Molybdenum disulfide

Molybdenum disulfide (or moly) is an inorganic compound composed of molybdenum and sulfur. Its chemical formula is MoS2. The compound is classified as a transition metal dichalcogenide. It is a silvery black solid that occurs as the mineral molybdenite, the principal ore for molybdenum. MoS2 is relatively unreactive. It is unaffected by dilute acids and oxygen. In appearance and feel, molybdenum disulfide is similar to graphite. It is widely used as a dry lubricant because of its low friction and robustness. Bulk MoS2 is a diamagnetic, indirect bandgap semiconductor similar to silicon, with a bandgap of 1.23 eV.

Production MoS2 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 MoS2. The main contaminant is carbon. MoS2 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

Crystalline phases All forms of MoS2 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 MoS2. Bulk MoS2 consists of stacked monolayers, which are held together by weak van der Waals interactions. Crystalline MoS2 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 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-MoS2 was discovered by intercalating 2H-MoS2 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 MoS2 are known.

Exfoliated MoS2 flakes While bulk MoS2 in the 2H-phase is known to be an indirect-band gap semiconductor, monolayer MoS2 has a direct band gap. The layer-dependent optoelectronic properties of MoS2 have promoted much research in 2-dimensional MoS2-based devices. 2D MoS2 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 MoS2 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 MoS2 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 MoS2 in solution. A few methods include lithium intercalation to delaminate the layers and sonication in a high-surface tension solvent.

Mechanical properties MoS2 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 MoS2 in various atmospheres. The shear strength of MoS2 increases as the coefficient of friction increases. This property is called superlubricity. At ambient conditions, the coefficient of friction for MoS2 was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa. Direct methods of measuring the shear strength indicate that the value is closer to 25.3 MPa. The wear resistance of MoS2 in lubricating applications can be increased by doping MoS2 with Cr. Microindentation experiments on nanopillars of Cr-doped MoS2 found that the yield strength increased from an average of 821 MPa for pure MoS2 (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 MoS2 nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant. The widely used method of micromechanical exfoliation has been carefully studied in MoS2 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. In recent years, MoS2 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 MoS2 flakes that were deposited on a holey substrate. The Young's modulus of monolayer flakes was 270 GPa, while the thicker flakes were stiffer, with a Young's modulus of 330 GPa. Molecular dynamic simulations found the in-plane Young's modulus of MoS2 to be 229 GPa, which matches the experimental results within error. 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 MoS2 is 23 GPa, which is close to the theoretical fracture strength for defect-free MoS2. The band structure of MoS2 is sensitive to strain.

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