Julia A. Baimova

Discrete breather clusters in strained graphene

Molecular-dynamics simulations based on many-body interatomic potentials are conducted to investigate the clusters of discrete breathers in graphene under in-plane homogeneous strain. It is found that the discrete breather clusters can easily be excited when the gap in the phonon density of states is introduced by the homogeneous strain. Clusters of up to four discrete breathers were studied. We demonstrate that such clusters are robust with respect to small perturbations and can have the lifetime of order of 103 oscillation periods. Partial energy exchange between discrete breathers in the clusters was observed under certain conditions. The possible role of discrete breather clusters in formation of lattice defects is discussed.

Discrete breathers in hydrogenated graphene

Discrete breathers (DBs) in graphane (fully hydrogenated graphene) are investigated using molecular dynamics simulations. It is found that the DB can be excited by applying an out-of-plane displacement on a single hydrogen atom of graphane. The vibration frequency of the DB lies either within the gap of the phonon spectrum of graphane or beyond its upper spectrum bound. Both soft and hard types of anharmonicity of the DB, which have not been found in the same system, are observed in graphane. The study shows that the DB is robust and its lifetime is affected by various factors including its anharmonicity type, its amplitude and frequency, and the force on the hydrogen atom that forms it, whose competition results in a complex mechanism for the lifetime determination. The investigation of the maximum kinetic energy of DBs reveals that they may function to activate or accelerate dehydrogenation of hydrogenated graphene at high temperatures.

Effect of strain on gap discrete breathers at the edge of armchair graphene nanoribbons

Linear and nonlinear vibration modes of strained armchair graphene nanoribbons with free edges are investigated by means of atomistic modeling. It is shown that phonon modes can be divided into two groups, the XY-modes with the displacements of atoms in the nanoribbon plane, and the Z-modes with atomic displacements normal to the nanoribbon plane. Strained nanoribbons possess a sufficiently wide gap in the phonon spectrum of the XY-modes so that a gap discrete breather (DB) can be excited. Large-amplitude DBs exist within the strain range 0.125?<??xx?<?0.20. At larger strains an attempt to excite a DB results in breaking of the nanoribbon, while at smaller strains the gap in the phonon spectrum of the XY-modes is either too narrow or absent. DBs can have energy up to 1 eV and the maximal DB energy is larger for smaller strain of the nanoribbon. A possible role of DBs in the fracture of strained graphene nanoribbons at finite temperatures is discussed.

Interfacial thermal conductance of a silicene/graphene bilayer heterostructure and the effect of hydrogenation.

van der Waals heterostructures, obtained by stacking layers of isolated two-dimensional atomic crystals like graphene (GE) and silicene (SE), are one of emerging nanomaterials for the development of future multifunctional devices. Thermal transport behaviors at the interface of these heterostructures play a pivotal role in determining their thermal properties and functional performance. Using molecular dynamics simulations, the interfacial thermal conductance G of an SE/GE bilayer heterostructure is studied. Simulations show that G of a pristine SE/GE bilayer at room temperature is 11.74 MW/m(2)K when heat transfers from GE to SE, and is 9.52 MW/m(2)K for a reverse heat transfer, showing apparent thermal rectification effects. In addition, G increases monotonically with both the temperature and the interface coupling strength. Furthermore, hydrogenation of GE is efficient in enhancing G if an optimum hydrogenation pattern is adopted. By changing the hydrogen coverage f, G can be controllably manipulated and maximized up to five times larger than that of pristine SE/GE. This study is helpful for understanding the interface thermal transport behaviors of novel van der Waals heterostructures and provides guidance for the design and control of their thermal properties.

Morphology and in-plane thermal conductivity of hybrid graphene sheets

This paper investigates the morphology and in-plane thermal conductivity of hybrid graphene sheets (HGSs), which consist of un-hydrogenated and single-side or double-side hydrogenated strips, via molecular dynamics simulation. The study shows that the hydrogenation styles and hydrogen coverage significantly affect the morphology and thermal conductivity of HGSs. The thermal conductivity of HGSs decreases dramatically, compared to that of pure graphene sheets, and the magnitude falls in the range of 30%-75%. Such differences are explained by conducting the phonon spectra analysis.

Thermal conductivity of silicene nanosheets and the effect of isotopic doping

This paper investigates the thermal conductivity of silicene nanosheets (SiNSs) via molecular dynamic simulation. It shows that the thermal conductivity of pristine SiNSs is about 25–30 W mK −1 and exhibits anisotropic behaviour. Moreover, it is found that isotopic doping is efficient in reducing the thermal conductivity of SiNSs. When SiNSs are randomly doped with 30 Si at the doping percentage of 50%, a maximum reduction of about 20% is obtained. This reduction can be increased when the dopants are arranged into a superlattice pattern. The thermal conductivity of these superlattice-structured SiNSs changes non-monotonically as the thickness of their lattice layers decreases. Detailed analysis of the phonon spectra demonstrates that the competing mechanism between the interface scattering and confinement effects of the phonon modes is responsible for this non-monotonical behaviour.

Thermal transport in a graphene–MoS2 bilayer heterostructure: a molecular dynamics study

With the availability of various types of two-dimensional materials such as graphene (GE) and MoS2, intensive efforts have been devoted to their van der Waals heterostructures obtained by vertically stacking them together for novel functionalities and applications. The thermal transport behavior of these heterostructures plays a pivotal role in determining their functional performance. This work studies the thermal transport in a GE–MoS2 bilayer heterostructure via molecular dynamics simulation. It is found that the in-plane thermal conductivity λB of the GE–MoS2 bilayer can be approximated by that of an isolated monolayer GE. The λB of an infinitely long GE–MoS2 bilayer is calculated to be 1037 W m−1 K−1, while its out-of-plane interface thermal conductance G is obtained as 5.81 MW m−2 K−1. The increase in the interface coupling strengths can dramatically increase G but has little effect on λB. On the other hand, G also increases with temperature because of the enhanced phonon coupling between GE and MoS2. This study is helpful for understanding the interface thermal transport behaviors of novel van der Waals heterostructures and could provide guidance for optimal design and control of their thermal properties.

Effect of Stone-Thrower-Wales defect on structural stability of graphene at zero and finite temperatures

Classical molecular dynamics with the AIREBO potential is used to investigate the effect of two types of the Stone-Thrower-Wales (STW) defects in an infinite graphene sheet on structural stability of graphene. The borders of the structural stability of graphene in the two-dimensional space of the strain components () are found for the two types of defects at 0?K. It is shown that the STW defects practically do not affect critical strain of graphene when tension is applied along the zigzag direction, while for tension along the armchair direction the defects reduce the graphene strength noticeably. The effect of temperature on the time until fracture for the defected graphene is studied for different values of strains.

Mechanical properties of crumpled graphene under hydrostatic and uniaxial compression

A molecular dynamics study is undertaken to estimate the mechanical responses of crumpled graphene subjected to hydrostatic compression and uniaxial compression, respectively. The crumpled graphene is found to be a non-Hookean medium showing a non-linear stress–strain relation even for small strain and this is explained by structural changes that start to occur when a small stress is applied. Analysis of the unloading curves suggests that the elasticity limit is achieved at smaller densities under hydrostatic compression rather than under uniaxial compression. Corners of double-folded graphene flakes are formed under hydrostatic compression, while uniaxial compression results mainly in formation of single folds with less damage of the graphene lattice.

Discrete breathers in graphane: Effect of temperature

The discrete breathers in graphane in thermodynamic equilibrium in the temperature range 50–600 K are studied by molecular dynamics simulation. A discrete breather is a hydrogen atom vibrating along the normal to a sheet of graphane at a high amplitude. As was found earlier, the lifetime of a discrete breather at zero temperature corresponds to several tens of thousands of vibrations. The effect of temperature on the decay time of discrete breathers and the probability of their detachment from a sheet of graphane are studied in this work. It is shown that closely spaced breathers can exchange energy with each other at zero temperature. The data obtained suggest that thermally activated discrete breathers can be involved in the dehydrogenation of graphane, which is important for hydrogen energetics.