Structure-dependent mechanical properties of self-folded two-dimensional nanomaterials

The design of self-folded two-dimensional nanomaterials (SF-2DNMs) has been proposed to greatly enhance the ductility of two-dimensional material assemblies. However, the dependences of the mechanical properties of SF-2DNMs on the folded geometries have not been fully clarified. In this paper, we develop a theoretical model to describe the mechanical properties of SF-2DNMs based on the shear-lag analysis. With this model, the load transfer behaviors in SF-2DNMs are demonstrated. The Young's modulus and tensile strength of SF-2DNMs are found to increase and then converge with the fold length, which agree well with the results of molecular dynamics simulations. Moreover, the phase diagrams of failure modes are obtained for SF-2DNMs and their stacked assemblies, providing design criteria for the geometries of SF-2DNMs. The structure-property relationship revealed in our study will provide useful guidelines for the structure design and property optimization of SF-2DNMs.

Geometrical nonlinear elasticity of axon under tension: A coarse-grained computational study

Axon bundles cross-linked by microtubule (MT) associate proteins and bounded by a shell skeleton are critical for normal function of neurons. Understanding effects of the complexly geometrical parameters on their mechanical properties can help gain a biomechanical perspective on the neurological functions of axons and thus brain disorders caused by the structural failure of axons. Here, the tensile mechanical properties of MT bundles cross-linked by tau proteins are investigated by systematically tuning MT length, axonal cross-section radius, and tau protein spacing in a bead-spring coarse-grained model. Our results indicate that the stress-strain curves of axons can be divided into two regimes, a nonlinear elastic regime dominated by rigid-body like inter-MT sliding, and a linear elastic regime dominated by affine deformation of both tau proteins and MTs. From the energetic analyses, first, the tau proteins dominate the mechanical performance of axons under tension. In the nonlinear regime, tau proteins undergo a rigid-body like rotating motion rather than elongating, whereas in the nonlinear elastic regime, tau proteins undergo a flexible elongating deformation along the MT axis. Second, as the average spacing between adjacent tau proteins along the MT axial direction increases from 25 to 125 nm, the Young's modulus of axon experiences a linear decrease whereas with the average space varying from 125 to 175 nm, and later reaches a plateau value with a stable fluctuation. Third, the increment of the cross-section radius of the MT bundle leads to a decrease in Young's modulus of axon, which is possibly attributed to the decrease in MT numbers per cross section. Overall, our research findings offer a new perspective into understanding the effects of geometrical parameters on the mechanics of MT bundles as well as serving as a theoretical basis for the development of artificial MT complexes potentially toward medical applications.

Mechanical anisotropy of two-dimensional metamaterials: a computational study

Metamaterials, rationally designed multiscale composite systems, have attracted extensive interest because of their potential for a broad range of applications due to their unique properties such as negative Poisson's ratio, exceptional mechanical performance, tunable photonic and phononic properties, structural reconfiguration, etc. Though they are dominated by an auxetic structure, the constituents of metamaterials also play an indispensable role in determining their unprecedented properties. In this vein, 2D materials such as graphene, silicene, and phosphorene with superior structural tunability are ideal candidates for constituents of metamaterials. However, the nanostructure-property relationship and composition-property relationship of these 2D material-based metamaterials remain largely unexplored. Mechanical anisotropy inherited from the 2D material constituents, for example, may substantially impact the physical stability and robustness of the corresponding metamaterial systems. Herein, classical molecular dynamics simulations are performed using a generic coarse-grained model to explore the deformation mechanism of these 2D material-based metamaterials with sinusoidally curved ligaments and the effect of mechanical anisotropy on mechanical properties, especially the negative Poisson's ratio. The results indicate that deformation under axial tensile load can be divided into two stages: bending-dominated and stretching-dominated, in which the rotation of junctions in the former stage results in auxetic behavior of the proposed metamaterials. In addition, the auxetic behavior depends heavily on both the amplitude/wavelength ratio of the sinusoidal ligament and the stiffness ratio between axial and transverse directions. The magnitude of negative Poisson's ratio increases from 0 to 0.625, with an associated increase of the amplitude/wavelength ratio from 0 to 0.225, and fluctuates at around 0.625, in good agreement with the literature, with amplitude/wavelength ratios greater than 0.225. More interestingly, the magnitude of negative Poisson's ratio increases from 0.47 to 0.87 with the increase of the stiffness ratio from 0.125 to 8, in good agreement with additional all-atom molecular dynamics simulations for phosphorene and molybdenum disulfide. Overall, these research findings shed light on the deformation mechanism of auxetic metamaterials, providing useful guidelines for designing auxetic 2D lattice structures made of 2D materials that can display a tunable negative Poisson's ratio.

A coarse-grained model for mechanical behavior of phosphorene sheets

The popularity of phosphorene (known as monolayer black phosphorus) in electronic devices relies on not only its superior electrical properties, but also its mechanical stability beyond the nanoscale. However, the mechanical performance of phosphorene beyond the nanoscale remains poorly explored owing to the spatiotemporal limitation of experimental observations, first-principles calculations, and atomistic simulations. To overcome this limitation, here a coarse-grained molecular dynamics (CG-MD) model is developed via a strain energy conservation approach to offer a new computational tool for the investigation of the mechanical properties of phosphorene beyond the nanoscale. The mechanical properties of a single phosphorene sheet are first characterized by all-atom molecular dynamics (AA-MD) simulations, followed by a force-field parameter optimization of the CG-MD model by matching these mechanical properties from AA-MD simulations. The intrinsic out-of-plane puckered feature is conserved in our CG-MD model, rendering mechanical anisotropy and heterogeneity in both the in-plane and out-of-plane directions preserved. The results indicate that our coarse-grained model is able to accurately capture the anisotropic in-plane mechanical performance of phosphorene and quantitatively reproduce Young's modulus, ultimate strength, and fracture strain under various environmental temperatures. Our CG-MD model can also capture the anisotropic out-of-plane bending stiffness of phosphorene. We demonstrate the applicability of our model in capturing the fracture toughness of phosphorene in both the armchair and zigzag directions by comparison with the results from AA-MD simulations. This CG-MD model proposed here offers greater capability to perform mechanical mesoscale simulations for phosphorene-based systems, allowing for a deeper understanding of the mechanical properties of phosphorene beyond the nanoscale, and the potential transferability of the developed force-field can help design hybrid phosphorene devices and structures.

van der Waals Graphene Kirigami Heterostructure for Strain-Controlled Thermal Transparency

Programming thermal transport across interfaces by engineering strain is of critical importance for exploring mechanical controllable and thermal manageable devices with multifunctionalities. Here, we report a van der Waals heterostructure that is composed of bilayer graphene kirigami with diverse layer cut patterns and assembly organizations and show that the thermal flow intensity across the van der Waals interfaces, named as thermal transparency, could be continuously regulated by applying an external in-plane tensile strain. The density of atomic interactions across the interfaces and the distribution of delocalized phonon modes in each graphene kirigami are elucidated to understand the underlying thermal transport mechanism and are also incorporated into a theoretical model for quantitative predictions of thermal conductance under mechanical strain. A proof-of-conceptual van der Waals graphene kirigami heterostructure by design is proposed and validated through extensive full-scale atomistic simulations on the feasibility and reliability of regulating the transparency ratio of thermal transport by mechanical strain, demonstrating its potential applications in thermal and electronic devices.

Abnormal linear elasticity in polycrystalline phosphorene

Unlike other two-dimension materials with planar crystalline structure, polycrystalline phosphorene sheets are almost linear elastic, resulting from its high bending stiffness due to its intrinsic buckled crystalline structure.