Role of defects in the mechanical properties of graphene-copper heterostructures.

Through molecular dynamics simulations of tensile tests, the role that vacancies and Stone-Wales defects play in the mechanical properties of sandwich-like heterostructures, composed by graphene and two symmetric copper layers at nanoscale, is studied. The dependence on the armchair and zigzag chiralities of the graphene layer is also investigated. During elastic deformation, defects negatively affect the mechanical response. However, defective systems can show an improvement of the plastic properties. Vacancies have a stronger impact compared to Stone-Wales defects. Elasticity, toughness, and ductility are enhanced along the zigzag chirality, while stiffness is improved along the armchair direction. The Poisson's ratio was calculated for all graphene-copper heterostructures. At a critical strain it becomes negative along the thickness direction, preserving the auxetic property at higher strains. In general, the behavior is governed by the graphene response. Our findings can be useful to understand the strengthening mechanism induced by this two-dimensional material in metals like copper and for the design of similar systems.

Effects of biaxial strain on the impurity-induced magnetism in P-doped graphene and N-doped silicene: a first principles study.

The effects of biaxial strain on the impurity-induced magnetism in P-doped graphene (P-graphene) and N-doped silicene (N-silicene) are studied by means of spin-polarized density functional calculations, using the supercell approach. The calculations were performed for three different supercell sizes 4 × 4, 5 × 5, and 6 × 6, in order to simulate three different dopant concentrations 3.1, 2.0 and 1.4%, respectively. For both systems, the calculated magnetic moment is 1.0 µ B per impurity atom for the three studied concentrations. From the analysis of the electronic structure and the total energy as a function of the magnetization, we show that a Stoner-type model describing the electronic instability of the narrow impurity band accounts for the origin of sp-magnetism in P-graphene and N-silicene. Under biaxial strain the impurity band dispersion increases and the magnetic moment gradually decreases, with the consequent collapse of the magnetization at moderate strain values. Thus, we found that biaxial strain induces a magnetic quantum phase transition in P-graphene and N-silicene.

Mechanical response of bilayer silicene nanoribbons under uniaxial tension.

Understanding the behaviour of nanoscale systems is of great importance to tailor their properties. To this aim, we investigate the Young's modulus (YM) of defect-free and defective armchair bilayer silicene nanoribbons (SNRs), at room temperature, as a function of length and distance between layers. In this study, we perform molecular dynamics simulations using the environment-dependent interatomic potential to describe the interaction of the Si atoms. We show that the Young's modulus of pristine and defective bilayer SNRs increases with the ribbon length exhibiting size dependence. In general, YM of defective bilayer SNRs is smaller than the value obtained for the defect-free case, as a result of the number of missing bonds. In all cases, as the interlayer distance increases YM decreases and the buckling increases. It is shown that the YM exhibits a quadratic interlayer distance dependence. Finally, when only one layer has a mono-vacancy defect, the atomic stress distribution of the pristine layer is affected by the presence of the vacancy. This effect can be considered as a "ghost vacancy" since the deterioration of the pristine layer is similar to that shown by the defective one. These results show that YM of pristine and defective bilayer SNRs could be tailored for a given length and interlayer distance. It is also found that the fracture stress and the fracture strain of defective bilayers are both smaller than those obtained for the defect-free ones.