Molybdenum disulfide

is naturally found as either molybdenite, a crystalline mineral, or jordisite, a rare low temperature form of molybdenite. Molybdenite ore is processed by flotation to give relatively pure . The main contaminant is carbon. also arises by thermal treatment of virtually all molybdenum compounds with hydrogen sulfide or elemental sulfur and can be produced by metathesis reactions from molybdenum pentachloride. == Structure and physical properties ==

of molybdenum disulfide. Scale bar: 1 nm. Crystalline phases All forms of have a layered structure, in which a plane of molybdenum atoms is sandwiched by planes of sulfide ions. These three strata form a monolayer of . Bulk consists of stacked monolayers, which are held together by weak van der Waals interactions. Crystalline exists in one of two phases, 2H- and 3R-, where the "H" and the "R" indicate hexagonal and rhombohedral symmetry, respectively. In both of these structures, each molybdenum atom exists at the center of a trigonal prismatic coordination sphere and is covalently bonded to six sulfide ions. Each sulfur atom has pyramidal coordination and is bonded to three molybdenum atoms. Both the 2H- and 3R-phases are semiconducting. A third, metastable crystalline phase known as 1T- was discovered by intercalating 2H- with alkali metals. This phase has trigonal symmetry and is metallic. The 1T-phase can be stabilized through doping with electron donors such as rhenium, or converted back to the 2H-phase by microwave radiation. The 2H/1T-phase transition can be controlled via the incorporation of sulfur (S) vacancies. Allotropes Nanotube-like and buckyball-like molecules composed of are known. Exfoliated flakes While bulk in the 2H-phase is known to be an indirect-band gap semiconductor, monolayer has a direct band gap. The layer-dependent optoelectronic properties of have promoted much research in 2-dimensional -based devices. 2D can be produced by exfoliating bulk crystals to produce single-layer to few-layer flakes either through a dry, micromechanical process or through solution processing. Micromechanical exfoliation, also pragmatically called "Scotch-tape exfoliation", involves using an adhesive material to repeatedly peel apart a layered crystal by overcoming the van der Waals forces. The crystal flakes can then be transferred from the adhesive film to a substrate. This facile method was first used by Konstantin Novoselov and Andre Geim to obtain graphene from graphite crystals. However, it can not be employed for a uniform 1-D layers because of weaker adhesion of to the substrate (either silicon, glass or quartz); the aforementioned scheme is good for graphene only. While Scotch tape is generally used as the adhesive tape, PDMS stamps can also satisfactorily cleave if it is important to avoid contaminating the flakes with residual adhesive. Liquid-phase exfoliation can also be used to produce monolayer to multi-layer in solution. A few methods include lithium intercalation to delaminate the layers and sonication in a high-surface tension solvent. Mechanical properties excels as a lubricating material (see below) due to its layered structure and low coefficient of friction. Interlayer sliding dissipates energy when a shear stress is applied to the material. Extensive work has been performed to characterize the coefficient of friction and shear strength of in various atmospheres. The shear strength of increases as the coefficient of friction increases. This property is called superlubricity. At ambient conditions, the coefficient of friction for was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa. The wear resistance of in lubricating applications can be increased by doping with Cr. Microindentation experiments on nanopillars of Cr-doped found that the yield strength increased from an average of 821 MPa for pure (at 0% Cr) to 1017 MPa at 50% Cr. The increase in yield strength is accompanied by a change in the failure mode of the material. While the pure nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant. In recent years, has been utilized in flexible electronic applications, promoting more investigation into the elastic properties of this material. Nanoscopic bending tests using AFM cantilever tips were performed on micromechanically exfoliated flakes that were deposited on a holey substrate. The Young's modulus of monolayer flakes was 270 GPa, Bertolazzi and coworkers also characterized the failure modes of the suspended monolayer flakes. The strain at failure ranges from 6 to 11%. The average yield strength of monolayer is 23 GPa, which is close to the theoretical fracture strength for defect-free . == Chemical reactions ==

Molybdenum disulfide is stable in air and attacked only by aggressive reagents. It reacts with oxygen upon heating forming molybdenum trioxide: : Chlorine attacks molybdenum disulfide at elevated temperatures to form molybdenum pentachloride: : Intercalation reactions Molybdenum disulfide is a host for formation of intercalation compounds. This behavior is relevant to its use as a cathode material in batteries. One example is a lithiated material, {{chem2|Li_{x}MoS2}}. With butyl lithium, the product is . == Applications ==

Lubricant Due to weak van der Waals interactions between the sheets of sulfide atoms, has a low coefficient of friction. in particle sizes in the range of 1–100 μm is a common dry lubricant. Few alternatives exist that confer high lubricity and stability at up to 350 °C in oxidizing environments. Sliding friction tests of using a pin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1. is often a component of blends and composites that require low friction. For example, it is added to graphite to improve sticking. and bullets. Other layered inorganic materials that exhibit lubricating properties (collectively known as solid lubricants (or dry lubricants)) includes graphite, which requires volatile additives and hexagonal boron nitride. Catalysis revealed by molybdenum disulfide is employed as a cocatalyst for desulfurization in petrochemistry, for example, hydrodesulfurization. The effectiveness of the catalysts is enhanced by doping with small amounts of cobalt or nickel. The intimate mixture of these sulfides is supported on alumina. Such catalysts are generated in situ by treating molybdate/cobalt or nickel-impregnated alumina with or an equivalent reagent. Catalysis does not occur at the regular sheet-like regions of the crystallites, but instead at the edge of these planes. finds use as a hydrogenation catalyst for organic synthesis. As it is derived from a common transition metal, rather than a group 10 metal, is chosen when price or resistance to sulfur poisoning are of primary concern. is effective for the hydrogenation of nitro compounds to amines and can be used to produce secondary amines via reductive amination. The catalyst can also effect hydrogenolysis of organosulfur compounds, aldehydes, ketones, phenols and carboxylic acids to their respective alkanes. However, it suffers from low activity, often requiring hydrogen pressures above 96 MPa and temperatures above 185 °C. == Research ==

plays an important role in condensed matter physics research. Hydrogen evolution and related molybdenum sulfides are efficient catalysts for hydrogen evolution, including the electrolysis of water; thus, are possibly useful to produce hydrogen for use in fuel cells. Oxygen reduction and evolution @Fe-N-C core/shell nanosphere with atomic Fe-doped surface and interface (/Fe-N-C) can be used as a used an electrocatalyst for oxygen reduction and evolution reactions (ORR and OER) bifunctionally because of reduced energy barrier due to Fe-N4 dopants and unique nature of /Fe-N-C interface. Microelectronics As in graphene, the layered structures of and other transition metal dichalcogenides exhibit electronic and optical properties that can differ from those in bulk. Bulk has an indirect band gap of 1.2 eV, while monolayers have a direct 1.8 eV electronic bandgap, supporting switchable transistors nanoflakes can be used for solution-processed fabrication of layered memristive and memcapacitive devices through engineering a {{chem2|MoO_{x}|}}/ heterostructure sandwiched between silver electrodes. -based memristors are mechanically flexible, optically transparent and can be produced at low cost. The sensitivity of a graphene field-effect transistor (FET) biosensor is fundamentally restricted by the zero band gap of graphene, which results in increased leakage and reduced sensitivity. In digital electronics, transistors control current flow throughout an integrated circuit and allow for amplification and switching. In biosensing, the physical gate is removed and the binding between embedded receptor molecules and the charged target biomolecules to which they are exposed modulates the current. has been investigated as a component of flexible circuits. In 2017, a 115-transistor, 1-bit microprocessor implementation was fabricated using two-dimensional . Valleytronics Due to the lack of spatial inversion symmetry, odd-layer MoS2 is a promising material for valleytronics because both the CBM and VBM have two energy-degenerate valleys at the corners of the first Brillouin zone, providing an exciting opportunity to store the information of 0s and 1s at different discrete values of the crystal momentum. The Berry curvature is even under spatial inversion (P) and odd under time reversal (T), the valley Hall effect cannot survive when both P and T symmetries are present. To excite valley Hall effect in specific valleys, circularly polarized lights were used for breaking the T symmetry in atomically thin transition-metal dichalcogenides. In monolayer , the T and mirror symmetries lock the spin and valley indices of the sub-bands split by the spin-orbit couplings, both of which are flipped under T; the spin conservation suppresses the inter-valley scattering. Therefore, monolayer MoS2 have been deemed an ideal platform for realizing intrinsic valley Hall effect without extrinsic symmetry breaking. Photonics and photovoltaics also possesses mechanical strength, electrical conductivity, and can emit light, opening possible applications such as photodetectors. has been investigated as a component of photoelectrochemical (e.g. for photocatalytic hydrogen production) applications and for microelectronics applications. Superconductivity of monolayers Under an electric field monolayers have been found to superconduct at temperatures below 9.4 K. Spintronics has been found to reduce damping in spintronics. == See also ==