MoS2

Two-dimensional (2D) nanomaterials have attracted increasing attention because of their unusual physical and chemical properties. Among these 2D nanomaterials, the monolayers of layered transition metal dichalcogenides exhibit intriguing physical and chemical properties. In contrast to the graphene, they are direct gap semiconductors with tunable band structures by controlling the composition, functionalizing, and applying external fields. In this review, the recent progress on the first-principles studies of MoS₂ is presented. The electronic and optical properties of MoS₂ monolayer, spin Hall Effect, lattice dynamics, functionalization, and H adsorption and diffusion, are systematically reviewed to provide a broad overview on its applications in electronics, optoelectronics, spintronics, sensor, and membrane. The recent advances on the MoS₂ nanoribbon, including edge stability, edge-dependent magnetism and electronic properties, and its application in renewable energy storage, are also presented. The first principles studies, supported by experimental results, show that their morphologies and properties are useful for nanodevices, catalysts, and energy storage applications.

Molybdenum disulfide (MoS₂) is a transition metal dichalcogenide which has been a subject of intense research mainly due to its catalytic properties. MoS₂ is formed by two-dimensional (2D) graphene-like S–Mo–S layers held together by weak noncovalent interactions. Recently, MoS₂ has been exfoliated into individual S–M–S monolayers and the electronic and optical properties of a single MoS₂ layer have been investigated. It was demonstrated that a single layer MoS₂ undergoes indirect to direct band gap transition, which enables a wide range of optoelectronic applications. I will review the electronic structure of exfoliated MoS₂ and discuss several ways in which it can be manipulated. I will also review potential applications of exfoliated MoS₂ in the exciting new field of valleytronics.

We report tunability in electronic and dielectric properties of a technologically promising nanomaterial MoS₂. The properties of MoS₂ can be tuned by varying the layer thickness, by applying mechanic strain, by tuning the interlayer distance, and by applying external electric field. Reducing the slab thickness systematically from bulk to monolayers causes blue shift in the band gap energies, thereby, resulting in tunability of the electronic band gap. By reducing the number of layers from bulk to monolayer limit, electron energy loss spectra (EELS) shows red shift in the energies of both \( {{\uppi}} \) and \( {{\uppi}} + {{\upsigma}} \) plasmons. Mechanical strains reduce the band gap of monolayer MoS₂ by causing a direct-to-indirect band gap transitions and finally rendering it into metal at critical values depending on the types of applied strain. Dielectric properties of monolayer MoS₂ too get influenced by the type of applied strain. Imaginary part of dielectric function (\( {{\upvarepsilon}}_{2} \)) shows redshift in the structure peak energy on the application of strains with significant dependence on the types of applied strain. In-plane strains also cause semiconductor-metal transitions (\( {\text{e}}_{T} \)) in bilayer sheets of MoS₂. The energy gap of semiconducting bilayer MoS₂ gets reduced continuously by reducing the bilayer separation, eventually rendering it metallic at critical value of interlayer distance. Electrically gated semiconducting bilayer MoS₂ is also found to show reduction in the band gap on increasing the magnitude of electric field and results in band gap closure at a critical value of the field.

In this chapter, recent progresses on the theoretical study of low-dimensional semiconducting transition metal dichalcogenides (TMD) MX₂ (M = Mo, W; X = S, Se, Te) are reviewed. The chemical trends in basic structural and electronic properties are discussed, and the band offsets between MX₂ monolayers are calculated. A simple model is proposed to interpret the chemical trends of the band offsets. Moreover, the suitable band edge position of MoS₂ monolayer makes it a good candidate for the photo-splitting of water. The cluster expansion method and special quasi-random structure approach are employed to study the properties of MX₂ alloys. It is demonstrated that in (S, Se) alloys, there exist stable-ordered alloy structures even at 0 K, whereas in (Se,Te) and (S,Te) alloys, phase separation into the two constituents will occur at 0 K. Nevertheless, a complete miscibility in these alloys can be achieved by increasing temperature. Finally, we show that the bandgap of MX₂ nanostructures can be efficiently modulated by strain, electronic field, and alloying. By increasing strain or electric field strength, the bandgap of MX₂ can be reduced, and gap closure is achieved when the strain/field strength reaches a critical value. In MX₂ alloys, the bandgap and band edge position varies as the composition changes, and exhibits bowing effect, which is a joint effect of volume deformation, chemical difference, and structure relaxation. More importantly, the direct gap character of MX₂ monolayer is retained in the alloys, making them good candidates for 2D optoelectronics.

The structural, electronic, and vibrational properties of MoS₂ together with other semiconducting transition metal dichalcogenides (TMDCs) are investigated based on first principles calculations. Due to its layered structure, single-layer MoS₂ can be fabricated by the mechanical exfoliation method. The band structure of MoS₂ shows an indirect-to-direct semiconductor transition from bulk to single layer because of a lack of interlayer interaction. Giant spin splitting at the K point of the Brillouin zone is predicted for single-layer MoS₂ and other TMDCs, due to the intrinsic strong spin–orbit coupling and the absence of inversion symmetry. Moreover, enhancement of the Rashba splitting is predicted for polar single-layer TMDCs. Two-dimensional dilute magnetic semiconductors are proposed for substitution of Mo by other transition metal atoms, such as Mn, Fe, and Co. Experimentally observed anomalous vibrational properties can be attributed to reduction of the interlayer interaction and strengthening of the intralayer interaction from bulk to single layer. It is demonstrated that strain plays an important role for the electronic and vibrational properties of single-layer MoS₂. The electronic states of one-dimensional structures (nanoribbon, nanotube, etc.) are sensitive to the edge structure, charity, and strain. Under sulfur-rich conditions, zero-dimensional MoS₂ shows a Mo-edge triangular structure with sulfur saturation.

Two-dimensional atomically thin crystals have recently emerged as a broad and an interesting field of research due to their technological applicability, due to their exceptional electrical and mechanical properties, and to the challenge they represent for theoretical condensed matter physics. Although graphene has been the most widely studied prototype of 2D material [1, 2], in the last few years other two-dimensional crystals (MoS₂ among them [3‐13]), have been investigated in search of different properties, which could lead to a broadening of their technological applicability.

We review the relevant vibrational and electronic properties of a single and a few layer MoS₂ to understand their resonant and nonresonant Raman scattering results. In particular, the optical modes and low frequency shear and layer breathing modes show significant dependence on the number of MoS₂ layers. Further, the electron doping of the MoS₂ single layer achieved using top-gating in a field effect transistor renormalizes the two optical modes A _1g and \( E_{2g}^{1} \) differently due to symmetry-dependent electron–phonon coupling. The issues related to carrier mobility, the Schottky barrier at the MoS₂–metal contact pads and the modifications of the dielectric environment are addressed. The direct optical transitions for single-layer MoS₂ involve two excitons at K-point in the Brillouin zone and their stability with temperature and pressure is reviewed. Finally, the Fermi level dependence of spectral shift for a quasiparticle, called trion, is discussed.

In recent years, the dichalcogenide MoS₂ has gained attention as an interesting material system for basic research and possible optoelectronic applications. Here, we report on optical spectroscopy of few- and single-layer MoS₂ flakes. We use Raman spectroscopy to characterize our samples. The energy of the characteristic phonon modes in MoS₂ depends on the number of layers, so that the thickness of individual flakes can be mapped in scanning Raman experiments. While bulk MoS₂ is an indirect-gap semiconductor, single-layer MoS₂ has a direct band gap and emits strong photoluminescence. We investigate the photoluminescence in single-layer MoS₂ for different experimental conditions. Additionally, we study the photocarrier dynamics in time-resolved photoluminescence experiments.

A series of environmental problems have emerged owing to the excess consumption of fossil fuels. Development of clean alternative energy has turned into an urgent issue facing to all the nations. Nanostructured MoS₂, with particular chemical and physical properties, has been studied extensively and intensively over the past years. A comprehensive overview of the progress achieved within the application of MoS₂ in energy storage and conversion will be given, which is composed of lithium ion batteries, Mg ion batteries, dye-sensitized solar cells and photocatalytic hydrogen evolution.

Degenerate valleys of energy bands well separated in momentum space constitute a discrete degrees of freedom for low-energy electrons. This has led to the emergence of valleytronics, a conceptual electronics based on manipulating valley index. Transition-metal dichalcogenides monolayers have been theoretically proposed as a good platform to realize manipulation of valley degrees of freedom through nonzero Berry curvatures at high symmetric points in Brillouin zone. Very recently several groups reported the selective occupation of the degenerate but inequivalent valleys by circularly polarized optical pumping in transition-metal dichalcogenides monolayers. These experimental evidences reveal the viability of optical valley control in group-VI transition-metal dichalcogenides, and form the basis for integrated valleytronics and spintronics application.