Numerical distortion and effects of thermostat in molecular dynamics simulations of single-walled carbon nanotubes

In this paper, single-walled carbon nanotubes （SWCNTs） are studied through molecular dynamics （MD） simulation. The simulations are performed at temperatures of 1 and 300K separately, with atomic interactions characterized by the second Reactive Empirical Bond Order （REBO） potential, and temperature controlled by a certain thermostat, i.e. by separately using the velocity scaling, the Berendsen scheme, the Nose-Hoover scheme, and the generalized Langevin scheme. Results for a （5,5） SWCNT with a length of 24.5 nm show apparent distortions in nanotube configuration, which can further enter into periodic vibrations, except in simulations using the generalized Langevin thermostat, which is ascribed to periodic boundary conditions used in simulation. The periodic boundary conditions may implicitly be applied in the form of an inconsistent constraint along the axis of the nanotube. The combination of the inconsistent constraint with the cumulative errors in calculation causes the distortions of nanotubes. When the generalized Langevin thermostat is applied, inconsistently distributed errors are dispersed by the random forces, and so the distortions and vibrations disappear. This speculation is confirmed by simulation in the case without periodic boundary conditions, where no apparent distortion and vibration occur. It is also revealed that numerically induced distortions and vibrations occur only in simulation of nanotubes with a small diameter and a large length-to-diameter ratio. When MD simulation is applied to a system with a particular geometry, attention should be paid to avoiding the numerical distortion and the result infidelity.

Influence of chirality on the thermal conductivity of single-walled carbon nanotubes

The influence of chirality on the thermal conductivity of single-walled carbon nanotubes （SWNTs） is discussed in this paper, using a non-equilibrium molecular dynamics （NEMD） method. The tube lengths of the SWNTs studied here are 20, 50, and 100 nm, respectively, and at each length the relationship between chiral angle and thermal conductivity of a SWNT is revealed. We find that if the tube length is relatively short, the influence of chirality on the thermal conductivity of a SWNT is more obvious and that a SWNT with a larger chiral angle has a greater thermal conductivity. Moreover, the thermal conductivity of a zigzag SWNT is smaller than that of an armchair one. As the tube length becomes longer, the thermal conductivity increases while the influence of chirality on the thermal conductivity decreases.

Energy Absorption Characteristics of Single-walled Carbon Nanotubes

The excellent mechanical properties of carbon nanotubes make them potential candidates for engineering application. In this paper, the impact and failure behaviors of single-walled carbon nanotubes （SWCNTs） are investigated. The effects of diameter, length, and chirality on their energy absorption characteristics under lateral impact and axial crush are studied. By integrating the principle of molecular structural mechanics （MSM） into finite element method （FEM）, the locations and directions of fracture process can be predicted. It is shown that the specific energy absorption （SEA） of SWCNTs is 1-2 order of magnitude higher than that of the ordinary metallic materials and composites in axial impact, indicating that carbon nanotubes are promising energy absorption materials for engineering applications.

Hexagon-Islands Density and Size Distribution on Growing Single-Walled Carbon Nanotubes in the Kinetic 5-Vertex Model

A kinetic 5-vertex model is used to investigate hexagon-islands formation on growing single-walled carbon nanotubes (SWCNT). In the model, carbon atoms adsorption and migration processes on the SWCNT edge are considered. These two dynamic processes are assumed to be mutually independent as well as mutually dependent as far as the whole growth of the nanotube is concerned. Key physical parameters of the model are the growth time t, the diffusion length Γ defined as the ratio of the diffusion rate D to the carbon atomic flux F and the SWCNT chiral angle. The kinetic equation that describes the nanotube edge dynamics is solved using kinetic Monte Carlo simulations with the Bortz, Kalos and Lebowitz update algorithm. The behaviors of islands density and size distribution are investigated within the growth parameters’ space. Our study revealed key mechanisms that enable the formation of a new ring of hexagons at the SWCNT edge. The growth occurs either by pre-existing steps propagation or by hexagon-islands growth and coalescence on terraces located between dislocation steps, depending on values of model parameters. This should offer a road map for edge design in nanotubes production. We also found that in appropriate growth conditions, the islands density follows Gaussian and generalized Wigner distributions whereas their size distribution at a given growth time shows a decreasing exponential trend.

MD Simulation of Structural and Mechanical Transformation of Single-Walled Carbon Nanotubes Under Pressure

We investigate the structural and mechanical properties of single-walled carbon nanotubes(SWNTs)under hydrostatic pressure,using constant-pressure molecular dynamics(MD)simulations.We observed that all the SWNTs,independent of their size and chirality,behave like a classical elastic ring exhibiting a buckling transition transforming their cross-sectional shape from a circle to an ellipse.The simulated critical transition pressure agrees well with the prediction from continuum mechanics theory,even for the smallest SWNT with a radius of 0.4nm.Accompanying the buckling shape transition,there is a mechanical hardness transition,upon which the radial moduli of the SWNTs decrease by two orders of magnitude.Further increase of pressure will eventually lead to a second transition from an elliptical to a peanut shape.The ratio of the second shape transition pressure over the first one is found to be very close to a constant of∼1.2,independent of the tube size and chirality.

Single-Wall Carbon Nanotube Growth from Graphite Layers-a Tight Binding Molecular Dynamics Simulation

The growth of single-wall carbon nanotube from graphite layers is studied by tight binding molecular dynamics simulation. Given temperature of 2500 K or 3500 K and an interval of 0.25 nm for the two layers of graphite, a single-wall carbon nanotube with a zigzag shell will be produced. On the other conditions the carbon nanotube cannot grow or grows with too many defects. All carbon nanotube ends have pentagons which play an important role during the tube ends closing.

Nonlocal material properties of single-walled carbon nanotubes

Natural frequencies of single-walled carbon nanotubes(SWCNTs)obtained using a model based on Eringen’s nonlocal continuum mechanics and the Timoshenko beam theory are compared with those obtained by molecular dynamics simulations.The goal was to determine the values of the material constant,considered here as a nonlocal property,as a function of the length and the diameter of SWCNTs.The present approach has the advantage of eliminating the SWCNT thickness from the computations.A sensitivity analysis of natural frequencies to changes in the nonlocal material constant is also carried out and it shows that the influence of the nonlocal effects decreases with an increase in the SWCNT dimensions.The matching of natural frequencies shows that the nonlocal material constant varies with the natural frequency and the SWCNT length and diameter.

Asymmetry of the water flux induced by the deformation of a nanotube

The behavior of nano-confined water is expected to be fundamentally different from the behavior of bulk water.At the nanoscale,it is still unclear whether water flows more easily along the convergent direction or the divergent one,and whether a hourglass shape is more convenient than a funnel shape for water molecules to pass through a nanotube.Here,we present an approach to explore these questions by changing the deformation position of a carbon nanotube.The results of our molecular dynamics simulation indicate that the water flux through the nanotube changes significantly when the deformation position moves away from the middle region of the tube.Different from the macroscopic level,we find water flux asymmetry（water flows more easily along the convergent direction than along the divergent one）,which plays a key role in a nano water pump driven by a ratchet-like mechanism.We explore the mechanism and calculate the water flux by means of the Fokker-Planck equation and find that our theoretical results are well consistent with the simulation results.Furthermore,the simulation results demonstrate that the effect of deformation location on the water flux will be reduced when the diameter of the nanochannel increases.These findings are helpful for devising water transporters or filters based on carbon nanotubes and understanding the molecular mechanism of biological channels.

H2O分子的微观特性在碳纳米管中的MD研究

通过采用分子动力学模拟的计算机模拟方法,研究了水分子在(6,6)碳纳米管中的运动现象,分析了H2O分子分别在温度为75 K、150 K和300 K下,受限于碳纳米管中的32个SPC/E模型中H2O分子的O—O的径向分布曲线图.模拟实验得到水分子随着温度的升高,热运动越来越剧烈,有序化程度大大的降低.RDF曲线的最高峰出现在2.8处,说明在该点出现其他H2O分子的可能性最大.

Aggregation of Glycerol Induced by Carbon Nanotubes in Aqueous Solution and Its Influencing Factors

Carbon nanotubes（CNTs） have received wide application and investigation because of their unique electronic, chemical and mechanical properties. But the self-aggregation of CNTs limits their practical application and study. In order to disperse CNTs effectively, polymers, such as polyglycerol and its derivatives, are adopted as dispersants in view of their strong interaction with CNTs. In order to understand the interaction between CNTs and glycerol in water in detail, a series of simulations has been conducted to investigate the interaction between them and analyze the influences of CNTs diameter and temperature. All the analyses indicate that the glycerol molecules are prone to aggregate around CNTs with the addition of CNTs. This is mainly due to hydrophobic interaction. It is confirmed that this aggregation is influenced by CNTs diameter and the temperature to some degree. This work will establish the basis for the exploration of polyglycerol and its derivatives interacting with CNTs and provide an invaluable guide to seek for emergent dispersants for CNTs.

低能碳原子通过单壁碳纳米管的计算机模拟研究

利用分子动力学(MD)模拟对低能碳原子通过单壁碳纳米管的运动轨迹进行了研究。通过改变不同的入射条件,得出粒子在横向平面和三维空间中的运动轨迹,结果表明:在满足一定的入射条件时,粒子在单壁碳纳米管中作受制约的运动或作折线或作螺旋线式的运动。

纳米材料的螺旋生长（英文）

因其独特的结构特点,纳米材料的螺旋生长一直备受关注。在本综述中,主要采用分子动力学方法来模拟指定材料间的相互作用。模拟结果证明,石墨烯纳米带能够自发螺旋嵌入碳纳米管,亦能螺旋缠绕在碳纳米管外壁,而这一结果已被实验证实。同时,封闭的石墨烯纳米环可填充到碳纳米管空腔中形成类似DNA的双螺旋结构,亦能螺旋塌陷在碳纳米管外壁形成大螺距双层螺旋结构。模拟发现,石墨烯纳米带与石墨烯纳米环均可螺旋缠绕在金属纳米线外壁形成碳-金属壳核结构,这一机理可用于制备壳核复合结构。另外,我们论述了硅纳米粒子在碳纳米锥与碳纳米管表面的螺旋形核机制。通过相互作用机制和热力学模型的建立来进一步研究纳米材料的螺旋生长。据推断,螺旋结构具有最低的能量且具有最高的空间利用率。同时,实验证明,极性面以及螺旋位错的存在也可驱动晶体结晶过程中的螺旋生长。到目前,尽管涉及纳米材料螺旋生长的研究非常广泛,但还需要更详实的后续研究来解释螺旋生长的作用机制。

多壁碳纳米管与基体之间界面热输运分子动力学模拟

碳纳米管与基体之间的热耦合对纳米结构界面热输运起主导作用。采用非平衡态分子动力学模拟了垂直生长多壁碳纳米管与Si基体之间的热耦合,得到界面热导的温度效应和作用力效应并利用声子输运理论进行了分析。低温下多壁碳纳米管与Si之间声子的态密度匹配较好,界面热输运能力主要取决于界面两侧低频声子的耦合振动,高温下近界面区域内热输运能力受高频声子的非弹性散射影响而逐渐增强。随着范德瓦尔斯力增强,界面热导线性增大。

论分子在受限碳纳米管中的模拟研究

21世纪得益于计算机技术的快速发展,分子动力学模拟技术也得到了很大的进步。相对于实验,分子动力学模拟的优越性不可替代。分子动力学模拟能够为实验提供更加准确的理论依据,有助于避免在实验过程中可能会遇到的各种无效操作,能够使实验更加有效。对于一些假设,尽管其提出有充分的理论依据,但在实验过程中依然不可避免的会遇到各种困难,而利用MD模拟的方法去验证这些假设则相对容易的多。该文主要通过对现阶段受限碳纳米管中分子动力学模拟的研究,从理论上分析了受限碳纳米管的性质和模拟方向,归纳总结了受限碳纳米管的一些性质和应用,进一步为以后的研究做准备。

Water and Ion Permeation through Electrically Charged Nanopore

The behaviour of water and small solutes in confined geometries is important to a variety of chemical and nanofluidic applications. Here we investigate the permeation and distribution of water and ions in electrically charged carbon cylindrical nanopore during the osmotic process using molecular dynamics simulations. In the simulations, charges are distributed uniformly on the pores with diameter of 0.9 nm. For nanopores with no charge or a low charge, ions are difficult to enter. With the increasing of charge densities on the pores, ions will appear inside the nanopores because of the large electronic forces between the ions and the charged pores. Different ion entries induce varying effects on osmotic water flow. Our simulations reveal that the osmotic water can flow through the negatively charged pore occupied by K^＋ ions, while water flux through the positively charged pores will be disrupted by Cl^- ions inside the pores. This may be explained by the different radial distributions of K^＋ ions and Cl^- ions inside the charged nanopores.