2D Transition Metal Dichalcogenides Research Articles

Electroluminescence from Megasonically Solution-Processed MoS2 Nanosheet Films.

Due to their superior optoelectronic properties, monolayer two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted significant attention for electroluminescent devices. However, challenges in isolating optoelectronically active TMD monolayers using scalable liquid phase exfoliation have precluded electroluminescence in large-area, solution-processed TMD films. Here, we overcome these limitations and demonstrate electroluminescence from molybdenum disulfide (MoS2) nanosheet films by employing a monolayer-rich MoS2 ink produced by electrochemical intercalation and megasonic exfoliation. Characteristic monolayer MoS2 photoluminescence and electroluminescence spectral peaks at 1.88-1.90 eV are observed in megasonicated MoS2 films, with the emission intensity increasing with film thickness over the range 10-70 nm. Furthermore, employing a vertical light-emitting capacitor architecture enables uniform electroluminescence in large-area devices. These results indicate that megasonically exfoliated MoS2 monolayers retain their direct bandgap character in electrically percolating thin films even following multistep solution processing. Overall, this work establishes megasonicated MoS2 inks as an additive manufacturing platform for flexible, patterned, and miniaturized light sources that can likely be expanded to other TMD semiconductors.

Crystal-Phase Engineering of Two-Dimensional Transition-Metal Dichalcogenides for Surface-Enhanced Raman Scattering: A Perspective.

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) have emerged as promising materials for surface-enhanced Raman scattering (SERS) due to their unique electronic, optical, and mechanical properties. In this Perspective, we briefly introduce the fundamental properties, crystal-phase configurations, and phase transition strategies of TMDs materials. We then discuss the importance of the crystal phase in determining the SERS effect of TMDs, highlighting recent advances in phase-engineering approaches to affording remarkable SERS performance. By considering the current challenges and future directions for improving the crystal-phase engineering of TMDs in SERS, we also offer new insights into the design and synthesis of more promising TMD-based SERS substrates.

Two-dimensional transition metal dichalcogenides beyond MoS2 for the catalytic reduction of nitroarenes: MoSe2 exhibits enhanced performance

Two-dimensional (2D) MoSe2 nanosheets were investigated as a catalyst for nitroarene reduction in the context of water treatment and demonstrated to exhibit a higher activity than their more studied MoS2 counterpart. Density functional theory (DFT) calculations disclosed a feasible nitroarene reduction pathway on 2D MoSe2, which involved passivation of the active site by oxygen from the nitro group and its reactivation by a reducing hydride. DFT analysis also provided an explanation for the isomer selectivity displayed by the 2D MoSe2 catalyst (towards, e.g., 2-, 3- and 4-nitroaniline). To facilitate their practical implementation, the bare MoSe2 nanosheets were coated with colloidal stabilizers, and were also immobilized on polymer foam, where they could be re-used for several consecutive catalytic cycles with no significant loss of activity. Overall, the present results open the prospect of 2D transition metal dichalcogenides beyond MoS2 as competitive nanocatalysts for the reduction of nitroarenes and other organic compounds.

Reinforcing Synaptic Plasticity of Defect-Tolerant States in Alloyed 2D Artificial Transistors.

While two-dimensional (2D) materials possess the desirable future of neuromorphic computing platforms, unstable charging and de-trapping processes, which are inherited from uncontrollable states, such as the interface trap between nanocrystals and dielectric layers, can deteriorate the synaptic plasticity in field-effect transistors. Here, we report a facile and effective strategy to promote artificial synaptic devices by providing physical doping in 2D transition-metal dichalcogenide nanomaterials. Our experiments demonstrate that the introduction of niobium (Nb) into 2D WSe2 nanomaterials produces charge trap levels in the band gap and retards the decay of the trapped charges, thereby accelerating the artificial synaptic plasticity by encouraging improved short-/long-term plasticity, increased multilevel states, lower power consumption, and better symmetry and asymmetry ratios. Density functional theory calculations also proved that the addition of Nb to 2D WSe2 generates defect tolerance levels, thereby governing the charging and de-trapping mechanisms of the synaptic devices. Physically doped electronic synapses are expected to be a promising strategy for the development of bioinspired artificial electronic devices.

Centimeter-level MoS2 films with controllable number of layers by face-to-face chemical vapor deposition strategy

Two-dimensional layered transition metal dichalcogenides (TMDCs) are highly promising in field-effect-transistors (EFTs), photodetectors, flexible devices, but the limitation of uncontrollable size and number of layers is a huge challenge for the development of these devices. Although some breakthroughs like wafer-level MoS2 have been achieved on specific substrates such as copper foil or sapphire using advanced chemical vapor deposition, electrical and optoelectronic testing requires the transfer of material from these substrates to SiO2/Si substrates. In this paper, we developed a face-to-face confined space CVD scheme for the preparation of centimeter-level MoS2 thin films on amorphous SiO2/Si substrate. In contrast to traditional confined-space CVD scheme where the MoO3 powder is often piled like a small hill in the small quartz boat, The MoO3 powder is evenly spread in the small quartz boat, facing directly up to the growing substrate with a “face-to-face” growth mode. Therefore, the nucleation centers are uniformly distributed on the growing substrate, finally resulting in a uniform and clean continuous film originating from the coalescence of numerous MoS2 domains grown up from these nucleation centers. The layer number can be well controlled by adjusting growing parameters like growing temperature, gas flow rate and growth time. Our facile but robust face-to-face growth strategy may be a great supplementary to the wafer-level growth of 2D TMDCs.

Transition Metal Dichalcogenide WS2 Films Prepared with a Combination of Spin/Dip Coating and CVD

Recently, transition metal dichalcogenides (TMDCs) with 2D structure have attracted interest due to their many unique optical and electrical properties. The primary preparation methods for 2D materials are chemical vapor deposition (CVD), exfoliation, and other vacuum technologies. Large-scale synthesis of WS2 via solution-process is rare due to the higher temperature needed for tungsten-based precursors. Combination of spin coating or dip coating with CVD have been studied recently to make large-area 2D TMDC with good electrical properties. Here, we report a new synthetic route for large WS2 crystal that combined solution coatings and CVD process. A solution of sodium tungstate and hydrazine hydrate with sodium thiosulphate was coated on a silicon wafer via dip and spin coating. The films were then treated with CVD at various positions and temperatures to facilitate crystallization. The double coating conditions and CVD parameters were modified to obtain WS2 crystals. Triangular shaped 44 ± 4 µm WS2 crystals could be obtained with simple annealing above 900oC without gas treatment. The synthesized WS2 was found to be bulk with a triangular shape, as confirmed by Raman and AFM analyses. A PL peak of WS2 at 643 nm was observed at an early crystallization stage.

Dilute Rhenium Doping and its Impact on Defects in MoS2.

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS2, indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS2. This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

The influence of MoS2 on perpendicular magnetic anisotropy and spin–orbit torque of Pt/Co/Pt multilayer

Two-dimensional (2D) materials provide a wide choice for the next-generation spintronic devices at the post-Moore era. Here, we reveal the effect of the 2D transition metal dichalcogenide MoS2 on the perpendicular magnetic anisotropy (PMA) and spin–orbit torque (SOT) of the Pt/Co/Pt multilayer. MoS2 can distinctly enhance the PMA of Pt/Co/Pt multilayer, which shows no obvious dependence on the thickness of MoS2. Moreover, the magnetization of monolayer MoS2/Pt/Co/Pt can be switched under a small external magnetic field around 300 Oe and maintained up to 4000 Oe, verifying the enhanced PMA by MoS2. Interestingly, the SOT efficiency can also be increased about 9.65% by monolayer MoS2, which can be attributed to the spin absorption at the MoS2/Pt interface. Comparing the SOT behaviors of devices with different MoS2 thickness, the monolayer MoS2 is proved to be an ideal candidate for high-performance SOT devices.

Atmospheric Degradation and Performance Recovery of Two‐Dimensional MoS2 Field Effect Transistor†

Comprehensive SummaryTwo‐dimensional (2D) transition metal dichalcogenides (TMDCs) showed great potentials in 2D nanoelectronic devices due to their abundant and unique properties. The performance stability of the 2D TMDCs devices turns into one of the keys for their practical applications but has been rarely explored. Here, we investigated stability of MoS2 devices in ambient condition and contributed the device performance degradation to the surface oxidation of the contact metals with low work function, which increased the contact barrier and hindered the electron injection. We developed a new approach to recover the performance of the aged devices through the selective doping of contacts with organolithium, which prolonged the lifetime of MoS2 devices. Our work not only provides important insights into the stability of 2D TMDCs devices, but also opens up a new avenue for optimizing the performance of 2D MoS2 devices.

The Effect of Sonication Parameters on the Thickness of the Produced MoS2 Nano-Flakes

Liquid Phase Exfoliation (LPE) is a common route to produce two-dimensional MoS2 nanosheets. In this research, MoS2 powder is exfoliated by an ultrasonic probe (sonicator) in a water-ethanol solution. It is reported that MoS2 as a prototype 2D Transition Metal Dichalcogenide, has a band gap that increases with a decreasing number of layers. There are some factors that affect the average band gap energy value and the thickness of the exfoliated flakes. We varied different parameters of the ultrasonic probe like power, pulse percentage and time duration of sonication to investigate the effects on the number of MoS2 layers. Our findings from the UV-Visible spectra, SEM, FESEM and TEM images indicate that the minimum thickness for these samples was acquired at 50% of the input power of the sonicator we used (65 W) and the optimum pulse percentage is 50%. The current study also found that the average amount of band gap increased with an increase in sonication time, and then remained unchanged after 60 minutes.

Natural overlaying behaviors push the limit of planar cyclic deformation performance in few‐layer MoS2 nanosheets

AbstractAs a typical two‐dimensional (2D) transition metal dichalcogenides (TMDCs) material with nonzero band gap, MoS2 has a wide range of potential applications as building blocks in the field of nanoelectronics. The stability and reliability of the corresponding nanoelectronic devices depend critically on the mechanical performance and cyclic reliability of 2D MoS2. Although an in situ technique has been used to analyze the mechanical properties of 2D materials, the cyclic mechanical behavior, that is, fatigue, remains a major challenge in the practical application of the devices. This study was aimed at analyzing the planar cyclic performance and deformation behavior of three‐layer MoS2 nanosheets (NSs) using an in situ transmission electron microscopy (TEM) variable‐amplitude uniaxial low‐frequency and cyclic loading–unloading tensile acceleration test. We also elucidated the strengthening effect of the natural overlaying affix fragments (other external NSs) or wrinkle folds (internal folds from the NS itself) on cycling performances and service life of MoS2 NSs by delaying the whole process of fatigue crack initiation, propagation, and fracture. The results have been confirmed by molecular dynamics (MDs) simulations. The overlaying enhancement effect effectively ensures the long‐term reliability and stability of nanoelectronic devices made of few‐layer 2D materials.image

Epitaxial growth of bilayer MoS2/MoSe2 heterostructure with high-responsivity enhanced by photogating effect

Van der Waals (vdW) semiconductors heterostructure based on transition metal dichalcogenides (TMDs) is a promising candidate for two-dimensional (2D) ultra-thin optoelectronic devices due to the strongly interlayer coupling and efficient carrier separation. However, the directly epitaxial growth of 2D TMDs vdW heterostructures with a clean interface and high photoresponsivity still remains a challenging. In this work, the epitaxial growth of bilayer MoS2/MoSe2 vdW heterostructure with atomic-layer clean interface is prepared via an iodine-assisted growth strategy. The electrical properties and optoelectronic characterization proves that the heterostructure device presents an enhanced photoresponsivity of 680 A/W, indicting the 21.25 times higher compared with pure MoS2, which benefited from the photogating effect formed by the localized electrons in this heterostructure. This study offers a well reference for the controlled growth of the TMDs vdW heterostructure, and the achieved heterostructure will prove promising applications in future integrated optoelectronic devices and systems.

Diatomite-Templated Synthesis of Single-Atom Cobalt-Doped MoS2 /Carbon Composites to Boost Sodium Storage.

2D transition metal dichalcogenides (TMDCs) and single-atom catalysts (SACs) are promising electrodes for energy conversion/storage because of the layered structure and maximum atom utilization efficiency. However, the integration of such two type materials and the relevant sodium storage applications remain daunting challenges. Here, an ingenious diatomite-templated synthetic strategy is designed to fabricate single-atom cobalt-doped MoS2 /carbon (SA Co-MoS2 /C) composites toward the high-performance sodium storage. Benefiting from the unique hierarchical structure, high electron/sodium-ion conductivity, and abundant active sites, the obtained SA Co-MoS2 /C reveals remarkable specific capacity (≈604.0 mAh g-1 at 0.1 A g-1 ), high rate performance, and outstanding long cyclic stability. Particularly, the sodium-ion full cell composed of SA Co-MoS2 /C anode and Na3 V2 (PO4 )3 cathode demonstrates unexpected stability with the cycle number exceeded 1200. The internal sodium storage mechanism is clarified with the aid of density functional theory calculations and in situ experimental characterizations. This work not only represents a substantial leap in terms of synthesizing SACs on 2D TMDCs but also provides a crucial step toward the practical sodium-ion battery applications.

Local Manipulation of the Energy Levels of 2D TMDCs on the Microscale Level via Microprinted Self‐Assembled Monolayers

Abstract2D transition metal dichalcogenides (TMDCs) are atomically‐thick semiconductors with great potential for next‐generation optoelectronic applications, such as transistors and sensors. Their large surface‐to‐volume ratio makes them energy‐efficient but also extremely sensitive to the physical‐chemical surroundings. The latter must be carefully considered when predicting the electronic behavior, such as their energy level alignment, which ultimately affects the charge carrier injection and transport in devices. Here, local doping is demonstrated and thus adjusting the opto‐electronic properties of monolayer TMDCs (WSe2 and MoS2) by chemically engineering the surface of the supporting substrate. This is achieved by decorating the substrate by microcontact printing with patterns of two different self‐assembled monolayers (SAMs). The SAMs posses distinct molecular dipoles and dielectric constants, significantly influencing the TMDCs electronic and optical properties. By analyzing (on various substrtates), it is confirmed that these effects arise solely from the interaction between SAMs and TMDCs. Understanding the diverse dielectric environments experienced by TMDCs allows for a correlation between electronic and optical behaviours. The changes primarily involve alteration in the electronic band gap width, which can be calculated using the Schottky‐Mott rule, incorporating the dielectric screening of the TMDCs surroundings. This knowledge enables accurate prediction of the (opto‐)electronic behavior of monolayer TMDCs for advanced device design.

Oriented lateral growth of two-dimensional materials on c-plane sapphire.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) represent the ultimate thickness for scaling down channel materials. They provide a tantalizing solution to push the limit of semiconductor technology nodes in the sub-1 nm range. One key challenge with 2D semiconducting TMD channel materials is to achieve large-scale batch growth on insulating substrates of single crystals with spatial homogeneity and compelling electrical properties. Recent studies have claimed the epitaxy growth of wafer-scale, single-crystal 2D TMDs on a c-plane sapphire substrate with deliberately engineered off-cut angles. It has been postulated that exposed step edges break the energy degeneracy of nucleation and thus drive the seamless stitching of mono-oriented flakes. Here we show that a more dominant factor should be considered: in particular, the interaction of 2D TMD grains with the exposed oxygen-aluminium atomic plane establishes an energy-minimized 2D TMD-sapphire configuration. Reconstructing the surfaces of c-plane sapphire substrates to only a single type of atomic plane (plane symmetry) already guarantees the single-crystal epitaxy of monolayer TMDs without the aid of step edges. Electrical results evidence the structural uniformity of the monolayers. Our findings elucidate a long-standing question that curbs the wafer-scale batch epitaxy of 2D TMD single crystals-an important step towards using 2D materials for future electronics. Experiments extended to perovskite materials also support the argument that the interaction with sapphire atomic surfaces is more dominant than step-edge docking.

Uncovering the Role of Crystal Phase in Determining Nonvolatile Flash Memory Device Performance Fabricated from MoTe2-Based 2D van der Waals Heterostructures.

Although the crystal phase of two-dimensional (2D) transition metal dichalcogenides (TMDs) has been proven to play an essential role in fabricating high-performance electronic devices in the past decade, its effect on the performance of 2D material-based flash memory devices still remains unclear. Here, we report the exploration of the effect of MoTe2 in different phases as the charge-trapping layer on the performance of 2D van der Waals (vdW) heterostructure-based flash memory devices, where a metallic 1T'-MoTe2 or semiconducting 2H-MoTe2 nanoflake is used as the floating gate. By conducting comprehensive measurements on the two kinds of vdW heterostructure-based devices, the memory device based on MoS2/h-BN/1T'-MoTe2 presents much better performance, including a larger memory window, faster switching speed (100 ns), and higher extinction ratio (107), than that of the device based on the MoS2/h-BN/2H-MoTe2 heterostructure. Moreover, the device based on the MoS2/h-BN/1T'-MoTe2 heterostructure also shows a long cycle (>1200 cycles) and retention (>3000 s) stability. Our study clearly demonstrates that the crystal phase of 2D TMDs has a significant impact on the performance of nonvolatile flash memory devices based on 2D vdW heterostructures, which paves the way for the fabrication of future high-performance memory devices based on 2D materials.

Barrier-free semimetallic PtSe2 contact formation in two-dimensional PtSe2/PtSe2 homostructure for high-performance field-effect transistors

The search for low-resistance metal contacts on two-dimensional (2D) layered transition metal dichalcogenide (TMDC) materials for high-performance electronic devices remains challenging owing to the lack of interfacial bonding on the surface and a strong Fermi-level pinning effect. In this study, we demonstrate a high-performance 2D large-area homostructured PtSe2/PtSe2 field-effect transistor (FET) by introducing a Schottky-barrier-free and semimetallic PtSe2 film (top layer) as an ohmic contact to semiconducting 2D PtSe2 films (bottom layer) via the wet-transfer method. We successfully improved the current on/off ratio of homostructured 2D/2D PtSe2/PtSe2 FET by more than approximately twofold increase compared to the PtSe2 FET with Pt contacts owing to the barrier-free homojunction PtSe2 layer. Our finding represents a significant achievement in obtaining high-performance electronic devices with barrier-free contacts on homostructured PtSe2 FETs and paves the way toward a promising strategy for wafer-scale 2D TMDC electronic devices.

Covalently Modified MoS2 Bearing a Hamilton-Type Receptor for Recognizing a Redox-Active Ferrocene-Barbiturate Guest via Multiple H-Bonds.

The covalent modification of the metallic phase of MoS2 with a Hamilton-type ligand is presented, transforming MoS2 to a recognition platform which is able to embrace barbiturate moieties via hydrogen bonding. The successful hydrogen bonding formation is easily monitored by simple electrochemical assessments, if a ferrocene-labeled barbiturate analogue is utilized as a proof of concept. Full spectroscopic, thermal, and electron microscopy imaging characterization is provided for the newly formed recognition system, along with valuable insights concerning the electrochemical sensing. The given methodology expands beyond the sensing applications, confidently entering the territory of supramolecular interactions on the surface of 2D transition metal dichalcogenides. The well-designed host-guest chemistry presented herein, constitutes a guide and an inspiration for hosting customized-structured functional building blocks on MoS2 and its relatives via hydrogen bonding, opening up new opportunities regarding potential applications.

Augmented photocatalytic water splitting performance of CdS nanorods encased with Co-C@WS2 nanostructures

Hydrogen production via photocatalytic water-splitting process is a favourable and eco-friendly technique which addresses the energy-environmental issues. However, the development of affordable, highly efficient as well as stable photocatalysts continues to be a challenging task. Here, we propose a photocatalyst based on CdS nanorods that are coupled with WS2 nanosheets and metal-organic framework (MOF) derived Co-C nanostructures to promote photo-assisted H2 evolution in the aqueous medium. The experimental outcomes demonstrate that the fabricated nanocomposite is highly active to produce H2 at a rate of 214.96 mmol. g−1. h−1, indicating highest values among CdS-based photocatalysts. Additionally, the nanocomposite displayed an excellent long-term stability for 40 h. Overall, the present work introduces a photocatalytic system devoid of noble-metals, illustrating the functionalities of MOF-derived materials along with 2D transition metal dichalcogenides for accelerating the H2 evolution rate of semiconductor photocatalysts.

Controllable Thermochemical Generation of Active Defects in the Horizontal/Vertical MoS2 for Enhanced Hydrogen Evolution

AbstractAs for 2D transition metal dichalcogenides, the creation of proper active defects concentrations is considered as the efficient strategy for improving hydrogen evolution performance. However, the synthesis methods of large‐area MoS2 catalysts with controllable active defects are limited, also for its working mechanism. Herein, thermochemical generation of active defects for MoS2 catalysts has established by annealing sodium hypophosphite, in which the phosphine is spontaneously generated and chemically tailors the MoS2 lattice. The defects formation is confirmed by the investigation of slightly‐changed surface structure and unpaired electrons for the annealed samples. The hydrogen evolution reaction performances of horizontally/vertically grown MoS2 films are improved by controlling reaction conditions, indicating the active defects could form in the basal plane and edges with retained crystal structure. The overpotential of MoS2 samples converted from 10 nm Mo reduces from −520 to −265 mV with largely decreased Tafel slope. The electrochemical microreactor studies reveal the protons adsorption of active sites shows much more significant contribution, than interfacial charge transfer with the enhanced remarkable performance (−100 mV at 10 mA cm−2). This study presents the large‐area synthesized strategy for MoS2 based catalysts with controllable defects concentration and helps establish rational design principles for future MoS2 family electrocatalysts.