Transforming graphene nanoribbons into nanotubes by use of point defects

Buckling kinetics of graphene membranes under uniaxial compression

Despite past investigations of the buckling instability, the kinetics of the buckling process is not well understood. We develop a generic framework for determining the buckling kinetics of membranes under compressive stress (σ_{b}) via molecular dynamics simulations. The buckling time (t_{b}) is modeled by an extended Boltzmann-Arrhenius-Zhurkov equation accounting for temperature (T) and scale-dependent bending rigidity. We discern three regimes: (I) t_{b} decreases with T; (II) t_{b} increases with T; (III) t_{b} is T independent. Regime II coheres with the predictions of the theory of fluctuating sheets (TFS). Regime I is seen at small scales due to fluctuations about equilibrium and is not predicted by the TFS.

Width Dependent Elastic Properties of Graphene Nanoribbons

The mechanical response of graphene nanoribbons under uniaxial tension, as well as its dependence on the nanoribbon width, is presented by means of numerical simulations. Both armchair and zigzag edged graphene nanoribbons are considered. We discuss results obtained through two different theoretical approaches, viz. density functional methods and molecular dynamics atomistic simulations using empirical force fields especially designed to describe interactions within graphene sheets. Apart from the stress-strain curves, we calculate several elastic parameters, such as the Young’s modulus, the third-order elastic modulus, the intrinsic strength, the fracture strain, and the Poisson’s ratio versus strain, presenting their variation with the width of the nanoribbon.

Efficient Mechanical Stress Transfer in Multilayer Graphene with a Ladder-like Architecture

We report that few graphene flakes embedded into polymer matrices can be mechanically stretched to relatively large deformation (>1%) in an efficient way by adopting a particular ladder-like morphology consisting of consecutive mono-, bi-, tri-, and four-layer graphene units. In this type of flake architecture, all of the layers adhere to the surrounding polymer inducing similar deformation on the individual graphene layers, preventing interlayer sliding and optimizing the strain transfer efficiency. We have exploited Raman spectroscopy to quantify this effect from a mechanical standpoint. The finite element method and molecular dynamics simulations have been used to interpret the above experimental findings. The results suggest that a step pyramid-like architecture of a flake can be ideal for efficient loading of layered materials embedded into a polymer and that there are two prevailing mechanisms that govern axial stress transfer, namely, interfacial shear transfer and axial transmission through the ends. This concept can be easily applied to other two-dimensional materials and related van der Waals heterostructures fabricated either by mechanical exfoliation or chemical vapor deposition by appropriate patterning. This work opens new perspectives in numerous applications, including high volume fraction composites, flexible electronics, and straintronic devices.

Chaotic dynamics of graphene and graphene nanoribbons

We study the chaotic dynamics of graphene structures, considering both a periodic, defect free, graphene sheet and graphene nanoribbons (GNRs) of various widths. By numerically calculating the maximum Lyapunov exponent, we quantify the chaoticity for a spectrum of energies in both systems. We find that for all cases, the chaotic strength increases with the energy density and that the onset of chaos in graphene is slow, becoming evident after more than 10⁴ natural oscillations of the system. For the GNRs, we also investigate the impact of the width and chirality (armchair or zigzag edges) on their chaotic behavior. Our results suggest that due to the free edges, the chaoticity of GNRs is stronger than the periodic graphene sheet and decreases by increasing width, tending asymptotically to the bulk value. In addition, the chaotic strength of armchair GNRs is higher than a zigzag ribbon of the same width. Furthermore, we show that the composition of ¹²C and ¹³C carbon isotopes in graphene has a minor impact on its chaotic strength.

Compressive response and buckling of graphene nanoribbons

We examine the mechanical response of single layer graphene nanoribbons (GNR) under constant compressive loads through molecular dynamics simulations. Compressive stress-strain curves are presented for GNRs of various lengths and widths. The dependence of GNR's buckling resistance on its size, aspect ratio, and chiral angle is discussed and approximate corresponding relations are provided. A single master curve describing the dependence of the critical buckling stress of GNRs on their aspect ratio is presented. Our findings were compared to the continuum elasticity theories for wide plates and wide columns. In the large width limit, the response of the GNRs agrees with the predictions of the wide plates theory and thus, with that of wide graphenes. In the small width limit, the behavior of graphene nanoribbons deviates from that of periodic graphenes due to various edge related effects which govern the stiffness and the stability of the graphene membranes, but it qualitatively agrees with the theory of wide columns. In order to assess the effect of thermal fluctuations on the critical buckling stress a wide range of temperatures is examined. The findings of the current study could provide important insights regarding the feasibility and the evaluation of the performance of graphene-based devices.