Mechanical Properties of Single-Walled （5,5） Carbon Nanotubes with Vacancy Defects

First-principles simulation is used to investigate the structural and mechanical properties of vacancy defective single-walled （5,5） carbon nanotubes. The relations of the defect concentration, distribution and characteristic of defects to Young＇s modulus of nanotubes are quantitatively studied. It is found that each dangling-bond structure （per supercell） decreases Young＇s modulus of nanotube by 6.1% for symmetrical distribution cases. However the concentrative vacancy structure with saturated atoms has less influence on carbon nanotubes. It is suggested that the mechanical properties of carbon nanotubes depend strongly upon the structure and relative position of vacancies in a certain defect concentration.     

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.     

Novel single-walled carbon nanotubes periodically embedded with four- and eight-membered rings

Based on experimental results, we obtain five types of single-walled carbon nanotube (SWNT) clusters with different chirality indices and diameters using density functional theory (DFT). We then obtain the corresponding SWNTs by using periodic boundary conditions. Studies of the stability and electronic properties show that the stability of the novel SWNTs is independent of the chirality index and relates only to the tube diameter; larger diameters correspond to more stable SWNTs. The electronic properties all show metallic characteristics independent of the chirality indices and tube diameters, thereby promoting the application of metallic-type SWNTs.     

单壁BN纳米管和碳纳米管物理吸附储氢性能的理论对比研究

采用巨正则蒙特卡罗方法(GCMC)研究了单壁氮化硼纳米管(SWBNNTs)和单壁碳纳米管(SWCNTs)的物理吸附储氢性能,主要对比研究了纳米管的管径、温度和手性对二者物理吸附储氢量的影响. 研究结果表明：在低温下,SWBNNTs的物理吸附储氢性能优于相应的SWCNTs;但是随着温度的升高,二者的物理吸附储氢性能差别越来越小,在常温下,SWBNNTs不具备有比SWCNTs更强的物理吸附储氢性能,而是和相同条件下的SWCNTs相差不大,只是在高压下的物理吸附储氢量稍稍大于SWCNTs,并给出了合理的理论解释 关键词： 巨正则蒙特卡罗方法(GCMC) 单壁氮化硼纳米管(SWBNNTs) 单壁碳纳米管(SWCNTs) 储氢     

单、双原子空位缺陷对扶手椅型单层碳纳米管屈曲性能的不同影响

采用分子动力学方法,对完善和含缺陷扶手椅型单层碳纳米管进行轴向压缩的数值模拟,对比研究三种不同的温度环境下单、双原子空位缺陷对碳纳米管轴压变形性能的特殊影响.研究结果表明管壁缺陷显著降低了纳米管低温时的承载能力,由于单原子空位缺陷造成的特殊应力集中效应会引发纳米管过早发生局部屈曲,单原子缺陷管的屈曲强度反而小于双原子管的屈曲强度. 关键词： 分子动力学 碳纳米管 屈曲 缺陷     

Effects of vacancy percentage on the energy gap of zigzag single-wall carbon nanotubes

The influence of vacancy percentage on the energy gap of zigzag single-wall carbon nanotube is investigated by the Green's function method in coherent potential approximation. Our probes for various kinds of zigzag single-wall carbon nanotubes show that by increasing vacancy percentage the energy gap is also increased, so for metallic single-wall carbon nanotubes, a metallic to semi-metallic transition is occurred. However, any transition does not appear for semiconductor carbon nanotubes. So by controlling on concentration of vacancies, one can make a semiconductor SWCNT with a predetermined energy gap which is useful in nanoelectronic devices.     

Geometry and electronic properties of single vacancies in achiral carbon nanotubes

An ideal single vacancy can be formed by removing one carbon atom from a hexagonal network. The vacancy is one of the most important defect structures in carbon nanotubes (CNTs). Vacancies can affect the mechanical, chemical, and electronic properties of CNTs. We have systematically investigated single vacancies and their related point defects for achiral, single-walled carbon nanotubes (SWNTs) using first-principles calculations. The structures around single vacancies undergo reconstruction without constraint, forming ground-stateor metastable-state structures. The 5-1DB and 3DB point defects can be formed in armchair CNTS, while the 5-1DB-P and 5-1DB-T point defects can be formed in zigzag CNTs. The related point defects can transform into each other under certain conditions. The formation energies of armchair CNTs change smoothly with the tube radius, while in the case of the 3DB defect, as the radius get larger, the formation energies tend towards a constant value.     

First principles modeling of boron-doped carbon nanotube sensors

We investigated the interactions between two different geometrical configurations of single-walled carbon nanotubes and boron atoms using first-principle calculations within the framework of the density functional theory. With the aid of ab initio calculations, we introduced a new type of toxic gas sensor that can detect the presence of CO, NO and H₂ molecules. We proved that the dopant concentration on the surface of the nanotube plays a crucial role in the sensitivity of this device. Furthermore, we showed that small concentrations of dopants can modify the transport and electronic properties of the single-walled carbon nanotube and can lend metallic properties to the nanotube. Band-gap narrowing occurs when the nanotube is doped with boron atoms. The emerged new energy level near the Fermi level upon boron doping clearly indicates the coupling between the p orbital of the boron atom and the large p bond of the carbon nanotube. We also predicted a weak hybridization between the boron atoms and the nanotube for the valence-band edge states; this weak coupling leads to conducting states around the band gap.     

Polythiophene encapsulated inside(13, 0) CNT: A nano-hybrid system

We present a first-principles calculation on the electronic and optical properties of a hybrid nanotube system consisting of a （13, 0） single-walled carbon nanotube encapsulated by polythiophene. This hybrid-system represents a complete new type of matter and is known as the peapod system. We analyze bow the polythiophene changes the electronic and optical properties of the nanotube. In particular, we examine new features in the dielectric function due to the transitions between the states of the polymer and the nanotube. The electronic structure of the combined system appears to be a simple superposition of the individual constituents. The density functional theory calculations demonstrate van der Waals interaction as the bonding mechanism between the tube and the encapsulated molecule.     

Electronic transport properties of a diarylethene-based molecular switch with single-walled carbon nanotube electrodes: The effect of chirality

We have studied the electronic transport properties of an optical molecular switch based on the diarylethene molecule with two single-walled carbon nanotube (SWCNT) electrodes using first-principles transport calculations. It is shown that the closed form shows an overall higher conductance than the open form at low bias which is independent of the SWCNTs’ chirality. Meanwhile, the conductance of the molecular switch can be tuned by changing the chirality of the SWCNTs.     

Molecular dynamics study of effects of nickel coating on torsional behavior of single-walled carbon nanotube

The effects of nickel coating on the torsional behaviors of single-walled carbon nanotubes (SWCNTs) subject to torsion motion are investigated using the molecular dynamics (MD) simulation method. The simulation results show that regardless of chirality, defect or radius, nickel coating can considerably enhance the critical torque of SWCNTs. However, by comparing the critical torsion angle between nickel-coated SWCNTs and corresponding pristine SWCNTs, it is found that nickel coating in small-radius nanotubes does induce a reduction in the critical torsion angle. The results also show that the structural failure of nickel coated imperfect (9,0) SWCNT occurs at an obviously higher critical torque in comparison with uncoated (9,0) SWCNT with a vacancy defect. Furthermore, we also find that the critical torque of a short nickel coated SWCNT is bigger than that of a long one, while the critical torsion angle for a short tube is smaller than that for a long one.     

A nonlocal Levinson beam model for free vibration analysis of zigzag single-walled carbon nanotubes including thermal effects

A nonlocal Levinson beam model is developed to study the free vibrations of a zigzag single-walled carbon nanotube (SWCNT) in thermal environments. The equivalent Young’s modulus and shear modulus for a zigzag SWCNT are derived using an energy-equivalent model. The present study illustrates that the vibration characteristics of an SWCNT are strongly dependent on the temperature change and on the chirality of a zigzag carbon nanotube. The investigation of the chirality and temperature effects on free vibration of carbon nanotubes may be used as a useful reference for the application and the design of nanoelectronic and nanodrive devices, nano-oscillators, and nanosensors, in which carbon nanotubes act as basic elements.     

半导体性单壁碳纳米管与其石墨衬底表面相互作用的第一性原理计算

利用基于密度泛函理论的计算方法,研究了处于石墨（HOPG）表面的半导体性单壁碳纳米管,发现碳纳米管下面的石墨受碳管的作用向下凹陷,而纳米管本身虽然保持其形状不变,但它的电子态受石墨衬底影响,造成导带下移,禁带宽度明显减小. 关键词： 单壁碳纳米管 密度泛函理论 局域密度近似方法     

Aharonov-Bohm interference and beating in single-walled carbon-nanotube interferometers

Relatively low magnetic fields applied parallel to the axis of a chiral single-walled carbon nanotube are found causing large modulations to the p channel or valence band conductance of the nanotube in the Fabry-Perot interference regime. Beating in the Aharonov-Bohm type of interference between two field-induced nondegenerate subbands of spiraling electrons is responsible for the observed modulation with a pseudoperiod much smaller than that needed to reach the flux quantum Phi0 = h/e through the nanotube cross section. We show that single-walled nanotubes represent the smallest cylinders exhibiting the Aharonov-Bohm effect with rich interference and beating phenomena arising from well-defined molecular orbitals reflective of the nanotube chirality.     

Influencing range of vacancy defects in zigzag single-walled carbon nanotubes

The influencing range of a vacancy defect in a zigzag single-walled nanotube is characterized with both structural deformation and variation in bandstructure. This paper proposes a microscopic explanation to relate the structural deformation to the bandstructure variation. With an increasing defect density, the nanotubes become oblate and the energy gap between the deep localized gap state and the conducting band minimum state decreases. Theoretical results shed some light on the local energy gap engineering via vacancy density for future potential applications.     

Adsorption on the carbon nanotubes

Adsorption on single walled carbon nanotubes (SWCNTs) is a subject of growing experimental and theoretical interest. The possible adsorbed patterns of atoms and molecules on the single-walled carbon nanotubes vary with the diameters and chirality of the tubes due to the confinement. The curvature of the carbon nanotube surface enlarges the distance of the adsorbate atoms and thus enhances the stability of high coverage structures of adsorbate. There exist two novel high-coverage stable structures of potassium adsorbed on SWCNTs, which are not stable on graphite. The electronic properties of SWCNTs can be modified by adsorbate atoms and metal-semiconductor and semiconductor-semi-conductor transitions can be achieved by the doping of alkali atoms.     

水分子链受限于单壁碳纳米管结构的密度泛函理论研究

本文采用第一性原理的密度泛函理论,主要以(6,6)Armchair型,(11,0)Zigzag型单壁碳纳米管为研究对象,研究了水分子链在碳纳米管内部吸附的稳定结构,以及结合能随其结构的变化.结果表明:当水分子链受限于碳纳米管内部时,引起碳纳米管直径收缩,这主要是由于水分子链与碳纳米管之间的氢键作用以及范德华弱相互作用所引起的.随着碳纳米管半径的增加,两种单体之间的结合能逐渐减小,但当碳纳米管半径增加至6.78时,其结合能又有所增加,这是由于在优化过程中,水分子链单体之间的氢键作用大于水分子链与碳纳米管之 关键词： 水分子链/单壁碳纳米管 密度泛函理论 结构稳定性     

First-principles calculations of carbon nanotubes adsorbed on Si(001)

We report first-principles calculations on the adsorption of a metallic (6,6) single-walled carbon nanotube (SWCN) on the Si(001) surface. We find stable geometries for the nanotube between two consecutive dimer rows where C-Si chemical bonds are formed. The binding energy in the most stable geometry is found to be 0.2 eV/A. Concerning the electronic properties, the most stable structure shows an increase in the density of states near the Fermi level due to the formation of C-Si bonds enhancing the metallic character of the nanotube by the contact with the surface. These properties may lead one to consider metallic SWCNs adsorbed on Si substrates for interconnections and contacts on future nanoscale devices. Finally, the nature of the nanotube-surface interaction for nanotubes of larger diameters is also discussed.     

First-principles study of the switching characteristics of the 15,16-dinitrile DDP/CPD-based optical molecular switch with carbon nanotube electrodes

We have studied the switching characteristics of an optical molecular switch based on the 15,16-dinitrile dihydropyrene/cyclophanediene (DDP/CPD) molecule with two single-walled carbon nanotube (SWCNT) electrodes using first-principles transport calculations. It is shown that the DDP shows an overall higher conductance than the CPD at low bias which is independently of the SWCNT chirality. Meantime, the conductance of the molecular switch can be tuned by the chirality of the SWCNT.     

Electronic properties of multi-defected zigzag carbon nanotubes

Electronic properties of multi-defected zigzag single-walled carbon nanotubes are investigated by use of the tight-binding Green's function method. The Stone-Wales defects and the vacancies are considered. We find that the conductance sensitively depends on the realistic defect configurations for the metallic zigzag carbon nanotubes. Interestingly, the electronic transport properties of the nanotubes with three vacancies can be considered as the sum effect of two double-vacancies, while those with Stone-Wal...