Large‐Area Synthesis of Continuous and Uniform MoS2 Monolayer Films on Graphene

Heterostructures composed of multiple layers of different atomically thin materials are of interest due to their unique properties and potential for new device functionality. MoS₂‐graphene heterostructures have shown promise as photodetectors and vertical tunnel transistors. However, progress is limited by the typically micrometer‐scale devices and by the multiple alignments required for fabrication when utilizing mechanically exfoliated material. Here, the synthesis of large‐area, continuous, and uniform MoS₂ monolayers directly on graphene by chemical vapor deposition is reported, resulting in heterostructure samples on the centimeter scale with the possibility for even larger lateral dimensions. Atomic force microscopy, photoluminescence, X‐ray photoelectron, and Raman spectroscopies demonstrate uniform single‐layer growth of stoichiometric MoS₂. The ability to reproducibly generate large‐area heterostructures is highly advantageous for both fundamental investigations and technological applications.     

Atomically Thin MoS2: A Versatile Nongraphene 2D Material

Two‐dimensional inorganic materials are emerging as a premiere class of materials for fabricating modern electronic devices. The interest in 2D layered transition metal dichalcogenides is especially high. Particularly, 2D MoS₂ is being heavily researched due to its novel functionalities and its suitability for a wide range of electronic and optoelectronic applications. In this article, the progress in mono/few layer(s) MoS₂ research is reviewed by focusing primarily on the layer dependent evolution of crystal, phonon, and electronic structure. The review includes extensive detail into the methodologies adapted for single or few layer(s) MoS₂ growth. Further, the review covers the versatility of 2D MoS₂ for a broad range of device applications. Recent advancements in the field of van der Waals heterostructures are also highlighted at the end of the review.     

0D/2D Heterostructures Vertical Single Electron Transistor

Mixed-dimensional heterostructures formed by the stacking of 2D materials with nanostructures of distinct dimensionality constitute a new class of nanomaterials that offers multifunctionality that goes beyond those of single dimensional systems. An unexplored architecture of single electron transistor (SET) is developed that employs heterostructures made of nanoclusters (0D) grown on a 2D molybdenum disulfide (MoS₂) channel. Combining the large Coulomb energy of the nanoclusters with the electronic capabilities of the 2D layer, the concept of 0D–2D vertical SET is unveiled. The MoS₂ underneath serves both as a charge tunable channel interconnecting the electrode, and as bottom electrode for each v-SET cell. In addition, its atomic thickness makes it thinner than the Debye screening length, providing electric field transparency functionality that allows for an efficient electric back gate control of the nanoclusters charge state. The Coulomb diamond pattern characteristics of SET are reported, with specific doping dependent nonlinear features arising from the 0D/2D geometry that are elucidated by theoretical modeling. These results hold promise for multifunctional single electron device taking advantage of the versatility of the 2D materials library, with as example envisioned spintronics applications while coupling quantum dots to magnetic 2D material, or to ferroelectric layers for neuromorphic devices.     

Improved Electrical Contact Properties of MoS2‐Graphene Lateral Heterostructure

2D materials have been extensively investigated in view of their excellent electrical/optical properties, with particular attention directed at the fabrication of vertical or lateral heterostructures. Although such heterostructures exhibit unexpected or enhanced properties compared to those of singly used 2D materials, their fabrication is challenged by the difficulty of realizing spatial control and large area integration. Herein, MoS₂ is grown on patterned graphene at variable temperatures, combining the concept of lateral heterostructure with chemical vapor deposition to realize large area growth with precise spatial control, and probe the spatial distribution of graphene and MoS₂ by a number of instrumental techniques. The prepared MoS₂‐graphene lateral heterostructure is employed to construct field effect transistors with graphene as the source/drain and MoS₂ as the channel, and the performance of these transistors (on/off ratio ≈10⁹, maximum field effect mobility = 8.5 cm² V^?1 s^?1) is shown to exceed that of their MoS₂‐only counterparts.     

Atomic Insight into Thermolysis‐Driven Growth of 2D MoS2

Understanding and controlling the transformations of transition metal dichalcogenides (TMDs) from amorphous precursors into two‐dimensional (2D) materials is important for guiding synthesis, directing fabrication, and tailoring functional properties. Here, the combined effects of thermal energy and electron beam irradiation are explored on the structural evolution of 2D MoS₂ flakes through the thermal decomposition of a (NH₄)₂MoS₄ precursor inside an ultrahigh vacuum (10^?9 Torr) scanning transmission electron microscope (STEM). The influence of reaction temperature, growth substrate, and the initial precursor morphology on the resulting 2D MoS₂ flake morphology, edge structures, and point defects are explored. Although thermal decomposition occurs extremely fast at elevated temperatures and is difficult to capture using current STEM techniques, electron beam irradiation can induce local transformations at lower temperatures, enabling direct observation and interpretation of critical growth steps including oriented attachment and transition from single‐ to multilayer structures at atomic resolution. An increase in the number of layers of the MoS₂ flakes from island growth is investigated using electron beam irradiation. These findings provide insight into the growth mechanisms and factors that control the synthesis of few‐layer MoS₂ flakes through thermolysis and toward the prospect of atomically precise control and growth of 2D TMDs.     

Electric Field Tunable Interlayer Relaxation Process and Interlayer Coupling in WSe2/Graphene Heterostructures

Transition metal dichalcogenides van der Waals (vdWs) heterostructures present fascinating optical and electronic phenomena, and bear tremendous significance for electronic and optoelectronic applications. As the significant merits in vdWs heterostructures, the interlayer relaxation of excitons and interlayer coupling at the heterointerface reflect the dynamic behavior of charge transfer and the coupled electronic/structural characteristics, respectively, which may give rise to new physics induced by quantum coupling. In this work, upon tuning the photoluminescence (PL) properties of WSe₂/graphene and WSe₂/MoS₂/graphene heterostructures by virtue of electric field, it is demonstrated that the interlayer relaxation of excitons at the heterointerface in WSe₂/graphene, which is even stronger than that in MoS₂/graphene and WSe₂/MoS₂ , plays a dominant role in PL tuning in WSe₂/graphene, while the carrier population in WSe₂ induced by electric field has a minor contribution. In addition, it is discovered that the interlayer coupling between monolayer WSe₂ and graphene is enhanced under high electric field, which breaks the momentum conservation of first order Raman‐allowed phonons in graphene, yielding the enhanced Raman scattering of defects in graphene. The interplay between electric field and vdWs heterostructures may provide versatile approaches to tune the intrinsic electronic and optical properties of the heterostructures.     

Three‐Dimensional Structures of MoS2 Nanosheets with Ultrahigh Hydrogen Evolution Reaction in Water Reduction

The hydrogen evolution reaction in an alkaline environment using a non‐precious catalyst with much greater efficiency represents a critical challenge in research. Here, a robust and highly active system for hydrogen evolution reaction in alkaline solution is reported by developing MoS₂ nanosheet arrays vertically aligned on graphene‐mediated 3D Ni networks. The catalytic activity of the 3D MoS₂ nanostructures is found to increase by 2 orders of magnitude as compared to the Ni networks without MoS₂. The MoS₂ nanosheets vertically grow on the surface of graphene by employing tetrakis(diethylaminodithiocarbomato)molybdate(IV) as the molybdenum and sulfur source in a chemical vapor deposition process. The few‐layer MoS₂ nanosheets on 3D graphene/nickel structure can maximize the exposure of their edge sites at the atomic scale and present a superior catalysis activity for hydrogen production. In addition, the backbone structure facilitates as an excellent electrode for charge transport. This precious‐metal‐free and highly efficient active system enables prospective opportunities for using alkaline solution in industrial applications.     

Patterned Growth of P‐Type MoS2 Atomic Layers Using Sol–Gel as Precursor

2D layered MoS₂ has drawn intense attention for its applications in flexible electronic, optoelectronic, and spintronic devices. Most of the MoS₂ atomic layers grown by conventional chemical vapor deposition techniques are n‐type due to the abundant sulfur vacancies. Facile production of MoS₂ atomic layers with p‐type behavior, however, remains challenging. Here, a novel one‐step growth has been developed to attain p‐type MoS₂ layers in large scale by using Mo‐containing sol–gel, including 1% tungsten (W). Atomic‐resolution electron microscopy characterization reveals that small tungsten oxide clusters are commonly present on the as‐grown MoS₂ film due to the incomplete reduction of W precursor at the reaction temperature. These omnipresent small tungsten oxide clusters contribute to the p‐type behavior, as verified by density functional theory calculations, while preserving the crystallinity of the MoS₂ atomic layers. The Mo containing sol–gel precursor is compatible with the soft‐lithography techniques, which enables patterned growth of p‐type MoS₂ atomic layers into regular arrays with different shapes, holding great promise for highly integrated device applications. Furthermore, an atomically thin p–n junction is fabricated by the as‐prepared MoS₂, which shows strong rectifying behavior.     

Wafer Scale Synthesis and High Resolution Structural Characterization of Atomically Thin MoS2 Layers

Synthesis of atomically thin MoS₂ layers and its derivatives with large‐area uniformity is an essential step to exploit the advanced properties of MoS₂ for their possible applications in electronic and optoelectronic devices. In this work, a facile method is reported for the continuous synthesis of atomically thin MoS₂ layers at wafer scale through thermolysis of a spin coated‐ammonium tetrathiomolybdate film. The thickness and surface morphology of the sheets are characterized by atomic force microscopy. The optical properties are studied by UV–Visible absorption, Raman and photoluminescence spectroscopies. The compositional analysis of the layers is done by X‐ray photo­emission spectroscopy. The atomic structure and morphology of the grains in the polycrystalline MoS₂ atomic layers are examined by high‐angle annular dark‐field scanning transmission electron microscopy. The electron mobilities of the sheets are evaluated using back‐gate field‐effect transistor configuration. The results indicate that this facile method is a promising approach to synthesize MoS₂ thin films at the wafer scale and can also be applied to synthesis of WS₂ and hybrid MoS₂‐WS₂ thin layers.     

Novel and Enhanced Optoelectronic Performances of Multilayer MoS2–WS2 Heterostructure Transistors

Van der Waals heterostructures designed by assembling isolated two‐dimensional (2D) crystals have emerged as a new class of artificial materials with interesting and unusual physical properties. Here, the multilayer MoS₂–WS₂ heterostructures with different configurations are reported and their optoelectronic properties are studied. It is shown that the new heterostructured material possesses new functionalities and superior electrical and optoelectronic properties that far exceed the one for their constituents, MoS₂ or WS₂. The vertical transistor exhibits a novel rectifying and bipolar behavior, and can also act as photovoltaic cell and self‐driven photodetector with photo‐switching ratio exceeding 10³. The planar device also exhibits high field‐effect ON/OFF ratio (>10⁵), high electron mobility of 65 cm²/Vs, and high photo­responsivity of 1.42 A/W compared to that in isolated multilayer MoS₂ or WS₂ nanoflake transistors. The results suggest that formation of MoS₂–WS₂ heterostructures could significantly enhance the performance of optoelectronic devices, thus open up possibilities for future nanoelectronic, photovoltaic, and optoelectronic applications.     

Tuning the Excitonic States in MoS2/Graphene van der Waals Heterostructures via Electrochemical Gating

The behavior of excitons in van der Waals (vdWs) heterostructures depends on electron–electron interactions and charge transfer at the hetero‐interface. However, what still remains to be unraveled is to which extent the carrier densities of both counterparts and the band alignment in the vdWs heterostructures determine the photoluminescence properties. Here, we systematically study the photoluminescence properties of monolayer MoS₂/graphene heterostructures by modulating the carrier densities and contact barrier at the interface via electrochemical gating. It is shown that the PL intensities of excitons can be tuned by more than two orders of magnitude, and a blue‐shift of the exciton peak of up to 40 meV is observed. By extracting the carrier density of MoS₂ using an electric potential distribution model, and the Schottky barrier using first‐principle calculations, we find that the controllable carrier density in MoS₂ plays a dominant role in the PL tuning at negative gate bias, whereas the interlayer relaxation of excitons induced by the Schottky barrier has a major contribution at positive gate bias. This is further verified by controlling the tunneling barrier and screening field across MoS₂ by inserting self‐assembled monolayers (SAMs) at the interface. These findings will benefit to better understand the effect of many‐body interactions and hetero‐interfaces on the optical and optoelectronic properties in vdWs heterostructures.     

Nonvolatile Floating‐Gate Memories Based on Stacked Black Phosphorus–Boron Nitride–MoS2 Heterostructures

Research on van der Waals heterostructures based on stacked 2D atomic crystals is intense due to their prominent properties and potential applications for flexible transparent electronics and optoelectronics. Here, nonvolatile memory devices based on floating‐gate field‐effect transistors that are stacked with 2D materials are reported, where few‐layer black phosphorus acts as channel layer, hexagonal boron nitride as tunnel barrier layer, and MoS₂ as charge trapping layer. Because of the ambipolar behavior of black phosphorus, electrons and holes can be stored in the MoS₂ charge trapping layer. The heterostructures exhibit remarkable erase/program ratio and endurance performance, and can be developed for high‐performance type‐switching memories and reconfigurable inverter logic circuits, indicating that it is promising for application in memory devices completely based on 2D atomic crystals.     

2D TMD Channel Transistors with ZnO Nanowire Gate for Extended Nonvolatile Memory Applications

2D transition metal dichalcogenides (TMDs) have been extensively studied due to their excellent physical properties. Mixed dimensional devices including 2D materials have also been studied, motivated by the possibility of any synergy effect from unique structures. However, only few such studies have been conducted. Here, semiconducting 1D ZnO nanowires are used as thin gate material to support 2D TMD field effect transistors (FETs) and 2D stack‐based interface trap nonvolatile memory. For the trap memory, deep level electron traps formed at the first MoS₂/second MoS₂ stack interface are exploited, since the first MoS₂ is treated in an atomic layer deposition chamber for a short while. On the one hand, a complementary inverter type memory device can also be achieved using a long single ZnO wire as a common gate to simultaneously support both n‐ and p‐channel TMD FETs. In addition, it is found that the semiconducting ZnO nanowire itself operates as an n‐type channel when the TMD materials can become a top‐gate to charge the ZnO channel. It means that 2D (bottom gated) and 1D channel (top gated) FETs are respectively operational in a single device structure. The 1D–2D mixed devices seem deserving broad attention in both aspects of novelty and functionality.     

A MoS2 and Graphene Alternately Stacking van der Waals Heterostructure for Li+/Mg2+ Co-Intercalation

Owing to the low-cost, dendrite-free formation, and high volumetric capacity, rechargeable Li⁺/Mg²⁺ hybrid-ion batteries (LMIBs) have attracted great attention and are regarded as promising energy storage devices. However, due to the strong Coulombic interaction of Mg²⁺ with host materials, the traditional “Daniell Type” LMIBs with only Li⁺ intercalation usually cannot ensure a satisfactory energy density. Herein, graphene monolayers are arranged intercalating into MoS₂ interlamination to construct van der Waals heterostructures (MoS₂/G VH). This operation transforms the construction of ion channels from pristine interlamination of two MoS₂ monolayers to the interlamination of MoS₂ monolayer with graphene monolayer, thereby greatly reducing ion diffusion energy barriers. Compared with pristine MoS₂, the MoS₂/G VH can obviously reduce the migration energy barriers of Li⁺ (from 0.67 to 0.09 eV) and Mg²⁺ (from 1.01 to 0.21 eV). Moreover, it is also demonstrated that MoS₂/G VHs realize Li⁺/Mg²⁺ co-intercalation even at a rate current of 1000 mA g⁻¹. As expected, the MoS₂/G VH exhibits superior electrochemical performance with a reversible capacity of 145.8 mAh g⁻¹ at 1000 mA g⁻¹ after 2200 cycles, suggesting the feasibility of potential applications for high-performance energy storage devices.     

2D/2D 1T‐MoS2/Ti3C2 MXene Heterostructure with Excellent Supercapacitor Performance

2D/2D heterostructures can combine the collective advantages of each 2D material and even show improved properties from synergistic effects. 2D Transition metal carbide Ti₃C₂ MXene and 2D 1T‐MoS₂ have emerged as attractive prototypes in electrochemistry due to their rich properties. Construction of these two 2D materials, as well as investigation about synergistic effects, is absent due to the instability of 1T‐MoS₂. Here, 3D interconnected networks of 1T‐MoS₂/Ti₃C₂ MXene heterostructure are constructed by magneto‐hydrothermal synthesis, and the electrochemical storage mechanisms are investigated. Improved extra capacitance is observed due to enlarged ion storage space from a synergistically interplayed effect in 3D interconnected networks. Outstanding rate performance is realized because of ultrafast electron transport originating from Ti₃C₂ MXene. This work provides an archetype to realize excellent electrochemical properties in 2D/2D heterostructures.     

Optoelectronics based on 2D TMDs and heterostructures

2D materials including graphene and TMDs have proven interesting physical properties and promising optoelectronic applications. We reviewed the growth, characterization and optoelectronics based on 2D TMDs and their heterostructures, and demonstrated their unique and high quality of performances. For example, we observed the large mobility, fast response and high photo-responsivity in MoS₂, WS₂ and WSe₂ phototransistors, as well as the novel performances in vdW heterostructures such as the strong interlayer coupling, am-bipolar and rectifying behaviour, and the obvious photovoltaic effect. It is being possible that 2D family materials could play an increasingly important role in the future nano- and opto-electronics, more even than traditional semiconductors such as silicon.     

Methane-Sensing Performance Enhancement in Graphene Oxide/Mg:ZnO Heterostructure Devices

Methane-sensing using reduced doped graphene oxide (rGO)/Mg:ZnO heterostructure devices is reported here. All samples are tested with CH₄ in dry air ambient by a gas-analyzing set-up. Crystallinity of the sensing film is improved through annealing treatment (at 800°C). The active device area (i.e., the rGO and rGO/Mg:ZnO heterostructures) are characterized using scanning electron microscope imaging, x-ray diffraction, and x-ray spectroscopy measurements. Electrical performance of the fabricated device is optimized. rGO/Mg:ZnO heterostructures are substantially more sensitive and have better transient response than bare rGO-based sensor devices. All fundamental parameters such as sensitivity and response–recovery time are examined and reported in detail.     

Phototransistors with Negative or Ambipolar Photoresponse Based on As‐Grown Heterostructures of Single‐Walled Carbon Nanotube and MoS2

A facile synthesis method for the heterostructures of single‐walled carbon nanotubes (SWCNTs) and few‐layer MoS₂ is reported. The heterostructures are realized by in situ chemical vapor deposition of MoS₂ on individual SWCNTs. Field effect transistors based on the heterostructures display different transfer characteristics depending on the formation of MoS₂ conduction channels along SWCNTs. Under light illumination, negative photoresponse originating from charge transfer from MoS₂ to SWCNT is observed while positive photoresponse is observed in MoS₂ conduction channels, leading to ambipolar photoresponse in devices with both SWCNT and MoS₂ channels. The heterostructure phototransistor, for negative photoresponse, exhibits high responsivity (100–1000 AW^?1) at low bias voltages (0.1 V) in the visible spectrum (500–700 nm) by combining high mobility conduction channel (SWCNT) with efficient light absorber (MoS₂).     

A Bottom‐Up Approach to Build 3D Architectures from Nanosheets for Superior Lithium Storage

Two‐dimensional (2D) atomic layers such as graphene, and metal chalcogenides have recently attracted tremendous attention due to their unique properties and potential applications. Unfortunately, in most cases, the free‐standing nanosheets easily re‐stack due to their van der Waals forces, and lose the advantages of their separated atomic layer state. Here, a bottom‐up approach is developed to build three‐dimensional (3D) architectures by 2D nanosheets such as MoS₂ and graphene oxide nanosheets as building blocks, the thin nature of which can be well retained. After simply chemical reduction, the resulting 3D MoS₂‐graphene architectures possess high surface area, porous structure, thin walls and high electrical conductivity. Such unique features are favorable for the rapid diffusions of both lithium ions and electrons during lithium storage. As a consequence, MoS₂‐graphene electrodes exhibit high reversible capacity of ≈1200 mAh g^?1, with very good cycling performance. Moreover, such a simple and low‐cost assembly protocol can provide a new pathway for the large‐scale production of various functional 3D architectures for energy storage and conversions.     

Morphology Engineering in Monolayer MoS2‐WS2 Lateral Heterostructures

In recent years, heterostructures formed in transition metal dichalcogenides (TMDs) have attracted significant attention due to their unique physical properties beyond the individual components. Atomically thin TMD heterostructures, such as MoS₂‐WS₂, MoS₂‐MoSe₂, MoS₂‐WSe₂, and WSe₂‐WS₂, are synthesized so far via chemical vapor deposition (CVD) method. Engineering the morphology of domains including size and shape, however, still remains challenging. Here, a one‐step CVD strategy on the morphology engineering of MoS₂ and WS₂ domains within the monolayer MoS₂‐WS₂ lateral heterostructures through controlling the weight ratio of precursors, MoO₃ and WO₃, as well as tuning the reaction temperature is reported. Not only can the size ratio in terms of area between WS₂ and MoS₂ domains be easily controlled from less than 1 to more than 20, but also the overall heterostructure size can be tuned from several to hundreds of micrometers. Intriguingly, the quantum well structure, a WS₂ stripe embedded in the MoS₂ matrix, is also observed in the as‐synthesized heterostructures, offering opportunities to study quantum confinement effects and quantum well applications. This approach paves the way for the large‐scale fabrication of MoS₂‐WS₂ lateral heterostructures with controllable domain morphology, and shall be readily extended to morphology engineering of other TMD heterostructures.