Mechanical property of carbon nanotubes with intramolecular junctions: Molecular dynamics simulations

Intramolecular junctions (IMJs) of carbon nanotubes hold a promise of potential applications in nano-electromechanical systems. However, their structure-property relation is still unclear. Using the revised second-generation Tersoff-Brenner potential, molecular dynamics simulations were performed to study the mechanical properties of single-walled to four-walled carbon nanotubes with IMJs under uniaxial tension. The dependence of deformation and failure behaviors of IMJs on the geometric parameters was examined. It was found that the rupture strength of a junction is close to that of its thinner carbon nanotube segment, and the rupture strain and Young's modulus show a significant dependence on its geometry. The simulations also revealed that the damage and rupture of multi-walled carbon nanotube junctions take place first in the innermost layer and then propagate consecutively to the outer layers. This study is helpful for optimal design and safety evaluation of IMJ-based nanoelectronics.     

Integration and characterization of aligned carbon nanotubes on metal/silicon substrates and effects of water

We report here a facile way to grow aligned multi-walled carbon nanotubes (MWCNTs) on various metal (e.g. gold, tungsten, vanadium and copper)/silicon electrically conductive substrates by aerosol-assisted chemical vapor deposition (AACVD). Without using any buffer layers, integration of high quality MWCNTs to the conductive substrates has been achieved by introducing appropriate amount of water vapor into the growth system. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) determination indicate tidy morphology and narrow diameter distribution of the nanotubes as well as promising growth rate suitable for industrial applications. Raman spectra analysis illustrates that the structural order and purity of the nanotubes are significantly improved in the presence of water vapor. The growth mechanism of the nanotubes has been discussed. It is believed that water vapor plays a key role in the catalyst-substrate interaction and nucleation of the carbon nanotubes on the conductive substrates. This synthesis approach is expected to be extended to other catalyst-conductive substrate systems and provide some new insight in the direct integration of carbon nanotubes onto conductive substrates, which promises great potential for applications in electrical interconnects, contacts for field emitters, and other electronic nanodevices.     

Structural properties of junctions between two carbon nanotubes

Received: 9 November 1998 / Accepted: 25 November 1998     

Catalytic oxidation of carbon nanotubes with noble metal nanoparticles

Catalytic oxidation of multi-walled carbon nanotubes (MWNCTs) with some noble metal nanoparticles was observed by environmental transmission electron microscopy (E-TEM). Amoeba-like movement of the nanoparticles was observed even at a temperature of ∼400 °C, which is much lower than the melting points of any of the metals. In particular, rhodium particles reacted intensely with MWCNTs, and assumed a droplet-like shape. On the other hand, gold particles caused very little erosion of the MWCNTs under the conditions of this study.     

Molecular-dynamics simulations of carbon nanotubes as gigahertz oscillators

Recently, Zheng and Jiang [Phys. Rev. Lett. 88, 045503 (2002)]] have proposed that multiwalled carbon nanotubes could be the basis for a new generation of nano-oscillators in the several gigahertz range. In this Letter, we present the first molecular dynamics simulation for these systems. Different nanotube types were considered in order to verify the reliability of such devices as gigahertz oscillators. Our results show that these nano-oscillators are dynamically stable when the radii difference values between inner and outer tubes are of approximately 3.4 A. Frequencies as large as 38 GHz were observed, and the calculated force values are in good agreement with recent experimental investigations.     

Electronic transport properties of junctions between carbon nanotubes and graphene nanoribbons

Using the tight-binding model and Green’s function method, we studied the electronic transport of four kinds of nanotube-graphene junctions. The results show the transport properties depend on both types of the carbon nanotube and graphene nanoribbon, metal or semiconducting. Moreover, the defect at the nanotube-graphene interface did not affect the conductance of the whole system at the Fermi level. In the double junction of nanotube/nanoribbon/nanotube, quasibound states are found, which cause antiresonance and result in conductance dips.     

单壁及双壁纳米碳管径向压缩特性的分子动力学研究

采用Tersoff-Brenner势与L-J势的分子动力学方法,研究了双石墨层作用下(13,0)(、22,0)锯齿形碳管以及它们组合的(13,0)/(22,0)双壁碳管的径向压缩力学特性.根据计算结果,讨论了三种碳管压缩过程中的构型、能量、压缩载荷等的变化及其差异.研究表明,压缩过程中,(22,0)碳管出现了明显的“塌陷”现象,“塌陷”时,能量及外载一度下降;(13,0)/(22,0)碳管的能量吸收能力及承受压缩载荷能力最大;(22,0)碳管的能量吸收能力与承受压缩载荷能力最差,且体积最容易压缩;当压缩应变小于12%时,(13,0)/(22,0)碳管的内管的构型、体积及其能量变化量均很小;内壁与外壁之间的Van der Waals能在整个压缩(13,0)/(22,0)碳管能量变化中仅占很小的份额.     

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.     

Growth of carbon nanotubes on metal nanoparticles: a microscopic mechanism from ab initio molecular dynamics simulations

We report on ab initio molecular dynamics simulations of the early stages of single-walled carbon nanotube (SWCNT) growth on metal nanoparticles. Our results show that a sp2 bonded cap is formed on an iron catalyst, following the diffusion of C atoms from hydrocarbon precursors on the nanoparticle surface. The weak adhesion between the cap and iron enables the graphene sheet to "float" on the curved surface, as additional C atoms covalently bonded to the catalyst "hold" the tube walls. Hence the SWCNT grows capped. At the nanoscale, we did not observe any tendency of C atoms to penetrate inside the catalyst, consistent with total energy calculations showing that alloying of Fe and C is very unlikely for 1 nm particles. Root growth was observed on Fe but not on Au, consistent with experiment.     

Stability and electronic properties of carbon nanotubes doped with transition metal impurities

We apply first-principles method to investigate the effect of the diameter on the stability and electronic properties of zigzag carbon nanotubes doped with iron, nickel and manganese impurity atoms. In this contribution we follow the evolution of the electronic and structural properties as a function of the nanotube diameter. As a general result, we found that the binding energy decreases with the increasing nanotube radius. Additionally, depending on the interaction of transition metal impurity with the tubular carbon structure, it is observed that the total magnetization varies with the tube diameter due to hybridization and confinement effects. It is also shown that such magnetization varies with the curvature radius, increasing for manganese impurity atoms and decreasing for iron and nickel.     

Peculiarities of the decoration of carbon nanotubes with transition metal atoms

Carbon nanotubes decorated with transition metal, in particular, scandium, titanium, and vanadium, atoms offer promise for use in various applied science fields. We report the results of quantum-chemical calculations of the structure of the metallic layer of atoms of these metals coating the surface of (9, 0) and (10, 0) carbon nanotubes. It was shown that uniform one-layer coating by scandium and titanium could form on nanotubes with diameters no less than the diameter of (10, 0) nanotubes. Vanadium atoms could not uniformly cover nanotubes irrespective of their diameters.     

Selective alignment of carbon nanotubes on sapphire surfaces: bond formation between nanotubes and substrates

We present our first-principles total-energy calculations performed for carbon nanotubes (CNTs) on sapphire substrates. We find that the formation of covalent and partly ionic bonds between Al and C atoms on the Al-rich surfaces causes the selective alignment of CNTs, this being the principal reason for the CNT growth along particular crystallographic directions. We also find that the van der Waals interaction which is important on the stoichiometric surfaces produces no directional preference. The characteristic features in the electron states of the CNT on the substrate are clarified.     

Electron-energy characteristics of double-walled carbon nanotubes doped with alkali metal atoms

A computational scheme based on density functional theory adapted for systems with translational symmetry is applied to calculating the electronic structure and energy spectrum of double-walled carbon nanotubes doped with alkali metal atoms (Li, Na, and K). The specific features of the electron-energy characteristics, including the potential curves for the interaction of nanotubes with “guest” atoms moving in the radial direction or along the circumference of the tube, are established. It is shown that doped double-walled nanotubes have an increased electron emissivity.     

Resistance vs. pressure of single-wall carbon nanotubes

Received: 23 January 1998     

Growth of carbon nanotubes on stainless steel substrates by DC-PECVD

We report on the fabrication of carbon nanotubes (CNTs) on Ni-coated stainless steel (SUS) substrates by using dc plasma enhanced chemical vapor deposition. The synthesized CNTs have the diameter of about 30 nm and the length of about 1.2 μm. To verify the effects of SUS substrates on the growth of CNTs, CNTs had also been grown on Ni-coated Si substrates. CNTs grown on the SUS substrates were more uniform compared with those grown on the Si substrates. Field emission properties of the CNT films were measured in the diode configuration, and the turn-on electric field of 3.87 V/μm and field enhancement factor β of about 1737 were obtained from the synthesized CNTs at the gap of 500 μm between the SUS substrate and the anode. These results have not only clarified the effects of the substrate on the growth of CNTs, but also shown the potential of CNTs in field emission applications, especially CNT-based cold-cathode X-ray tubes.     

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.     

Nucleation of single wall carbon nanotubes of various chiralities

A simple model for the nucleation and growth of single wall carbon nanotubes from a graphene sheet at the surface of a metallic catalyst saturated in carbon is developed. It enables to predict the geometry and energy of tube embryos of all possible chiralities, as well as the way that they can grow. It is shown that armchair-like chiralities are energy preferred for geometrical reasons. This result is discussed and compared to experimental literature.     

Real-time ab initio simulations of excited carrier dynamics in carbon nanotubes

Combining time-dependent density functional calculations for electrons with molecular dynamics simulations for ions, we investigate the dynamics of excited carriers in a (3,3) carbon nanotube at different temperatures. Following an hnu=6.8 eV photoexcitation, the carrier decay is initially dominated by efficient coupling to electronic degrees of freedom. At room temperature, the excitation gap is reduced to nearly half its initial value after approximately 230 fs, where coupling to ionic motion starts dominating the decay. We show that the onset point and damping rate in the phonon regime change with initial ion velocities, a manifestation of temperature-dependent coupling between electronic and ionic degrees of freedom.