Metal Dichalcogenides Research Articles

Vertically aligned two-dimensional SnS2 nanosheets with a strong photon capturing capability for efficient photoelectrochemical water splitting

Two-dimensional (2D) metal dichalcogenides have emerged as attractive materials for application in photoelectrochemical (PEC) water splitting due to their unique structure and strong interaction with light.

Nanoparticle – Based Materials for Various Applications

The paper reviews recent scientific developments at the Institute of Combustion Problems. The hydrophobic sponges were obtained by coating polyurethane and melamine sponges with carbon nanomaterials. They are excellent water-resistant sorbents for oil, petroleum products and other organic liquids of various densities. Another interesting development is concerned to the synthesis of multiwalled carbon nanotubes on a glass-cloth by use of cobalt oxide catalyst nanoparticles obtained by solution combustion and production of the smart-textile on its basis. A model of soldier with heated jacket based on electroconductive smart-textile was made. The textile showed good electroconductive properties and effective Joule heating by externally applied current. Studies on the development of nanostructured carbon materials and their application as high-performance active components for the electrodes of advanced energy storage systems, in particular electric double layer capacitors were carried out. 2D heterostructures based on graphene nd dichalcogenides of transition metals were derived. The epitaxial and single crystals of graphene were synthesized by the CVD-method separately on a copper foil. Two dimension WS2 layers were synthesized using sulfurization of thin WO3 films deposited by thermal evaporating on the FTO substrate. A setting time of concrete mass, which could be used as a construction material for 3D printing technology was determined. It was found that calcium chloride decreased the setting time up to 10 min, which is sufficient for the intended application.

(Invited) Mechanism of Low Temperature ALD of Al2O3 on Graphene Terraces

2-D materials are single or trilayer networks of atoms, such as carbon, hBN, and MoS2. 2D materials are attractive for future electronic devices because they provide ballistic transport of charge carriers as well as carrier confinement. These novel devices require ultrathin and defect-free dielectric layers as gate or channel insulators. However 2D semiconductors such as graphene, boron nitride, and metal dichalcogenides (MDCs) are un-reactive; thus, the dielectric layer selectively nucleates on defect sites or step edges. In the conventional atomic layer deposition (ALD) process on graphene or other 2D semiconductors, such non-uniformly grown oxides can create large leakage currents. In the present report, polar metal phthalocyanine molecules have been employed as an ALD template for high nucleation density deposition of dielectric layers. By combining in-situ scanning tunneling microscopy (STM) and fabrication graphene devices, the insulating property and growth mechanism of the dielectric layer have been elucidated. Metal Ptholocyanines have been used to deposit gate oxides on graphene, MoS2, and WSe2. To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy (MBE) as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with four-fold symmetry as measured by scanning tunneling microscopy (STM). ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistor (FETs) formed using this process have a leakage current of 0.046 pA/µm2 at 1 V gate bias with 3.0 nm EOT, which is lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.

High-Performance Molybdenum Diselenide Electrodes Used in Dye-Sensitized Solar Cells and Supercapacitors

Metal dichalcogenide has a layered structure, which is endowed with versatile and tunable photoelectrochemical properties. In this paper, molybdenum diselenide (MoSe2) is synthesized by the solvothermal method, and the as-synthesized MoSe2 shows microsphere hierarchical architecture composed of 2-D nanosheets. The electrochemical characterization indicates that the MoSe2 electrode has lower resistances and better electrocatalytic activity toward triiodide/iodide redox reaction than the traditional Pt electrode. Using the MoSe 2 as a counter electrode in the dye-sensitized solar cell (DSSC), a high-power-conversion efficiency of 9.80% is achieved, outperforming the DSSC-based traditional Pt counter electrode (8.17%). Moreover, using the MoSe2 as an electrode in the supercapacitor, it displays a high specific capacitance of 272 F⋅g−1.

ChemInform Abstract: Solution‐Processed Two‐Dimensional Metal Dichalcogenide‐Based Nanomaterials for Energy Storage and Conversion

AbstractReview: [167 refs.

Electronic structures and enhanced optical properties of blue phosphorene/transition metal dichalcogenides van der Waals heterostructures

As a fast emerging topic, van der Waals (vdW) heterostructures have been proposed to modify two-dimensional layered materials with desired properties, thus greatly extending the applications of these materials. In this work, the stacking characteristics, electronic structures, band edge alignments, charge density distributions and optical properties of blue phosphorene/transition metal dichalcogenides (BlueP/TMDs) vdW heterostructures were systematically studied based on vdW corrected density functional theory. Interestingly, the valence band maximum and conduction band minimum are located in different parts of BlueP/MoSe2, BlueP/WS2 and BlueP/WSe2 heterostructures. The MoSe2, WS2 or WSe2 layer can be used as the electron donor and the BlueP layer can be used as the electron acceptor. We further found that the optical properties under visible-light irradiation of BlueP/TMDs vdW heterostructures are significantly improved. In particular, the predicted upper limit energy conversion efficiencies of BlueP/MoS2 and BlueP/MoSe2 heterostructures reach as large as 1.16% and 0.98%, respectively, suggesting their potential applications in efficient thin-film solar cells and optoelectronic devices.

The role of collective motion in the ultrafast charge transfer in van der Waals heterostructures.

The success of van der Waals heterostructures made of graphene, metal dichalcogenides and other layered materials, hinges on the understanding of charge transfer across the interface as the foundation for new device concepts and applications. In contrast to conventional heterostructures, where a strong interfacial coupling is essential to charge transfer, recent experimental findings indicate that van der Waals heterostructues can exhibit ultrafast charge transfer despite the weak binding of these heterostructures. Here we find, using time-dependent density functional theory molecular dynamics, that the collective motion of excitons at the interface leads to plasma oscillations associated with optical excitation. By constructing a simple model of the van der Waals heterostructure, we show that there exists an unexpected criticality of the oscillations, yielding rapid charge transfer across the interface. Application to the MoS2/WS2 heterostructure yields good agreement with experiments, indicating near complete charge transfer within a timescale of 100 fs.

Solution-Processed Two-Dimensional Metal Dichalcogenide-Based Nanomaterials for Energy Storage and Conversion.

The development of renewable energy storage and conversion devices is one of the most promising ways to address the current energy crisis, along with the global environmental concern. The exploration of suitable active materials is the key factor for the construction of highly efficient, highly stable, low-cost and environmentally friendly energy storage and conversion devices. The ability to prepare two-dimensional (2D) metal dichalcogenide (MDC) nanosheets and their functional composites in high yield and large scale via various solution-based methods in recent years has inspired great research interests in their utilization for renewable energy storage and conversion applications. Here, we will summarize the recent advances of solution-processed 2D MDCs and their hybrid nanomaterials for energy storage and conversion applications, including rechargeable batteries, supercapacitors, electrocatalytic hydrogen generation and solar cells. Moreover, based on the current progress, we will also give some personal insights on the existing challenges and future research directions in this promising field.

Materials properties of out-of-plane heterostructures of MoS2-WSe2 and WS2-MoSe2

Based on first-principles calculations, the materials properties (structural, electronic, vibrational, and optical properties) of out-of-plane heterostructures formed from the transition metal dichalcogenides, specifically MoS2-WSe2 and WS2-MoSe2, were investigated. The heterostructures of MoS2-WSe2 and WS2-MoSe2 are found to be direct and indirect band gap semiconductors, respectively. However, a direct band gap in the WS2-MoSe2 heterostructure can be achieved by applying compressive strain. Furthermore, the excitonic peaks in both monolayer and bilayer heterostructures are calculated to understand the optical behavior of these systems. The suppression of the optical spectrum with respect to the corresponding monolayers is due to interlayer charge transfer. The stability of the systems under study is confirmed by performing phonon spectrum calculations.

Graphene/GaSe-Nanosheet Hybrid: Towards High Gain and Fast Photoresponse

While high photoconductive gain has been recently achieved in graphene-based hybrid phototransistors using semiconductor two-dimensional transition/post-transition metal dichalcogenides or quantum dots sensitizers, obtaining fast photoresponse simutaneously remains a challenge that must be addressed for practical applications. In this paper we report a graphene/GaSe nanosheets hybrid photodetector, in which GaSe nanosheets provide a favorable geometric link to graphene conductive layer through van Der Waals force. After a vacuum annealing process, a high gain in exceeding 107 has been obtained simitaneously with a dynamic response time of around 10 ms for both light on and off. We attribute the high performance to the elimination of possible deep charge traps, most probably at the graphene/GaSe nanosheets interface. This result demonstrates high photoconductive gain and fast photoresponse can be achieved simultaneously and a clean interface is the key to the high performance of these hybrid devices.

(Invited) Vertical Field Effect Transistor Based on Graphene/Transition Metal Dichalcogenide Van Der Waals Heterostructure

We demonstrated exfoliated-graphene/transition metal dichalcogenide (TMD)/metal vertical field-effect transistor (vFET). The van der Waals junction between graphene and TMD exhibit Schottky barrier. Owing to the low density of state of graphene, the position of graphene’s Fermi level can be modulated by the external electric field, thus enable us electric field modulation of Schottky barrier height at graphene/TMD interface. Based on this mechanism, a vertical transport in the heterostructure exhibits large current modulation of >105, and at the same time the heterostructure reveal operation current density of ~104 A/cm2. These results reveal potential high performance of vFET for electronics applications.

Controlled Layer-by-Layer Etching of MoS₂.

Two-dimensional (2D) metal dichalcogenides like molybdenum disulfide (MoS2) may provide a pathway to high-mobility channel materials that are needed for beyond-complementary metal-oxide-semiconductor (CMOS) devices. Controlling the thickness of these materials at the atomic level will be a key factor in the future development of MoS2 devices. In this study, we propose a layer-by-layer removal of MoS2 using the atomic layer etching (ALET) that is composed of the cyclic processing of chlorine (Cl)-radical adsorption and argon (Ar)(+) ion-beam desorption. MoS2 etching was not observed with only the Cl-radical adsorption or low-energy (<20 eV) Ar(+) ion-beam desorption steps; however, the use of sequential etching that is composed of the Cl-radical adsorption step and a subsequent Ar(+) ion-beam desorption step resulted in the complete etching of one monolayer of MoS2. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) indicated the removal of one monolayer of MoS2 with each ALET cycle; therefore, the number of MoS2 layers could be precisely controlled by using this cyclical etch method. In addition, no noticeable damage or etch residue was observed on the exposed MoS2.

(Invited) Two-Dimensional Layered Materials/Silicon Heterojunctions for Energy and Optoelectronic Applications

Two-dimensional (2D) layered materials such as graphene, metal dichalcogenides, and topological insulators have attracted much attention due to their extraordinary properties. However, their applications in optoelectronic devices are hindered by the relatively low light absorption of monolayer materials as well as difficulties in controllable doping and p-n junction fabrications. In comparison to the formation of homojunctions or heterojunctions, respectively, from one or two different 2D layered materials, the formation of heterojunctions by combining a 2D layered material with a semiconductor offers a feasible and easier way to construct high-performance optoelectronic devices by harnessing the advantages of both materials. Herein, we report a systematic investigation on 2D layered material/silicon heterojunction based photovoltaic and optoelectronic devices. It was found that interface charge recombination and graphene conductivity play important roles in determining solar cell performance. By suppressing surface recombination and optimizing graphene layers and doping level, we demonstrate power conversion efficiencies (PCEs) over 14% for graphene/silicon heterojunctions. A new type of solar cell based on the graphene quantum dots (GQDs)/silicon heterojunctions was also developed. Owing to the extraordinary optical and electrical properties of GQDs, the GQDs/Si heterojunctions show a promising efficiency over 12%. Further, ultra-fast photodetectors were constructed from the MoS2/Si, MoSe2/Si, and Bi2SeTe2/Si heterojunctions. Our work reveals the great potential of two-dimensional layered materials/silicon heterojunctions for high-performance photovoltaic and optoelectronic devices.

Hybrid Materials Based on Magnetic Layered Double Hydroxides: A Molecular Perspective.

Design of functional hybrids lies at the very core of synthetic chemistry as it has enabled the development of an unlimited number of solids displaying unprecedented or even improved properties built upon the association at the molecular level of quite disparate components by chemical design. Multifunctional hybrids are a particularly appealing case among hybrid organic/inorganic materials. Here, chemical knowledge is used to deploy molecular components bearing different functionalities within a single solid so that these properties can coexist or event interact leading to unprecedented phenomena. From a molecular perspective, this can be done either by controlled assembly of organic/inorganic molecular tectons into an extended architecture of hybrid nature or by intercalation of organic moieties within the empty channels or interlamellar space offered by inorganic solids with three-dimensional (MOFs, zeolites, and mesoporous hosts) or layered structures (phosphates, silicates, metal dichalcogenides, or anionic clays). This Account specifically illustrates the use of layered double hydroxides (LDHs) in the preparation of magnetic hybrids, in line with the development of soft inorganic chemistry processes (also called "Chimie Douce"), which has significantly contributed to boost the preparation hybrid materials based on solid-state hosts and subsequent development of applications. Several features sustain the importance of LDHs in this context. Their magnetism can be manipulated at a molecular level by adequate choice of constituting metals and interlayer separation for tuning the nature and extent of magnetic interactions across and between planes. They display unparalleled versatility in accommodating a broad range of anionic species in their interlamellar space that encompasses not only simple anions but chemical systems of increasing dimensionality and functionalities. Their swelling characteristics allow for their exfoliation in organic solvents with high dielectric strength, to produce two-dimensional nanosheets with atomic thickness that can be used as macromolecular building blocks in the assembly of nanocomposites. We describe how these advantageous properties turn LDHs into excellent vehicles for the preparation of multifunctional materials with increasing levels of complexity. For clarity, the reader will first find a succinct description of the most relevant aspects controlling the magnetism of LDHs followed by their use in the preparation of magnetic hybrids from a molecular perspective. This includes the intercalation anionic species of increasing nuclearity like paramagnetic mononuclear complexes, stimulus-responsive molecular guests, one- and two-dimensional coordination polymers, or even preassembled 2D networks. This approach allows us to evolve from "dual-function" materials with coexistence, for example, of magnetism and superconductivity, to smart materials in which the magnetic or structural properties of the LDH layers can be tuned by applying an external stimulus like light or temperature. We will conclude with a brief look into the promising features offered by magnetic nanocomposites based on LDHs and our views on the most promising directions to be pursued in this context.

Constitutive modeling of some 2D crystals: Graphene, hexagonal BN, MoS2, WSe2 and NbSe2

We lay down a nonlinear elastic constitutive framework for the modeling of some 2D crystals of current interest. The 2D crystals we treat are graphene, hexagonal boron nitride and some metal dichalcogenides: molybdenium disulfide (MoS2), tungsten selenium (WSe2), and niobium diselenide (NbSe2). We first find their arithmetic symmetries by using the theory of monoatomic and diatomic 2-nets. Then, by confinement to weak transformation neighborhoods and by applying the Cauchy–Born rule we are able to use the symmetries continuum mechanics utilizes: geometric symmetries. We give the complete and irreducible representation for energies depending on an in-plane measure, the curvature tensor and the shift vector. This is done for the symmetry hierarchies that describe how symmetry changes at the continuum level: C6ν→C2ν→C1 for monoatomic 2-nets, and C6ν→C1ν→C1 for diatomic two nets. We stress that we do not take into account how energy behaves at the transition regime. Having these energies at hand we are able to evaluate stresses and couple stresses for each symmetry regime. These quantities participate to the field equations: the momentum equation, the moment of momentum equation and the equation ruling the shift vector. By making specific assumption for the loading histories that correspond to in-plane biaxial tension/compression as well as in-plane simple shear, we obtain necessary expressions for the shift vector components in order all field equations to be satisfied. When the algebraic equations ruling the shift vector render infinite solutions, we use the Cauchy–Kowalevski theorem-when it applies-for the momentum equation viewed as a quasilinear system in order to single out the unique solution.

The importance of gap states for energy level alignment at hybrid interfaces

Energy level alignment and electronic structure at organic semiconductor interfaces must be controlled to ensure efficient carrier harvesting or injection in next-generation organic optoelectronic technologies. In this context, hybrid organic/inorganic semiconductor interfaces exhibit particularly rich physics. Here, we show that states in the band gap of the inorganic layered van der Waals dichalcogenide SnS2 play an important role in determining energy level alignment at the hybrid interface with copper phthalocyanine (CuPc). By taking advantage of the closely related CuPc film growth on SnS2 and the well-studied interface of CuPc/HOPG, we are able to trace spectroscopic differences to the fundamentally different electronic interactions across the two interfaces. We provide a detailed picture of the role of gap states at the hybrid interface and shed light on the electronic properties of inorganic semiconductors in general and metal dichalcogenides in particular.

Nanostructured catalyst for hydrogen electrochemical reduction based on molybdenum diselenide thin films

Thin-filmed dichalcogenides of transition metals, in particular, MoSe2, are considered as potentially active materials in the hydrogen evolution reaction, which can compete with expensive platinum. It has been established that the laser deposition technique ensures the formation of nanocomposite films containing a high density of Mo nanoparticles in the MoSex shell. Deposition of Mo nanoparticles increase roughness and induce activation of a surface. This manifests itself in formation of “edge” sites in the MoSex shell and on edges of the basis planes in MoSex nanocrystals oriented normally to the film surface. In a 0.5 M H2SO4 solution, on carbon cathodes coated with MoSex films, the hydrogen overvoltage dropped to −0.17 V and the current density doubled.

Two-dimensional materials under electron irradiation

Abstract

High intrinsic mobility and ultrafast carrier dynamics in multilayer metal-dichalcogenideMoS2

The ultimate limitations on carrier mobilities in metal dichalcogenides, and the dynamics associated with carrier relaxation, are unclear. We present measurements of the frequency-dependent conductivity of multilayer dichalcogenide ${\mathrm{MoS}}_{2}$ by optical-pump terahertz-probe spectroscopy. We find mobilities in this material approaching 4200 ${\mathrm{cm}}^{2}$ ${\mathrm{V}}^{\ensuremath{-}1}$ ${\mathrm{s}}^{\ensuremath{-}1}$ at low temperatures. The temperature dependence of scattering indicates that the mobility, an order of magnitude larger than previously reported for ${\mathrm{MoS}}_{2}$, is intrinsically limited by acoustic phonon scattering at THz frequencies. Our measurements of carrier relaxation reveal picosecond cooling times followed by recombination lasting tens of nanoseconds and dominated by Auger scattering into defects. Our results provide a useful context in which to understand and evaluate the performance of ${\mathrm{MoS}}_{2}$-based electronic and optoelectronic devices.

Phase diagram of electron-doped dichalcogenides

Using first principle calculations, we examine the sequence of phases in electron doped dichalcogenides, such as recently realized in field-gated MoS$_2$. Upon increasing the electron doping level, we observe a succession of semiconducting, metallic, superconducting and charge density wave regimes, i.e. in different order compared to the phase diagram of (semi-)metallic dichalcogenides such as TiSe$_2$. Both instabilities trace back to a softening of phonons which couple the electron populated conduction band minima. The superconducting dome, calculated using Eliashberg theory, is found to fit the experimentally observed phase diagram, obtained from resistivity measurements. The charge density wave phase at higher electron doping concentrations as predicted from instabilities in the phonon modes is further corroborated by detecting the accompanying lattice deformation in density functional based supercell relaxations. Upon charge density wave formation, doped MoS$_2$ remains metallic but undergoes a Lifschitz transition, where the number of Fermi pockets is reduced.