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Mos2 lattice constant9/15/2023 The widely used method of micromechanical exfoliation has been carefully studied in MoS 2 to understand the mechanism of delamination in few-layer to multi-layer flakes. While the pure MoS 2 nanopillar fails through a plastic bending mechanism, brittle fracture modes become apparent as the material is loaded with increasing amounts of dopant. The increase in yield strength is accompanied by a change in the failure mode of the material. Microindentation experiments on nanopillars of Cr-doped MoS 2 found that the yield strength increased from an average of 821 MPa for pure MoS 2 (at 0% Cr) to 1017 MPa at 50% Cr. The wear resistance of MoS 2 in lubricating applications can be increased by doping MoS 2 with Cr. Direct methods of measuring the shear strength indicate that the value is closer to 25.3 MPa. At ambient conditions, the coefficient of friction for MoS 2 was determined to be 0.150, with a corresponding estimated shear strength of 56.0 MPa (mega pascals). The shear strength of MoS 2 increases as the coefficient of friction increases. Extensive work has been performed to characterize the coefficient of friction and shear strength of MoS 2 in various atmospheres. Interlayer sliding dissipates energy when a shear stress is applied to the material. MoS 2 excels as a lubricating material (see below) due to its layered structure and low coefficient of friction. A few methods include lithium intercalation to delaminate the layers and sonication in a high-surface tension solvent. Liquid-phase exfoliation can also be used to produce monolayer to multi-layer MoS 2 in solution. While Scotch tape is generally used as the adhesive tape, PDMS stamps can also satisfactorily cleave MoS 2 if it is important to avoid contaminating the flakes with residual adhesive. The aforementioned scheme is good for graphene only. However, it can not be employed for a uniform 1-D layers because of weaker adhesion of MoS 2 to the substrate (either Si, glass or quartz). This facile method was first used by Konstantin Novoselov and Andre Geim to obtain graphene from graphite crystals. The crystal flakes can then be transferred from the adhesive film to a substrate. 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. 2D MoS 2 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. The layer-dependent optoelectronic properties of MoS 2 have promoted much research in 2-dimensional MoS 2-based devices. While bulk MoS 2 in the 2H-phase is known to be an indirect-band gap semiconductor, monolayer MoS 2 has a direct band gap. Nanotube-like and buckyball-like molecules composed of MoSĢ are known. The 2H/1T-phase transition can be controlled via the incorporation of S vacancies. The 1T-phase can be stabilized through doping with electron donors such as rhenium, or converted back to the 2H-phase by microwave radiation. This phase has tetragonal symmetry and is metallic. Ī third, metastable crystalline phase known as 1T-MoS 2 was discovered by intercalating 2H-MoS 2 with alkali metals. Both the 2H- and 3R-phases are semiconducting. Each sulfur atom has pyramidal coordination and is bonded to three molybdenum atoms. 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. Bulk MoS 2 consists of stacked monolayers, which are held together by weak van der Waals interactions.Ĭrystalline MoS 2 exists in one of two phases, 2H-MoS 2 and 3R-MoS 2, where the "H" and the "R" indicate hexagonal and rhombohedral symmetry, respectively. These three strata form a monolayer of MoS 2. Crystalline phases Ģ have a layered structure, in which a plane of molybdenum atoms is sandwiched by planes of sulfide ions. Electron microscopy of antisites (a, Mo substitutes for S) and vacancies (b, missing S atoms) in a monolayer of molybdenum disulfide.
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