Nanosphere and carbon nanotube as motor and gun controlled by light

We propose a system like motor and gun molecular that has the open extremities of external nanotube using the internal C60 nanosphere as a probe. The system consists of a rigid and static nanotube opened and the internal C60 probe that allows relaxation between them. The initial position of C60 is out of symmetry that permits the probe to start the system movement due to van der Waals force acting in the probe. The simulation was made by classic molecular dynamics with standard parameterization. We calculated thermodynamics properties of these two devices as molar specific heat and molar entropy variation. Properties as probe speed were obtained like molecular motor and gun versus time. The nanotube has 360 carbon atoms with up to almost 0.7 ns of simulation. These facts can be useful for building new molecular machines.     

Curvature induced L-defects in water conduction in carbon nanotubes

We conduct molecular dynamics simulations to study the effect of the curvature induced static dipole moment of small open-ended single-walled carbon nanotubes (CNTs) immersed in water. This dipole moment generates a nonuniform electric field, changing the energy landscape in the CNT and altering the water conduction process. The CNT remains practically filled with water at all times, whereas intermittent filling is observed when the dipole term is not included. In addition, the dipole moment induces a preferential orientation of the water molecules near the end regions of the nanotube, which in turn causes a reorientation of the water chain in the middle of the nanotube. The most prominent feature of this reorientation is an L-defect in the chain of water molecules inside the CNT. The analysis of the water energetics and structural characteristics inside and in the vicinity of the CNT helps to identify the role of the dipole moment and to suggest possible mechanisms for controlled water and proton transport at the nanoscale.     

碳纳米管/聚乙烯复合物分子动力学模拟研究

采用分子动力学方法模拟了碳纳米管/聚乙烯复合物的结构、热力学和力学特性,分析其随模拟温度和碳纳米管填充率的变化。模拟结果表明,碳纳米管/聚乙烯复合物为各向同性的无定形结构,聚乙烯和碳纳米管通过较强的范德华作用结合在一起,在聚乙烯基体作用下,碳纳米管壁上的碳原子排列的周期性下降,出现弯曲和褶皱。从能量上看,填充率较高的复合物更加稳定。碳纳米管/聚乙烯复合物具有比聚乙烯体系更高的等容热容和与聚乙烯体系相反的负值热压力系数,热容随碳纳米管填充率的变化较小,但随温度的升高而明显减小,具有显著的温度效应;热压力系数随温度的变化较小,温度稳定性比聚乙烯更好,但随填充率增加而减小。碳纳米管/聚乙烯复合物的力学特性表现出各向同性材料的弹性常数张量,弹性模量和泊松比比纯聚乙烯体系高得多,并且都随温度的升高和碳纳米管含量的降低而减小,说明加入碳纳米管可显著改善聚乙烯的力学性质。     

Adhesion between two radially collapsed single-walled carbon nanotubes

Continuum mechanics analysis and molecular mechanics simulations are performed to study adhesion between two identical, radially collapsed single-walled carbon nanotubes. Not only the inter-adhesion energy between nanotubes but also the inner adhesion energy in a nanotube is considered. A closed-form solution to the adhesion configuration is achieved, which is well consistent with our molecular mechanics simulation. Comparing the potential energy of the adhesion structures formed by two identical single-walled carbon nanotubes, three types of configurations, i.e., circular, deformed, and collapsed shape, will be formed with increasing carbon nanotubes radius and separated by two critical radii of the single-walled carbon nanotube. Furthermore, it is found that the collapsed adhesion structure possesses the highest interfacial adhesion energy. The results demonstrate that, as a potential application in carbon nanotube reinforced composites, arrays formed by collapsed carbon nanotubes will be optimal due to the strong interface strength.     

Nanoscale fluid transport: size and rate effects

The transport behavior of water molecules inside a model carbon nanotube is investigated by using nonequilibrium molecular dynamcis (NMED) simulations. The shearing stress between the nanotube wall and the water molecules is identified as a key factor in determining the nanofluidic properties. Due to the effect of nanoscale confinement, the effective shearing stress is not only size sensitive but also strongly dependent on the fluid flow rate. Consequently, the nominal viscosity of the confined water decreases rapidly as the tube radius is reduced or when a faster flow rate is maintained. An infiltration experiment on a nanoporous carbon is performed to qualitatively validate these findings.     

An Investigation into the Mechanical Behavior of Single-Walled Carbon Nanotubes under Uniaxial Tension Using Molecular Statics and Molecular Dynamics Simulations

This study performs a series of Molecular Dynamics (MD) and Molecular Statics (MS) simulations to investigate the mechanical properties of single-walled carbon nanotubes (SWCNTs) under a uniaxial tensile strain. The simulations focus specifically on the effects of the nanotube helicity, the nanotube diameter and the percentage of vacancy defects on the bond length, bond angle and tensile strength of zigzag and armchair SWCNTs. In this study, a good agreement is observed between the MD and MS simulation results for the stress-strain response of the SWCNTs in both the elastic and the plastic deformation regimes. The MS simulations reveal that in the plastic deformation regime, the tensile strength of the armchair and zigzag SWCNTs increases with an increasing wrapping angle. In addition, it is shown that the tensile strength reduces significantly at larger values of the nanotube diameter. Moreover, it is observed that the tensile strength of both SWCNTs reduces as the percentage of defects within the nanotube structure increases. Finally, it is found that the results obtained from the molecular statics method are relatively insensitive to instabilities in the atomic structure, particularly in the absence of thermal fluctuations, and are in good agreement with the predictions obtained from the molecular dynamics method.     

Nanopumping using carbon nanotubes

A new "nanopumping" effect consisting of the activation of an axial gas flow inside a carbon nanotube by producing Rayleigh traveling waves on the nanotube surface is predicted. The driving force for the new effect is the friction between the gas particles and the nanotube walls. A molecular dynamics simulation of the new effect was carried out showing macroscopic flows of atomic and molecular hydrogen and helium gases in a carbon nanotube.     

Vibrating carbon nanotubes as water pumps

Nanopumps conducting fluids directionally through nanopores and nanochannels have attracted considerable interest for their potential applications in nanofiltration, water purification, and hydroelectric power generation. Here, we demonstrate by molecular dynamics simulations that an excited vibrating carbon nanotube (CNT) cantilever can act as an efficient and simple nanopump. Water molecules inside the vibrating cantilever are driven by centrifugal forces and can undergo a continuous flow from the fixed to free ends of the CNT. Further extensive simulations show that the pumping function holds good not only for a single-file water chain in a narrow (6,6) CNT, but also for bulk-like water columns inside wider CNTs, and that the water flux increases monotonically with increasing diameter of the nanotube.     

Structure and dynamics of confined water inside narrow carbon nanotubes

We have performed atomistic molecular dynamics (MD) simulations of water molecules inside narrow, open-ended carbon nanotubes placed in a bath of water molecules. The radius of the tube is such that only a single file of water molecules is allowed inside the tube. The confined water molecules are shown to be positionally ordered even at a temperature of 300 K. The calculated mean-square displacement (MSD) of the confined water molecules reveals that initially the water molecules undergo ballistic motion that crosses over to normal (Fickian) diffusion at longer times. We also develop a random-walk model in 1D for the motion of a cluster of water molecules inside the nanotube. The agreement of the MSD calculated from the MD simulation and from the 1 D random-walk model establishes the occurrence of normal diffusion of water molecules even in a tube where single-file diffusion is expected.     

Comparative MD simulation study on the mechanical properties of a zigzag single-walled carbon nanotube in the presence of Stone-Thrower-Wales defects

Using molecular dynamics simulation, we investigate the influence of Stone-Thrower-Wales defects in the mechanical behavior of a zigzag (5, 0) single-walled carbon nanotube considering two different interatomic potential functions, the Tersoff–Brenner bond order potential and the Tight-Binding potential. The nanotube is subjected to axial stretch and the potential energy is computed for gradually increasing values of strain. From the energy–strain curve the mechanical characteristics like Young’s modulus, tensile strength and ductility are computed using both the potentials, firstly with a perfect lattice and then by introducing an increasing number of Stone-Thrower-Wales defects. Significant reduction in the values of the mechanical properties is observed with changes in the plastic deformation pattern. Experimental data compares reasonably well with our calculated values of the mechanical constants. Such investigations will help designing carbon nanotube based composites.     

On the effective elastic moduli of carbon nanotubes for nanocomposite structures

A critical review on the validity of different experimental and theoretical approaches to the mechanical properties of carbon nanotubes for advanced composite structures is presented. Most research has been recently conducted to study the properties of single-walled and multi-walled carbon nanotubes. Special attention has been paid to the measurement and modeling of tensile modulus, tensile strength, and torsional stiffness. Theoretical approaches such as molecular dynamic (MD) simulations, finite element analysis, and classical elastic shell theory were frequently used to analyze and interpret the mechanical features of carbon nanotubes. Due to the use of different fundamental assumptions and boundary conditions, inconsistent results were reported. MD simulation is a well-known technique that simulates accurately the chemical and physical properties of structures at atomic-scale level. However, it is limited by the time step, which is of the order of 10⁻¹⁵ s. The use of finite element modeling combined with MD simulation can further decrease the processing time for calculating the mechanical properties of nanotubes. Since the aspect ratio of nanotubes is very large, the elastic rod or beam models can be adequately used to simulate their overall mechanical deformation. Although many theoretical studies reported that the tensile modulus of multi-walled nanotubes may reach 1 TPa, this value, however, cannot be directly used to estimate the mechanical properties of multi-walled nanotube/polymer composites due to the discontinuous stress transfer inside the nanotubes.     

Spectroscopic evidence and molecular simulation investigation of the pi-pi interaction between pyrene molecules and carbon nanotubes

The pi-pi interaction between pyrene molecules and single-walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs) was studied by fluorescence, FTIR, Raman spectroscopy and molecular simulation. The carbon nanotubes were incubated in pyrene solution and dried for characterization. A broadband fluorescence emission at 463 nm of the incubated samples was observed, which is similar to that of pyrene excimers but shifts to shorter wavelength. The typical FTIR bands of pyrene shift to lower wavenumbers in the incubated samples. D- and G-bands in Raman spectra of SWNTs also shift to low frequencies. All these spectroscopic evidences reveal the stronger pi-pi stacking interaction between the nanotubes and pyrene molecules over the pyrene dimers, which leads to the formation of pyrene-carbon nanotube complexes. The systems of SWNTs and pyrene molecules were also studied with molecular simulation. It was found from the binding energy calculation that a stronger interaction presents between the pyrene molecule and the nanotube. In addition, the simulation gives some structural information about the pyrene-nanotube complex, such as the staggered conformation of pyrene on nanotube. The effect of defects in carbon nanotube sidewall was also discussed.     

Molecular dynamics study of nano mass transfer in a vibrating cantilevered carbon-nanotube

We investigate the nano mass transfer in an ultrahigh frequency carbon-nanotube-resonator encapsulating a nanocluster via classical molecular dynamics simulations. When the carbon-nanotube-resonator vibrated, the encapsulated copper nanocluster more rapidly approached the end of the cantilevered carbon-nanotube-resonator. Such phenomena were due to the migration of the encapsulated copper nanocluster due to the centrifugal force induced by the vibrating nanotube resonator. So the resonance frequency change could be time-dependently found. For the movable copper nanocluster in carbon nanotube resonator, the vibrational spectra when the copper nanocluster inside the carbon nanotube resonator rapidly settled at the capped edge were different from those obtained when the copper nanocluster continuously oscillated inside the carbon nanotube resonator. Such results showed that the frequency of the carbon-nanotube-resonator encapsulating the movable copper nanocluster could be adjusted by controlling the mean position of the oscillating copper nanocluster. The movable nanocluster inside a carbon-nanotube can be applied to a nanotube-based data storage media by sensing the position of the nanocluster.     

Self-filtering oscillations in carbon nanotube hetero-junctions

We evaluate the vibrational properties of single-wall carbon nanotube (SWCNT) hetero-junction (HJ) oscillators using a hybrid atomistic-continuum approach validated by molecular mechanics/molecular dynamics simulations. The SWCNT-HJs show a broken symmetry topology of their mode shapes, with striction effects caused on the bending and radial modes by the combined effect of the HJ and the tube with the thinner radius. The single-wall nanotube HJs also show selective mass sensing properties based solely on the geometry and type of the boundary conditions of the specific nanostructure. This unusual behaviour has not been observed so far in classical SWCNT systems.     

Molecular dynamics and atomistic based continuum studies of the interfacial behavior of nanoreinforced epoxy

In this study, we investigate the interfacial mechanical characteristics of carbon nanotube (CNT) reinforced epoxy composite using molecular dynamics (MD) simulations. The second-generation polymer consistent force field (PCFF) is used in the MD simulations. In particular, we compare MD results with those obtained by atomistic-based continuum (ABC) multiscale modeling technique, which makes use of the appropriate constitutive relations derived solely from interatomic potentials. The results of our comparative investigation suggest that (i) the ABC multiscale model and MD simulation provides almost identical predictions for the interfacial properties of the nanocomposite for smaller diameter of CNTs, (ii) the ABC model slightly over predict the interfacial properties of the nanocomposite for larger diameter of CNTs, and (iii) the MD simulations represents the real nanocomposite structure with the minimum assumptions compared to that of the ABC multiscale model but with much greater computer requirements and limited length scale.     

The oscillatory characteristics of a 2C60/CNT oscillator system

The authors have studied, using molecular dynamic (MD) simulations, the oscillatory characteristics of a 2C60/CNT oscillator system, in which two C60 fullerenes oscillate inside a single walled carbon nanotube (CNT) in two basic modes, i.e., the symmetric and non-symmetric motions. In the symmetric mode, with each oscillation the two fullerenes move symmetrically from the CNT ends towards the CNT center where they bounce off each other and head back towards the ends. In the non-symmetric mode, the two fullerenes move back and forth inside the CNT crossing the center point of the CNT together with each oscillation. The simulations show that the non-symmetric oscillation mode is stable for the prescribed initial (maximum) velocities up to 300 m/s, while the symmetric oscillation mode however, experiences dynamic instabilities for a prescribed initial (maximum) velocity larger than 250 m/s. The instability takes place as a result of the transfer of energy from the translational to the rotational motion of the fullerenes. This characteristic differentiates 2C60/CNT oscillators from double-walled CNT oscillators. The rotation is primarily caused by the inter-colliding of the two fullerenes, which subjects the fullerenes to large van der Waals repelling forces. These repelling forces are not necessarily aligned perfectly along the CNT axis nor precisely pointing towards the mass centers of the fullerenes. These misalignments cause the fullerenes to rock around the CNT's axis, while their offsets from the mass centers cause the fullerenes to rotate. The rocking motion, being severely confined by the CNT, does not gain much energy itself, but instead, channels energy from translational to rotational motion. The energy channeling is found to be reversed in some very short time intervals, but the rotational motion always gains energies from the translational motion over a time interval that is long enough at the MD time scale. This feature, contrary to our experiences in the macroscopic world, appears to be unique for such nanoscopic mechanical systems.     

Mechanical properties of carbon nanotube reinforced polymer nanocomposites: A coarse-grained model

In this work, a coarse-grained (CG) model of carbon nanotube (CNT) reinforced polymer matrix composites is developed. A distinguishing feature of the CG model is the ability to capture interactions between polymer chains and nanotubes. The CG potentials for nanotubes and polymer chains are calibrated using the strain energy conservation between CG models and full atomistic systems. The applicability and efficiency of the CG model in predicting the elastic properties of CNT/polymer composites are evaluated through verification processes with molecular simulations. The simulation results reveal that the CG model is able to estimate the mechanical properties of the nanocomposites with high accuracy and low computational cost. The effect of the volume fraction of CNT reinforcements on the Young's modulus of the nanocomposites is investigated. The application of the method in the modeling of large unit cells with randomly distributed CNT reinforcements is examined. The established CG model will enable the simulation of reinforced polymer matrix composites across a wide range of length scales from nano to mesoscale.     

3<Emphasis Type="Italic">d</Emphasis> Transition metal decorated B-C-N composite nanostructures for efficient hydrogen storage: A first-principles study

Ti decorated BC_4N nanotube has been studied using first-principles density functional approach, to explore the storage of molecular hydrogen. It combines the advantages of carbon nanotube, together with the thermal stability of BN nanotube. The local structural unit of BN₃ and NB₃ linked with B-N bonds are responsible for the extra stability of BC_4N nanotube as compared with CNT. While the host carbon nanotube is metallic, the substitutional doping of B and N with a large enough concentration (33%) turns it to semiconducting. Endohedral decoration, although energetically favourable, encounters a rather high barrier height of ∼4 eV, as obtained from our nudge elastic band calculation of the minimum energy path. Exohedral Ti@BC₄N can bind up to four H₂ molecules. For full Ti coverage, the system can absorb up to 5·6 wt% of hydrogen. Ab initio molecular dynamics simulation reveals that at 500 K hydrogen gets released in molecular form. We believe that this novel composite nanotube, functionalized by Ti atoms from outside, serves as a promising system for hydrogen storage.     

Effects of feed gas composition and catalyst thickness on carbon nanotube and nanofiber synthesis by plasma enhanced chemical vapor deposition

Many engineering applications require carbon nanotubes with specific characteristics such as wall structure, chirality and alignment. However, precise control of nanotube properties grown to application specifications remains a significant challenge. Plasma-enhanced chemical vapor deposition (PECVD) offers a variety of advantages in the synthesis of carbon nanotubes in that several important synthesis parameters can be controlled independently. This paper reports an experimental study of the effects of reacting gas composition (percentage methane in hydrogen) and catalyst film thickness on carbon nanotube (CNT) growth and a computational study of gas-phase composition for the inlet conditions of experimentally observed carbon nanotube growth using different chemical reaction mechanisms. The simulations seek to explain the observed effects of reacting gas composition and to identify the precursors for CNT formation. The experimental results indicate that gas-phase composition significantly affects the synthesized material, which is shown to be randomly aligned nanotube and nanofiber mats for relatively methane-rich inlet gas mixtures and non-tubular carbon for methane-lean incoming mixtures. The simulation results suggest that inlet methane-hydrogen mixture coverts to an acetylene-methane-hydrogen mixture with minor amounts of ethylene, hydrogen atom, and methyl radical. Acetylene appears to be the indicator species for solid carbon formation. The simulations also show that inlet methane-hydrogen mixture does not produce enough gas-phase precursors needed to form quality CNTs below 5% CH4 concentrations in the inlet stream.     

Toughening Mechanisms in Carbon Nanotube-Reinforced Amorphous Carbon Matrix Composites

Crack deflection and penetration at the interface of multi-wall carbon nanotube/amorphous carbon composites were studied via molecular dynamics simulations. In-situ strength of double-wall nanotubes bridging a matrix crack was calculated under various interfacial conditions. The structure of the nanotube reinforcement -ideal multi-wall vs. multi-wall with interwall sp³ bonding - influences the interfacial sliding and crack penetration. When the nanotube/matrix interface is strong, matrix crack penetrates the outermost layer of nanotubes but it deflects within the nanotubes with certain sp³ interwall bond density, resulting in inner wall pullout. With increasing the sp³ interwall bond density, the fracture mode becomes brittle; the fracture energy decrease while the bridging strength increases and then decreases. Our results suggest that the outermost nanotube wall can serve as a sacrificial layer such that the interface may be designed by effectively putting it inside the nanotubes. Controlling the density of sp³ interwall bond within the multiwall carbon nanotube makes the transition from brittle to tough failure modes in the composites even when the matrix/nanotube interface is strong.