Bulk 2H-MoS2 Research Articles

Investigation of the dispersion and the effective masses of excitons in bulk2H−MoS2using transition electron energy-loss spectroscopy

We have investigated the electronic excitations in bulk 2H-MoS2 using electron energy-loss spectroscopy. The electron energy-loss spectra in the {\Gamma}M and {\Gamma}K directions were measured for various momentum transfer values. The results allow the identification of the A1 and B1 exciton peaks and in particular their energy-momentum dispersion. The dispersions exhibit approximately quadratic upward trends and slight anisotropies in the {\Gamma}M and {\Gamma}K directions. The fitted energy-momentum transfer functions allow the estimation of the effective masses of the excitons which are in close proximity to predicted values.

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.

The Effect of Morphology on the Tribological Properties of MoS2 in Liquid Paraffin

The tribological properties of liquid paraffin (LP) containing molybdenum disulfide (MoS2) additives, including nano-balls, nano-slices, and bulk 2H-MoS2, are evaluated using a four-ball tribometer. Results show that all MoS2 additives used can improve the tribological properties of LP, and that nanosized MoS2 particles function as lubrication additives in LP better than micro-MoS2 particles do. The LP with nano-balls presents the best antifriction and antiwear properties at the MoS2 content of 1.5 wt%. This is ascribed to the chemical stability of the layer-closed spherical structure of nano-balls. The Stribeck curves confirm that the rotation speed of 1,450 rpm used is located at the mixed lubrication region under 300 N. MoS2 nano-slices have small sizes and easily enter into the interface of the friction pair with a roughness of 0.032 μm, functioning as a lubricant in LP better than nano-balls do at the MoS2 content of 1.0 wt%. The Stribeck curves also show that the differences between the two nano samples were magnified at high rotation speeds in hydrodynamic lubrication region. The application of nano-slices in high sliding speeds will be more advantageous. This work furthers the understanding of the relationship between the tribological properties and morphology of MoS2.

The effect of morphology and size on the photocatalytic properties of MoS2

The photocatalytic properties of MoS2 samples including nano-ball, nano-slice and bulk 2H-MoS2 were evaluated and compared with that of the anatase TiO2 using the degradation reaction of methyl orange under visible light. The catalytic behaviors of the samples were also characterized using transmission electron microscopy, Brunauer-Emmett-Teller (BET) surface area, ultraviolet-visible spectroscopy, and FTIR spectroscopy. The results show that the bulk MoS2 and the anatase TiO2 were almost inactive under visible light. The nano-slice presented the most positive catalytic effect because it has a wide absorption at 400–700 nm and a high BET surface area. Though the BET surface area of the nano-ball was lower than that of the nano-slice, it has an active curved basal surface and presented a catalytic activity close to that of the nano-slice. Moreover, the MoS2 nano-slice catalyst could be conveniently regenerated after filtration and drying.

A magnetic resonance study of MoS2 fullerene-like nanoparticles

We report on the first nuclear magnetic resonance (NMR) and electronparamagnetic resonance (EPR) investigation of inorganic fullerene-likeMoS2 nanoparticles.Spectra of bulk 2H-MoS2 samples have also been measured for comparison. The similarity between the measured quadrupolecoupling constants and chemical shielding anisotropy parameters for bulk and fullerene-likeMoS2 reflects the nearly identical local crystalline environments of the Moatoms in these two materials. EPR measurements show that fullerene-likeMoS2 exhibits a larger density of dangling bonds carrying unpaired electrons, indicative ofthem having a more defective structure than the bulk sample. The latterobservation explains the increase in the spin–lattice relaxation rate observed inthe NMR measurements for this sample in comparison with the bulk 2H-MoS2 ones.

Ab initio study of 2H-MoS2using Hay and Wadt effective core pseudo-potentials for modelling the (101̄0) surface structure

Periodic Hartree–Fock and DFT methods were employed to calculate the geometric and electronic properties of bulk 2H-MoS2 and of its catalytically important (100) edge structure. The core electrons of molybdenum and sulfur were represented by the effective core pseudo-potentials, developed by Hay and Wadt. For the calculations (100) type surface structures were generated by cutting sections from an 2H-MoS2 crystal, which consists of four, six or eight rows of molybdenum and sulfur atoms. The calculated elastic constants follow the experimental constants and show that 2H-MoS2 is an anisotropic covalent compound held together by weak dispersion interactions between neighbouring S-Mo-S units. The relaxation of the (100) surface of 2H-MoS2 leads to an inner relaxation of the Mo atoms and the formation of weakly coupled surface S-S species. As a result of the surface relaxation, the electronic charge of the surface states is different to that of the bulk states: empty Mo d states move into the band gap, and the surface becomes a better electron acceptor. In all calculations the model consisting of six rows of sulfur and molybdenum atoms is the most favourable one, because it provides an accurate description of the 2H-MoS2 surface structure and enables calculations at reasonable computation time. The calculations based on pseudo-potentials are in good agreement with all-electron calculations and confirm that, in contrast to the semiconducting bulk, the (100) 2H-MoS2 surface has metallic properties.