Tension-induced phase transition of single-layer molybdenum disulphide (MoS2) at low temperatures

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 ≈166 GPa above 3 nm width, but is inversely proportional to the width below 3 nm, 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.

Mechanical Behaviors in Janus Transition-Metal Dichalcogenides: A Molecular Dynamics Simulation

In this work, molecular dynamics simulations are performed to investigate the mechanical properties of Janus WSSe and MoSSe monolayers considering the effects of structural anisotropy, temperature, and tensile strain rates. The results demonstrate that Janus WSSe and MoSSe monolayers show strong mechanical anisotropy under tension along the armchair and zigzag directions, respectively. This anisotropy displays distinct temperature dependence. When the coupled effects of the temperature and anisotropy are considered for the tensions along the zigzag direction, there is a transition of ductile-to-brittle failure in the Janus WSSe monolayer at the critical temperature range of 80~90 K due to the competition between atomic thermal vibrations and structural bending/wrinkles. This phenomenon is further confirmed by both stress–strain curves and structural evolutions of the systems. Finally, a strain rate hardening mechanism is found when various strain rates are applied, and it demonstrates that the Janus monolayer could maintain stable mechanical properties under different loading conditions. Our investigations provide a helpful reference for subsequent theoretical and experimental studies on the mechanical properties of Janus monolayer structures and could shed some light on the design of promising nanoscale functional devices based on Janus transition-metal dichalcogenides.

Self-Assembly of MoS2 Monolayer Sheets by Desulfurization

Self-assembled structures of two-dimensional (2D) materials exhibit novel physical properties distinct from those of their parent materials. Herein, the critical role of desulfurization on the self-assembled structural morphologies of molybdenum disulfide (MoS₂) monolayer sheets is explored using molecular dynamics (MD) simulations. MD results show that there are differences in the atomic energetics of MoS₂ monolayer sheets with different desulfurization contents. Both free-standing and substrate-hosted MoS₂ monolayer sheets show diversity in structural morphologies, for example, flat plane structures, wrinkles, nanotubes, and folds, depending on the desulfurization contents, planar dimensions, and ratios of length to width of MoS₂ sheets. Particularly, at the critical desulfurization content, they can roll up into nanotubes, which is in good agreement with previous experimental observations. Importantly, these observed differences in the molecular structural morphologies between free-standing and substrate-hosted MoS₂ monolayer sheets can be attributed to interatomic interactions and interlayer van der Waals interactions. Furthermore, MD results have demonstrated that the surface-driven stability of MoS₂ structures can be indicated by the desulfurization contents on one surface of MoS₂ monolayer sheets, and the self-assembly of MoS₂ monolayer sheets by desulfurization can emerge to adjust their surface-driven stability. The study provides important atomic insights into tuning the self-assembling structural morphologies of 2D materials through defect engineering in the future science and engineering applications.

A 2D Rashba electron gas with large spin splitting in Janus structures of SnPbO2

For realizing two-dimensional (2D) spintronic devices, controllable manipulation of different spin channels is needed. In order to achieve this goal, the materials should have well-separated spin energies induced by spin-orbit coupling (SOC). However, a negligible SOC effect in 2D SnO limits its application in this prospect. Herein, we demonstrate that the stable Janus structure of SnPbO₂ can exhibit large spin splitting (67 meV) at the valence band maximum (VBM) due to the breakdown of inverse symmetry. This spin splitting is larger than the thermal fluctuation at room temperature as well as some other 2D systems. In addition, the splitting can be further enhanced by strains (82 meV) or hole doping (147 meV). More importantly, there are no other electronic states except for the Rashba states at the VBM, which make it ideal for practical applications. Our study provides alternative ways to regulate the electronic structure of SnO and the feasibility for spin manipulation in this interesting 2D system.

Mechanical properties and superconductivity in two-dimensional B2O under extreme strain

Because of the particular characteristics of layered materials, searching for new two-dimensional (2D) structures has become the foundation of future device manufacture. Using first-principles calculations and theoretical analysis, we identified a new monolayer B2O and systematically investigated its fundamental mechanical and electronic properties, especially under strain. Due to the complicated puckered hinge structure of monolayer B2O, it possesses an intrinsic negative Poisson's ratio of -0.021. Moreover, we found that the superconducting state of monolayer B2O can be improved, where strain plays an effective role in regulating the properties of the 2D material. The current results elucidate the structure and corresponding properties of monolayer B2O, which may stimulate related fundamental research and potential nanoscale applications.

Graphynes for Water Desalination and Gas Separation

Selective transport of mass through membranes, so-called separation, is fundamental to many industrial applications, e.g., water desalination and gas separation. Graphynes, graphene analogs yet containing intrinsic uniformly distributed pores, are excellent candidates for highly permeable and selective membranes owing to their extreme thinness and high porosity. Graphynes exhibit computationally determined separation performance far beyond experimentally measured values of commercial state-of-the-art polyamide membranes; they also offer advantages over other atomically thin membranes like porous graphene in terms of controllability in pore geometry. Here, recent progress in proof-of-concept computational research into various graphynes for water desalination and gas separation is discussed, and their theoretically predicted outstanding permeability and selectivity are highlighted. Challenges associated with the future development of graphyne-based membranes are further analyzed, concentrating on controlled synthesis of graphyne, maintenance of high structural stability to withstand loading pressures, as well asthe demand for accurate computational characterization of separation performance. Finally, possible directions are discussed to align future efforts in order to push graphynes and other 2D material membranes toward practical separation applications.

Intrinsic Electric Field-Induced Properties in Janus MoSSe van der Waals Structures

Focusing on two-dimensional (2D) Janus MoSSe monolayers, we show that simultaneously existing in-plane and out-of-plane intrinsic electric fields cause Zeeman- and Rashba-type spin splitting, respectively. In MoSSe van der Waals (vdW) structures, intrinsic electric field results in a large interlayer band offset. Therefore, large interlayer band offset, being the driving force for interlayer excitons, endows ultralong lifetimes to excitons and might dissociate excitons into free carriers. In comparison to its parent structure (i.e., MoS₂), MoSSe vdW structures are rather appealing for new concepts in light-electricity interconversion. In addition, the Rashba effects could be tuned by changing the interlayer distances due to the competition between the intralayer and interlayer electric field. Due to the large band offset, valley polarization relaxation is markedly reduced, promising enhanced valley polarization and ultralong valley lifetimes. As a result, MoSSe vdW structures harbor strong valley-contrasting physics, making them competitive systems to their parent structures.

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 chirality-dependent elastic properties and crack propagation behavior of single and bilayer stanene

Mechanical properties of stanene, a promising quasi-two-dimensional honeycomb-like nanostructure of tin belonging to the family of 2D-Xenes (X = Si, Ge, Sn), have been investigated in this paper.

Structure, Stability, and Kinetics of Vacancy Defects in Monolayer PtSe2: A First-Principles Study

The recent epitaxial growth of monolayer PtSe₂ has raised hope for its novel applications in valleytronic, spintronic, and energy-harvesting devices. Compared with 2H-phase transition-metal dichalcogenides, the 1T-phase PtSe₂ is much less studied and this is especially true for its defects behaviors and their influence on electronic properties. In this article, we systemically explore the structure, stability, and kinetics of both Pt and Se vacancies in monolayer PtSe₂ using first-principles calculations. By examining the relative energies of these vacancies, we identify the most stable Se/Pt single and double vacancies. In particular, we reveal a new type of Se double vacancy structure with the lowest energy. Energetically, both Se and Pt single vacancies prefer to combine to form double vacancies. All Se and Pt vacancies have remarkable influence on the electronic properties. Moreover, Pt single and double vacancies can introduce strong spin polarization in PtSe₂, which may be promising for spintronic applications. These findings not only enrich the fundamental understanding of 1T-phase PtSe₂ but also provide useful guidance to design PtSe₂ for its optoelectronic and spintronic applications.

Effects of atomic vacancies and temperature on the tensile properties of single-walled MoS2nanotubes

Using molecular dynamics simulations, we study the effects of Mo and S atomic vacancies and different temperatures on the tensile properties of single-walled MoS₂nanotubes through a series of tensile tests.

Fracture behaviors of pre-cracked monolayer molybdenum disulfide: A molecular dynamics study

The fracture strength and crack propagation of monolayer molybdenum disulfide (MoS2) sheets with various pre-existing cracks are investigated using molecular dynamics simulation (MDS). The uniaxial tensions of pre-cracked monolayer MoS2 sheets with different crack tips, different locations of crack, different crack lengths and angled cracks are simulated and studied. The results show that the configuration of crack tip can influence significantly the fracture behaviors of monolayer MoS2 sheets while the location of crack does not influence the fracture strength. With the increase of crack length, the fracture strength of monolayer MoS2 sheets reduces almost linearly, and the fracture of monolayer MoS2 sheets is transformed from almost brittle to ductile. By making comparison between the MDS results and the predictions of continuum fracture mechanics theories, including Inglis' model, Griffith's model with and without finite size effect, it is found that MDS results agree well with the predictions of Griffith's model with finite size effect, differ from the predictions of Inglis' model and Griffith's model without finite size effect. Finally, the MDS results of monolayer MoS2 sheets with different angled crack are also analyzed based on the continuum fracture mechanics model.

Superhigh moduli and tension-induced phase transition of monolayer gamma-boron at finite temperatures

Two dimensional (2D) gamma-boron (γ-B₂₈) thin films have been firstly reported by the experiments of the chemical vapor deposition in the latest study. However, their mechanical properties are still not clear. Here we predict the superhigh moduli (785 ± 42 GPa at 300 K) and the tension-induced phase transition of monolayer γ-B₂₈ along a zigzag direction for large deformations at finite temperatures using molecular dynamics (MD) simulations. The new phase can be kept stable after unloading process at these temperatures. The predicted mechanical properties are reasonable when compared with our results from density functional theory. This study provides physical insights into the origins of the new phase transition of monolayer γ-B₂₈ at finite temperatures.

Room Temperature Semiconductor-Metal Transition of MoTe2 Thin Films Engineered by Strain

We demonstrate a room temperature semiconductor-metal transition in thin film MoTe2 engineered by strain. Reduction of the 2H-1T' phase transition temperature of MoTe2 to room temperature was realized by introducing a tensile strain of 0.2%. The observed first-order SM transition improved conductance ∼10 000 times and was made possible by an unusually large temperature-stress coefficient, which results from a large volume change and small latent heat. The demonstrated strain-modulation of the phase transition temperature is expected to be compatible with other TMDs enabling the 2D electronics utilizing polymorphism of TMDs along with the established materials.

Highly stretchable MoS2 kirigami

We report the results of classical molecular dynamics simulations focused on studying the mechanical properties of MoS2 kirigami. Several different kirigami structures were studied based upon two simple non-dimensional parameters, which are related to the density of cuts, as well as the ratio of the overlapping cut length to the nanoribbon length. Our key findings are significant enhancements in tensile yield (by a factor of four) and fracture strains (by a factor of six) as compared to pristine MoS2 nanoribbons. These results, in conjunction with recent results on graphene, suggest that the kirigami approach may be generally useful for enhancing the ductility of two-dimensional nanomaterials.