Structure Of Transition Metal Dichalcogenides Research Articles

Plasma Assisted Low Temperature Synthesis of WSe2

The two-dimensional layered structure of transition metal dichalcogenides (TMDs) materials has attracted much attention due to their particular structure. The electrical properties show large differences depending on the choice of the metal employed. These TMD layered materials may exhibit metallic, superconducting, or semiconducting properties. Particularly, tungsten disulphide (WSe2) shows a semiconducting electronic structure that varies with their thickness. Previous studies using exfoliated WSe2, have demonstrated a particular ambipolar behavior with large on/off ration and large mobilities. Despite the exceptional electrical and optical properties of WSe2 reliable synthesis methods on large area are still in early stages. The use of large processing temperatures (over 800°C) restricts the use of substrates used for synthesis, which are able to bear high temperature. Furthermore, the films have to be transferred to an additional substrate due to the damage caused to the original substrate used for synthesis. In this work, we present a method to synthesize WSe2 from the reduction and later selenization of WO3. The oxygen atoms are replaced selenium in a plasma assisted chemical vapor deposition (CVD) system. By controlling the plasma power the process temperature can be further lowered from 800°C to 250°C. The thickness can be defined by precisely controlling the thickness of predeposited WO3. The plasma assisted synthesis of WSe2 develops a new approach that can be extended to other TMD layered materials; and due to the low temperature does not requires further transfer and is compatible with current silicon semiconductor processes (300°C)

Electronic structure and optical signatures of semiconducting transition metal dichalcogenide nanosheets.

CONSPECTUS: Two-dimensional (2D) crystals derived from transition metal dichalcogenides (TMDs) are intriguing materials that offer a unique platform to study fundamental physical phenomena as well as to explore development of novel devices. Semiconducting group 6 TMDs such as MoS2 and WSe2 are known for their large optical absorption coefficient and their potential for high efficiency photovoltaics and photodetectors. Monolayer sheets of these compounds are flexible, stretchable, and soft semiconductors with a direct band gap in contrast to their well-known bulk crystals that are rigid and hard indirect gap semiconductors. Recent intense research has been motivated by the distinct electrical, optical, and mechanical properties of these TMD crystals in the ultimate thickness regime. As a semiconductor with a band gap in the visible to near-IR frequencies, these 2D MX2 materials (M = Mo, W; X = S, Se) exhibit distinct excitonic absorption and emission features. In this Account, we discuss how optical spectroscopy of these materials allows investigation of their electronic properties and the relaxation dynamics of excitons. We first discuss the basic electronic structure of 2D TMDs highlighting the key features of the dispersion relation. With the help of theoretical calculations, we further discuss how photoluminescence energy of direct and indirect excitons provide a guide to understanding the evolution of the electronic structure as a function of the number of layers. We also highlight the behavior of the two competing conduction valleys and their role in the optical processes. Intercalation of group 6 TMDs by alkali metals results in the structural phase transformation with corresponding semiconductor-to-metal transition. Monolayer TMDs obtained by intercalation-assisted exfoliation retains the metastable metallic phase. Mild annealing, however, destabilizes the metastable phase and gradually restores the original semiconducting phase. Interestingly, the semiconducting 2H phase, metallic 1T phase, and a charge-density-wave-like 1T' phase can coexist within a single crystalline monolayer sheet. We further discuss the electronic properties of the restacked films of chemically exfoliated MoS2. Finally, we focus on the strong optical absorption and related exciton relaxation in monolayer and bilayer MX2. Monolayer MX2 absorbs as much as 30% of incident photons in the blue region of the visible light despite being atomically thin. This giant absorption is attributed to nesting of the conduction and valence bands, which leads to diversion of optical conductivity. We describe how the relaxation pathway of excitons depends strongly on the excitation energy. Excitation at the band nesting region is of unique significance because it leads to relaxation of electrons and holes with opposite momentum and spontaneous formation of indirect excitons.

Transition metal dichalcogenides and beyond: synthesis, properties, and applications of single- and few-layer nanosheets.

CONSPECTUS: In the wake of the discovery of the remarkable electronic and physical properties of graphene, a vibrant research area on two-dimensional (2D) layered materials has emerged during the past decade. Transition metal dichalcogenides (TMDs) represent an alternative group of 2D layered materials that differ from the semimetallic character of graphene. They exhibit diverse properties that depend on their composition and can be semiconductors (e.g., MoS2, WS2), semimetals (e.g., WTe2, TiSe2), true metals (e.g., NbS2, VSe2), and superconductors (e.g., NbSe2, TaS2). The properties of TMDs can also be tailored according to the crystalline structure and the number and stacking sequence of layers in their crystals and thin films. For example, 2H-MoS2 is semiconducting, whereas 1T-MoS2 is metallic. Bulk 2H-MoS2 possesses an indirect band gap, but when 2H-MoS2 is exfoliated into monolayers, it exhibits direct electronic and optical band gaps, which leads to enhanced photoluminescence. Therefore, it is important to learn to control the growth of 2D TMD structures in order to exploit their properties in energy conversion and storage, catalysis, sensing, memory devices, and other applications. In this Account, we first introduce the history and structural basics of TMDs. We then briefly introduce the Raman fingerprints of TMDs of different layer numbers. Then, we summarize our progress on the controlled synthesis of 2D layered materials using wet chemical approaches, chemical exfoliation, and chemical vapor deposition (CVD). It is now possible to control the number of layers when synthesizing these materials, and novel van der Waals heterostructures (e.g., MoS2/graphene, WSe2/graphene, hBN/graphene) have recently been successfully assembled. Finally, the unique optical, electrical, photovoltaic, and catalytic properties of few-layered TMDs are summarized and discussed. In particular, their enhanced photoluminescence (PL), photosensing, photovoltaic conversion, and hydrogen evolution reaction (HER) catalysis are discussed in detail. Finally, challenges along each direction are described. For instance, how to grow perfect single crystalline monolayer TMDs without the presence of grain boundaries and dislocations is still an open question. Moreover, the morphology and crystal structure control of few-layered TMDs still requires further research. For wet chemical approaches and chemical exfoliation methods, it is still a significant challenge to control the lateral growth of TMDs without expansion in the c-axis direction. In fact, there is plenty of room in the 2D world beyond graphene. We envisage that with increasing progress in the controlled synthesis of these systems the unusual properties of mono- and few-layered TMDs and TMD heterostructures will be unveiled.

Interplay between structure and electronic properties of layered transition-metal dichalcogenides: Comparing the loss function of1Tand2Hpolymorphs

Transition-metal dichalcogenides (TMD) share the same global layered structure, but distinct polymorphs are characterized by different local coordinations of the transition-metal atoms. Here we compared the $1T$ and $2H$ families of metallic TMD, both in the bulk and in the two-dimensional forms. By means of first-principles time-dependent density functional calculations of the loss function, we established the direct connection between the low-energy plasmon properties and the crystal-structure symmetry. The different atomic environments affect the $d\ensuremath{-}d$ electron-hole excitations, which are prominent at low energies, resulting in distinct in-plane plasmon dispersions in the two families. Conversely, the different periodicity of the plasmon reappearance along the $c$ axis perpendicular to the layers can be used to distinguish the various crystal structures of TMD.

Momentum dependence of spin–orbit interaction effects in single-layer and multi-layer transition metal dichalcogenides

One of the main characteristics of the new family of two-dimensional crystals of semiconducting transition metal dichalcogenides (TMDs) is the strong spin–orbit interaction, which makes them very promising for future applications in spintronics and valleytronics devices. Here we present a detailed study of the effect of spin–orbit coupling (SOC) on the band structure of single-layer and bulk TMDs, including explicitly the role of the chalcogen orbitals and their hybridization with the transition metal atoms. To this aim, we combine density functional theory (DFT) calculations with a Slater–Koster tight-binding (TB) model. Whereas most of the previous TB models have been restricted to the K and K’ points of the Brillouin zone (BZ), here we consider the effect of SOC in the whole BZ, and the results are compared to the band structure obtained by DFT methods. The TB model is used to analyze the effect of SOC in the band structure, considering separately the contributions from the transition metal and the chalcogen atoms. Finally, we present a scenario where, in the case of strong SOC, the spin/orbital/valley entanglement at the minimum of the conduction band at Q can be probed and be of experimental interest in the most common cases of electron-doping reported for this family of compounds.

Large-area single-layer MoSe2 and its van der Waals heterostructures.

Layered structures of transition metal dichalcogenides stacked by van der Waals interactions are now attracting the attention of many researchers because they have fascinating electronic, optical, thermoelectric, and catalytic properties emerging at the monolayer limit. However, the commonly used methods for preparing monolayers have limitations of low yield and poor extendibility into large-area applications. Herein, we demonstrate the synthesis of large-area MoSe2 with high quality and uniformity by selenization of MoO3 via chemical vapor deposition on arbitrary substrates such as SiO2 and sapphire. The resultant monolayer was intrinsically doped, as evidenced by the formation of charged excitons under low-temperature photoluminescence analysis. A van der Waals heterostructure of MoSe2 on graphene was also demonstrated. Interestingly, the MoSe2/graphene heterostructures show strong quenching of the characteristic photoluminescence from MoSe2, indicating the rapid transfer of photogenerated charge carriers between MoSe2 and graphene. The development of highly controlled heterostructures of two-dimensional materials will further promote advances in the physics and chemistry of reduced dimensional systems and will provide novel applications in electronics and optoelectronics.

The influence of carbon content on the microtribological performance of W-S-C films

The layered structure of transition metal dichalcogenides makes them promising materials for self-lubricating films. Transition metal dichalcogenide films can be considered as substitute for carbon-based self-lubricating films in several varieties of environmental conditions. The tribological properties of these films at high load have been studied extensively. However, the tribological behaviour of these films in the milli-Newton load range relevant for micro-electro-mechanical systems has hardly been reported. In the present work, the microtribological response of W-S-C coatings deposited by reactive sputtering is investigated. For that purpose, W-S-C coatings with various concentrations of carbon were deposited on steel substrates using magnetron sputtering. The friction and wear of these coatings are determined as function of applied load and carbon content. The results show that, even though the wear of these films increases with applied load and decreases with carbon content, the friction coefficient is minimum for the films containing the highest amount of carbon at low and intermediate load. It is maximum for the films containing minimum carbon at the highest load. There is no evidence for a transfer layer on the worn surfaces.

Metallization and superconductivity of BeH2 under high pressure

Pressure-induced metallization and potential superconductivity of BeH2 has been a topic of interest. In the present study, we extensively explored the crystal structures of BeH2 in a wide pressure range of 0-300 GPa using an unbiased structure searching method coupled with first-principles density functional calculations. A series of pressure-induced structural transformations are predicted for BeH2, as Ibam (α phase) → P-3m1 (phase II) → R-3m (phase III) → Cmcm (phase IV). Calculated pressures of phase transition are 25, 140, and 202 GPa, respectively. The phase II is isostructural to the well-known 1T structure of transition metal dichalcogenides, which is composed of covalent bonded BeH2 slabs stacked along the perpendicular direction by van der Waals forces. The phase III is constructed by the same BeH2 slabs, but differs from the phase II in the stacking sequence. The α phase, phase II, and phase III all have insulating electronic states while their band gaps decrease as pressure increases. We predicted that BeH2 reaches a metallic state by a III → IV phase transition, instead of a direct band gap closure in phase III. The phase IV has a three-dimensional extended Be-H network formed by edge-sharing BeH8 polyhedrons with delocalized electrons. Electron-phonon coupling calculations implemented using linear response theory on the metallic BeH2 predict a large electron-phonon coupling parameter of 0.63, leading to an estimation of superconducting transition temperature (Tc) of ∼38 K at 250 GPa.

Growth and Electronic Structure Studies of Metal Intercalated Transition Metal Dichalcogenides MxNbSe2 (M: Fe and Cu)

There has been renewed interest in transition metal dichalcogenides (TMDs) as electronic materials. The modification of the electronic structures of TMDs by means of metal intercalation would be of interest in that respect. With this motivation, we have synthesized and performed angle-resolved photoemission studies on Fe- and Cu-intercalated TMD FexNbSe2 and CuxNbSe2 with various x values. FexNbSe2 (x = 0, 0.25) and CuxNbSe2 (x = 0, 0.06) were grown by the iodine vapor transport method. Intercalation and doping were confirmed by changes in c-axis lattice constant and electronic structure, respectively. A significant amount of electron doping from Fe and Cu was observed, as indicated in the ARPES data measured as the rigid downshift of bands. In addition, band folding was observed in FexNbSe2, for which the unit cell is doubled owing to the Fe order. However, the extra band folding expected from the antiferromagnetic order of Fe was not observed.

The chemistry of two-dimensional layered transition metal dichalcogenide nanosheets

Ultrathin two-dimensional nanosheets of layered transition metal dichalcogenides (TMDs) are fundamentally and technologically intriguing. In contrast to the graphene sheet, they are chemically versatile. Mono- or few-layered TMDs - obtained either through exfoliation of bulk materials or bottom-up syntheses - are direct-gap semiconductors whose bandgap energy, as well as carrier type (n- or p-type), varies between compounds depending on their composition, structure and dimensionality. In this Review, we describe how the tunable electronic structure of TMDs makes them attractive for a variety of applications. They have been investigated as chemically active electrocatalysts for hydrogen evolution and hydrosulfurization, as well as electrically active materials in opto-electronics. Their morphologies and properties are also useful for energy storage applications such as electrodes for Li-ion batteries and supercapacitors.

Two-Dimensional Transition Metal Dichalcogenides under Electron Irradiation: Defect Production and Doping

Using first-principles atomistic simulations, we study the response of atomically thin layers of transition metal dichalcogenides (TMDs)--a new class of two-dimensional inorganic materials with unique electronic properties--to electron irradiation. We calculate displacement threshold energies for atoms in 21 different compounds and estimate the corresponding electron energies required to produce defects. For a representative structure of MoS2, we carry out high-resolution transmission electron microscopy experiments and validate our theoretical predictions via observations of vacancy formation under exposure to an 80 keV electron beam. We further show that TMDs can be doped by filling the vacancies created by the electron beam with impurity atoms. Thereby, our results not only shed light on the radiation response of a system with reduced dimensionality, but also suggest new ways for engineering the electronic structure of TMDs.

Self-adaptive low friction coatings based on transition metal dichalcogenides

This paper deals with three fundamentally different concepts of self-lubricant coatings based on the transition metal dichalcogenides (TMD) and deposited by magnetron sputtering. The first two designs could be considered as traditional: the TMD films doped by carbon or nitrogen. The main attention is aimed at qualitative description of surface and sub-surface modification of the films as the result of the sliding process. Based on a thorough analysis of the worn surfaces, two features emerge: self-adaptability, where originally randomly oriented TMD phase becomes well oriented (i.e. with basal planes parallel to the surface), and limited role of carbon or nitrogen in the contact. The films doped with carbon are the best solution for humid environment, while those doped with nitrogen are ideal for use in dry environment or vacuum. The last concept deals with the combination of a hard TiN matrix deposited by reactive magnetron sputtering with fullerene-like TMD nanoclusters, which are directly injected into the growing TiN film. Despite the embryonic stage of the process development limiting in many ways the final properties of the film, the mechanical properties seem to be promising. The fullerene-like structure of TMD is well maintained and the composite hardness is relatively high compared to the competitive self-lubricant coating designs. On the other hand, the tribological tests did not show any effect of the embedded TMD nanoclusters in the sliding process.

Pyrite, marcasite, and arsenopyrite type minerals: Crystal chemical and structural principles

Quantitative molecular orbital (MO) calculations and qualitative perturbational MO arguments are used to interpret the spectra and structure of transition metal dichalcogenides and related compounds. Competition between pyrite (FeS2), marcasite (FeS2) and loellingite (FeAs2) structure types is explained in terms of the number of electrons occupying a set of MO's obtained from the mixing of dianion (A 2) orbitals and metal (M) dσ orbitals. Direct metal-metal d orbital interaction is argued to be small. Attention is focused upon the M - A - M angles which differ substantially among the three structure types as a consequence of varying numbers of electrons in orbitals which closely resemble the pπ* orbitals of the dianions. Variations in M - A and A - A distances can also be understood in terms of the occupations of this set of MO's. Disulfide valence region photo-emission spectra are interpreted in terms of calculations on MS6 and S6 molecular clusters. M3d orbitals are found to progressively approach the S3p orbitals with increasing atomic number of M from Fe to Ni. For CuS2 comparison of calculation and experiment supports an approximate electron configuration of Cu+1 S 2 −1 .