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.

Formation of Coherent 1H-1T Heterostructures in Single-Layer MoS2 on Au(111)

Heterojunctions of semiconductors and metals are the fundamental building blocks of modern electronics. Coherent heterostructures between dissimilar materials can be achieved by composition, doping, or heteroepitaxy of chemically different elements. Here, we report the formation of coherent single-layer 1H-1T MoS₂ heterostructures by mechanical exfoliation on Au(111), which are chemically homogeneous with matched lattices but show electronically distinct semiconducting (1H phase) and metallic (1T phase) character, with the formation of these heterojunctions attributed to a combination of lattice strain and charge transfer. The exfoliation approach employed is free of tape residues usually found in many exfoliation methods and yields single-layer MoS₂ with millimeter (mm) size on the Au surface. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS) have collectively been employed to elucidate the structural and electronic properties of MoS₂ monolayers on Au substrates. Bubbles in the MoS₂ formed by the trapping of ambient adsorbates beneath the single layer during deposition, have also been observed and characterized. Our work here provides a basis to produce two-dimensional heterostructures which represent potential candidates for future electronic devices.

Structure and Properties of Single-Layer MoS₂ for Nano-Photoelectric Devices

To meet the need for preparing high-performance nano-optoelectronic devices based on single-layer MoS₂, the effects of the heating method (one-step or two-step heating) and the temperature of the MoO₃ source on the morphology, size, structure, and layers of an MoS₂ crystal grown on a sapphire substrate using chemical vapor deposition are studied in this paper. The results show that MoS₂ prepared by two-step heating (the heating of the S source starts when the temperature of the MoO₃ source rises to 837 K) is superior over that of one-step heating (MoO₃ and S are heated at the same time). One-step heating tends to form a mixture of MoO₂ and MoS₂. Neither too low nor too high of a heating temperature of MoO₃ source is conducive to the formation of MoS₂. When the temperature of MoO₃ source is in the range of 1073 K to 1098 K, the size of MoS₂ increases with the rise in temperature. A uniform large-sized triangle with a side length of 100 μm is obtained when the heating temperature of MoO₃ is 1098 K. The triangular MoS₂ crystals grown by the two-step heating method have a single-layer structure.

Generating Sub-nanometer Pores in Single-Layer MoS2 by Heavy-Ion Bombardment for Gas Separation: A Theoretical Perspective

Single-layer molybdenum disulfide (MoS₂) filters with nanometer-size pores have attracted great attention recently due to their promising performance for membrane separation. Generating nanopores in MoS₂ controllably, however, is still a challenging task, which greatly limits the real application of MoS₂ filters. In this work, the pore forming process in single-layer MoS₂ by heavy-ion bombardment was investigated in detail using molecular dynamics simulations. We found that pores with sub-nanometer size (0.6-1.2 nm) can be created in the MoS₂ sheet by single-ion bombardment, with a probability as high as 0.8 pores per incident ion. The size and shape of the nanopore can be tuned controllably by adjusting bombardment parameters. Furthermore, the performance of the MoS₂ filter with these sub-nanometer-size pores for separation of He, Ne, H₂, Ar, and Kr gases was evaluated by density functional theory-based first-principles calculations. The MoS₂ filter was found to show much higher selectivity for separating H₂/He and He/Ne than that reported for graphene and other membranes. Such high selectivity was attributed to the interaction between gases and the charged edge of pores in MoS₂. Our results suggest the potential application of ion beam technology in single-layer MoS₂ for membrane separation.

2D Monolayer MoS₂-Carbon Interoverlapped Superstructure: Engineering Ideal Atomic Interface for Lithium Ion Storage

A novel strategy for the controlled synthesis of 2D MoS2/C hybrid nanosheets consisting of the alternative layer-by-layer interoverlapped single-layer MoS2 and mesoporous carbon (m-C) is demonstrated. Such special hybrid nanosheets with a maximized MoS2 /m-C interface contact show very good performance for lithium-ion batteries in terms of high reversible capacity, excellent rate capability, and outstanding cycling stability.

Single-layer MoS(2) mechanical resonators

Mechanical resonators are fabricated from freely suspended single-layer MoS2 . Their dynamics  have been studied by optical interferometry. These resonators behave as membranes with resonance frequencies in between 10 and 30 MHz and quality factors in between 16 and 109. We also demonstrate clear signatures of nonlinear resonance in these atomically thin resonators.