WS2 Domains Research Articles

Epitaxial Growth of Monolayer WS2 Single Crystals on Au(111) Toward Direct Surface-Enhanced Raman Spectroscopy Detection.

The epitaxial growth of wafer-scale two-dimensional (2D) semiconducting transition metal dichalcogenides (STMDCs) single crystals is the key premise for their applications in next-generation electronics. Despite significant advancements, some fundamental factors affecting the epitaxy growth have not been fully uncovered, e.g., interface coupling strength, adlayer-substrate lattice matching, substrate step-edge-guiding effects, etc. Herein, we develop a model system to tackle these issues concurrently, and realize the epitaxial growth of wafer-scale monolayer tungsten disulfide (WS2) single crystals on the Au(111) substrate. This epitaxial system is featured with good adlayer-substrate lattice matching, obvious step-edge-guiding effect for the unidirectionally aligned nucleation/growth, and relatively weaker interfacial interaction than that of monolayer MoS2/Au(111), as evidenced by the evolution of a uniform Moiré pattern and an intrinsic band gap, according to on-site scanning tunneling microscopy/spectroscopy (STM/STS) characterizations and density functional theory calculations. Intriguingly, the unidirectionally aligned monolayer WS2 domains along the Au(111) steps can behave as ultrasensitive templates for surface-enhanced Raman scattering detection of organic molecules, due to the obvious charge transfer occurred at substrate step edges. This work should hereby deepen our understanding of the epitaxy mechanism of 2D STMDCs on single-crystal substrates, and propel their wafer-scale production and applications in various cutting-edge fields.

Few layer chemical vapor deposited two dimensional WS2 for low voltage resistive switching application

Optimizing the fabrication method gives rise to control over the deposition of WS2 sheets in a CVD chamber. The deposition was executed on Pt-coated silicon substrate so that electrical behavior of the WS2 domains could be observed. Utilizing two types of electrode material helped in understanding the role of metal diffusion mediated switching failure. The Au/WS2/Pt/Ti/SiO2/Si device exhibited resistive switching characteristics at very low operating voltages. A high on/off ratio (>104), high retention time (104 s), and a long endurance (1500 cycles) were respectively recorded to establish the non-volatile memory characteristics in a device with thin layer of WS2. The switching mechanism was established by performing conducting atomic force microscopy. This work also explained the mechanism of resistance switching in CVD grown tungsten disulfide in two steps and pave the way for the development of large-scale devices.

Nucleation and coalescence of tungsten disulfide layers grown by metalorganic chemical vapor deposition

The nucleation of tungsten disulfide WS2 crystallites and coalescence behaviour of monolayer WS2 films grown on C-plane sapphire have been studied in a commercial AIXTRON close coupled showerhead metalorganic chemical vapor deposition (MOCVD) reactor using tungsten hexacarbonyl and di-tert-butyl sulfide as precursors. The influence of sapphire substrates offcut (both orientation and angle), as well as the substrate annealing conditions and growth temperature, on the nucleation and size of the WS2 domains has been investigated. A higher nucleation temperature combined with increased substrate annealing temperatures was found to promote an increase in the WS2 domain size. The dependence of surface coverage and coalescence behaviour of the WS2 domains on the growth duration has also been determined, allowing monolayer WS2 films with < 10 % bilayer to be obtained. Crystalline quality of the coalesced WS2 films has been assessed by grazing incidence XRD and misorientation mapping using 4D-scanning transmission electron microscopy, showing a slight in-plane misorientation of the crystallites in the WS2 monolayer. The electrical performance of the coalesced WS2 films was assessed using backgated TLM structures demonstrating mobility values of up to 5 cm2/Vs and a high Ion/Ioff ratio greater than 108.

Vapor-Liquid-Solid Growth of Morphology-Tailorable WS2 toward P-Type Monolayer Field-Effect Transistors.

Although substantial efforts have been made, controllable synthesis of p-type WS2 remains a challenge. In this work, we employ NaCl as a seeding promoter to realize vapor-liquid-solid (VLS) growth of p-type WS2. Morphological evolution, including a one-dimensional (1D) nanowire to two-dimensional (2D) planar domain and 2D shape transition of WS2 domains, can be well-controlled by the growth temperature and sulfur introduction time. A high growth temperature is required to enable planar growth of 2D WS2, and a sulfur-rich environment is found to facilitate the growth of high-quality WS2. Raman and photoluminescence (PL) mappings demonstrate uniform crystallinity and high quantum efficiency of VLS-grown WS2. Moreover, monolayer WS2-based field-effect transistors (FETs) are fabricated, showing p-type conducting behavior, which is different from previous reported n-type FETs from WS2 grown by other methods. First-principles calculations show that the p-type behavior originates from the substitution of Na at the W site, which will form an additional acceptor level above the valence band maximum (VBM). This facile VLS growth method opens the avenue to realize the p-n WS2 homojunctions and p/n-WS2-based heterojunctions for monolayer wearable electronic, photonic, optoelectronic, and biosensing devices and should also be a great benefit to the development of 2D complementary metal-oxide-semiconductor (CMOS) circuit applications.

Atomically sharp jagged edges of chemical vapor deposition-grown WS2 for electrocatalysis

Chemical vapor deposition (CVD)-grown 2D transition metal dichalcogenides can adopt faceted edges. To investigate how sharp these sites can be, we utilize aberration-corrected annular dark-field scanning transmission electron microscopy (ADF-STEM) to resolve the atomic structure of two-dimensional (2D) WS2 domains that show jagged edges. Nanoscale triangular edge structures with S zigzag terminations are observed. Both the peak and valley regions exhibit near-atomic sharpness. The peaks are as sharp as two atoms in width. Highly ordered valley sites display a minimum width of three atoms, and prospective single-atom valleys appear as two-atom-wide sites in the ADF-STEM contrast. Regarding the kinetics, density-functional theory (DFT) calculations indicate that the 2-W-atom peak would not evolve into a single-W-atom peak even though the latter configuration is also stable with a high enough binding energy under the growth conditions. These results help deepen our understanding of the possible structuring at the nanoscale and the atomic-scale limits of peaks and valleys formed via intersection of two zigzag edges. The enriched edge sites lead to higher catalytic activities for the hydrogen evolution reaction (HER).

Characterization of high quality, monolayer WS2 domains via chemical vapor deposition technique

Characterization of high quality, monolayer WS2 domains via chemical vapor deposition technique

Understanding the Role of 2D Nature on the Junction Properties in WS2 Layers; Effect of AFM Tip Induced Loading Force on Spatially Varying Contact

In the present study, AFM metal tip induced loading force dependent electrical transport behaviour of WS2 domains synthesized using chemical vapour deposition technique has been investigated. Raman spectroscopy, optical microscopy and atomic force microscopy (AFM) techniques have been employed to characterize the synthesized two-dimensional (2D) domains. The local nanoscale I-V measurements have been carried out as a function of loading force (53 – 300 nN) using conductive atomic force microscopy to probe the interface formed between AFM tip (Pt coated) and 2D domains of WS2 layers. An interesting phenomena of decreasing junction current with increasing loading force obtained at the centre of single layer WS2 sample is opposed by an increasing junction current at the edges of domain. Loading force induced spatially varying, compressive and tensile strain resulting in charge polarization causing an opposite flow of majority electron carriers away and towards the tip-WS2 domain has been explained as the possible mechanism for the applied opposing uniaxial strains. This study reports the influence of single layer nature of WS2 constituting polar edges and wrinkle terminations on electronic properties of metal-semiconductor contacts. These results are crucial for understanding the junction properties and application of 2D material based devices in next-generation electronics.

Controllable Epitaxial Growth of Large-Area MoS2 /WS2 Vertical Heterostructures by Confined-Space Chemical Vapor Deposition.

The controllable large-area growth of single-crystal vertical heterostructures based on 2D transition metal dichalcogenides (TMDs) remains a challenge. Here, large-area vertical MoS2 /WS2 heterostructures are synthesized using single-step confined-space chemical vapor epitaxy. The heterostructures can evolve into two different kinds by switching the H2 flow on and off: MoS2 /WS2 heterostructures with multiple WS2 domains can be achieved without introducing the H2 flow due to the numerous nucleation centers on the bottom MoS2 monolayer during the transition stage between the MoS2 and WS2 monolayer growth. In contrast, isolated MoS2 /WS2 heterostructures with single WS2 domain can be obtained with introducing the H2 flow due to the reduced nucleation centers on the bottom MoS2 monolayer arising from the hydrogen etching effect. Both the two kinds of the vertical MoS2 /WS2 heterostructures feature high quality. The photodetectors based on the isolated MoS2 /WS2 heterostructures exhibit a high responsivity of 68 mA W-1 and a short response time of 35ms. This single-step chemical vapor epitaxy can be used to synthesize vertical MoS2 /WS2 heterostructures with high production efficiency. The new epitaxial growth approach may open new pathways to fabricate large-area heterostructures made of different 2D TMDs monolayers of interest to electronics, optoelectronics, and other applications.

Wafer-Scale Epitaxial Growth of Unidirectional WS2 Monolayers on Sapphire.

Realization of wafer-scale single-crystal films of transition metal dichalcogenides (TMDs) such as WS2 requires epitaxial growth and coalescence of oriented domains to form a continuous monolayer. The domains must be oriented in the same crystallographic direction on the substrate to inhibit the formation of inversion domain boundaries (IDBs), which are a common feature of layered chalcogenides. Here we demonstrate fully coalesced unidirectional WS2 monolayers on 2 in. diameter c-plane sapphire by metalorganic chemical vapor deposition using a multistep growth process to achieve epitaxial WS2 monolayers with low in-plane rotational twist (0.09°). Transmission electron microscopy analysis reveals that the WS2 monolayers are largely free of IDBs but instead have translational boundaries that arise when WS2 domains with slightly offset lattices merge together. By regulating the monolayer growth rate, the density of translational boundaries and bilayer coverage were significantly reduced. The unidirectional orientation of domains is attributed to the presence of steps on the sapphire surface coupled with growth conditions that promote surface diffusion, lateral domain growth, and coalescence while preserving the aligned domain structure. The transferred WS2 monolayers show neutral and charged exciton emission at 80 K with negligible defect-related luminescence. Back-gated WS2 field effect transistors exhibited an ION/OFF of ∼107 and mobility of 16 cm2/(V s). The results demonstrate the potential of achieving wafer-scale TMD monolayers free of inversion domains with properties approaching those of exfoliated flakes.

Growth-Etch Metal-Organic Chemical Vapor Deposition Approach of WS2 Atomic Layers.

Metal-organic chemical vapor deposition (MOCVD) is one of the main methodologies used for thin-film fabrication in the semiconductor industry today and is considered one of the most promising routes to achieve large-scale and high-quality 2D transition metal dichalcogenides (TMDCs). However, if special measures are not taken, MOCVD suffers from some serious drawbacks, such as small domain size and carbon contamination, resulting in poor optical and crystal quality, which may inhibit its implementation for the large-scale fabrication of atomic-thin semiconductors. Here we present a growth-etch MOCVD (GE-MOCVD) methodology, in which a small amount of water vapor is introduced during the growth, while the precursors are delivered in pulses. The evolution of the growth as a function of the amount of water vapor, the number and type of cycles, and the gas composition is described. We show a significant domain size increase is achieved relative to our conventional process. The improved crystal quality of WS2 (and WSe2) domains wasis demonstrated by means of Raman spectroscopy, photoluminescence (PL) spectroscopy, and HRTEM studies. Moreover, time-resolved PL studies show very long exciton lifetimes, comparable to those observed in mechanically exfoliated flakes. Thus, the GE-MOCVD approach presented here may facilitate their integration into a wide range of applications.

(Invited) Wafer-Scale Epitaxy of Transition Metal Dichalcogenides By MOCVD

Monolayer semiconducting transition metal dichalcogenides (MX2, M=W, Mo and X=S, Se) exhibit novel electronic and optical properties. Availability of epitaxial single-crystal monolayers over a wafer scale is a major hurdle in integrating these materials with existing device fabrication processes. Our approach focusses on synthesis of these monolayers using gas source precursors, like those used in traditional III-V semiconductor industry. The precursors are kept in temperature and pressure-controlled bubblers outside the growth chamber and can be metered independently into the growth zone with precision. Uniform epitaxial binary TMD monolayers including MoS2, WS2, WSe2 and MoSe2 have been deposited on 2” sapphire wafers in a cold-wall CVD reactor using metal hexacarbonyl and hydride chalcogen precursors. A multi-step growth process comprising of precursor modulation was developed for WSe2 to independently control nucleation density and the lateral growth rate of monolayer domains on the sapphire substrate [1]. For the sulfides, additional temperature modulation was introduced to increase crystalline quality. Using this approach, uniform, coalesced monolayer and few-layer TMD films were obtained on 2” sapphire substrates. In-plane X-ray diffraction demonstrates that the films are epitaxially oriented with respect to sapphire with narrow X-ray full-width-at-half-maximum indicating minimal rotational misorientation of domains within the basal plane [2]. Deposition of these materials in the same reactor provides insight into the factors controlling their growth, which in essential for the growth of heterostructures and/or alloys. Transmission electron microscopy analysis of these materials revealed additional information about the coalesced film structure. For instance, ~95 % single oriented WS2 domains were observed when the film growth rate was modulated. For WS2, it was also confirmed that despite translational line defects present in the film, mirror boundaries were predominantly absent. WS2 also showed no defect related photoluminescence peak at 80 K, indicating its high quality.In this talk the factors affecting the growth of TMDs (MX2, M=W, Mo and X=S, Se) will be presented. The role of the substrate surface in the nucleation and growth of these TMDs will be discussed. In addition, the structural and optoelectronic characteristics for these films will also be presented.The authors acknowledge financial support of the U.S. National Science Foundation through the Penn State 2D Crystal Consortium – Materials Innovation Platform (2DCC-MIP) under NSF cooperative agreement DMR-1539916 and EFRI 2-DARE Grant EFRI-1433378.[1] Zhang X, Choudhury TH, Chubarov M et al., 2018. Nano Lett. 18(2):1049–56[2] Chubarov M, Choudhury TH, Zhang X et al., 2018. Nanotechnology. 29(5):55706

Study of the Properties of Two-Dimensional MoS2 and WS2 Films Synthesized by Chemical-Vapor Deposition

Molybdenum- and tungsten-disulfide films are synthesized by chemical-vapor deposition. The set of optimal synthesis parameters (temperature, time, and amount and ratio of precursors) is established at which MoS2 domains with maximum lateral sizes of up to 250 μm on sapphire and MoS2 and WS2 domains up to 80 μm in size on SiO2 can be grown. Domain intergrowth leads to the formation of homogeneous single-layer MoS2 films. The Raman spectra of the synthesized films contain two characteristic peaks corresponding to the atomic vibrations in MoS2 and WS2. Photoluminescence of the single-layer and bilayer MoS2 films with a maximum intensity of 670 ± 2 nm and of the single-layer WS2 films with a maximum intensity of 630 ± 2 nm is detected. The photoluminescence spectral maps (the dependences of the photoluminescence intensity on the luminescence and excitation-light wavelengths) are measured. According to the measured data, the photoluminescence excitation spectrum of MoS2 has a maximum at 350 ± 5 nm and the photoluminescence excitation spectrum of WS2 has a maximum at 330 ± 5 nm. The I–V characteristics of the synthesized films are photosensitive in the visible spectral range.

A Bilayer 2D-WS2/Organic-Based Heterojunction for High-Performance Photodetectors.

Two-dimensional (2D) tungsten disulfide (WS2) has inspired great efforts in optoelectronics, such as in solar cells, light-emitting diodes, and photodetectors. However, chemical vapor deposition (CVD) grown 2D WS2 domains with the coexistence of a discontinuous single layer and multilayers are still not suitable for the fabrication of photodetectors on a large scale. An emerging field in the integration of organic materials with 2D materials offers the advantages of molecular diversity and flexibility to provide an exciting aspect on high-performance device applications. Herein, we fabricated a photodetector based on a 2D-WS2/organic semiconductor materials (mixture of the (Poly-(N,N′-bis-4-butylphenyl-N,N′-bisphenyl) benzidine and Phenyl-C61-butyric acid methyl ester (Poly-TPD/PCBM)) heterojunction. The application of Poly-TPD/PCBM organic blend film enhanced light absorption, electrically connected the isolated WS2 domains, and promoted the separation of electron-hole pairs. The generated exciton could sufficiently diffuse to the interface of the WS2 and the organic blend layers for efficient charge separation, where Poly-TPD was favorable for hole carrier transport and PCBM for electron transport to their respective electrodes. We show that the photodetector exhibited high responsivity, detectivity, and an on/off ratio of 0.1 A/W, 1.1 × 1011 Jones, and 100, respectively. In addition, the photodetector showed a broad spectral response from 500 nm to 750 nm, with a peak external quantum efficiency (EQE) of 8%. Our work offers a facile solution-coating process combined with a CVD technique to prepare an inorganic/organic heterojunction photodetector with high performance on silicon substrate.

Lateral Bilayer MoS2–WS2 Heterostructure Photodetectors with High Responsivity and Detectivity

Abstract2D heterostructures combining different layered semiconductors have received great interest due to their intriguing electrical and optical properties. However, the arbitrary growth of layers in a lateral heterostructure remains a challenge. Here, the synthesis of large‐scale lateral bilayer (LBL) WS2–MoS2 heterostructures is reported by a two‐step chemical vapor deposition route. Raman, photoluminescence, and second‐harmonic generation images show the sharp boundaries between WS2 and MoS2 domains in the heterostructure. Atomically resolved scanning transmission electron microscopy further reveals that sharp boundaries are formed by seamless connections via a lateral zigzag epitaxy between WS2 and MoS2. Notably, the photodetector device based on the LBL WS2–MoS2 heterostructure exhibits ultrahigh photoresponsivity and detectivity (6.72 × 103 A W−1 and 3.09 × 1013 Jones for 457 nm laser light, respectively), orders of magnitude higher than those of MoS2 and WS2 monocrystals. These excellent performances render LBL WS2–MoS2 heterostructures as promising candidates for next‐generation optoelectronics.

WS2 Nanotubes, 2D Nanomeshes, and 2D In-Plane Films through One Single Chemical Vapor Deposition Route.

We demonstrate a versatile, catalyst free chemical vapor deposition process on insulating substrates capable of producing in one single stream one-dimensional (1D) WO3–x suboxides leading to a wide range of substrate-supported 2H-WS2 polymorphs: a tunable class of out-of-plane (of the substrate) nanophases, with 1D nanotubes and a pure WS2, two-dimensional (2D) nanomesh (defined as a network of webbed, micron-size, few-layer 2D sheets) at its extremes; and in-plane (parallel to the substrate) mono- and few-layer 2D domains. This entails a two-stage approach in which the 2WO3 + 7S → 2WS2 + 3SO2 reaction is intentionally decoupled. First, various morphologies of nanowires or nanorods of high stoichiometry, WO2.92/WO2.9 suboxides (belonging to the class of Magnéli phases) were formed, followed by their sulfurization to undergo reduction to the aforementioned WS2 polymorphs. The continuous transition of WS2 from nanotubes to the out-of-plane 2D nanomesh, via intermediary, mixed 1D-2D phases, delivers tunable functional properties, for example, linear and nonlinear optical properties, such as reflectivity (linked to optical excitations in the material), and second harmonic generation (SHG) and onset of saturable absorption. The SHG effect is very strong across the entire tunable class of WS2 nanomaterials, weakest in nanotubes, and strongest in the 2D nanomesh. Furthermore, a mechanism via suboxide (WO3–x) intermediate as a possible path to 2D domain growth is demonstrated. 2D, in-plane WS2 domains grow via “self-seeding and feeding” where short WO2.92/WO2.9 nanorods provide both the nucleation sites and the precursor feedstock. Understanding the reaction path (here, in the W–O–S space) is an emerging approach toward controlling the nucleation, growth, and morphology of 2D domains and films of transition-metal dichalcogenides.

2D-Layer-Dependent Behavior in Lateral Au/WS2/Graphene Photodiode Devices with Optical Modulation of Schottky Barriers

We investigate the 2D-layer-dependent electronic and optoelectronic properties of lateral Au/graphene/WS2 photodetecting diodes. All 2D materials used [graphene and WS2 domains [monolayer (1L) and ...

Facile access to shape-controlled growth of WS2 monolayer via environment-friendly method

2D materials with tailored morphologies exhibit distinctive shape-dependent properties especially in electrocatalysis and gas sensing, extending their applications in nanoelectronics. Atomic-layer tungsten disulfide (WS2) stands out as a promising candidate, but the controllable synthesis of WS2 still faces unavoidable obstacles, such as strict parameter requirements, low efficiency and serious pollution during the preparation. Here we report an elegant technique for growing homogeneous-luminescence WS2 monolayer with desired shapes and developed electrochemical properties, through the improved chemical vapor deposition method with a semi-closed airflow environment. It is an effective, economic and especially environment-friendly approach with atmospheric pressure and hydrogen-free condition, aiming at synthesizing high-quality and large-scale monolayer WS2 crystals with only one heating zone. Compared with the existing methods, this one significantly minimizes the sulphur contamination to the environment and equipment, enabling the reuse of quartz tube and reducing the cost. More importantly, different shapes of WS2 domains with high crystalline and excellent homogeneity of luminescent intensity are controllable by adjusting the growth temperatures. In addition to the good electrical transport performance of WS2 monolayer, the WS2 domains with different morphologies exhibit modulated properties for catalyzing the electrochemical hydrogen evolution. This systematic study on shape-controlled growth and shape-dependent electrochemical performances of WS2 will further push forward their practical applications.

Morphology Engineering in Monolayer MoS2‐WS2 Lateral Heterostructures

AbstractIn 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.

Controllable syntheses and energy related applications of MX&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; heterostructures

Two-dimensional (2D) layered materials have been a central focus of materials research since the discovery of graphene. Each layer in 2D layered materials consists of a covalently bonded and is weakly bound to neighboring layers by van der Waals interactions. This makes it feasible to mix and match highly disparate atomic layers to create a wide range of vertical heterostructures without the constraints of lattice matching and processing compatibility. In these heteros- tructures, different physical and chemical properties accompany with the stacked materials are combined, furthermore, many applications in electronic/optoelectronic devices and renewable energy storage/conversions are demonstrated. Notably, the controllable syntheses of large area, large domain size and intrinsic vertical heterostructures are the first step. In this review, recent research achievements towards the controllable syntheses and novel physical properties of MX2/ graphene and MX2/MX2 vertical heterostructures, as well as their applications in electrocatalytic/photocatalytic hydrogen evolution reaction are highlighted. Firstly, by using a facile chemical vapor deposition (CVD) approach, high quality and coverage-tunable monolayer MoS2/graphene vertical heterostructures on conventional Au foil substrates are demonstrated. The unique system is selected in view of the tunability of the coverage of MoS2, or the densities of active sites in hydrogen evolution reaction (HER), as well as its compatibility with scanning tunneling microscopy/spectroscopy (STM/STS) observations. Spectroscopic characterizations reveal a quasi-freestanding monolayer MoS2 feature, which is evidenced by a very weak n-doping effect and an intrinsic bandgap of MoS2 in the MoS2/graphene/Au sandwich, as obtained from STM/STS characterizations. Moreover, the exciton binding energy is also deduced by combining photoluminescence measurements. For monolayer MoS2 synthesized on graphene/Au foils, a dramatic decrease of the bandgap from 2.20 to 0.30 eV occurs at the domain edge within a lateral distance of 6 nm, as evidenced by STM/STS observations. The edges of monolayer MoS2 triangles on graphene/Au foils can thus be regarded as narrow-gap quantum wires considering of their reduced bandgaps. More intriguingly, the bandgap decrease at the domain edge is closely related to the rather high HER performance for MoS2/graphene/Au foils comparing with that of MoS2/Au foils. Secondly, a growth-temperature-mediated two-step CVD strategy is designed to synthesize either MoS2/WS2 or WS2/MoS2 vertical heterostructures on Au foils in a controllable manner. The Au foil substrate is selected in view of its concurrently compatible with the growth of MoS2 and WS2 domains and even complete layers at considerably different temperatures. This unique growth system is also suitable for on-site STM/STS observations and direct photocatalysis applications. A dominant A-A stacked MoS2/WS2 and A-B stacked WS2/MoS2 heterostructures are selectively achieved and convinced by spherical-aberration corrected scanning transmission electron microscope (STEM) characterizations. The quasiparticle bandgap of MoS2/WS2/Au foils is unambiguously determined as 1.60±0.08 eV by STM/STS. More intriguingly, the as-grown MoS2/WS2 stacks is found to possess higher photocatalytic activity in HER than that of MoS2, WS2, and WS2/MoS2, due to the effective electron-hole separation and the fast electron transfer kinetics. Motivated by those observations, we believe that significant research interests would be sparked about the oriented syntheses and the versatile applications of MX2-based heterostructures. Finally, remaining challenges for the controllable syntheses and the large-scale applications of MX2/graphene and MX2/MX2 vertical heterostructures are discussed, and the future research directions in the related fields are also proposed.

Photoluminescence Segmentation within Individual Hexagonal Monolayer Tungsten Disulfide Domains Grown by Chemical Vapor Deposition.

We show that hexagonal domains of monolayer tungsten disulfide (WS2) grown by chemical vapor deposition (CVD) with powder precursors can have discrete segmentation in their photoluminescence (PL) emission intensity, forming symmetric patterns with alternating bright and dark regions. Two-dimensional maps of the PL reveal significant reduction within the segments associated with the longest sides of the hexagonal domains. Analysis of the PL spectra shows differences in the exciton to trion ratio, indicating variations in the exciton recombination dynamics. Monolayers of WS2 hexagonal islands transferred to new substrates still exhibit this PL segmentation, ruling out local strain in the regions as the dominant cause. High-power laser irradiation causes preferential degradation of the bright segments by sulfur removal, indicating the presence of a more defective region that is higher in oxidative reactivity. Atomic force microscopy (AFM) images of topography and amplitude modes show uniform thickness of the WS2 domains and no signs of segmentation. However, AFM phase maps do show the same segmentation of the domain as the PL maps and indicate that it is caused by some kind of structural difference that we could not clearly identify. These results provide important insights into the spatially varying properties of these CVD-grown transition metal dichalcogenide materials, which may be important for their effective implementation in fast photo sensors and optical switches.