The Effect of VMoS3 Point Defect on the Elastic Properties of Monolayer MoS2 with REBO Potentials

Optimization of Monolayer MoS2 with Prescribed Mechanical Properties

Various technological challenges are essentially material problems in our times. New functional and functional graded nanomaterials are constructed of components with predefined properties. The design of nanostructures with predefined mechanical properties was considered in this paper. This study applies the evolutionary algorithm (EA) to the optimization problem in the design of nanomaterials. The optimal design combined EA with molecular dynamics to identify the size of the void for the prescribed elastic properties in monolayer 2D MoS₂ nanostructures. The numerical results show that the proposed EA and the use of optimization method allowed accurately obtaining nanostructures with predefined mechanical material properties by introducing elliptical voids in the 2D MoS₂ nanosheets.

Impact of ion beam irradiation on two-dimensional MoS2: a molecular dynamics simulation study

Two-dimensional (2D) materials such as MoS₂have extraordinary properties and significant application potential in electronics, optoelectronics, energy storage, bioengineering, etc. To realize the numerous application potential, it is needed to modulate the structure and properties of these 2D materials, for which ion beam irradiation has obvious advantages. This research adopted classical molecular dynamics simulations to study the sputtering of atoms in 2D MoS₂, defect formation and the control rule under Ar ion beam irradiation, considering the influence of ion irradiation parameters (i.e., ion beam energy, ion dose), layer number of 2D MoS₂, substrate. Furthermore, the uniaxial mechanical performance of the ion-irradiated nanostructures was investigated for actual applications loading with mechanical stress/strain. This research could provide important theoretical support for fabricating high-performance 2D MoS₂-based nanodevices by ion beam irradiation method.

Remarkably high thermal-driven MoS2 grain boundary migration mobility and its implications on defect healing

Two-dimensional (2D) transition-metal dichalcogenides (TMDs) hold great potential for many important device applications, such as field effect transistors and sensors, which require a robust control of defect type, density, and distribution. However, how to control the defect type, density, and distribution in these materials is still a challenge. In this study, we explore the kinetics and dynamics of four types of grain boundaries (GBs) in monolayer MoS₂, which are composed of S-polar dislocation (S5|7), Mo-polar dislocation (Mo5|7), dislocation-double S vacancy complex (S4|6), and dislocation-double S interstitial complex (S6|8), respectively. Our study shows that these four GBs in monolayer MoS₂ exhibit a great disparity in their migration behavior. More specifically, the S4|6 and S6|8 GBs possess a much higher migration mobility than the S5|7 and Mo5|7 GBs under the same thermal fluctuations or temperature gradient. Interestingly, the S4|6 and S6|8 GBs follow an abnormal relationship with temperature, due to the change in defect configurations with temperature. Our study further shows that the remarkably high mobilities of the S4|6 and Mo6|8 GBs may enable the reactions of GBs, leading to the annihilation and reduction of defect density. In addition, the movement of GBs in MoS₂ under a temperature gradient field can cause defect redistribution, which in turn changes the thermal conductivity. The present study not only deepens our understanding of the dynamic evolution of GBs in TMDs, but also presents new opportunities to engineer GBs for novel electronic applications.

Mechanical Properties of Monolayer MoS2 with Randomly Distributed Defects

The variation of elastic constants stiffness coefficients with respect to different percentage ratios of defects in monolayer molybdenum disulfide (MLMoS2) is reported for a particular set of atomistic nanostructural characteristics. The common method suggested is to use conventional defects such as single vacancy or di vacancy, and the recent studies use stone-walled multiple defects for highlighting the differences in the mechanical and electronic properties of 2D materials. Modeling the size influence of monolayer MoS2 by generating defects which are randomly distributed for a different percentage from 0% to 25% is considered in the paper. In this work, the geometry of the monolayer MoS2 defects modeled as randomized over the domain are taken into account. For simulation, the molecular static method is adopted and study the effect of elastic stiffness parameters of the 2D MoS2 material. Our findings reveals that the expansion of defects concentration leads to a decrease in the elastic properties, the sheer decrease in the elastic properties is found at 25%. We also study the diffusion of Molybdenum (Mo) in Sulphur (S) layers of atoms within MoS2 with Mo antisite defects. The elastic constants dwindle in the case of antisite defects too, but when compared to pure defects, the reduction was to a smaller extent in monolayer MoS2. Nevertheless, the Mo diffusion in sulfur gets to be more and more isotropic with the increase in the defect concentrations and elastic stiffness decreases with antisite defects concentration up to 25%. The distribution of antisite defects plays a vital role in modulating Mo diffusion in sulfur. These results will be helpful and give insights in the design of 2D materials.

Defect-Mediated Alloying of Monolayer Transition-Metal Dichalcogenides

Alloying plays a central role in tailoring the material properties of 2D transition-metal dichalcogenides (TMDs). However, despite widespread reports, the details of the alloying mechanism in 2D TMDs have remained largely unknown and are yet to be further explored. Here, we combine a set of systematic experiments with ab initio density functional theory (DFT) calculations to unravel a defect-mediated mechanism for the alloying of monolayer TMD crystals. In our alloying approach, a monolayer MoSe₂ film serves as a host crystal in which exchanging selenium (Se) atoms with sulfur (S) atoms yields a MoS_{2 x}Se_{2(1- x)} alloy. Our study reveals that the driving force required for the alloying of CVD-grown films with abundant vacancy-type defects is significantly lower than that required for the alloying of exfoliated films with fewer vacancies. Indeed, we show that pre-existing Se vacancies in the host MoSe₂ lattice mediate the replacement of chalcogen atoms and facilitate the synthesis of MoS_{2 x}Se_{2(1- x)} alloys. Our DFT calculations suggest that S atoms can bind to Se vacancies and then diffuse throughout the host MoSe₂ lattice via exchanging the position with Se vacancies, further supporting our proposed defect-mediated alloying mechanism. Beside native vacancy defects, we show that the existence of large-scale defects in CVD-grown MoSe₂ films causes the fracture of alloys under the alloying-induced strain, while no such effect is observed in exfoliated MoSe₂ films. Our study provides a deep insight into the details of the alloying mechanism and enables the synthesis of 2D alloys with tunable properties.

Phase Transition of Single-Layer Molybdenum Disulfide Nanosheets under Mechanical Loading Based on Molecular Dynamics Simulations

The single-layer molybdenum disulfide (SLMoS2) nanosheets have been experimentally discovered to exist in two different polymorphs, which exhibit different electrical properties, metallic or semiconducting. Herein, molecular dynamics (MD) simulations of nanoindentation and uniaxial compression were conducted to investigate the phase transition of SLMoS2 nanosheets. Typical load-deflection curves, stress-strain curves, and local atomic structures were obtained. The loading force decreases sharply and then increases again at a critical deflection under the nanoindentation, which is inferred to the phase transition. In addition to the layer thickness, some related bond lengths and bond angles were also found to suddenly change as the phase transition occurs. A bell-like hollow, so-called residual deformation, was found to form, mainly due to the lattice distortion around the waist of the bell. The effect of indenter size on the residual hollow was also analyzed. Under the uniaxial compression along the armchair direction, a different phase transition, a uniformly quadrilateral structure, was observed when the strain is greater than 27.7%. The quadrilateral structure was found to be stable and exhibit metallic conductivity in view of the first-principle calculation.

Probing the shear modulus of two-dimensional multiplanar nanostructures and heterostructures

Generalized high-fidelity closed-form formulae have been developed to predict the shear modulus of hexagonal graphene-like monolayer nanostructures and nano-heterostructures based on a physically insightful analytical approach. Hexagonal nano-structural forms (top view) are common for nanomaterials with monoplanar (such as graphene and hBN) and multiplanar (such as stanene and MoS₂) configurations. However, a single-layer nanomaterial may not possess a particular property adequately, or multiple desired properties simultaneously. Recently, a new trend has emerged to develop nano-heterostructures by assembling multiple monolayers of different nanostructures to achieve various tunable desired properties simultaneously. Shear modulus assumes an important role in characterizing the applicability of different two-dimensional nanomaterials and heterostructures in various nanoelectromechanical systems such as determining the resonance frequency of vibration modes involving torsion, wrinkling and rippling behavior of two-dimensional materials. We have developed mechanics-based closed-form formulae for the shear modulus of monolayer nanostructures and multi-layer nano-heterostructures. New results of shear modulus are presented for different classes of nanostructures (graphene, hBN, stanene and MoS₂) and nano-heterostructures (graphene-hBN, graphene-MoS₂, graphene-stanene and stanene-MoS₂), which are categorized on the basis of fundamental structural configurations. The numerical values of shear modulus are compared with the results from the scientific literature (as available) and separate molecular dynamics simulations, wherein a good agreement is noticed. The proposed analytical expressions will enable the scientific community to efficiently evaluate shear modulus of a wide range of nanostructures and nanoheterostructures.

Grain-Size-Controlled Mechanical Properties of Polycrystalline Monolayer MoS2

Pristine monocrystalline molybdenum disulfide (MoS₂) possesses high mechanical strength comparable to that of stainless steel. Large-area chemical-vapor-deposited monolayer MoS₂ tends to be polycrystalline with intrinsic grain boundaries (GBs). Topological defects and grain size skillfully alter its physical properties in a variety of materials; however, the polycrystallinity and its role played in the mechanical performance of the emerging single-layer MoS₂ remain largely unknown. Here, using large-scale atomistic simulations, GB structures and mechanical characteristics of realistic single-layered polycrystalline MoS₂ of varying grain size prepared by confinement-quenched method are investigated. Depending on misorientation angle, structural energetics of polar-GBs in polycrystals favor diverse dislocation cores, consistent with experimental observations. Polycrystals exhibit grain-size-dependent thermally induced global out-of-plane deformation, although defective GBs in MoS₂ show planar structures that are in contrast to the graphene. Tensile tests show that presence of cohesive GBs pronouncedly deteriorates the in-plane mechanical properties of MoS₂. Both stiffness and strength follow an inverse pseudo Hall-Petch relation to grain size, which is shown to be governed by the weakest link mechanism. Under uniaxial tension, transgranular crack propagates with small deflection, whereas upon biaxial stretching, the crack grows in a kinked manner with large deflection. These findings shed new light in GB-based engineering and control of mechanical properties of MoS₂ crystals toward real-world applications in flexible electronics and nanoelectromechanical systems.

First-Principles Study on the Structural and Electronic Properties of Monolayer MoS₂ with S-Vacancy under Uniaxial Tensile Strain

Monolayer molybdenum disulfide (MoS₂) has obtained much attention recently and is expected to be widely used in flexible electronic devices. Due to inevitable bending in flexible electronic devices, the structural and electronic properties would be influenced by tensile strains. Based on the density functional theory (DFT), the structural and electronic properties of monolayer MoS₂ with a sulfur (S)-vacancy is investigated by using first-principles calculations under uniaxial tensile strain loading. According to the calculations of vacancy formation energy, two types of S-vacancies, including one-sulfur and two-sulfur vacancies, are discussed in this paper. Structural analysis results indicate that the existence of S-vacancies will lead to a slightly inward relaxation of the structure, which is also verified by exploring the change of charge density of the Mo layer and the decrease of Young’s modulus, as well as the ultimate strength of monolayer MoS₂. Through uniaxial tensile strain loading, the simulation results show that the band gap of monolayer MoS₂ decreases with increased strain despite the sulfur vacancy type and the uniaxial tensile orientation. Based on the electronic analysis, the band gap change can be attributed to the π bond-like interaction between the interlayers, which is very sensitive to the tensile strain. In addition, the strain-induced density of states (DOS) of the Mo-d orbital and the S-p orbital are analyzed to explain the strain effect on the band gap.

Abnormal nonlocal scale effect on static bending of single-layer MoS2

The nonlocal scale parameter of nonlocal Euler-Bernoulli beam theory is evaluated for the static bending of single-layer molybdenum disulfide (SLMoS₂) without predetermined bending rigidity. The evaluation is performed by matching the fitted curve between the maximum deflection and the beam length obtained from molecular mechanics simulations. It was observed that the fitted curves have an abnormal sign in the second-order term of the maximum deflection for SLMoS₂, opposite to that for graphene and regardless of the interatomic interaction potentials used. Based on the nature of 'nonlocal' and the phenomenological point of view, a modified nonlocal constitutive relation with a positive sign in front of the higher-order term is suggested for SLMoS₂. The nonlocal parameter and the bending rigidity of SLMoS₂ are finally extracted, and the effect of the nonlocal scale parameter on the bending response for SLMoS₂ is found to be significant for beam length less than a critical length, depending on both the interatomic interaction potentials and the boundary conditions. Our new perspective should be useful for researchers who are interested in the engineering application of graphene-like quasi-two-dimensional nanostructures using nonlocal beam theories.