Two-dimensional (2D), layered transition metal dichalcogenides (TMDCs) can grow in two different growth directions, that is, horizontal and vertical. In the horizontal growth, 2D TMDC layers grow in planar direction with their basal planes parallel to growth substrates. In the vertical growth, 2D TMDC layers grow standing upright on growth substrates exposing their edge sites rather than their basal planes. The two distinct morphologies present unique materials properties suitable for specific applications, such as horizontal TMDCs for optoelectronics and vertical TMDCs for electrochemical reactions. Precise control of the growth orientation is essential for realizing the true potential of these 2D materials for large-scale, practical applications. In this Letter, we investigate the transition of vertical-to-horizontal growth directions in 2D molybdenum (or tungsten) disulfide and study the underlying growth mechanisms and parameters that dictate such transition. We reveal that the thickness of metal seed layers plays a critical role in determining their growth directions. With thick (>∼ 3 nm) seed layers, the vertical growth is dominant, while the horizontal growth occurs with thinner seed layers. This finding enables the synthesis of novel 2D TMDC heterostructures with anisotropic layer orientations and transport properties. The present study paves a way for developing a new class of 2D TMDCs with unconventional materials properties.

Transition metal dichalcogenides (TMDCs) are a promising class of two-dimensional (2D) materials for use in applications such as 2D electronics, optoelectronics, and catalysis. Due to the van der Waals (vdW) bonding between layers, vdW heterostructures can be constructed between two different species of TMDCs. Most studies employ exfoliation or co-vapor growth schemes, which are limited by the small size and uneven distribution of heterostructures on the growth substrate. In this work we demonstrate a one-step synthesis procedure for large-area vdW heterostructures between horizontal TMDCs MoS2 and WS2. The synthesis procedure is scalable and provides patterning ability, which is critical for electronic applications in integrated circuits. We demonstrate rectification characteristics of large-area MoS2/WS2 stacks. In addition, hydrogen evolution reaction performance was measured in these horizontal MoS2 and WS2 thin films, which indicate that, in addition to the catalytically active sulfur edge sites, defect sites may serve as catalyst sites.

1 of 9) 1605928applications. [8][9][10][11][12][13][14] A spin quantum Hall state is also predicted in the distorted octahedral phase (1T′) of MX 2 in the monolayer limit, further extending applications of TMDs into spintronics and lowdissipation electronics. [13] As a part of the TMDs family, WTe 2 has recently attracted great interest due to its giant, nonsaturating magnetoresistance (MR) observed in bulk crystals, [15] and its predicted Weyl state. [16] Pressure-induced superconductivity and large spin-orbit coupling are also observed. [17,18] In addition, the lattice thermal conductivity of WTe 2 is predicted to be smaller than that of WSe 2 due to the heavier atom mass and the lower in-plane crystal symmetry. [19] Studies on WTe 2 have so far been carried out using bulk crystals or mechanically exfoliated flakes. Although mechanical exfoliation can produce high-quality flakes down to a monolayer, scaling it to obtain large-area thin films for practical applications is challenging. Thus, direct synthesis of WTe 2 thin films is desirable for potential electronic and thermal propertyrelated applications, but has yet to be realized due to the low bonding energy of W-Te. Synthesizing WTe 2 directly into largescale thin films is challenging due to its very small standard Gibbs free energy of reaction (−26.2 kJ mol −1 ) compared to that of WSe 2 (−135.0 kJ mol −1 ). [20,21] Additionally, the low melting point of the forming Te-W binary eutectic and high melting point of W (3422 °C) restrict the reaction efficiency between W and Te. Only recently, direct synthesis of MoTe 2 thin films, another interesting TMD [22] with a lower standard Gibbs free energy of reaction (−64.3 kJ mol −1 ) than WTe 2 , has been demonstrated via chemical vapor deposition synthesis (all values of standard Gibbs free energy of reaction are taken at 1100 K). [21,23,24] So far, no direct synthesis of large-area, highly crystalline WTe 2 thin films has been reported.Here, we demonstrate a large-area, facile synthesis of WTe 2 and MoTe 2 thin films by reacting sputtered metal films with H 2 Te, an intermediate vapor phase formed from Te vapor and H 2 carrier gas, through atmospheric pressure chemical vapor reaction. The synthesized films are polycrystalline whose grain size increases with increasing metal film thickness. Based on time-domain thermoreflectance (TDTR), [25,26] the in-plane thermal conductivity of our polycrystalline WTe 2 thin film is less than 2 W m −1 K −1 , at least 7.5 times smaller than that of single-crystalline exfoliated flakes (15 ± 3 W m −1 K −1 ) at room temperature. Through-plane thermal conductivity of our WTe 2 thin films was measured to be 0.8 W m −1 K −1 at room temperature, which is lower than that of the recently reported Large-scale, polycrystalline WTe 2 thin films are synthesized by atmospheric chemical vapor reaction of W metal films with Te vapor catalyzed by H 2 Te intermediates, paving a way to understanding the synthesis mechanism for low bonding energy tellurides and toward synthesis of single-crystallin...

For the electrochemical hydrogen evolution reaction (HER), the electrical properties of catalysts can play an important role in influencing the overall catalytic activity. This is particularly important for semiconducting HER catalysts such as MoS , which has been extensively studied over the last decade. Herein, on-chip microreactors on two model catalysts, semiconducting MoS and semimetallic WTe , are employed to extract the effects of individual factors and study their relations with the HER catalytic activity. It is shown that electron injection at the catalyst/current collector interface and intralayer and interlayer charge transport within the catalyst can be more important than thermodynamic energy considerations. For WTe , the site-dependent activities and the relations of the pure thermodynamics to the overall activity are measured and established, as the microreactors allow precise measurements of the type and area of the catalytic sites. The approach presents opportunities to study electrochemical reactions systematically to help establish rational design principles for future electrocatalysts.

Chemical vapor deposition (CVD) is used widely to synthesize monolayer and few-layer transition metal dichalcogenide molybdenum disulfide (MoS2), a two-dimensional (2D) material with various applications in nanoelectronics, catalysis, and optoelectronics. However, the CVD synthesis of 2D MoS2 is highly sensitive to small changes in growth parameters and the growth mechanism has not been extensively studied. This work systematically investigates the effect of sulfur concentration on CVD synthesis of MoS2 using molybdenum trioxide (MoO3) and sulfur precursors. We find that with increasing concentration of sulfur vapor, intermediate products of molybdenum dioxide (MoO2) and molybdenum oxysulfide (MoOS2) can form during a stepwise sulfurization of MoO3 to the final product of MoS2. The intermediate MoOS2, formed due to sulfur vapor deficiency, can be fully converted to MoS2 with further sulfurization. We show that the local sulfur to molybdenum vapor ratio at the growth substrate critically determines the growth products. This study thus highlights the importance of keeping the molar ratio of sulfur to molybdenum vapor well in excess of the stoichiometrically required ratio of 3.5:1 in order to grow 2D MoS2.

Recent renewed interest in layered transition metal dichalcogenides stems from the exotic electronic phases predicted and observed in the single- and few-layer limit. Realizing these electronic phases requires preserving the desired transport properties down to a monolayer, which is challenging. Surface oxides are known to impart Fermi level pinning or degrade the mobility on a number of different systems, including transition metal dichalcogenides and black phosphorus. Semimetallic WTe exhibits large magnetoresistance due to electron-hole compensation; thus, Fermi level pinning in thin WTe flakes could break the electron-hole balance and suppress the large magnetoresistance. We show that WTe develops an ∼2 nm thick amorphous surface oxide, which shifts the Fermi level by ∼300 meV at the WTe surface. We also observe a dramatic suppression of the magnetoresistance for thin flakes. However, due to the semimetallic nature of WTe, the effects of Fermi level pinning are well screened and are not the dominant cause for the suppression of magnetoresistance, supported by fitting a two-band model to the transport data, which showed the electron and hole carrier densities are balanced down to ∼13 nm. However, the fitting shows a significant decrease of the mobilities of both electrons and holes. We attribute this to the disorder introduced by the amorphous surface oxide layer. Thus, the decrease of mobility is the dominant factor in the suppression of magnetoresistance for thin WTe flakes. Our study highlights the critical need to investigate often unanticipated and sometimes unavoidable extrinsic surface effects on the transport properties of layered dichalcogenides and other 2D materials.

Using the MoS2‐WTe2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS2. Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large‐area heterojunctions between MoS2 and WTe2 and effective screening of mirror charges due to the semimetallic nature of WTe2. The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS2 through large‐area heterojunctions. Based on this understanding, a MoS2/WTe2 hybrid catalyst is fabricated with an HER overpotential of −140 mV at 10 mA cm−2, a Tafel slope of 40 mV dec−1, and long stability. These results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions.

The intercalation-induced phase transition of MoS2 from the semiconducting 2H to the semimetallic 1T' phase has been studied in detail for nearly a decade; however, the effects of a heterointerface between MoS2 and other two-dimensional (2D) crystals on the phase transition have largely been overlooked. Here, ab initio calculations show that intercalating Li at a MoS2hexagonal boron nitride (hBN) interface stabilizes the 1T phase over the 2H phase of MoS2 by ~ 100 mJ m -2 , suggesting that encapsulating MoS2 with hBN may lower the electrochemical energy needed for the intercalation-induced phase transition. However, in situ Raman spectroscopy of hBN-MoS2-hBN heterostructures during electrochemical intercalation of Li + shows that the phase transition occurs at the same applied voltage for the heterostructure as for bare MoS2. We hypothesize that the predicted thermodynamic stabilization of the 1T'-MoS2-hBN interface is counteracted by an energy barrier to the phase transition imposed by the steric hindrance of the heterointerface. The phase transition occurs at lower applied voltages upon heating the heterostructure, which supports our hypothesis. Our study highlights that interfacial effects of 2D heterostructures can go beyond modulating electrical properties and can modify electrochemical and phase transition behaviors. Main TextVan der Waals heterostructures comprised of different two-dimensional (2D) materials 1,2 can exhibit novel electronic and optical properties [3][4][5] , in which the effect of heterointerfaces is important. Heterointerfaces may also play a central role in determining electrochemical properties and phase transitions of 2D materials. For MoS2, intercalation of alkali metal ions such as Li + into