Molecular dynamics simulations of electrowetting in double-walled carbon nanotubes

On the basis of the atomistic simulations of electrowetting in single-walled carbon nanotubes, electrowetting of double-walled carbon nanotubes by mercury is studied using classical molecular dynamics simulations. Wetting of double-walled carbon nanotubes by mercury occurs above a threshold size of inner tube when the voltage is applied on the outer tube, but no wetting phenomenon appears when the voltage is applied on the inner tube. The filling rate increases greatly with enlarging the inner tube size. The space between the two walls of double-walled carbon nanotubes cannot be filled by mercury during electrowetting process.     

液体水银在碳纳米管中传输的压力控制模型

碳纳米管在纳米技术中有一个很重要的应用,就是它可以作为纳米管道传输液体. 本文用分子动力学模拟方法,结合一个液体压力控制模型,对液体水银由于外部压力作用在碳纳米管中传输的现象进行研究. 研究结果表明,当液体水银的内部压强超过一个临界值时,液体水银能够浸入到碳纳米管内部;不断增大液体内部的压强,碳纳米管可以连续地传输液体;而当对液体水银进行循环加压时,碳纳米管可以间断地传输液体水银. 关键词： 碳纳米管 流体传输 分子动力学     

液体水银在碳纳米管中传输的压力控制模型

碳纳米管在纳米技术中有一个很重要的应用，就是它可以作为纳米管道传输液体. 本文用分子动力学模拟方法，结合一个液体压力控制模型，对液体水银由于外部压力作用在碳纳米管中传输的现象进行研究. 研究结果表明，当液体水银的内部压强超过一个临界值时，液体水银能够浸入到碳纳米管内部；不断增大液体内部的压强，碳纳米管可以连续地传输液体；而当对液体水银进行循环加压时，碳纳米管可以间断地传输液体水银.     

双壁碳纳米管电浸润现象的分子动力学模拟

在单壁碳纳米管电浸润现象原子模拟的基础上,对双壁碳纳米管的电浸润现象进行了计算机模拟.运用经典分子动力学方法结合一个宏观的电毛细管模型模拟了双壁碳纳米管在水银中的电浸润过程,对不同内管尺寸情况下的浸润现象作了研究和比较.计算结果表明双壁碳管和单壁碳管的电浸润过程存在很大的不同,双壁碳管的内管在电浸润过程中起到重要的作用：当改变双壁碳管中内管的尺寸时,浸润现象会产生很大的改变. 关键词： 双壁碳纳米管 电浸润 分子动力学     

Specific features of low-frequency vibrational dynamics and low-temperature heat capacity of double-walled carbon nanotubes

A continuous model has been constructed for low-frequency dynamics of a double-walled carbon nanotube. The formation of the low-frequency part of the phonon spectrum of a double-walled nanotube from phonon spectra of its constituent single-walled nanotubes has been considered in the framework of the proposed approach. The influence of the environment on the phonon spectrum of a single double-walled carbon nanotube has been analyzed. A combined method has been proposed for estimating the coefficients of the van der Waals interaction between the walls of the nanotube from the spectroscopic data and the known values of the elastic moduli of graphite. The low-temperature specific heat has been calculated for doublewalled carbon nanotubes, which in the field of applicability of the model (T < 35 K) is substantially less than the sum of specific heats of two individual single-walled nanotubes forming it.     

Codoping of Potassium and Bromine in Carbon Nanotubes： A Density Functional Theory Study

We investigate the co-doping of potassium and bromine in single-walled carbon nanotubes （SWCNTs） and doublewalled carbon nanotubes （DWCNTs） based on density functional theory. In the co-doped （6,0） SWCNTs, the 4s electron of potassium is transferred to nanotube and Br, leading to the n-type feature of SWCNTs. When potassium is intercalated into inner tube and bromine is put on outer tube, the positive and negative charges reside on the outer and inner tubes of the （7,0）@（16,0） DWCNT, respectively. It is expected that DWCNTs would be an ideal candidate for p-n junction and diode applications.     

On the role of interband surface plasmons in carbon nanotubes

We review the properties of collective surface excitations—excitons and interband plasmons—in single-walled and double-walled carbon nanotubes. We show that an electrostatic field applied perpendicular to the nanotube axis can control the exciton-plasmon coupling in individual small-diameter (≲nm) singlewalled nanotubes, both in the linear excitation regime and in the non-linear excitation regime with the photoinduced biexcitonic states formation. For double-walled carbon nanotubes, we report a profound effect of interband surface plasmons on the inter-tube Casimir force at tube separations similar to their equilibrium distances. Strong overlapping plasmon resonances from both tubes warrant their stronger attraction. Nanotube chiralities possessing such collective excitation features will result in forming the most favorable innerouter tube combination in double-walled carbon nanotubes. These findings pave the way for the development of new generation of tunable optoelectronic and nano-electromechanical device applications with carbon nanotubes.     

Thermal conductivity of multi-walled carbon nanotubes:Molecular dynamics simulations

Heat conduction in single-walled carbon nanotubes(SWCNTs) has been investigated by using various methods, while less work has been focused on multi-walled carbon nanotubes(MWCNTs). The thermal conductivities of the double-walled carbon nanotubes(DWCNTs) with two different temperature control methods are studied by using molecular dynamics(MD) simulations. One case is that the heat baths(HBs) are imposed only on the outer wall, while the other is that the HBs are imposed on both the two walls. The results show that the ratio of the thermal conductivity of DWCNTs in the first case to that in the second case is inversely proportional to the ratio of the cross-sectional area of the DWCNT to that of its outer wall. In order to interpret the results and explore the heat conduction mechanisms, the inter-wall thermal transport of DWCNTs is simulated. Analyses of the temperature profiles of a DWCNT and its two walls in the two cases and the interwall thermal resistance show that in the first case heat is almost transported only along the outer wall, while in the second case a DWCNT behaves like parallel heat transport channels in which heat is transported along each wall independently.This gives a good explanation of our results and presents the heat conduction mechanisms of MWCNTs.     

多壁碳纳米管储氢的物理吸附特性

采用巨正则蒙特卡罗方法 ,模拟常温、1 0MPa下氢在扶手椅型多壁壁碳纳米管中的物理吸附过程 .氢分子之间、氢分子与碳原子之间的相互作用采用Lennard Jones势能模型 .研究了双壁碳纳米管外 (内 )径固定而内 (外 )径改变时的物理吸附储氢情况 ,发现氢分子主要储存在双壁碳纳米管的管壁附近 ,当双壁碳纳米管的内外管壁间距由 0 .34nm增大到 0 .6 1或 0 .88nm时可有效增加物理吸附储氢量 ,并给出了相应的理论解释 .在此基础上 ,计算了管壁间距为 0 .34、0 .6 1和 0 .88nm时的三壁碳纳米管的物理吸附储氢量 ,并与相同条件下单壁和双壁碳纳米管的物理吸附储氢量作了比较 ,发现多壁碳纳米管的物理吸附储氢量随碳管层数的增加而减小 .     

Molecular dynamic investigation of mechanical properties of armchair and zigzag double-walled carbon nanotubes under various loading conditions

Using molecular dynamic simulation (MDS), effects of chirality and Van der Waals interaction on Young's modulus, elastic compressive modulus, bending, tensile, and compressive stiffness, and critical axial force of double-walled carbon nanotube (DWCNT) and its inner and outer tubes are considered. Achieving the highest safety factor, mechanical properties have been investigated under applied load on both inner and outer tubes simultaneously and on each one of them separately. Results indicate that as a compressive element, DWCNT is more beneficial than single-walled carbon nanotube (SWCNT) since it carries two times higher compression before buckling. Except critical axial pressure and tensile stiffness, in other parameters zigzag DWCNT shows higher amounts than armchair type. Outer tube has lower strength than inner tube; therefore, most reliable design of nanostructures can be attained if the mechanical properties of outer tube taken as the properties of DWCNT.     

Curvature-induced metallization of double-walled semiconducting zigzag carbon nanotubes

We report total-energy electronic-structure calculations that provide energetics and electronic structures of double-walled carbon nanotubes consisting of semiconducting (n,0) nanotubes. We find that optimum spacing between the walls of the nanotubes is slightly larger than the interlayer spacing of the graphite. We also find that the electronic structures of the double-walled nanotubes with the inner (7,0) nanotube are metallic with multicarrier characters in which electrons and holes exist on inner and outer nanotubes, respectively. Interwall spacing and curvature difference are found to be essential for the electron states around the Fermi level.     

Frequency change by inter-walled length difference of double-wall carbon nanotube resonator

Ultrahigh frequency nanomechanical resonators based on double-walled carbon nanotubes with different wall lengths were investigated via classical molecular dynamics simulations. For a double-walled carbon nanotube resonator with a short outer wall, the free edge of the short outer wall plays an important role in the vibration of the long inner wall. For a double-walled carbon nanotube resonator with a short inner wall, the short inner wall can be considered as a flexible core, thus, the fundamental frequency is influenced by its length. By controlling the length of the inner or outer wall, various frequency devices can be realized by a single type of double-walled carbon nanotube with walls of equal length.     

Nanotube-based nanoelectromechanical systems

Nanoelectromechanical systems based on multiwalled carbon nanotubes are considered. Control of motion and modes of operation of these systems are discussed. The structure of double-walled carbon nanotubes with atomic structural defects that can be used as bolt-nut pairs is analyzed. Energy barriers and threshold forces for relative motion of walls along and across the “thread” are computed for double-walled nanotubes with various types of defects. It is found that the qualitative characteristics of the thread are independent of the type of defect. Feasibility of fabricating double-walled nanotubes for use as bolt-nut pairs by self-organization is discussed.     

Double-walled carbon nanotube with surrounding elastic medium under axial pressure

In this paper, the buckling behavior and critical axial pressure of double-walled carbon nanotubes (DWCNTs) with surrounding elastic medium are investigated. A double-shell (circular cylindrical shell) model is presented and the effects of surrounding elastic medium on the outer tube and the van der Waals forces between two adjacent tubes are taken into account. The analysis and the numerical solution method are based on the classical theory of plates and shells and the Galerkin method. Equations are derived for the critical axial forces and pressures of DWCNTs; the critical axial forces and pressures are calculated for different axial half sine wavenumbers and circumferential sine wavenumbers and compared with those for single-walled carbon nanotubes (SWCNTs).Results indicate that the critical axial force of a DWCNT is higher than that of an SWCNT, but the critical axial pressure of a DWCNT is lower than the critical axial pressure of a SWCNT. Although the critical axial force of a DWCNT decreases as the axial half sine wavenumbers increase, it rises as the circumferential sine wavenumbers increase.     

Observation of fluid layering and reverse motion in double-walled carbon nanotubes

Most modelling-based research in the field of carbon nanotube-related nano-fluidics has been concerned with the fluid flow in single-walled carbon nanotubes (SWCNTs), showing that the dynamics of the channel affect the structure and behaviour of the fluid. We have extended this work by modelling the flow of Ar in a double-walled carbon nanotube, and have modelled the flow in both the inner shell and the outer annular region of such a nanotube. We have found that the flows in these channels are strongly correlated, such that the fluid moves in opposite directions in these two regions. This phenomenon can give rise to a circulatory motion which can be exploited in nano-fluidic devices. Fluid layering phenomenon, that is usually associated with the flow of fluids in nano-scale channels, is also observed. Furthermore, we have also found that the fluid velocity in dynamic channels is smaller than in static channels, in line with the findings reported for single-walled carbon nanotubes.     

Radial breathing vibration of double-walled carbon nanotubes subjected to pressure

A theoretical vibrational analysis of the radial breathing mode (RBM) of double-walled carbon nanotubes (DWCNTs) subjected to pressure is presented based on an elastic continuum model. The results agree with reported experimental results obtained under different conditions. Frequencies of the RBM in DWCNTs subjected to increasing pressure depend strongly on circumferential wave numbers, but weakly on the aspect ratio and axial half-wave numbers. For the inner and outer tubes of DWCNTs, the frequency of the RBM increases obviously as the pressure increases under different conditions. The range of variation is smaller for the inner tube than the outer tube.     

氢在多壁碳纳米管中的吸附储存

利用巨正则系综蒙特卡罗(GCMC)的方法模拟了氢在多壁碳纳米管中的吸附,氢气分子之间、氢气分子和碳原子之间的相互作用势能采用Lennard-Jones势能模型。模拟了不同结构参数(管内径、管壁数、管壁间距)的多壁碳纳米管在77K和298K下的吸附等温线,分析了多壁碳纳米管的管内径、管壁数以及管壁间距对吸附性能的影响。模拟结果表明:多壁碳纳米管的管壁数和管壁间距对吸附性能的影响较明显;管壁数越少,管壁间距越大,其吸附性能越好;多壁碳纳米管的管内径对其吸附性能的影响甚微。     

X-ray irradiation effect of double walled carbon nanotube

We have studied double-walled carbon nanotube (DWNT) irradiated by soft X-ray by Raman scattering spectroscopy and the spectral characteristics are compared to single-walled carbon nanotube (SWNT) irradiated under the same condition. We proved that DWNT is more stable for the X-ray induced defect formation than SWNT. Moreover, we found that the outer tube of DWNT was more sensitive on X-ray irradiation than the inner tube. The defect was recovered by annealing in Ar at lower temperature than that of SWNT. Based on these results, we inferred that X-ray irradiation leads to formation of interstitial-vacancy pairs, Frenkel defects, in carbon nanotube. The interstitial-vacancy separation on the inner tube of DWNT is conceivably shorter than that of the outer tube.     

Nanotube nanoscience: A molecular-dynamics study

Carbon nanotubes, fullerenes, and other nanostructured carbon materials are now the most important material phases in the field of nanoscience and nanotechnology. We study the structural stabilities and the interconversion of carbon nanotubes and various other carbon nanostructured phases at elevated temperatures as well as under high pressure using the molecular dynamics method combined with a newly parametrized transferable tight-binding model. The model can deal with not only sp² and sp³ covalent bonds but also the interaction between sp² layers, which plays an important role in the structural and electronic properties of carbon nanostructured materials. It is found that, during a thermal transformation process of carbon nanotubes with C₆₀ fullerenes trapped inside into double-walled carbon nanotubes, the outer carbon-nanotube wall is chemically active and forms covalent bonds with inner carbon atoms, and that most vacancies on the initially imperfect outer tube wall are eventually filled with atoms migrated from inner fullerenes. It is also found that external pressure of about 20 GPa induces a variety of structural transformations in carbon nanostructures. On the other hand, pressure of 30 GPa or higher usually results in sp³-rich amorphous carbon materials. Finally, the rotational interlayer friction force in double-walled carbon nanotubes is studied for the system of (4,4)@(9,9), and the torque of the friction force per unit area acting on each nanotube of the system is found to be as small as . This small value indicates the importance of carbon nanostuctured materials not only for nanoelectronics but also for nanometer-scale machines in the future.     

单壁碳纳米管微分电导在高压和强磁场下的实验研究

单壁碳纳米管在高压下会发生结构相变，导致金属型的碳纳米管变成半导体.相变后碳纳米管中电子的库仑关联的表现形式发生变化，从Luttinger liquid行为转变成环境量子涨落行为.同时，相变后电子波函数的相位关联导致弱局域化行为的出现.为了研究库仑关联和相位关联之间是否有相互影响，使用金刚石对顶砧和液压自锁高压包在0—10 GPa准静压范围内测量了单层碳纳米管样品在低温和不同磁场下的微分电导随偏压的依赖关系.实验结果表明，相位关联和库仑关联是两种独立的效应，各自影响着电子的输运行为. 关键词： 单层碳纳米管 高压 微分电导