Revisiting the Role of Active Sites for Hydrogen Evolution Reaction through Precise Defect Adjusting

2D transition metal dichalcogenides (TMDs) have presented outstanding potential for efficient hydrogen evolution reaction (HER) to replace traditional noble metal catalysts. Here, to achieve enhanced HER performance, specific areas of the few‐layer 1T'‐MoTe₂ film are precisely controlled with a focused ion beam to create particular active sites. Electrochemical measurements indicate that the HER performance, although inconspicuous in pristine 1T'‐MoTe₂ ultrathin films prepared through the chemical vapor deposition method, can be greatly enhanced after patterning and precisely controlled by the morphologies as well as the amounts of the defects, reaching a small onset potential and a record‐low Tafel slope of 44 mV per decade for few‐layer TMDs. Conductivity tests, visualized copper electrodeposition, and density functional theory calculations also confirm that the enhancement of HER performance comes from the exposed edges by patterning. In this pioneering work, not only is the catalysis mechanism of the edge active sites of 1T'‐MoTe₂ unveiled, but also a universal route to study the properties of 2D materials is demonstrated.     

Inversion Symmetry Broken 2D 3R‐MoTe2

Inversion symmetry broken 3R phase semiconducting transition metal dichalcogenides (TMDC) have huge potential applications in many novel fields, such as valleytronics and nonlinear optics for the strong spin–orbit coupling and particularly the persistent noncentrosymmetric structure regardless the layer numbers, in stark contrast to the strict layer number requirement in other phases. Unfortunately, the fabrication of 3R phase TMDC is still a huge task to date. Molybdenum telluride (MoTe₂) attracts increasing interest in recent years due to the easy transition between its various phases and its narrow bandgap close to silicon. However, the weak Mo–Te bond and the small energy imparity among phases make it a big challenge to obtain pure‐phase single crystalline MoTe₂, especially; it is still a virgin land to obtain two‐dimensional (2D) 3R‐MoTe₂. Here, by rational controlling the deposition temperature and tellurization velocity, for the first time high quality 2D 3R‐MoTe₂ flakes are synthesized via chemical vapor deposition from a MoCl₅ precursor. Scanning transmission electron microscopy unambiguously reveals the 3R stacking mode of as‐synthesized MoTe₂. Second harmonic generation measurement confirms the excellent odd/even layer‐independent frequency conversion efficiency. Besides, the outstanding intrinsic infrared detection ability of as‐synthesized 3R‐MoTe₂ is demonstrated as well.     

Carrier Modulation of Ambipolar Few‐Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping

Semiconducting molybdenum ditelluride (2H‐MoTe₂), a fast‐emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field‐effect transistors and is the focus of substantial research interest, showing hole‐dominated ambipolar characteristics. Here, carrier modulation of ambipolar few‐layer MoTe₂ transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe₂ layers, the carrier polarity of MoTe₂ transistors from p‐type to n‐type can be reversely controlled. The electron mobility of MoTe₂ is significantly enhanced from 0.1 to 20 cm² V^?1 s^?1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high‐performance complementary inverters with high DC gain of >25 and photodetectors based on few‐layer MoTe₂ flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition‐metal dichalcogenide semiconductors.     

Space‐Confined Chemical Vapor Deposition Synthesis of Ultrathin HfS2 Flakes for Optoelectronic Application

Due to the predicted excellent electronic properties superior to group VIB (Mo and W) transition metal dichalcogenides (TMDs), group IVB TMDs have enormous potential in nanoelectronics. Here, the synthesis of ultrathin HfS₂ flakes via space‐confined chemical vapor deposition, realized by an inner quartz tube, is demonstrated. Moreover, the effect of key growth parameters including the dimensions of confined space and deposition temperature on the growth behavior of products is systematically studied. Typical as‐synthesized HfS₂ is a hexagonal‐like flake with a smallest thickness of ≈1.2 nm (bilayer) and an edge size of ≈5 µm. The photodetector based on as‐synthesized HfS₂ flakes demonstrates excellent optoelectronic performance with a fast photoresponse time (55 ms), which is attributed to the high‐quality crystal structure obtained at a high deposition temperature and the ultraclean interface between HfS₂ and the mica substrate. With such properties HfS₂ holds great potential for optoelectronics applications.     

Sulfur‐Mastery: Precise Synthesis of 2D Transition Metal Dichalcogenides

Chemical vapor deposition (CVD) has been developed as the most promising method for the growth of transition metal dichalcogenides (TMDs). In this work, the key factor determining the growth of TMDs is ascertained. A straightforward method is devised to directly achieve a holistic control of thickness, shape, and size of WS₂ flakes via a single parameter control, namely, the status of the S‐precursor. The thickness‐dependent growth of WS₂ flakes from mono‐ to quad‐layers is achieved by precise control of the feeding rate of elemental S‐precursor. Moreover, the explicit control over amount and exposure time of S‐precursor determines the most optimum combination of these parameters to tune the shape of the crystals from triangular to hexagonal with appropriate size. Hence, the experimental findings provide a promising strategy to engineer the growth evolution of WS₂ atomic layers by fine tuning of the sulfur supply, paving a pathway to scalable electronic and photonic devices.     

Synthesis of 2H‐1T′ WS2‐ReS2 Heterophase Structures with Atomically Sharp Interface via Hydrogen‐Triggered One‐Pot Growth

2D transition metal chalcogenides (TMDs) with different compositions, phase structures, and properties offer giant opportunities for building novel 2D lateral heterostructures. However, the studies to date have been largely limited to homophase TMD heterostructures, while the construction of heterophase TMD heterostructures remains a challenge. Herein, the synthesis of 2H‐1T′ WS₂‐ReS₂ heterophase junctions with high‐quality interface structure via a hydrogen‐triggered one‐pot growth approach is reported. Sequential introduction of hydrogen during growth system, which acts as a “switch” to selectively turn off the growth of ReS₂ while turning on the growth of WS₂, allows WS₂ to seamlessly grow around ReS₂ to form the WS₂‐ReS₂ heterojunction. Moreover, WS₂ prefers to nucleate at the vertices of ReS₂ grain with fixed lattice orientation, which makes the surrounding WS₂ grains merge into single crystal. Scanning transmission electron microscopy reveals high crystal quality of the heterojunction with an atomically sharp 2H‐1T′ heterophase interface. Transient absorption spectroscopy indicates that the photocarriers can effectively separate at the heterophase interface. Based on the high quality heterophase junction, prominent rectification characteristics and polarization‐dependent photodiode properties are achieved. This study provides a robust way for the controlled synthesis of 2D heterophase structures, which is essential for their fundamental studies and device applications.     

Mass Production of High‐Quality Transition Metal Dichalcogenides Nanosheets via a Molten Salt Method

2D transition metal dichalcogenides (TMDs) are well suited for energy storage and field–effect transistors because of their thickness‐dependent chemical and physical properties. However, as current synthetic methods for 2D TMDs cannot integrate both advantages of liquid‐phase syntheses (i.e., massive production and homogeneity) and chemical vapor deposition (i.e., high quality and large lateral size), it still remains a great challenge for mass production of high‐quality 2D TMDs. Here, a molten salt method to massively synthesize various high‐crystalline TMDs nanosheets (MoS₂, WS₂, MoSe₂, and WSe₂) with the thicknesses less than 5 nm is reported, with the production yield over 68% with the reaction time of only several minutes. Additionally, the thickness and size of the as‐synthesized nanosheets can be readily controlled through adjusting reaction time and temperature. The as‐synthesized MoSe₂ nanosheets exhibit good electrochemical performance as pseudocapacitive materials. It is further anticipates that this work will provide a promising strategy for rapid mass production of high‐quality nonoxides nanosheets for energy‐related applications and beyond.     

Intercalation on Transition Metal Trichalcogenides via a Quasi-Amorphous Phase with 1D Order

Intercalation into 1D transition metal trichalcogenides (TMTs) in which fibers are bonded by a weak van der Waals force can be expected to create various intercalation compounds and develop unique physical properties according to the combination of the host materials and guest ions. However, structural changes via intercalation into 1D TMTs are not as simple as those in 2D transition metal dichalcogenides (TMDs) and are still not understood comprehensively. ZrTe₃: a typical compound with a 1D trigonal prismatic structure, belongs to TMTs. Herein, through the Ag introduction to ZrTe₃ via solid-state intercalation, a novel crystal phase with a 1D octahedral structure and a quasi-amorphous (QA) phase during the structural transition are discovered; the QA phase is a novel state of matter in which long-range order is lost while retaining 1D order. Based on the Ag concentration, the transport properties are flexibly modulated from superconductivity to semiconductivity. Density functional theory calculations indicate the attraction between Ag ions and the pair diffusion due to their attraction. Furthermore, judging the attraction or repulsion between guest ions predicts whether to induce a QA phase or simple lattice expansion like the intercalation into 2D TMDs.     

Role of Charge Density Wave in Monatomic Assembly in Transition Metal Dichalcogenides

The charge density wave (CDW) in transition metal dichalcogenides (TMDs) has drawn tremendous interest due to its potential for tailoring their surface electronic and chemical properties. Due to technical challenges, however, how the CDW could modulate the chemical behavior of TMDs is still not clear. Here, this work presents a study of applying the CDW of NbTe₂, with a high transition temperature above room temperature, to generate the assembling adsorption of Sn adatoms on the surface. It is shown that highly ordered monatomic Sn adatoms with a quasi‐1D structure can be obtained under regulation by the single‐axis CDW of the substrate. In addition, the CDW modulated superlattices could in turn change the surface electronic properties from semimetallic to metallic. These results demonstrate an effective approach for tuning the surface chemical properties of TMDs by their CDWs, which could be applied in exploring them for various practical applications, such as heterogeneous catalysis, epitaxial growth of low‐dimensional materials, and future nanoelectronics.     

Sub‐Millimeter‐Scale Monolayer p‐Type H‐Phase VS2

2D H‐phase vanadium disulfide (VS₂) is expected to exhibit tunable semiconductor properties as compared with its metallic T‐phase structure, and thus is of promise for future electronic applications. However, to date such 2D H‐phase VS₂ nanostructures have not been realized in experiment likely due to the polymorphs of vanadium sulfides and thermodynamic instability of H‐phase VS₂. Preparation of H‐phase VS₂ monolayer with lateral size up to 250 µm, as a new member in the 2D transition metal dichalcogenides (TMDs) family, is reported. A unique growth environment is built by introducing the molten salt‐mediated precursor system as well as the epitaxial mica growth platform, which successfully overcomes the aforementioned growth challenges and enables the evolution of 2D H‐phase structure of VS₂. The honeycomb‐like structure of H‐phase VS₂ with broken inversion symmetry is confirmed by spherical aberration‐corrected scanning transmission electron microscopy and second harmonic generation characterization. The phase structure is found to be ultra‐stable up to 500 K. The field‐effect device study further demonstrates the p‐type semiconducting nature of the 2D H‐phase VS₂. The study introduces a new phase‐stable 2D TMDs materials with potential features for future electronic devices.     

Low‐Temperature Eutectic Synthesis of PtTe2 with Weak Antilocalization and Controlled Layer Thinning

Metallic transition metal dichalcogenides (TMDs) have exhibited various exotic physical properties and hold the promise of novel optoelectronic and topological devices applications. However, the synthesis of metallic TMDs is based on gas‐phase methods and requires high‐temperature condition. As an alternative to the gas‐phase synthetic approach, lower temperature eutectic liquid‐phase synthesis presents a very promising approach with the potential for larger‐scale and controllable growth of high‐quality thin metallic TMD single crystals. Here, the first realization of low‐temperature eutectic liquid‐phase synthesis of type‐II Dirac semimetal PtTe₂ single crystals with thickness ranging from 2 to 200 nm is presented. The electrical measurement of synthesized PtTe₂ reveals a record‐high conductivity of as high as 3.3 × 10⁶ S m⁻¹ at room temperature. Besides, the weak antilocalization behavior is identified experimentally in the type‐II Dirac semimetal PtTe₂ for the first time. Furthermore, a simple and general strategy is developed to obtain atomically thin PtTe₂ crystal by thinning as‐synthesized bulk samples, which can still retain highly crystalline and exhibits excellent electrical conductivity. The results of controllable and scalable low‐temperature eutectic liquid‐phase synthesis and layer‐by‐layer thinning of high‐quality thin PtTe₂ single crystals offer a simple and general approach for obtaining different thickness metallic TMDs with high melting‐point transition metal.     

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.     

Anomalous Dimensionality-Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

Phase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface-to-volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality-driven phase transition of molybdenum ditelluride (MoTe₂) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single-crystal 2H-MoTe₂ transformed into polycrystalline T_d-MoTe₂ with tilt-angle grain boundaries of 60°-glide-reflection and 120°-twofold rotation. In contrast to conventional nanomaterials, the hBN-encapsulated MoTe₂ exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe₂ with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless T_d contacts for 2H-MoTe₂ transistors can be fabricated by using the dimensionality-driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi-phase 2D electronics.     

Ultrahigh Speed and Broadband Few‐Layer MoTe2/Si 2D–3D Heterojunction‐Based Photodiodes Fabricated by Pulsed Laser Deposition

2D transition metal dichalcogenides are promising candidates for high‐performance photodetectors. However, the relatively low response speed as well as the complex transfer process hinders their wide applications. Herein, for the first time, the fabrication of a few‐layer MoTe₂/Si 2D–3D vertical heterojunction for high‐speed and broadband photodiodes by a pulsed laser deposition technique is reported. Owing to the high junction quality, ultrathin MoTe₂ film thickness, and unique vertical n–n heterojunction structure, the photodiode exhibits excellent device performance in terms of a high responsivity of 0.19 A W^?1 and a large detectivity of 6.8 × 10¹³ Jones. The device is also capable of detecting a broadband light with wavelength spanning from 300 to 1800 nm. More importantly, the device possesses an ultrahigh response speed up to 150 ns with a 3‐dB electrical bandwidth approaching 0.12 GHz. This work paves the way toward the fabrication of novel 2D–3D heterojunctions for high‐performance, ultrafast photodetectors.     

Role of Molecular Sieves in the CVD Synthesis of Large‐Area 2D MoTe2

The synthesis of high‐quality 2D MoTe₂ with a desired phase on SiO₂/Si substrate is crucial to its diverse applications. A side reaction of Te with the substrate Si leading to SiTe and Si₂Te₃ tends to happen during growth, resulting in the failure to obtain MoTe₂. It has been found that molecular sieves can adsorb the silicon telluride byproducts and eliminate the influence of the side reaction during the chemical vapor deposition synthesis of MoTe₂. With the help of molecular sieves, few‐layer 1T′ MoTe₂ can be grown from the MoO_x precursor. Pure 1T′ MoTe₂ and 2H MoTe₂ regions in centimeter‐sized areas synthesized on the same piece of SiO₂/Si substrate can be obtained by using an overlapped geometry. The strategy provides a new method to controllably synthesize MoTe₂ with desired phases and can be generalizable to the synthesis of other tellurium‐based layered materials.     

Contact Engineering of Layered MoS2 via Chemically Dipping Treatments

The performance of electronic/optoelectronic devices is governed by carrier injection through metal–semiconductor contact; therefore, it is crucial to employ low‐resistance source/drain contacts. However, unintentional introduction of extrinsic defects, such as substoichiometric oxidation states at the metal–semiconductor interface, can degrade carrier injection. In this report, controlling the unintentional extrinsic defect states in layered MoS₂ is demonstrated using a two‐step chemical treatment, (NH₄)₂S(aq) treatment and vacuum annealing, to enhance the contact behavior of metal/MoS₂ interfaces. The two‐step treatment induces changes in the contact of single layer MoS₂ field effect transistors from nonlinear Schottky to Ohmic behavior, along with a reduction of contact resistance from 35.2 to 5.2 kΩ. Moreover, the enhancement of I_ON and electron field effect mobility of single layer MoS₂ field effect transistors is nearly double for n‐branch operation. This enhanced contact behavior resulting from the two‐step treatment is likely due to the removal of oxidation defects, which can be unintentionally introduced during synthesis or fabrication processes. The removal of oxygen defects is confirmed by scanning tunneling microscopy and X‐ray photoelectron spectroscopy. This two‐step (NH₄)₂S(aq) chemical functionalization process provides a facile pathway to controlling the defect states in transition metal dichalcogenides (TMDs), to enhance the metal‐contact behavior of TMDs.     

Layered Platinum Dichalcogenides (PtS2, PtSe2, and PtTe2) Electrocatalysis: Monotonic Dependence on the Chalcogen Size

Presently, research in layered transition metal dichalcogenides (TMDs) for numerous electrochemical applications have largely focused on Group 6 TMDs, especially MoS₂ and WS₂, whereas TMDs belonging to other groups are relatively unexplored. This work unravels the electrochemistry of Group 10 TMDs: specifically PtS₂, PtSe₂, and PtTe₂. Here, the inherent electroactivities of these Pt dichalcogenides and the effectiveness of electrochemical activation on their charge transfer and electrocatalytic properties are thoroughly examined. By performing density functional theory (DFT) calculations, the electrochemical and electrocatalytic behaviors of the Pt dichalcogenides are elucidated. The charge transfer and electrocatalytic attributes of the Pt dichalcogenides are strongly associated with their electronic structures. In terms of charge transfer, electrochemical activation has been successful for all Pt dichalcogenides as evident in the faster heterogeneous electron transfer (HET) rates observed in electrochemically reduced Pt dichalcogenides. Interestingly, the hydrogen evolution reaction (HER) performance of the Pt dichalcogenides adheres to a trend of PtTe₂ > PtSe₂ > PtS₂ whereby the HER catalytic property increases down the chalcogen group. Importantly, the DFT study shows this correlation to their electronic property in which PtS₂ is semiconducting, PtSe₂is semimetallic, and PtTe₂ is metallic. Furthermore, Pt dichalcogenides are effectively activated for HER. Distinct electronic structures of Pt dichalcogenides account for their different responses to electrochemical activation. Among all activated Pt dichalcogenides, PtS₂ shows most accentuated improvement as a HER electrocatalyst with an exceptional 50% decline in HER overpotential. Knowledge on Pt dichalcogenides provides valuable insights in the field of TMD electrochemistry, in particular, for the currently underrepresented Group 10 TMDs.     

Significantly Increased Raman Enhancement on MoX2 (X = S,Se) Monolayers upon Phase Transition

2D transition metal dichalcogenide (TMD) materials have been recognized as active platforms for surface‐enhanced Raman spectroscopy (SERS). Here, the effect of crystal structure (phase) transition is shown, which leads to altered electronic structures of TMD materials, on the Raman enhancement. Using thermally evaporated copper phthalocyanine, solution soaked rhodamine 6G, and crystal violet as typical probe molecules, it is found that a phase transition from 2H‐ to 1T‐phase can significantly increase the Raman enhancement effect on MoX₂ (X = S, Se) monolayers through a predominantly chemical mechanism. First‐principle density functional theory calculations indicate that the significant enhancement of the Raman signals on metallic 1T‐MoX₂ can be attributed to the facilitated electron transfer from the Fermi energy level of metallic 1T‐MoX₂ to the highest occupied molecular orbital level of the probe molecules, which is more efficient than the process from the top of valence band of semiconducting 2H‐MoX₂. This study not only reveals the origin of the Raman enhancement and identifies 1T‐MoSe₂ and 1T‐MoS₂ as potential Raman enhancement substrates, but also paves the way for designing new 2D SERS substrates via phase‐transition engineering.     

Efficient Photocarrier Transfer and Effective Photoluminescence Enhancement in Type I Monolayer MoTe2/WSe2 Heterostructure

Artificial van der Waals heterostructures of 2D layered materials are attractive from the viewpoint of the possible discovery of new physics together with improved functionalities. Stacking various combinations of atomically thin semiconducting transition metal dichalcogenides, MX₂ (M = Mo, W; X = S, Se, Te) with a hexagonal crystal structure, typically leads to the formation of a staggered Type II band alignment in the heterostructure, where electrons and holes are confined in different layers. Here, the comprehensive studies are performed on heterostructures prepared from monolayers of WSe₂ and MoTe₂ using differential reflectance, photoluminescence (PL), and PL excitation spectroscopy. The MoTe₂/WSe₂ heterostructure shows strong PL from the MoTe₂ layer at ≈1.1 eV, which is different from the quenched PL from the WSe₂ layer. Moreover, enhancement of PL intensity from the MoTe₂ layer is observed because of the near‐unity highly efficient photocarrier transfer from WSe₂ to MoTe₂. These experimental results suggest that the MoTe₂/WSe₂ heterostructure has a Type I band alignment where electrons and holes are confined in the MoTe₂ layer. The findings extend the diversity and usefulness of ultrathin layered heterostructures based on transition metal dichalcogenides, leading to possibilities toward future optoelectronic applications.     

Dual Functionalization of Liquid‐Exfoliated Semiconducting 2H‐MoS2 with Lanthanide Complexes Bearing Magnetic and Luminescence Properties

Liquid exfoliated, atomically thin semiconducting transition metal dichalcogenides (TMDs), as inorganic equivalents of graphene, have attracted great interest due to their distinctive physical, optoelectronic, and chemical properties. Functionalization of 2D TMDs brings new prospects for applications in optoelectronics, quantum technologies, catalysis, and medicine. In this report, dual functionalization of 2D semiconducting 2H‐MoS₂ nanosheets through simultaneous incorporation of magnetic and luminescent properties is demonstrated. A facile method is proposed for tuning the properties of the TDM semiconductors and accessing multimodal platforms, consisting in covalent grafting of lanthanide complexes onto the surface of 2D TMDs. Dual functionalization of liquid‐exfoliated MoS₂ nanosheets is demonstrated simultaneously with both europium (III) and gadolinium (III) complexes to form a colloidally stable luminescent (with millisecond lifetimes) and paramagnetic MoS₂‐based nanohybrid material. This work is the first example of transition metal dichalcogenide nanosheets functionalized with preformed lanthanide complexes. These findings open new prospects for covalent functionalization of TMDs with molecular species bearing specific functionalities as a means to tune the optoelectronic properties of the semiconductors, in order to create advanced materials and devices with a wide range of functionalities.