Linear Dichroism and Nondestructive Crystalline Identification of Anisotropic Semimetal Few‐Layer MoTe2

Semimetal 1T′ MoTe₂ crystals have attracted tremendous attention owing to their anisotropic optical properties, Weyl semimetal, phase transition, and so on. However, the complex refractive indices (n‐ik) of the anisotropic semimetal 1T′ MoTe₂ still are not revealed yet, which is important to applications such as polarized wide spectrum detectors, polarized surface plasmonics, and nonlinear optics. Here, the linear dichroism of as‐grown trilayer 1T′ MoTe₂ single crystals is investigated. Trilayer 1T′ MoTe₂ shows obvious anisotropic optical absorption due to the intraband transition of d_z² orbits for Mo atoms and p_x orbits for Te atoms. The anisotropic complex refractive indices of few‐layer 1T′ MoTe₂ are experimentally obtained for the first time by using the Pinier equation analysis. Based on the linear dichroism of 1T′ MoTe₂, angle‐resolved polarized optical microscopy is developed to visualize the grain boundary and identify the crystal orientation of 1T′ MoTe₂ crystals, which is also an excellent tool toward the investigation of the optical absorption properties in the visible range for anisotropic 2D transition metal chalcogenides. This work provides a universal and nondestructive method to identify the crystal orientation of anisotropic 2D materials, which opens up an opportunity to investigate the optical application of anisotropic semimetal 2D materials.     

High Phase Purity of Large‐Sized 1T′‐MoS2 Monolayers with 2D Superconductivity

The development of transition metal dichalcogenides has greatly accelerated research in the 2D realm, especially for layered MoS₂. Crucially, the metallic MoS₂ monolayer is an ideal platform in which novel topological electronic states can emerge and also exhibits excellent energy conversion and storage properties. However, as its intrinsic metallic phase, little is known about the nature of 2D 1T′‐MoS₂, probably because of limited phase uniformity (<80%) and lateral size (usually <1 µm) in produced materials. Herein, solution processing to realize high phase‐purity 1T′‐MoS₂ monolayers with large lateral size is demonstrated. Direct chemical exfoliation of millimeter‐sized 1T′ crystal is introduced to successfully produce a high‐yield of 1T′‐MoS₂ monolayers with over 97% phase purity and unprecedentedly large size up to tens of micrometers. Furthermore, the large‐sized and high‐quality 1T′‐MoS₂ nanosheets exhibit clear intrinsic superconductivity among all thicknesses down to monolayer, accompanied by a slow drop of transition temperature from 6.1 to 3.0 K. Prominently, unconventional superconducting behavior with upper critical field far beyond the Pauli limit is observed in the centrosymmetric 1T′‐MoS₂ structure. The results open up an ideal approach to explore the properties of 2D metastable polymorphic materials.     

Misorientation‐Angle‐Dependent Phase Transformation in van der Waals Multilayers via Electron‐Beam Irradiation

Misorientation‐angle dependence on layer thickness is an intriguing feature of van der Waals materials, which causes stark optical gain and electrical transport modulation. However, the influence of misorientation angle on phase transformation is not determined yet. Herein, this phenomenon in a MoS₂ multilayer via in situ electron‐beam irradiation is reported. An AA′‐stacked MoS₂ bilayer undergoes structural transformation from the 2H semiconducting phase to the 1T′ metallic phase, similar to a MoS₂ monolayer, which is confirmed via in situ transmission electron microscopy. Moreover, non‐AA′ stacking, which has no local AA′ stacking order in the Moiré pattern, does not reveal such a phase transformation. While a collective sliding motion of chalcogen atoms easily occurs during the transformation in AA′ stacking, in non‐AA′ stacking it is suppressed by the weak van der Waals strength and by the chalcogen atoms interlocked at different orientations, which disfavor their kinetics by the increased entropy of mixing.     

Gram‐Scale Aqueous Synthesis of Stable Few‐Layered 1T‐MoS2: Applications for Visible‐Light‐Driven Photocatalytic Hydrogen Evolution

Most recently, much attention has been devoted to 1T phase MoS₂ because of its distinctive phase‐engineering nature and promising applications in catalysts, electronics, and energy storage devices. While alkali metal intercalation and exfoliation methods have been well developed to realize unstable 1T‐MoS₂, but the aqueous synthesis for producing stable metallic phase remains big challenging. Herein, a new synthetic protocol is developed to mass‐produce colloidal metallic 1T‐MoS₂ layers highly stabilized by intercalated ammonium ions (abbreviated as N‐MoS₂). In combination with density functional calculations, the X‐ray diffraction pattern and Raman spectra elucidate the excellent stability of metallic phase. As clearly depicted by high‐angle annular dark‐field imaging in an aberration‐corrected scanning transmission electron microscope and extended X‐ray absorption fine structure, the N‐MoS₂ exhibits a distorted octahedral structure with a 2a ₀ × a ₀ basal plane superlattice and 2.72 Å Mo–Mo bond length. In a proof‐of‐concept demonstration for the obtained material's applications, highly efficient photocatalytic activity is achieved by simply hybridizing metallic N‐MoS₂ with semiconducting CdS nanorods due to the synergistic effect. As a direct outcome, this CdS:N‐MoS₂ hybrid shows giant enhancement of hydrogen evolution rate, which is almost 21‐fold higher than pure CdS and threefold higher than corresponding annealed CdS:2H‐MoS₂.     

Epitaxial Growth of Rectangle Shape MoS2 with Highly Aligned Orientation on Twofold Symmetry a‐Plane Sapphire

Research on transition metal dichalcogenides (TMDs) has been accelerated by the development of large‐scale synthesis based on chemical vapor deposition (CVD) growth. However, in most cases, CVD‐grown TMDs are composed of randomly oriented grains, and thus contain many distorted grain boundaries (GBs), which seriously degrade their electrical and photoelectrical properties. Here, the epitaxial growth of highly aligned MoS₂ grains is reported on a twofold symmetry a‐plane sapphire substrate. The obtained MoS₂ grains have an unusual rectangle shape with perfect orientation alignment along the [1‐100] crystallographic direction of a‐plane sapphire. It is found that the growth temperature plays a key role in its orientation alignment and morphology evolution, and high temperature is beneficial to the initial MoS₂ seeds rotate to the favorable orientation configurations. In addition, the photoluminescence quenching of the well‐aligned MoS₂ grains indicates a strong MoS₂?substrate interaction which induces the anisotropic growth of MoS₂, and thus brings the formation of rectangle shape grains. Moreover, the well‐aligned MoS₂ grains splice together without GB formation, and thus that has negligible effect on its electrical transport properties. The progress achieved in this work could promote the controlled synthesis of large‐area TMDs single crystal film and the scalable fabrication of high‐performance electronic devices.     

Controlling Structural Anisotropy of Anisotropic 2D Layers in Pseudo‐1D/2D Material Heterojunctions

Chemical vapor deposition and growth dynamics of highly anisotropic 2D lateral heterojunctions between pseudo‐1D ReS₂ and isotropic WS₂ monolayers are reported for the first time. Constituent ReS₂ and WS₂ layers have vastly different atomic structure, crystallizing in anisotropic 1T′ and isotropic 2H phases, respectively. Through high‐resolution scanning transmission electron microscopy, electron energy loss spectroscopy, and angle‐resolved Raman spectroscopy, this study is able to provide the very first atomic look at intimate interfaces between these dissimilar 2D materials. Surprisingly, the results reveal that ReS₂ lateral heterojunctions to WS₂ produce well‐oriented (highly anisotropic) Re‐chains perpendicular to WS₂ edges. When vertically stacked, Re‐chains orient themselves along the WS₂ zigzag direction, and consequently, Re‐chains exhibit six‐fold rotation, resulting in loss of macroscopic scale anisotropy. The degree of anisotropy of ReS₂ on WS₂ largely depends on the domain size, and decreases for increasing domain size due to randomization of Re‐chains and formation of ReS₂ subdomains. Present work establishes the growth dynamics of atomic junctions between novel anisotropic/isotropic 2D materials, and overall results mark the very first demonstration of control over anisotropy direction, which is a significant leap forward for large‐scale nanomanufacturing of anisotropic systems.     

Aligned Heterointerface‐Induced 1T‐MoS2 Monolayer with Near‐Ideal Gibbs Free for Stable Hydrogen Evolution Reaction

1T‐phase molybdenum disulfide (1T‐MoS₂) exhibits superior hydrogen evolution reaction (HER) over 2H‐phase MoS₂ (2H‐MoS₂). However, its thermodynamic instability is the main drawback impeding its practical application. In this work, a stable 1T‐MoS₂ monolayer formed at edge‐aligned 2H‐MoS₂ and a reduced graphene oxide heterointerface (EA‐2H/1T/RGO) using a precursor‐in‐solvent synthesis strategy are reported. Theoretical prediction indicates that the edge‐aligned layer stacking can induce heterointerfacial charge transfer, which results in a phase transition of the interfacial monolayer from 2H to 1T that realizes thermodynamic stability based on the adhesion energy between MoS₂ and graphene. As an electrocatalyst for HER, EA‐2H/1T/RGO displays an onset potential of ?103 mV versus RHE, a Tafel slope of 46 mV dec^?1 and 10 h stability in acidic electrolyte. The unexpected activity of EA‐2H/1T/RGO beyond 1T‐MoS₂ is due to an inherent defect caused by the gliding of S atoms during the phase transition from 2H to 1T, leading the Gibbs free energy of hydrogen adsorption (ΔG_H*) to decrease from 0.13 to 0.07 eV, which is closest to the ideal value (0.06 eV) of 2H‐MoS₂. The presented work provides fundamental insights into the impressive electrochemical properties of HER and opens new avenues for phase transitions at 2D/2D hybrid interfaces.     

Pressure‐Induced Phase Transition in Weyl Semimetallic WTe2

Tungsten ditelluride (WTe₂) is a semimetal with orthorhombic T_d phase that possesses some unique properties such as Weyl semimetal states, pressure‐induced superconductivity, and giant magnetoresistance. Here, the high‐pressure properties of WTe₂ single crystals are investigated by Raman microspectroscopy and ab initio calculations. WTe₂ shows strong plane‐parallel/plane‐vertical vibrational anisotropy, stemming from its intrinsic Raman tensor. Under pressure, the Raman peaks at ≈120 cm^?1 exhibit redshift, indicating structural instability of the orthorhombic T_d phase. WTe₂ undergoes a phase transition to a monoclinic T′ phase at 8 GPa, where the Weyl states vanish in the new T′ phase due to the presence of inversion symmetry. Such T_d to T′ phase transition provides a feasible method to achieve Weyl state switching in a single material without doping. The new T′ phase also coincides with the appearance of superconductivity reported in the literature.     

Photocarrier‐Induced Active Control of Second‐Order Optical Nonlinearity in Monolayer MoS2

Atomically thin transition metal dichalcogenides (TMDs) in their excited states can serve as exceptionally small building blocks for active optical platforms. In this scheme, optical excitation provides a practical approach to control light‐TMD interactions via the photocarrier generation, in an ultrafast manner. Here, it is demonstrated that via a controlled generation of photocarriers the second‐harmonic generation (SHG) from a monolayer MoS₂ crystal can be substantially modulated up to ≈55% within a timeframe of ≈250 fs, a set of performance characteristics that showcases the promise of low‐dimensional materials for all‐optical nonlinear data processing. The combined experimental and theoretical study suggests that the large SHG modulation stems from the correlation between the second‐order dielectric susceptibility χ⁽²⁾ and the density of photoexcited carriers in MoS₂. Indeed, the depopulation of the conduction band electrons, at the vicinity of the high‐symmetry K/K′ points of MoS₂, suppresses the contribution of interband electronic transitions in the effective χ⁽²⁾ of the monolayer crystal, enabling the all‐optical modulation of the SHG signal. The strong dependence of the second‐order optical response on the density of photocarriers reveals the promise of time‐resolved nonlinear characterization as an alternative route to monitoring carrier dynamics in excited states of TMDs.     

Ruthenium Nanoparticles on Cobalt‐Doped 1T′ Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS₂ nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co–Ru–MoS₂ hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T′ phase MoS₂ nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm^?2 and Tafel slopes of 55 and 50 mV decade^?1 in 1.0 m KOH, respectively. Analysis of X‐ray photoelectron and absorption spectra of the catalysts show that the MoS₂ well retained its metallic 1T′ phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co‐doped MoS₂ greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO₂ nanoparticles that are produced via oxidation of Ru nanoparticles.     

Direct Bandgap Transition in Many‐Layer MoS2 by Plasma‐Induced Layer Decoupling

We report a robust method for engineering the optoelectronic properties of many‐layer MoS₂ using low‐energy oxygen plasma treatment. Gas phase treatment of MoS₂ with oxygen radicals generated in an upstream N₂–O₂ plasma is shown to enhance the photoluminescence (PL) of many‐layer, mechanically exfoliated MoS₂ flakes by up to 20 times, without reducing the layer thickness of the material. A blueshift in the PL spectra and narrowing of linewidth are consistent with a transition of MoS₂ from indirect to direct bandgap material. Atomic force microscopy and Raman spectra reveal that the flake thickness actually increases as a result of the plasma treatment, indicating an increase in the interlayer separation in MoS₂. Ab initio calculations reveal that the increased interlayer separation is sufficient to decouple the electronic states in individual layers, leading to a transition from an indirect to direct gap semiconductor. With optimized plasma treatment parameters, we observed enhanced PL signals for 32 out of 35 many‐layer MoS₂ flakes (2–15 layers) tested, indicating that this method is robust and scalable. Monolayer MoS₂, while direct bandgap, has a small optical density, which limits its potential use in practical devices. The results presented here provide a material with the direct bandgap of monolayer MoS₂, without reducing sample thickness, and hence optical density.     

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2‐Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS₂) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS₂ is introduced into the 2H‐MoS₂ nanosheets by two‐step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write‐once read‐many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H‐MoS₂‐polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H‐MoS₂‐PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS₂ nanosheets plays an important role on the resistive switching behaviors of the MoS₂‐based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.     

Contact‐Engineered Electrical Properties of MoS2 Field‐Effect Transistors via Selectively Deposited Thiol‐Molecules

Although 2D molybdenum disulfide (MoS₂) has gained much attention due to its unique electrical and optical properties, the limited electrical contact to 2D semiconductors still impedes the realization of high‐performance 2D MoS₂‐based devices. In this regard, many studies have been conducted to improve the carrier‐injection properties by inserting functional paths, such as graphene or hexagonal boron nitride, between the electrodes and 2D semiconductors. The reported strategies, however, require relatively time‐consuming and low‐yield transfer processes on sub‐micrometer MoS₂ flakes. Here, a simple contact‐engineering method is suggested, introducing chemically adsorbed thiol‐molecules as thin tunneling barriers between the metal electrodes and MoS₂ channels. The selectively deposited thiol‐molecules via the vapor‐deposition process provide additional tunneling paths at the contact regions, improving the carrier‐injection properties with lower activation energies in MoS₂ field‐effect transistors. Additionally, by inserting thiol‐molecules at the only one contact region, asymmetric carrier‐injection is feasible depending on the temperature and gate bias.     

Unveiling Active Sites for the Hydrogen Evolution Reaction on Monolayer MoS2

Here, the hydrogen evolution reaction (HER) activities at the edge and basal‐plane sites of monolayer molybdenum disulfide (MoS₂) synthesized by chemical vapor deposition (CVD) are studied using a local probe method enabled by selected‐area lithography. Reaction windows are opened by e‐beam lithography at sites of interest on poly(methyl methacrylate) (PMMA)‐covered monolayer MoS₂ triangles. The HER properties of MoS₂ edge sites are obtained by subtraction of the activity of the basal‐plane sites from results containing both basal‐plane and edge sites. The catalytic performances in terms of turnover frequencies (TOFs) are calculated based on the estimated number of active sites on the selected areas. The TOFs follow a descending order of 3.8 ± 1.6, 1.6 ± 1.2, 0.008 ± 0.002, and 1.9 ± 0.8 × 10^?4 s^?1, found for 1T′‐, 2H‐MoS₂ edges, and 1T′‐, 2H‐MoS₂ basal planes, respectively. Edge sites of both 2H‐ and 1T′‐MoS₂ are proved to have comparable activities to platinum (≈1–10 s^?1). When fitted into the HER volcano plot, the MoS₂ active sites follow a trend distinct from conventional metals, implying a possible difference in the reaction mechanism between transition‐metal dichalcogenides (TMDs) and metal catalysts.     

Strong and Tunable Electrical Anisotropy in Type‐II Weyl Semimetal Candidate WP2 with Broken Inversion Symmetry

A transition metal diphosphide, WP₂, is a candidate for type‐II Weyl semimetals (WSMs) in which spatial inversion symmetry is broken and Lorentz invariance is violated. As one of the prerequisites for the presence of the WSM state in WP₂, spatial inversion symmetry breaking in this compound has rarely been investigated. Furthermore, the anisotropy of the WP₂ electrical properties and whether its electrical anisotropy can be tuned remain elusive. Angle‐resolved polarized Raman spectroscopy, electrical transport, optical spectroscopy, and first‐principle studies of WP₂ are reported. The energies of the observed Raman‐active phonons and the angle dependences of the detected phonon intensities are consistent with results obtained by first‐principle calculations and analysis of the proposed crystal symmetry without spatial inversion, showing that spatial inversion symmetry is broken in WP₂. Moreover, the measured ratio (R_c /R_a ) between the crystalline c‐axis and a‐axis electrical resistivities exhibits a weak dependence on temperature (T) in the temperature range from 100 to 250 K, but increases abruptly at T ≤ 100 K, and then reaches the value of ≈8.0 at T = 10 K, which is by far the strongest in‐plane electrical resistivity anisotropy among the reported type‐II WSM candidates with comparable carrier concentrations. Optical spectroscopy study, together with the first‐principle calculations on the electronic band structure, reveals that the abrupt enhancement of the electrical resistivity anisotropy at T ≤ 100 K mainly arises from a sharp increase in the scattering rate anisotropy at low temperatures. More interestingly, the R_c /R_a of WP₂ at T = 10 K can be tuned from 8.0 to 10.6 as the magnetic field increases from 0 to 9 T. The so‐far‐strongest and magnetic‐field‐tunable electrical resistivity anisotropy found in WP₂ can serve as a degree of freedom for tuning the electrical properties of type‐II WSMs, which paves the way for the development of novel electronic applications based on type‐II WSMs.     

Threshold Voltage Control of Multilayered MoS2 Field‐Effect Transistors via Octadecyltrichlorosilane and their Applications to Active Matrixed Quantum Dot Displays Driven by Enhancement‐Mode Logic Gates

In recent past, for next‐generation device opportunities such as sub‐10 nm channel field‐effect transistors (FETs), tunneling FETs, and high‐end display backplanes, tremendous research on multilayered molybdenum disulfide (MoS₂) among transition metal dichalcogenides has been actively performed. However, nonavailability on a matured threshold voltage control scheme, like a substitutional doping in Si technology, has been plagued for the prosperity of 2D materials in electronics. Herein, an adjustment scheme for threshold voltage of MoS₂ FETs by using self‐assembled monolayer treatment via octadecyltrichlorosilane is proposed and demonstrated to show MoS₂ FETs in an enhancement mode with preservation of electrical parameters such as field‐effect mobility, subthreshold swing, and current on–off ratio. Furthermore, the mechanisms for threshold voltage adjustment are systematically studied by using atomic force microscopy, Raman, temperature‐dependent electrical characterization, etc. For validation of effects of threshold voltage engineering on MoS₂ FETs, full swing inverters, comprising enhancement mode drivers and depletion mode loads are perfectly demonstrated with a maximum gain of 18.2 and a noise margin of ≈45% of 1/2 V_DD. More impressively, quantum dot light‐emitting diodes, driven by enhancement mode MoS₂ FETs, stably demonstrate 120 cd m^?2 at the gate‐to‐source voltage of 5 V, exhibiting promising opportunities for future display application.     

In‐Plane 2H‐1T′ MoTe2 Homojunctions Synthesized by Flux‐Controlled Phase Engineering

The fabrication of in‐plane 2H‐1T′ MoTe₂ homojunctions by the flux‐controlled, phase‐engineering of few‐layer MoTe₂ from Mo nanoislands is reported. The phase of few‐layer MoTe₂ is controlled by simply changing Te atomic flux controlled by the temperature of the reaction vessel. Few‐layer 2H MoTe₂ is formed with high Te flux, while few‐layer 1T′ MoTe₂ is obtained with low Te flux. With medium flux, few‐layer in‐plane 2H‐1T′ MoTe₂ homojunctions are synthesized. As‐synthesized MoTe₂ is characterized by Raman spectroscopy and X‐ray photoelectron spectroscopy. Kelvin probe force microscopy and Raman mapping confirm that in‐plane 2H‐1T′ MoTe₂ homojunctions have abrupt interfaces between 2H and 1T′ MoTe₂ domains, possessing a potential difference of about 100 mV. It is further shown that this method can be extended to create patterned metal–semiconductor junctions in MoTe₂ in a two‐step lithographic synthesis. The flux‐controlled phase engineering method could be utilized for the large‐scale controlled fabrication of 2D metal–semiconductor junctions for next‐generation electronic and optoelectronic devices.     

Lowering the Schottky Barrier Height by Graphene/Ag Electrodes for High‐Mobility MoS2 Field‐Effect Transistors

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V^?1 s^?1, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W^?1, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.     

A Simple Method for Synthesis of High‐Quality Millimeter‐Scale 1T′ Transition‐Metal Telluride and Near‐Field Nanooptical Properties

The controlled synthesis of MoTe₂ and WTe₂ is crucial for their fundamental research and potential electronic applications. Here, a simplified ambient‐pressure chemical vapor deposition (CVD) strategy is developed to synthesize high‐quality and large‐scale monolayer and few‐layer 1T′‐phase MoTe₂ (length ≈ 1 mm) and WTe₂ (length ≈ 350 µm) crystals by using ordinary salts (KCl or NaCl) as the growth promoter combining with low‐cost (NH₄)₆Mo₇O₂₄·4H₂O and hydrate (NH₄)₁₀W₁₂O₄₁·xH₂O as the Mo and W sources, respectively. Atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy confirm the high‐quality nature and the atomic structure of the as‐grown 1T′ MoTe₂ and WTe₂ flakes. Variable‐temperature transport measurements exhibit their semimetal properties. Furthermore, near‐field nanooptical imaging studies are performed on the 1T′ MoTe₂ and WTe₂ flakes for the first time. The sub‐wavelength effects of 1T′‐phase MoTe₂ (λ_p ≈ 140 nm) and WTe₂ (λ_p ≈ 100 nm) are obtained. This approach paves the way for the growth of special transition‐metal dichalcogenides materials and boosts the future polaritonic research of 2D telluride compounds.     

Designer Shape Anisotropy on Transition‐Metal‐Dichalcogenide Nanosheets

MoS₂ and generally speaking, the wide family of transition‐metal dichalcogenides represents a solid nanotechnology platform on which to engineer a wealth of new and outperforming applications involving 2D materials. An even richer flexibility can be gained by extrinsically inducing an in‐plane shape anisotropy of the nanosheets. Here, the synthesis of anisotropic MoS₂ nanosheets is proposed as a prototypical example in this respect starting from a highly conformal chemical vapor deposition on prepatterend substrates and aiming at the more general purpose of tailoring anisotropy of 2D nanosheets by design. This is envisioned to be a suitable configuration for strain engineering as far as strain can be spatially redistributed in morphologically different regions. With a similar approach, both the optical and electronic properties of the 2D transition‐metal dichalcogenides can be tailored over macroscopic sample areas in a self‐organized fashion, thus paving the way for new applications in the field of optical metasurfaces, light harvesting, and catalysis.