Single-layer Molybdenum Disulfide Research Articles

Nonlinear Static Bending and Forced Vibrations of Single-Layer MoS2 with Thermal Stress.

Single-layer molybdenum disulfide (MoS2) has been a research focus in recent years owing to its extensive potential applications. However, how to model the mechanical properties of MoS2 is an open question. In this study, we investigate the nonlinear static bending and forced vibrations of MoS2, subjected to boundary axial and thermal stresses using modified plate theory with independent in-plane and out-of-plane stiffnesses. First, two nonlinear ordinary differential equations are obtained using the Galerkin method to represent the nonlinear vibrations of the first two symmetrical modes. Second, we analyze nonlinear static bending by neglecting the inertial and damping terms of the two equations. Finally, we explore nonlinear forced vibrations using the method of multiple scales for the first- and third-order modes, and their 1:3 internal resonance. The main results are as follows: (1) The thermal stress and the axial compressive stress reduce the MoS2 stiffness significantly. (2) The bifurcation points of the load at the low-frequency primary resonance are much smaller than those at high frequency under single-mode vibrations. (3) Temperature has a more remarkable influence on the higher-order mode than the lower-order mode under the 1:3 internal resonance.

Properties of Molybdenum Disulfide and Preparation Method

Behind the development of integrated circuits to the current brilliant era is the contribution of Moore's Law, which has guided the development prospects of the industry for a long time. But as transistors continue to shrink in size and thickness, silicon itself approaches its own physical limits and Moore's Law no longer applies. Therefore, scientists are eager to solve this puzzle by introducing new materials, including two-dimensional materials. Molybdenum disulfide has attracted much attention because of its special properties, and the preparation method needs to be studied deeply for large-scale use of molybdenum disulfide. There are many factors that affect the synthesis of single layer molybdenum disulfide, such as the amount of reaction source, air pressure, substrate, reaction temperature of two sources, source heating rate, holding time, relative position between molybdenum source and sulfur source substrate, gas flow, etc. In short, these reaction conditions are summarized, which mainly affect the concentration and reaction speed of sulfur source and molybdenum source in the reaction substrate position. In this paper, the effects of CVD on the quality of single layer disulfide were summarized and its development trend was prospected.

Diffusion and magnetization of metal adatoms on single-layer molybdenum disulfide at elevated temperatures

The present work models temperature-dependent (500−1300K) diffusion dynamics of Ag, Au, and Cu adatoms on MoS2 as well as electronic and magnetic properties of adatom (Ag, Au, and Cu)/MoS2 systems. Modeling is done by means of ab initio molecular dynamics (AIMD) simulations that account for van der Waals corrections and electronic spin degrees of freedom in the framework of density functional theory. It is found that Ag and Au adatoms exhibit super-diffusive motion on MoS2 at all temperatures, while Cu adatoms follow a random walk pattern of uncorrelated surface jumps. The observed behavior is consistent with AIMD-calculated effective migration barriers Ea (EaAg=190±50meV, EaAu=67±7meV, and EaCu=300±100meV) and can be understood on the basis of the considerably flatter potential energy landscapes encountered by Ag and Au adatoms on the MoS2 surface (corrugation of the order of tens of meV), as compared to Cu adatoms (corrugation >100meV). Moreover, evaluation of the electronic and magnetic properties of AIMD configurations suggest that Ag, Au, and Cu monomer adsorption induces semimetallic features in at least one spin channel of the adatom/MoS2 electronic structure at elevated temperatures. The overall results presented herein may provide insights into fabricating 2D-material-based heterostructure devices beyond graphene.

Effect of strain on electronic properties of tri-layer MoS2/h-BN/graphene van der Waals heterostructures

The two-dimensional materials have grabbed the attention of researchers because of their high functionality and widely controllable properties. Due to the dimensionality reduction, the external factors have a significant influence on their properties. The density functional theory calculations have been performed on the heterostructures of single-layer graphene (G), boron nitride (BN) and molybdenum disulfide (MoS2) to investigate the alteration in their electronic properties. Our calculations predict that electronic properties such as bandgap, work function, Schottky barrier height and tunnelling barrier probability can be modulated by applying mechanical strain. The application of strain resulted in a band gap opening of about 1 eV in trilayer heterostructures. At low values of strain, heterostructures maintained a low resistance ohmic contact. At −9% strain, the p-to-n-type transition of Schottky contact was observed indicating the flow of electrons from semiconductor to metal. The MoS2 monolayer was found to be more electron deficient and exhibited a greater charge accumulation. MoS2/BN/G heterostructure can be used for the fabrication of capacitive devices. Whereas BN/G/MoS2 and BN/MoS2/G heterostructures can act as potential candidates for trilayer Schottky devices due to the presence of low-barrier metal-semiconductor junction.

Stiffer Bonding of Armchair Edge in Single-Layer Molybdenum Disulfide Nanoribbons.

The physical and chemical properties of nanoribbon edges are important for characterizing nanoribbons and applying them in electronic devices, sensors, and catalysts. The mechanical response of molybdenum disulfide nanoribbons, which is an important issue for their application in thin resonators, is expected to be affected by the edge structure, albeit this result is not yet being reported. In this work, the width-dependent Young's modulus is precisely measured in single-layer molybdenum disulfide nanoribbons with armchair edges using the developed nanomechanical measurement based on a transmission electron microscope. The Young's modulus remains constant at ≈166GPa above 3nm width, but is inversely proportional to the width below 3nm, suggesting a higher bond stiffness for the armchair edges. Supporting the experimental results, the density functional theory calculations show that buckling causes electron transfer from the Mo atoms at the edges to the S atoms on both sides to increase the Coulomb attraction.

Supported Pt Nanoclusters on Single-Layer MoS2 for the Detection of Cortisol: From Atomistic Scale to Device Modeling

The development of nonenzymatic sensors is a challenge which requires, on the one hand, careful design of the sensing materials with respect to the chosen analyte, and on the other hand, suitable device architectures. In this work, we propose single-layer molybdenum disulfide (MoS2) decorated with subnanometer Pt clusters as the sensing platform for the detection of cortisol. The aim is to assess the suitability of such a sensing platform for the development of wearable and portable cortisol sensors. For this study, we performed multiscale computer simulations at the materials level up to device scale. First, ab initio simulations within the framework of density functional theory (DFT) allowed us to gain insights into the interaction, at the atomic level, between the analyte (cortisol) and the sensing platform (MoS2/Pt). Then, by carrying out technology computer-aided design (TCAD) simulations, we were able to consider a device architecture and investigate its performance as cortisol sensor. Following our multiscale simulation strategy, we were able to assess the proposed field-effect transistor (FET) sensor, whose channel is made of Pt-decorated MoS2. The sensing mechanism relies on the chemiresistive response of the device to the adsorption of cortisol on the channel, which leads to a sizable charge transfer from the analyte to the substrate and, consequently, to the measurable shift in the gate voltage threshold of the FET. Our findings suggest that both the choice of the sensing materials and the proposed FET architecture are suitable for detecting cortisol by non-enzymatic means. In the best case scenario, we predict theoretical gate voltage shifts between 76 and 780 mV with respect to the cluster concentration and between 27 and 780 mV when varying the cluster occupancy by cortisol. We may expect our results to provide the necessary basis to develop highly sensitive nonenzymatic cortisol sensors based on 2D materials decorated with Pt nanoclusters.

Energy transfer and patterning characteristics in pulsed-laser subtractive manufacturing of single layer of MoS2

The single-layer Molybdenum disulfide (MoS2) was subtracted by using pulsed lasers with different parameters. The machining results were tested by optical microscopy, Raman spectrum, photoluminescent (PL) spectrum, scanning electron microscopy (SEM), and atomic force microscopy (AFM), indicating that the femtosecond laser machining has a better boundary effect than the nanosecond laser. Molecular dynamics (MD) were used to simulate the subtractive manufacturing effect and thermal transfer under different pulse widths and energy conditions. It has been proved that the short-width pulsed laser is more suitable for patterning MoS2. During the MD simulation, wrinkles and defects were found during the laser machining of long-width pulsed laser. The band structure of MoS2 was calculated by density functional theory (DFT) to investigate the effect of different wrinkle and defect conditions on the nonlinear laser absorption. The effect of defects on MoS2 thermal transfer was explored by plasma construction defects. The energy transfer during long-width pulsed laser machining was obtained. This work provides fundamental understanding of laser patterning and machining of MoS2 for future device applications.

Enhancement and Sensing Application of Ultra-Narrowband Circular Dichroism in the Chiral Nanostructures Based on Monolayer MoS2 and a Distributed Bragg Reflector.

Narrowband circular dichroism (CD) has aroused wide concerns in high-sensitivity detections of chiral molecular and chiral catalysis. Nevertheless, the dynamical adjustment of ultra-narrowband (UNB) CD signals is hard to achieve. In this work, single-layer molybdenum disulfide (MoS2), vanadium dioxide (VO2), and a distributed Bragg reflector (DBR) are introduced into X-shaped chiral nanostructures (XCNs) for overcoming the above challenge. The simulation results show that XCNs can generate four strong UNB CD signals in the near-infrared band, and XCNs/MoS2 can further enhance the UNB CD signals. The full width at half-minimum of UNB CD signals can reach 0.14 nm. The electric field distributions of XCNs/MoS2 show that the four CD signals originate from the coupling between the guided mode resonances along the x and y axes in the VO2 layer and the Tamm plasmon polaritons along the x and y axes in the DBR layer. Four UNB CD peaks can be actively tuned by varying the structural parameters, the number of MoS2 layers, and the environmental temperature. The FOM of XCNs/MoS2 can reach 1487 by changing the refractive index of the DRB layers. These findings contribute to the design of UNB chiral devices and provide new possibilities for environmental monitoring and ultrasensitive detection of chiral molecules.

Growth Mechanism of Single-Domain Monolayer MoS2 Nanosheets on Au(111) Revealed by In Situ Microscopy: Implications for Optoelectronics Applications

The nucleation and growth of single-layer molybdenum disulfide single-domain nanosheets is investigated by in situ low-energy electron microscopy. We study the growth of micrometer-sized flakes and the correlated flattening process of the gold surface for three different elevated temperatures. Furthermore, the influence of surface step edges on the molybdenum disulfide growth process is revealed. We show that both nanosheet and underlying terrace grow simultaneously by pushing the surface step in the expansion process. Our findings point to an optimized growth procedure allowing for step-free, single-domain, single-layer islands of several micrometers in size, which is likely transferable to other transition-metal dichalcogenides (TMDs), offering a very fine degree of control over the TMD nanosheet structure and thickness.

Nonlinear Stiffness and Nonlinear Damping in Atomically Thin MoS2 Nanomechanical Resonators

We report on experimental measurements and quantitative analyses of nonlinear dynamic characteristics in ultimately thin nanomechanical resonators built upon single-layer, bilayer, and trilayer (1L, 2L, and 3L) molybdenum disulfide (MoS2) vibrating drumhead membranes. This synergistic study with calibrated measurements and analytical modeling on observed nonlinear responses has led to the determination of nonlinear damping and stiffness coefficients at cubic and quintic orders for these two-dimensional (2D) resonators operating in the very high frequency (VHF) band (up to ∼90 MHz). We find that the quintic force can be ∼20% of the Duffing force at larger amplitudes, and thus, it generally cannot be ignored in a nonlinear dynamics analysis. This study provides the first quantification of nonlinear damping and frequency detuning characteristics in 2D semiconductor nanomechanical resonators and elucidates their origins and dependency on engineerable parameters, setting a foundation for future exploration and utilization of the rich nonlinear dynamics in 2D nanomechanical systems.

Understanding the size and chirality dependence of bending stiffness of single-layer MoS2 by a spring-driven method

Single-layer molybdenum disulfide ($\mathrm{SL}{\mathrm{MoS}}_{2}$) is a promising candidate in next-generation flexible electronics owing to its atomic thickness and excellent semiconductor property. Bending stiffness is an essential property for assessing the deformation and function of flexible devices, but its accurate evaluation remains a challenge. In this study, a spring-driven bending method is proposed to predict the bending stiffness of $\mathrm{SL}{\mathrm{MoS}}_{2}$ based on molecular dynamics simulation. This method can effectively constrain $\mathrm{SL}{\mathrm{MoS}}_{2}$ to arbitrary curvatures without the limitation of characteristic size. The results indicate that the bending property of the small-size $\mathrm{SL}{\mathrm{MoS}}_{2}$ is highly size and chirality dependent, but that of the large-size $\mathrm{SL}{\mathrm{MoS}}_{2}$ is isotropic. The bending stiffness of $\mathrm{SL}{\mathrm{MoS}}_{2}$ with different characteristic sizes (1--50 nm) ranges 4.88--8.87 eV, which is ascribed to the competition from different regions of $\mathrm{SL}{\mathrm{MoS}}_{2}$ with different bending resistances. Furthermore, a theoretical formula is established to estimate the size- and chirality-dependent bending stiffness of $\mathrm{SL}{\mathrm{MoS}}_{2}$. The research findings provide a computational method for evaluating the bending property of ultrathin two-dimensional (2D) materials, which offers a valuable strategy for designing the 2D materials-based flexible components with adjustable bending performance.

Quantum dots bind nanosheet to promote nanomaterial stability and resist endotoxin-induced fibrosis and PM2.5-induced pneumonia

Endotoxin lipopolysaccharide (LPS) is a harmful substance commonly found in various environments that causes lung fibrosis. Exposure to PM2.5 also increases the risk of respiratory diseases. Through sulfur-carbon bonds and the edge S effect, GOQDs were used to bind in single-layer molybdenum disulfide (SLMoS2) nanosheets to synthesize SLMoS2@GOQDs heterojunction structures. GOQDs doping greatly increased the water solubility and stabilized of SLMoS2. SLMoS2@GOQDs with catalase-like activity protected cells from ultrastructural and cytomembrane damage and apoptosis induced by LPS. Moreover, the doping of GOQDs enhanced the escape of SLMoS2@GOQDs from cellular uptake and suppressed the release of Mo ions. Nanosheet-cell interface interactions that were regulated by quantum dots supported these positive effects. Immunofluorescence analysis and cell imaging confirmed that the nanomaterial protected against cell injury by regulating the canonical Wnt/β-catenin pathway and the secretion of relevant cytokines, such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α). Moreover, SLMoS2@GOQDs also mitigated pneumonia caused by PM2.5 in vivo. Collectively, our findings not only provide a simple and effective approach to control lung diseases (caused by LPS or PM2.5), but also reveal the potential value of heterojunction materials in the fields of toxicology and human health, boosting the application of nanotechnology in the fields of ecotoxicology and environmental safety.

Extracellular polymeric substances mediate defect generation and phytotoxicity of single-layer MoS2

Two-dimensional transition metal dichalcogenide (TMDC) nanomaterials have attracted tremendous research interest in various fields, but the effects of eco-corona formation on the transformation mechanisms and ecological risk of TMDCs remain largely unknown. The effect of eco-corona formation on TMDC reactivity was explored using extracellular polymeric substances (EPS) as the eco-corona constituents and single-layer molybdenum disulfide (SLMoS2) as the model TMDC. We found that EPS promoted lattice distortion and the formation of defects (sulfur vacancies and pores) on SLMoS2 after it was aged (precoated) with EPS under simulated visible-light irradiation. In addition, the EPS-corona induced higher free radical (especially hyperoxide radical) photogeneration by SLMoS2. Furthermore, compared to pristine SLMoS2, SLMoS2-EPS exhibited stronger developmental inhibition, oxidative stress, membrane damage, photosynthetic toxicity and metabolic perturbation effects on Chlorella vulgaris. However, the endocytosis pathway (especially macropinocytosis) of SLMoS2 entry into C. vulgaris was inhibited by EPS. Metabolic and transcriptomic analyses revealed that the enhanced toxicity of SLMoS2-EPS was associated with the downregulation of fatty acid metabolism and transcription related to photosynthesis, respectively. The present work provides mechanistic insights into the roles of the EPS-corona on the environmental transformation and phytotoxicity of TMDCs, which benefit environmental safety assessments and sustainable applications of engineered nanomaterials.

A Comparative Investigation of the Mechanical Properties of Single and Bi Layer MoS2 Structures: Influences of Defect, Strain Rate and Temperature

In this paper, the mechanical properties of single and bi layer molybdenum disulfide (MoS2) structures are investigated using uniaxial tensile molecular dynamics (MD) simulation. According to the results of MD simulations, these structures show superior mechanical properties (failure strain, ultimate tensile strength and Young’s modulus) for various applications of nanodevice. The mechanical properties of single and bi layer MoS2 structures are studied at four different temperatures between 300 K and 900 K and different strain rates from 107 s-1 to 109 s-1. As temperature increases up to 900 K, the mechanical properties of single and bi layer MoS2 structures gradually decrease, due to the high temprerature’s weakening effect. Also, changing of temperatures shows more effect on the bi layer MoS2 structure than single layer MoS2 structure. Furthermore, MD results show that the mechanical properties of single and bi layer MoS2 structures demonstrate increasing trend when the strain rate increases. Different strain rates indicate similar effects on the mechanical properties of single and bi layer MoS2 structures. On the other hand, the mechanical properties of these structures are adversely affected by structural defects. Accordingly, the influences of two different S atom types vacancy defect on the mechanical properties of single and bi layer MoS2 structures are examined. When the vacancy defect concentrations in MoS2 structures increase, the mechanical properties of these structures decrease significantly. In addition, S atom bi vacancy defects type exerts more effect on the mechanical properties of single and bi layer MoS2 structures than S atom single vacancy defect type do by increasing concentration. Additionally, vacancy defects indicate more influence on the bi layer MoS2 structure than single layer MoS2 structure. Finally, the results of this study make them excellent candidate for nano-mechanical systems.

First-Principles Study on the Effect of Strain on Single-Layer Molybdenum Disulfide.

By adopting the first-principles plane wave pseudopotential method based on density functional theory, the electronic structure properties of single-layer MoS2 (molybdenum disulfide) crystals under biaxial strain are studied. The calculation results in this paper show that when a small strain is applied to a single-layer MoS2, its band structure changes from a direct band gap to an indirect band gap. As the strain increases, the energy band still maintains the characteristics of the indirect band gap, and the band gap shows a linear downward trend. Through further analysis of the density of states, sub-orbital density of states, thermodynamic parameters and Raman spectroscopy, it revealed the variation of single-layer MoS2 with strain. This provides a theoretical basis for realizing the strain regulation of MoS2.

Single nucleotide discrimination with sub-two nanometer monolayer graphene pore

The effective sensing area is the key point of solid-state nanopore single-molecule detection, which is determined by the diameter of the hole and the thickness of the membrane. High spatial resolution can be achieved by using single-atom thickness two-dimensional materials, such as single-layer graphene and molybdenum disulfide. Helium ion beam microscopy provides an effective technique for manufacturing small-sized nanopores. Here, we show that the precise fabrication of sub-2 nanometer nanopores has been successfully achieved on a single-layer graphene film, which provides the best spatial resolution in room-temperature ionic liquid-assisted single-molecule detection. Many graphene nanopores of different sizes have been fabricated, with a minimum pore diameter of 1.4 nm. The application of these graphene nanopores manufactured by HIM shows that single-layer graphene nanopores can distinguish different homopolymer single-stranded DNA lengths and types, and can even identify four single nucleotides.

Tunable Smith–Purcell radiation from MoS2-based grating

Different from the traditional tunable Smith–Purcell (SP) radiation in the graphene-based gratings in the terahertz band, we propose a tunable SP radiation generated from an electron beam passing through a single-layer molybdenum disulfide (MoS2) based grating in the visible band. The comparison between the simulation and the theoretical results shows good agreement. By varying the Fermi energy of MoS2 from 0.025 eV to 0.125 eV for the MoS2-based grating, we can not only control the radiation frequency but also can change the radiation magnitude. The radiation frequency, angle, and magnitude varying with the Fermi energy are also discussed, respectively. These properties would have potential applications in developing tunable visible SP radiation.

Nanopore creation in MoS2 and graphene monolayers by nanoparticles impact: a reactive molecular dynamics study

In this work, extensive reactive molecular dynamics simulations are conducted to analyze the nanopore creation by nanoparticles impact over single-layer molybdenum disulfide (MoS2) with 1T and 2H phases. We also compare the results with graphene monolayer. In our simulations, nanosheets are exposed to a spherical rigid carbon projectile with high initial velocities ranging from 2 to 23 km/s. Results for three different structures are compared to examine the most critical factors in the perforation and resistance force during the impact. To analyze the perforation and impact resistance, kinetic energy and displacement time history of the projectile as well as perforation resistance force of the projectile are investigated. Interestingly, although the elasticity module and tensile strength of the graphene are by almost five times higher than those of MoS2, the results demonstrate that 1T and 2H-MoS2 phases are more resistive to the impact loading and perforation than graphene. For the MoS2nanosheets, we realize that the 2H phase is more resistant to impact loading than the 1T counterpart. Our reactive molecular dynamics results highlight that in addition to the strength and toughness, atomic structure is another crucial factor that can contribute substantially to impact resistance of 2D materials. The obtained results can be useful to guide the experimental setups for the nanopore creation in MoS2or other 2D lattices.

Toward miniaturizing microelectronics using covalent organic framework dielectric

Toward miniaturizing microelectronics using covalent organic framework dielectric

Nanocolloids, but Not Humic Acids, Augment the Phytotoxicity of Single-Layer Molybdenum Disulfide Nanosheets.

Engineered nanomaterials (ENMs), especially transition metal dichalcogenide (TMDC), have received great attention in recent years due to their advantageous properties and applications in various fields and are inevitably released into the environment during their life cycle. However, the effect of natural nanocolloids, widely distributed in the aquatic environment, on the environmental transformation and ecotoxicity of ENMs remains largely unknown. In this study, the effects of natural nanocolloids were compared to humic acid on the environmental transformation and ecotoxicity of single-layer molybdenum disulfide (SLMoS2), a representative TMDC. SLMoS2 with nanocolloids or humic acid (HA) enhanced their dispersion and Mo ion release in deionized water. Nanocolloids induced growth inhibition, reactive oxygen species (ROS) elevation, and cell permeability. Low-toxicity SLMoS2 combined with nanocolloids will enhance the above adverse effects. SLMoS2-nanocolloids induced serious damage (cell distortion and deformation), SLMoS2 internalization, and metabolic perturbation on Chlorella vulgaris (C. vulgaris). In contrast, the addition of HA induced the growth promotion and lower ROS level, inhibited the internalization of SLMoS2, and mitigated metabolic perturbation on C. vulgaris. This work provides insights into the effect of natural nanocolloids on the behaviors and biological risks of ENMs in aquatic environments, deserving substantial future attention.