magnetism, and charge density waves (CDW). However, further studies and practical applications are being slowed down by difficulties in synthesizing high-quality materials with a large grain size and well-determined thickness. In this work, we demonstrate epitaxial chemical vapor deposition (CVD) growth of 2D NbS 2 crystals on a sapphire substrate, with a thickness-dependent structural phase transition. NbS 2 crystals are epitaxially aligned by the underlying c-plane sapphire resulting in high-quality growth. The thickness of NbS 2 is well controlled by growth parameters to be between 1.5 and 10 nm with a large grain size of up to 500 μm. As the thickness increases, we observe in our NbS 2 a transition from a metallic 3R-polytype to a superconducting 2H-polytype, confirmed by Raman spectroscopy, aberration-corrected scanning transmission electron microscopy (STEM) and electrical transport measurements. A Berezinskii–Kosterlitz–Thouless (BKT) superconducting transition occurs in the CVD-grown 2H-phase NbS 2 below the transition temperature ( T c ) of 3 K. Our work demonstrates thickness and phase-controllable synthesis of high-quality superconducting 2D NbS 2 , which is imperative for its practical applications in next-generation TMDC-based electrical devices.

Having a direct optical band gap, monolayers of transition metal dichalcogenide (TMD) nanosheets have attracted great attention due to their exceptional optical properties and potential applications in spintronics and valleytronics. Recently, the stacking configuration of layered materials has been proved to offer an additional degree of freedom to control their physical properties. Unique physical properties, such as interlayer excitons and superconductivity, have been observed in homobilayers of TMDs by controlling their stacking orientation. Here, we use artificial stackings of chemical vapor deposition (CVD)-grown tungsten disulfide (WS 2 ) to fabricate homobilayers with various stacking angles. The artificial stacks showed a 60°periodic change of their photoluminescence (PL) spectra with the stacking angle. An additional low-energy PL peak was observed for the low-angle stacked bilayers, which was revealed by electro-optical measurements to originate in the indirect intralayer exciton relaxation. In addition, we found the high optical quality of our CVD-grown WS 2 to be a key factor in observing the intrinsic exciton dynamics free from defect-induced localized excitons. Our work sheds light on the controlled modulation of the optical properties of TMD homobilayers by tuning the stacking of high-quality monolayers.

Defects in solids are unavoidable and can create complex electronic states that can significantly influence the electrical and optical properties of semiconductors. With the rapid progress in the integration of 2D semiconductors in practical devices, it is imperative to understand and characterize the influence of defects in this class of materials. Here, we examine the electrical response of defect filling and emission using deep level transient spectroscopy (DLTS) and reveal defect states and their hybridization in a monolayer MOCVD-grown material deposited on CMOS-compatible substrates. Supported by aberration-corrected STEM imaging and theoretical calculations, we find that neighboring sulfur vacancy pairs introduce additional shallow trap states via hybridization of individual vacancy levels. Even though such vacancy pairs only represent ~10% of the total defect concentration, they can have a substantial influence on the off currents and switching slopes of field-effect transistors based on 2D semiconductors. Our technique, which can quantify the energy states of different defects and their interactions, allows rapid and nondestructive electrical characterization of defect states important for the defect engineering of 2D semiconductors.