The compound is classified as atransition metal dichalcogenide. It is a silvery black solid that occurs as the mineralmolybdenite, the principal ore for molybdenum.[6]MoS2 is relatively unreactive. It is unaffected by diluteacids andoxygen. In appearance and feel, molybdenum disulfide is similar tographite. It is widely used as adry lubricant because of its lowfriction and robustness. BulkMoS2 is adiamagnetic,indirect bandgap semiconductor similar tosilicon, with a bandgap of 1.23 eV.[2]
MoS2 is naturally found as eithermolybdenite, a crystalline mineral, or jordisite, a rare low temperature form of molybdenite.[7] Molybdenite ore is processed byflotation to give relatively pureMoS2. The main contaminant is carbon.MoS2 also arises by thermal treatment of virtually all molybdenum compounds withhydrogen sulfide or elemental sulfur and can be produced by metathesis reactions frommolybdenum pentachloride.[8]
Electron microscopy of antisites (a, Mo substitutes for S) and vacancies (b, missing S atoms) in amonolayer of molybdenum disulfide. Scale bar: 1 nm.[9]
All forms ofMoS2 have a layered structure, in which a plane of molybdenum atoms is sandwiched by planes of sulfide ions. These three strata form a monolayer ofMoS2. BulkMoS2 consists of stacked monolayers, which are held together by weakvan der Waals interactions.
CrystallineMoS2 exists in one of two phases, 2H-MoS2 and 3R-MoS2, 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 atrigonal prismaticcoordination 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.[10]
A third, metastable crystalline phase known as 1T-MoS2 was discovered by intercalating 2H-MoS2 withalkali metals.[11] This phase has trigonal symmetry and is metallic. The 1T-phase can be stabilized through doping with electron donors such asrhenium,[12] or converted back to the 2H-phase by microwave radiation.[13] The 2H/1T-phase transition can be controlled via the incorporation of sulfur (S)vacancies.[14]
While bulkMoS2 in the 2H-phase is known to be an indirect-band gap semiconductor, monolayerMoS2 has a direct band gap. The layer-dependent optoelectronic properties ofMoS2 have promoted much research in 2-dimensionalMoS2-based devices. 2DMoS2 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 byKonstantin Novoselov andAndre Geim to obtain graphene from graphite crystals. However, it can not be employed for a uniform 1-D layers because of weaker adhesion ofMoS2 to the substrate (either silicon, glass or quartz); the aforementioned scheme is good for graphene only.[16] While Scotch tape is generally used as the adhesive tape,PDMS stamps can also satisfactorily cleaveMoS2 if it is important to avoid contaminating the flakes with residual adhesive.[17]
Liquid-phase exfoliation can also be used to produce monolayer to multi-layerMoS2 in solution. A few methods include lithiumintercalation[18] to delaminate the layers andsonication in a high-surface tension solvent.[19][20]
MoS2 excels as a lubricating material (see below) due to its layered structure and lowcoefficient 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 ofMoS2 in various atmospheres.[21] Theshear strength ofMoS2 increases as the coefficient of friction increases. This property is calledsuperlubricity. At ambient conditions, the coefficient of friction forMoS2 was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa.[21] Direct methods of measuring the shear strength indicate that the value is closer to 25.3 MPa.[22]
The wear resistance ofMoS2 in lubricating applications can be increased bydopingMoS2 withCr. Microindentation experiments onnanopillars of Cr-dopedMoS2 found that the yield strength increased from an average of 821 MPa for pureMoS2 (at 0% Cr) to 1017 MPa at 50% Cr.[23] The increase in yield strength is accompanied by a change in the failure mode of the material. While the pureMoS2 nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant.[23]
The widely used method of micromechanical exfoliation has been carefully studied inMoS2 to understand the mechanism of delamination in few-layer to multi-layer flakes. The exact mechanism of cleavage was found to be layer dependent. Flakes thinner than 5 layers undergo homogenous bending and rippling, while flakes around 10 layers thick delaminated through interlayer sliding. Flakes with more than 20 layers exhibited a kinking mechanism during micromechanical cleavage. The cleavage of these flakes was also determined to be reversible due to the nature of van der Waals bonding.[24]
In recent years,MoS2 has been utilized in flexible electronic applications, promoting more investigation into the elastic properties of this material. Nanoscopic bending tests usingAFM cantilever tips were performed on micromechanically exfoliatedMoS2 flakes that were deposited on a holey substrate.[17][25] The Young's modulus of monolayer flakes was 270 GPa,[25] while the thicker flakes were stiffer, with a Young's modulus of 330 GPa.[17] Molecular dynamic simulations found the in-plane Young's modulus ofMoS2 to be 229 GPa, which matches the experimental results within error.[26]
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 monolayerMoS2 is 23 GPa, which is close to the theoretical fracture strength for defect-freeMoS2.[25]
The band structure ofMoS2 is sensitive to strain.[27][28][29]
Molybdenum disulfide is a host for formation ofintercalation compounds. This behavior is relevant to its use as a cathode material in batteries.[30][31] One example is a lithiated material,LixMoS2.[32] Withbutyl lithium, the product isLiMoS2.[6]
A tube of commercial graphite powder lubricant with molybdenum disulfide additive (called "molybdenum")[33]
Due to weakvan der Waals interactions between the sheets of sulfide atoms,MoS2 has a lowcoefficient of friction.MoS2 in particle sizes in the range of 1–100 μm is a commondry lubricant.[34] Few alternatives exist that confer high lubricity and stability at up to 350 °C in oxidizing environments. Sliding friction tests ofMoS2 using apin on disc tester at low loads (0.1–2 N) give friction coefficient values of <0.1.[35][36]
MoS2 is often a component of blends and composites that require low friction. For example, it is added to graphite to improve sticking.[33] A variety ofoils andgreases useMoS2, because they retain their lubricity even in cases of almost complete oil loss, thus finding a use in critical applications such asaircraft engines. When added toplastics,MoS2 forms acomposite with improved strength as well as reduced friction. Polymers that may be filled withMoS2 includenylon (trade nameNylatron),Teflon andVespel. Self-lubricating composite coatings for high-temperature applications consist of molybdenum disulfide andtitanium nitride, usingchemical vapor deposition.
Other layered inorganic materials that exhibit lubricating properties (collectively known assolid lubricants (or dry lubricants)) includes graphite, which requires volatile additives and hexagonalboron nitride.[39]
MoS2 is employed as acocatalyst for desulfurization inpetrochemistry, for example,hydrodesulfurization. The effectiveness of theMoS2 catalysts is enhanced bydoping with small amounts ofcobalt ornickel. The intimate mixture of these sulfides issupported onalumina. Such catalysts are generated in situ by treating molybdate/cobalt or nickel-impregnated alumina withH 2S 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.[40]
MoS2@Fe-N-C core/shell[47] nanosphere with atomic Fe-doped surface and interface (MoS2/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 ofMoS2/Fe-N-C interface.
MoS2 nanoflakes can be used for solution-processed fabrication of layeredmemristive and memcapacitive devices through engineering aMoOx/MoS2 heterostructure sandwiched between silver electrodes.[56]MoS2-basedmemristors are mechanically flexible, optically transparent and can be produced at low cost.
The sensitivity of a graphenefield-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.[57]
MoS2 has been investigated as a component of flexible circuits.[58][59]
In 2017, a 115-transistor, 1-bitmicroprocessor implementation was fabricated using two-dimensionalMoS2.[60]
Due to the lack of spatial inversion symmetry, odd-layer MoS2 is a promising material forvalleytronics 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. TheBerry 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.[62] In monolayerMoS2, 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.[63]
MoS2 also possesses mechanical strength, electrical conductivity, and can emit light, opening possible applications such as photodetectors.[64]MoS2 has been investigated as a component of photoelectrochemical (e.g. for photocatalytic hydrogen production) applications and for microelectronics applications.[53]
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