Structure Of Transition Metal Dichalcogenides Research Articles

Epitaxial Growth of Single-Phase 1T'-WSe2 Monolayer with Assistance of Enhanced Interface Interaction.

The WSe2 monolayer in 1T' phase is reported to be a large-gap quantum spin Hall insulator, but is thermodynamically metastable and so far the fabricated samples have always been in the mixed phase of 1T' and 2H, which has become a bottleneck for further exploration and potential applications of the nontrivial topological properties. Based on first-principle calculations in this work, it is found that the 1T' phase could be more stable than 2H phase with enhanced interface interactions. Inspired by this discovery, SrTiO3 (100) is chosen as substrate and WSe2 monolayer is successfully grown in a 100% single 1T' phase using the molecular beam epitaxial method. Combining in situ scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements, it is found that the in-plane compressive strain in the interface drives the 1T'-WSe2 into a semimetallic phase. Besides providing a new material platform for topological states, the results show that the interface interaction is a new approach to control both the structure phase stability and the topological band structures of transition metal dichalcogenides.

Microwave-Induced Structural Engineering and Pt Trapping in 6R-TaS2 for the Hydrogen Evolution Reaction.

The nanoengineering of the structure of transition metal dichalcogenides (TMDs) is widely pursued to develop viable catalysts for the hydrogen evolution reaction (HER) alternative to the precious metallic ones. Metallic group-5 TMDs have been demonstrated to be effective catalysts for the HER in acidic media, making affordable real proton exchange membrane water electrolysers. Their key-plus relies on the fact that both their basal planes and edges are catalytically active for the HER. In this work, the 6R phase of TaS2 is "rediscovered" and engineered. A liquid-phase microwave treatment is used to modify the structural properties of the 6R-TaS2 nanoflakes produced by liquid-phase exfoliation. The fragmentation of the nanoflakes and their evolution from monocrystalline to partly polycrystalline structures improve the HER-activity, lowering the overpotential at cathodic current of 10 mA cm-2 from 0.377 to 0.119 V. Furthermore, 6R-TaS2 nanoflakes act as ideal support to firmly trap Pt species, which achieve a mass activity (MA) up 10 000 AgPt -1 at overpotential of 50 mV (20 000 AgPt -1 at overpotentials of 72 mV), representing a 20-fold increase of the MA of Pt measured for the Pt/C reference, and approaching the state-of-the-art of the Pt mass activity.

Structural, electronic and optical properties of two-dimensional Janus transition metal oxides MXO (M=Ti, Hf and Zr; X=S and Se) for photovoltaic and opto-electronic applications

Two-dimensional Janus transition metal dichalcogenides structures have drawn increasing importance due to their remarkable properties for versatile applications in optoelectronic, photo-catalytic, thermo-electricity, piezoelectricity and spintronic. In this Paper, by employing the Ab-initio computations based on the density functional theory, the structural, electronic and optical properties of Janus MXO (M = Ti, Hf and Zr; X = S and Se) mono-layers are investigated utilizing full potential linearized augmented plane waves (FP-LAPW) method. The lattice parameters of the six Janus were computed, which are close to the previous theoretical results. The density of states and the electronic band structures were investigated for the first time using (GGA-PBE) approximation for the potential of the exchange and correlation. The optical parameters like complex dielectric function, refractive index, reflectivity, extinction and absorption coefficients of all Janus were performed. Our results reveal strong absorption coefficient and low reflectivity in the visible and ultraviolet regions, which make them candidates for opto-electronic and photovoltaic applications.

Structural Engineering and Coupling of Two-Dimensional Transition Metal Compounds for Micro-Supercapacitor Electrodes.

The development of portable, wearable, and miniaturized integrated electronics has significantly promoted the immense desire for planar micro-supercapacitors (MSCs) among the extremely competitive energy storage devices. However, their energy density is still insufficient owing to the low electrochemical performance of conventional electrode materials. Compared with their bulk counterparts, the large specific surface area and fast ion transport with efficient intercalation of two-dimensional (2D) transition metal compounds have spurred the research platforms for their exploitation in the creation of high-performance MSCs. This Outlook presents a systematic summary of cutting-edge research on atomically thin, layered structures of transition metal dichalcogenides, MXenes, and transition metal oxides/hydroxides. Special emphasis is given to the rapid and durable storage of ions, benefiting from the low ion diffusion barriers of host interlayer spaces. Moreover, various strategies have been described to circumvent the structural damage due to the volume change and simultaneously evincing remarkable electronic properties.

Topological structures of transition metal dichalcogenides: A review on fabrication, effects, applications, and potential

AbstractTransition metal dichalcogenides (TMDs) have received tremendous attention owing to their potential for optoelectronic applications. Topological structures, such as wrinkles, folds, and scrolls, have been generated on TMDs, thereby exhibiting novel physical properties with improved optoelectronic performance, making them attractive prospects for both basic understanding and advanced applications in optoelectronics. In this review, the methods for fabricating wrinkles, folds, and scrolls on TMDs are outlined, including modification of the fabrication and transfer processes, and manipulation via auxiliary polymers. The effects on their physical and electronic properties are also discussed, with particular paid to the energy band structure, single‐photon sources, second harmonic generation (SHG), and interlayer coupling. In comparison to pristine TMDs, these topologies exhibit great advantages in optoelectronic devices, such as field‐effect transistors and photodetectors. Finally, existing challenges and opportunities of wrinkled, folded, and scrolled TMDs are outlined and an outlook is presented.image

Colloidal MoS2 van der Waals Template for Growing Highly Uniform Nanomaterials.

The van der Waals (vdW) structures of transition metal dichalcogenides (TMDCs) have been studied extensively owing to the excellent electronic, optoelectronic, and catalytic performance with the atomic-scale sharpness of the interfaces. In addition, the presence of substitution and redox reactions in the vdW gaps also provides huge potential to be employed as a nanoscale reactor. Herein, the vdW gap of colloidal MoS2 is reported as a natural template for the formation of CoMo bimetallic oxide nanoparticles (BMONPs) with highly uniform size (∼4 nm), which resulted in further synthesis of the thin multiwalled carbon nanotubes with extremely narrow diameter distribution. This study not only pioneers a new application of TMDCs but also provides an efficient means to control the size of nanomaterials, which directly affect material properties.

(Keynote) Exciton Manipulation in Heterostructures of 2D Materials

The discovery of graphene marked the start of research in 2D electronic materials which was expanded in new directions with MoS2 and other layered semiconducting materials. They have a wide range of interesting fundamental properties and potential applications. New opportunities are enabled by the band structure of transition metal dichalcogenides (TMDCs) in which we could harness the valley degree of freedom for valleytronics and next-generation photonics. Long-lived interlayer excitons in van der Waals heterostructures based on TMDCs have recently emerged as a promising platform for this, allowing control over exciton diffusion length, energy and polarisation. I will show here how by using MoS2/WSe2 van der Waals heterostructures, we can realize excitonic transistors with switching action, confinement and control over diffusion length at room temperature in a reconfigurable potential landscape. Heterostructures with a long-range moiré potential such as in MoSe2/WSe2, on the other hand, offer the way to control polarization, emission and wavelength emitted by different optically active regions in the moiré.

Controlled vanadium doping of mos2 thin films through co-sputtering and thermal sulfurization

Recently, transition metal dichalcogenides (TMDs) have gained great attention owing to their remarkable properties. The electronic structure of TMDs can be modified by substitutional doping, which could give rise to novel and exciting properties. In this study, a strategy is presented for controlled vanadium (V) doping of MoS2, in which V doped MoS2 films with good uniformity are prepared by thermal sulfurization of V-Mo alloy films deposited using co-sputtering. The V incorporation in MoS2 induces p type doping, which enhances the electrical conductivity of MoS2 by a factor of 35-40. Such doping strategy and consequent conductivity improvement may be useful in many applications such as catalysis, nanoelectronics and optoelectronics.

Recent Developments on Emerging Properties, Growth Approaches, and Advanced Applications of Metallic 2D Layered Vanadium Dichalcogenides

AbstractThe electronic structures of transition metal dichalcogenides (TMDs) has a strong dependency on the d‐band electrons of the central metal (M) atom. In particular, trigonal prismatic coordinated (2H) and octahedral coordinated (1T) phases with distinct crystal structure‐property relationships. Extraordinarily diverse electrical properties ranging from semiconducting, semimetallic to intrinsic metallic basically underlie from different Fermi level locations. Amid all TMDs, most specifically metallic VX2 possess plenty of interesting physical properties among them superconductivity, electrical conductivity, charge density wave, optical and magnetism etc. have offered intriguing applications in the fields of condensed matter physics, materials and device physics over the last few decades. Further modification of these materials by defect engineering, Li intercalation, electron beam/light irradiation, alloying and dimensional tuning can lead several tunable device applications. Due to their superior mechanical flexibility, controllable electrical properties, planar fabrication and high surface to volume ratio etc., 2D metallic TMDs materials emerge as active material for sensing, energy storage, conversion and field emission applications. This review article aims to provide the insights on the recent developments of different emerging properties, growth approaches and applications of 2D metallic VX2 (X = S, Se and Te) and its heterojunctions.

Controlled edge dependent stacking of WS2-WS2 Homo- and WS2-WSe2 Hetero-structures: A Computational Study

Transition Metal Dichalcogenides (TMDs) are one of the most studied two-dimensional materials in the last 5–10 years due to their extremely interesting layer dependent properties. Despite the presence of vast research work on TMDs, the complex relation between the electro-chemical and physical properties make them the subject of further research. Our main objective is to provide a better insight into the electronic structure of TMDs. This will help us better understand the stability of the bilayer post growth homo/hetero products based on the various edge-termination, and different stacking of the two layers. In this regard, two Tungsten (W) based non-periodic chalcogenide flakes (sulfides and selenides) were considered. An in-depth analysis of their different edge termination and stacking arrangement was performed via Density Functional Theory method using VASP software. Our finding indicates the preference of chalcogenide (c-) terminated structures over the metal (m-) terminated structures for both homo and heterobilayers, and thus strongly suggests the nonexistence of the m-terminated TMDs bilayer products.

Research progress of two-dimensional transition metal dichalcogenide phase transition methods

Following traditional semiconductors such as silicon and GaAs, in recent years the two-dimensional materials have attracted attention in the field of optoelectronic devices, thermoelectric devices and energy storage and conversion due to their many peculiar properties. However, the normal two-dimensional materials such as graphene, cannot be well used in the field of optoelectronics due to the lack of a band gap, and the black phosphorus is also greatly limited in practical applications due to its instability in the air. The two-dimensional transition metal dichalcogenides have attracted more attention due to the different atomic structures, adjustable energy band and excellent photoelectric properties. There are different crystal phases in transition metal dichalcogenides, some of which are stable in the ground state, and others are instable. Different phases exhibit different characteristics, some of which have semiconductor properties and others have like metal in property. These stable and metastable phases of transition metal dichalcogenides can be transformed into each other under some conditions. In order to obtain these metastable phases, thereby modulating their photoelectric performance and improving the mobility of the devices, it is essential to obtain a phase transition method that enables the crystal phase transition of the transition metal dichalcogenides. In this article, first of all, we summarize the different crystal structures of transition metal dichalcogenides and their electrical, mechanical, and optical properties. Next, the eight phase transition methods of transition metal dichalcogenides are listed, these being chemical vapor deposition, doping, ion intercalation, strain, high temperature thermal treatment, laser inducing, plasma treatment, and electric field inducing. After that, the research progress of these phase transition methods and their advantages and disadvantages are introduced. Finally, we sum up all the phase transition methods mentioned in this article and then list some of the problems that have not been solved so far. This review elaborates all of the presently existing different phase transition methods of transition metal dichalcogenides in detail, which provides a good reference for studying the phase transition of transition metal dichalcogenides in the future, the electrical performance regulated by different phases, and the applications of memory devices and electrode manufacturing.

Topology of transition metal dichalcogenides: the case of the core-shell architecture.

Non-planar architectures of the traditionally flat 2D materials are emerging as an intriguing paradigm to realize nascent properties within the family of transition metal dichalcogenides (TMDs). These non-planar forms encompass a diversity of curvatures, morphologies, and overall 3D architectures that exhibit unusual characteristics across the hierarchy of length-scales. Topology offers an integrated and unified approach to describe, harness, and eventually tailor non-planar architectures through both local and higher order geometry. Topological design of layered materials intrinsically invokes elements highly relevant to property manipulation in TMDs, such as the origin of strain and its accommodation by defects and interfaces, which have broad implications for improved material design. In this review, we discuss the importance and impact of geometry on the structure and properties of TMDs. We present a generalized geometric framework to classify and relate the diversity of possible non-planar TMD forms. We then examine the nature of curvature in the emerging core-shell architecture, which has attracted high interest due to its versatility and design potential. We consider the local structure of curved TMDs, including defect formation, strain, and crystal growth dynamics, and factors affecting the morphology of core-shell structures, such as synthesis conditions and substrate morphology. We conclude by discussing unique aspects of TMD architectures that can be leveraged to engineer targeted, exotic properties and detail how advanced characterization tools enable detection of these features. Varying the topology of nanomaterials has long served as a potent methodology to engineer unusual and exotic properties, and the time is ripe to apply topological design principles to TMDs to drive future nanotechnology innovation.

Robust Room Temperature Valley Hall Effect of Interlayer Excitons.

The Berry curvature in the band structure of transition metal dichalcogenides (TMDs) introduces a valley-dependent effective magnetic field, which induces the valley Hall effect (VHE). Similar to the ordinary Hall effect, the VHE spatially separates carriers or excitons, depending on their valley index, and accumulates them at opposite sample edges. The VHE can play a key role in valleytronic devices, but previous observations of the VHE have been limited to cryogenic temperatures. Here, we report a demonstration of the VHE of interlayer excitons in a MoS2/WSe2 heterostructure at room temperature. We monitored the in-plane propagation of interlayer excitons through photoluminescence mapping and observed their spatial separation into two opposite transverse directions that depended on the valley index of the excitons. Our theoretical simulations reproduced the salient features of these observations. Our demonstration of the robust interlayer exciton VHE at room temperature, enabled by their intrinsically long lifetimes, will open up realistic possibilities for the development of opto-valleytronic devices based on TMD heterostructures.

Excellent photoresponse performances of graphene/metallic WSe2 nanosheet heterostructure films

The metallic (1T) structure of transition metal dichalcogenides is of many peculiar chemical and physical properties such as higher carrier mobility and narrower bandgap compared to 2H structure and so maybe beneficial in their photodetection application. Currently, efficient assembly of 1T-WSe2 in the one-step process is still an enormous challenge. Here we report a modified facile hydrothermal approach assembling 1T-WSe2 nanosheets and 1T-WSe2/reduced graphene oxide (RGO) hybrids. The photodetector based on the assembled 1T-WSe2 nanosheets showed very high photocurrent and extremely intense and fast light responses. The RGO incorporation introduced a remarkable charge transfer at the heterojunction interface and further dramatically heightened the photocurrent and light responsivity.

Lithographically defined synthesis of transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) promise to revolutionize optoelectronic applications. While monolayer exfoliation and vapor phase growth produce extremely high quality 2D materials, direct fabrication at wafer scale remains a significant challenge. Here, we present a method that we call ‘lateral conversion’, which enables the synthesis of patterned TMD structures, with control over the thickness down to a few layers, at lithographically predefined locations. In this method, chemical conversion of a metal-oxide film to TMD layers proceeds by diffusion of precursor propagating laterally between silica layers, resulting in structures where delicate chalcogenide films are protected from contamination or oxidation. Lithographically patterned WS2 structures were synthesized by lateral conversion and analyzed in detail by hyperspectral Raman imaging, scanning electron microscopy and transmission electron microscopy. The rate of conversion was investigated as a function of time, temperature, and thickness of the converted film. In addition, the process was extended to grow patterned MoS2, WSe2, MoSe2 structures, and to demonstrate unique WS2/SiO2 multilayer structures. We believe this method will be applicable to a variety of additional chalcogenide materials, and enable their incorporation into novel architectures and devices.

High Sensitivity Hybrid PbS CQD-TMDC Photodetectors up to 2 μm

Recent approaches to develop infrared photodetectors characterized by high sensitivities, broadband spectral coverage, easy integration with silicon electronics and low cost have been based on hybrid structures of transition metal dichalcogenides (TMDCs) and PbS colloidal quantum dots (CQDs). However, to date, such photodetectors have been reported with high sensitivity up to 1.5 $\mu$m. Here we extend the spectral coverage of this technology towards 2 $\mu$m demonstrating for the first time compelling performance with responsivities 1400 A/W at 1.8 $\mu$m with 1V bias and detectivities as high as $10^{12}$ Jones at room temperature. To do this we studied two TMDC materials as a carrier transport layer, tungsten disulfide (WS$_2$) and molybdenum disulfide (MoS$_2$) and demonstrate that WS$_2$ based hybrid photodetectors outperform those of MoS$_2$ due to a more adequate band alignment that favors carrier transfer from the CQDs.

Strain regulation of two-dimensional transition metal dichalcogenides

As layered materials beyond graphene discovered after 2004, two-dimensional (2D) transition metal dichalcogenides (TMDs) are vital to fundamental research and practical applications, owing to their unique crystal structures and excellent properties, as well as diversity of the electronic band structures. 2D TMDs have played an important role in electronics, optoelectronics, energy storage and catalysis. To meet with the increasing requirements of programmable and function-integrated devices, property modulation could be concerned as one of the most essential strategies for 2D TMDs. In comparison with conventional external electrical field induction, strain regulation could be much more efficient. In detail, external induction through the electrical field leads to the electron delocalization along the field direction and then induces the transformation of band structures, but electrical field exhibits small modulation for monolayer TMDs. On the contrary, the strain regulation shows excellent tuning efficiency for continuous and reversible modulation. As a result, the strain regulation has become a commonly-used strategy for property tuning of 2D TMDs. On the basis of the lattice transformation, the strain regulation of 2D TMDs can lead to different overlapping of d and p orbits of metal and chalcogen atoms, and then affect the electronic structures of 2D TMDs. Therefore, it can be applied to electronics, optoelectronics, magnetic devices and piezoelectronics. The strategies of introducing strains to 2D TMDs are classified into lattice induction, local deformation, macroscopic regulation and so on. Lattice induction is attributable to the structural distortions and mismatches, including atomic defect induction and lattice mismatch induction. The former one demonstrates that the microenvironment affected by atomic vacancies and doping atoms can introduce strain to 2D TMDs. The latter one means that the lattice mismatches between two materials (between two different TMDs in a heterostructure or between a TMD and the substrate) can result in lattice distortion and then induce the strain. However, strain introduced through lattice induction is fixed and difficult to achieve the reversible modulation. Local deformation refers to the morphological transformation at the scale of several micrometers, which can be induced by the bubbles and wrinkles of 2D TMDs, external forces of tips, as well as patterned substrates. Generally, large but nonuniform strain can be introduced to 2D TMDs through the local deformation, which results in the funnel effects and then induces large property variation. Furthermore, macroscopic regulation introduces the strains to the lattices, including the deformation of flexible substrates (bend, tension and compression), external pressure (applied by the diamond anvil cell) and thermal expansion coefficient mismatch. Macroscopic regulation would be compatible with industrial manufacture to achieve strain regulation on an extremely large scale in the future. There are some other ways to achieve strain regulation such as the design of special stack structures and the induction of the external electrical field. After summarizing the methods of introducing strain to 2D TMDs, we presented the applications based on strain regulation, such as field effect transistors, flexible photodetectors and strain sensors. Finally, we pointed out the further development and challenges of strain regulation of 2D TMDs.

Electrochemistry of Layered Semiconducting AIIIBVI Chalcogenides: Indium Monochalcogenides (InS, InSe, InTe)

AbstractLayered AIIIBVI chalcogenides represent an interesting class semiconductors, where most of adopting 2D structures. Unlike the typical sandwiched structure of transition metal dichalcogenides (TMDs), layered AIIIBVI chalcogenides like InSe and GaSe are composed of X−M−M−X motif where M is gallium/indium and X is sulfur/selenium/tellurium. The exception is InS, which adopt an orthorhombic 3D structure. Herein, we studied and compared the inherent electrochemical properties as well as the electrocatalytic performances towards hydrogen evolution (HER), oxygen evolution (OER) and oxygen reduction reaction (ORR) of indium monochalcogenides (InS, InSe and InTe). Inherent electrochemistry studies in phosphate buffered saline electrolyte showed that InS did not exhibit any inherent electrochemical signals when compared to bare glassy carbon electrode. However, InSe showed a reduction peak at −1.6 V while InTe had an oxidation peak at 0.2 V. The heterogeneous electron transfer (HET) rates of indium monochalcogenides were measured with [Fe(CN)6]3−/4− redox probe using cyclic voltammetry (vs. Ag/AgCl) at the scan rate of 100 mV s−1. It was found that InTe exhibited the best electrochemical performance with the fastest HET rate with highest obtained (3.7×10−3 cm s−1). InS showed the best electrocatalytic performance for HER with the lowest overpotential value of 0.92 V at current density of −10 mA cm−2. However, the performances of indium monochalcogenides were almost comparable to that of bare glassy carbon electrode and do not exhibit any improvements in electrocatalytic capabilities. This study provides insights into the electrochemical properties and electrocatalytic performances of layered AIIIBVI indium monochalcogenides which would influence potential applications.

Probing the active sites of newly predicted stable Janus scandium dichalcogenides for photocatalytic water-splitting

The Janus structures of transition metal dichalcogenides with intrinsic dipoles have recently drawn attention as efficient candidates in the class of non-precious metal photocatalysts for water splitting.

Molybdenum Disulfide Quantum Dots Attenuates Endothelial-to-Mesenchymal Transition by Activating TFEB-Mediated Lysosomal Biogenesis.

A defective lysosome-autophagy degradation pathway contributes to a variety of endothelial-to-mesenchymal transition (EndMT)-related cardiovascular diseases. Molybdenum disulfide quantum dots (MoS2 QDs) are nanoscale sizes in the planar dimensions and atomic structures of transition metal dichalcogenides (TMDs) materials with excellent physicochemical and biological properties, making them ideal for various biomedical applications. In this study, water-soluble MoS2 QDs with an average diameter of about 3.4 nm were synthesized by using a sulfuric acid-assisted ultrasonic method. The as-prepared MoS2 QDs exhibited low cytotoxicity of less than 100 μg/mL in both human umbilical vein endothelial cells and human coronary artery endothelial cells and showed novel biological properties to prevent EndMT and promote angiogenesis in vitro. We found that MoS2 QDs treatment-induced transcription factor (TFEB) mediated lysosomal biogenesis, which could cause autophagy activation. Importantly, using in vitro transforming growth factor (TGF)-β-induced EndMT model, we demonstrated that the cardiovascular protective effect of MoS2 QDs against EndMT acted through triggering TFEB nucleus translocation and restoring an impairment of autophagic flux, whereas genetic suppression of TFEB impaired the protective action of MoS2 QDs against EndMT. Taken together, these results gain novel insights into the mechanisms by which MoS2 QDs regulate EndMT and facilitate the development of MoS2-based nanoagents for the treatment of EndMT-related cardiovascular diseases.