Armchair Tubes Research Articles

Joule heating and thermoelectric properties in short single-walled carbon nanotubes: Electron-phonon interaction effect

The electron-phonon interaction (EPI) effect in single-walled carbon nanotube is investigated by the nonequilibrium Green’s function approach within the Born approximation. Special attention is paid to the EPI induced Joule heating phenomenon and the thermoelectric properties in both metallic armchair (10, 10) tube and semiconductor zigzag (10, 0) tube. For Joule heat in the metallic (10, 10) tube, the theoretical results for the breakdown bias voltage is quite comparable with the experimental value. It is found that the Joule heat can be greatly enhanced by increasing the chemical potential, while the role of the temperature is not so important for Joule heat. In the zigzag (10, 0) tube, the Joule heat is smaller than the armchair tube, resulting from nonzero bandgap in the electron band structure. For the electronic conductance Ge and electron thermal conductance σel, the EPI has important effect at higher temperature or higher chemical potential. Compared with ballistic transport, there is an opposite tendency for Ge to decrease with increasing temperature after EPI is considered. This is due to the dominant effect of the electron phonon scattering mechanism in the electron transport in this situation. There is an interesting “electron-drag” phenomenon for the phonon thermal conductance in case of low temperature and high chemical potential, where phonons are dragged by electrons from low temperature region into high temperature region through EPI effect.

Electro-optical properties of zigzag and armchair boron nitride nanotubes under a transverse electric field: Tight binding calculations

The electro-optical properties of zigzag and armchair BNNTs in a uniform transverse electric field are investigated within tight binding approximation. It is found that the electric field modifies the band structure and splits band degeneracy where these effects reflect in the DOS and JDOS spectra. A decrease in the band gap, as a function of the electric field, is observed. This gap reduction increases with the diameter and it is independent of chirality. An analytic function to estimate the electric field needed for band gap closing is proposed which is in good agreement with DFT results. In additional, we show that the larger diameter tubes are more sensitive than small ones. Number and position of peaks in DOS and JDOS spectra for armchair and zigzag tubes with similar radius are dependent on electric field strength.

Nanotube bundles and tube-tube orientation: A van der Waals density functional study

We study the binding energy, intertube distance and electronic structure of bundles consisting of single walled carbon nanotubes of the same chirality. We model various nanotube structures (chiralities) and orientations with van der Waals density functional theory. The orientation of the tubes in the bundle strongly influences the properties of the bundles if the chirality of the tubes shares symmetry with the trigonal bundle structure, meaning chiralities which have a $60^\circ$-rotational symmetry (C$_6$-axis), e.g. ($12,0$) bundles. The bundle structure breaks the symmetry depending on the arrangement of the neighboring tubes. Pseudogaps open in the electronic density of states and intertube distances ($\pm5$-10%) vary in dependence of the relative orientation of the tubes in the bundle. Bundles of C$_6$-axis armchair tubes have metallic configurations. A $15^\circ$ rotation off the high symmetry configuration (AA-stacked) of a ($6,6$) bundle shows metallic behavior and has higher binding energy than the high symmetry configuration. We find binding energies between $19 meV/atom$ and $35 meV/atom$, depending on the chirality of the tubes. The intertube distances are between $3.2 {\AA}$ and $3.4 {\AA}$ but independent of orientation for non C$_6$-axis tubes.

Evolution of DNA Sequences Toward Recognition of Metallic Armchair Carbon Nanotubes

The armchair carbon nanotube is an ideal system to study fundamental physics in one-dimensional metals and potentially a superb material for applications such as electrical power transmission. Synthesis and purification efforts to date have failed to produce a homogeneous population of such a material. Here we report evolutionary strategies to find DNA sequences for the recognition and subsequent purification of (6,6) and (7,7) armchair species from synthetic mixtures. The new sequences were derived by single-point scanning mutation and sequence motif variation of previously identified ones for semiconducting tubes. Optical absorption spectroscopy of the purified armchair tubes revealed well-resolved first- and second-order electronic transitions accompanied by prominent sideband features that have neither been predicted nor observed previously. Resonance Raman spectroscopy showed a single Lorentzian peak for the in-plane carbon-carbon stretching mode (G band) of the armchair tubes, repudiating the common practice of using such a line shape to infer the absence of metallic species. Our work demonstrates the exquisite sensitivity of DNA to nanotube metallicity and makes the long-anticipated pure armchair tubes available as seeds for their mass amplification.

Theoretical Studies on Interaction Between Methanol and Functionalized Single-Walled Carbon Nanotubes

We study the adsorption of a methanol molecule on single-walled carbon nanotubes (SWCNTs) with various diameters and chiral angles by using the density functional theory based calculations. We find that methanol prefers to be adsorbed physically on the exterior surface of chiral nanotubes in comparison to the armchair and zigzag tubes with binding energy of about −2.76 kcal/mol, which is consistent with recent experimental and theoretical investigation results. We further consider the adsorption of methanol on the exterior surface and edge site of functionalized SWCNTs. The obtained results indicate that the binding energy of methanol is significantly increased for adsorption on the sidewall of functionalized nanotubes. It is also found that the adsorption of methanol at the edge site of both functionalized and pristine SWCNT is remarkably different (chemisoption process) in comparison to the exterior sidewall of the tubes. Furthermore, the electronic structures and Mulliken charge population of the considered complexes at their ground state are discussed within the context.

Helicity effects on Rh adsorption behavior inside and outside the single-wall carbon nanotubes

The curvature and the helicity of single-wall carbon nanotube (SWCNT) are the important factors which influence the adsorption behaviors of metal atoms inside and outside carbon tubes. However, it is difficult to investigate the separate effects of SWCNT helicity on the adsorption behaviors of metal atoms. In the present work, the armchair (6, 6), zigzag (10, 0), and chiral (8, 4) tubes with similar curvature are selected, then the Rh adsorption behaviors inside and outside the tubes are systematically investigated using the density functional theroy. Due to the different SWCNT helicities, the stable configurations of Rh atoms on tubes are different. The neighbor carbon atoms interacting with Rh atoms vary with tube helicity, therefore, the Rh adsorption energies for a similar configuration are also different. It indicates that the outer charge density of SWCNT is higher than the inner one. Different helicities lead to different charge density variations along the radial direction. Charge density difference shows that the orbital orientations of Rh adatom and the electrons obtained and lost are slightly different due to the different helicities. The bandstructure indicates that the doping band appears near the Fermi energy level. The (6, 6) tube with Rh adatom still exhibits metallicity. When Rh atoms are adsorbed inside the (10, 0) tube, the nanotube transforms from the semiconducting into the metallic one. However, the band gap reduces when Rh atoms adsorbed outside the tube. After the Rh adsorption, the (8, 4) tube band gap reduces.

Edge effects, electronic arrangement, and aromaticity patterns on finite-length carbon nanotubes

Previous investigations have revealed that even long carbon nanotubes (CNTs) retain bond patterns that are characterized by the localization of Clar rings. Even for CNTs with 10 nm length, an alternated, oscillating structure of Clar and Kekulé patterning was also found, indicating that these arrangements may possibly persist for even longer nanotubes, given that they are finite. In the present work, we perform a detailed and comprehensive theoretical study of this phenomenon, in order to find the causes that give rise to these patterns. A complete set of CNTs with different chiralities, diameters (up to 2 nm), lengths (up to 10 nm) and endings (capped, uncapped, and tailored endings) was considered for such purposes. The results indicate that the Clar patterning appears not only on armchair CNTs, but also on those with chiral angle values close to 30°, and this results in a stabilization of the structure, when compared with the uniform, zigzag CNTs. This stabilizing effect points to the causes that underlie the three Nakamura CNT types, resulting as the superposition of structures with a maximal number of Clar rings. Although there is a strict dependence on the border shape, the main cause of the bond patterning in long tubes is to be found in the intrinsic wrapping of each CNT, because the type and number of oscillations present in the longest structures do not depend on the particular length. Nevertheless, the three Nakamura types of armchair tubes appear to subsist beyond the appearance of oscillations, because each of these sets evolves in a different manner, and energy properties that link them together. Apart from the geometry, Clar patterning was investigated through NICS (Nucleus Independent Chemical Shifts) measures, which reveal a connection between the Clar rings and a local concentration of aromaticity.

The calculation of energy gaps in small single-walled carbon nanotubes within a symmetry-adapted tight-binding model

This paper studies in detail the electronic properties of the semimetallic single-walled carbon nanotubes by applying the symmetry-adapted tight-binding model. It is found that the hybridization of π—σ states caused by the curvature produces an energy gap at the vicinity of the Fermi level. Such effects are obvious for the small zigzag and chiral single-walled carbon nanotubes. The energy gaps decrease as the diameters and the chiral angles of the tubes increase, while the top of the valence band and the bottom of the conduction band of armchair tubes cross at the Fermi level. The numeral results agree well with the experimental results.

Density functional theory study of the hydrogen chemisorption of single-walled carbon nanotubes with carbon ad-dimer defect

The structural and electronic properties of hydrogenated armchair and zigzag SWCNTs with carbon ad-dimer (CD) defect were investigated by means of the B3LYP hybrid density functional method using 6-31G∗ basis set. It is found that the chemisorptions of two hydrogen atoms inside and outside the CD defective SWCNTs are exothermic processes. Exohedral nanotube adsorption is energetically more favorable than endohedral adsorption. These results are in agreement with hydrogen on pristine nanotubes. The positional preference for the chemisorption of two hydrogen atoms is the same for the CD defective armchair and zigzag nanotubes. However, the reaction energy of two hydrogen atoms on the exterior sidewalls of CD defective SWCNTs is almost independent of the tube diameter. This is different from the results reported on pristine nanotubes. The calculated energy gaps indicate that the hydrogen-chemisorbed CD defective armchair tubes are always wide energy gap structures, while the hydrogen-chemisorbed CD defective zigzag tubes have significantly lower gaps. The HOMO–LUMO gap and reaction energy for the chemisorption of more hydrogen atoms on the exterior sidewalls of CD defective armchair SWCNTs were also explored.

Nitrogen‐doped SWCNT synthesis using ammonia and carbon monoxide

AbstractGreat effort has been directed into modifying the electrical properties of produced carbon nanotubes by chemical doping and – in parallel – into controlling their chirality. We present a new approach to synthesizing nitrogen‐doped single‐walled carbon nanotubes (SWCNTs), based on gas phase floating catalyst CVD. The nitrogen content in the tubes was verified by transmission electron microscopy/electron‐energy loss spectroscopy (TEM/EELS), and their chiral angle distribution found by electron diffraction (ED) to be skewed toward armchair tubes. Additionally, the optical properties of the material were probed by multi‐frequency Raman spectroscopy and optical absorption.

Axial strain and twist-induced changes in the electronic structure of carbon nanotubes

Based on the tight-binding model of generalized honeycomb lattice on cylinder, the band structure changes induced by axial strain and twist are present for the signal-wall carbon nanotubes (SWCNTs). The deformation-modified hopping amplitudes are dominated by the metric tensor and the curvature tensor. The analytical relations of the energy bands and the band gaps to both armchair and zigzag tubes are obtained. For the chiral nanotubes, we have successfully investigated the deformation-induced changes in band structure of individual nanotubes of known chiral index. The results in our model, returning to the undeformed tubes, are in good agreement with the previous experiments and theories.

Effects of diameter, length, chirality and defects on the scavenging action of single-walled carbon nanotubes for OH radicals: A quantum computational study

Binding of an OH radical at the outer surfaces of the open-ended pristine armchair (5, 5), (7, 7) single-walled carbon nanotubes of varying lengths, zigzag (9, 0), (10, 0) tubes, and Stone–Wales and single vacancy defected (5, 5) tubes was investigated theoretically. Binding of two OH radicals with a zigzag (10, 0) nanotube was also investigated. The semiempirical PM6 method was used for geometry optimization which was followed by single-point energy calculations at B3LYP/6-31G(d,p) level of density functional theory. Binding energies of the adducts of armchair nanotubes with an OH radical each were found to decrease with increase in diameter, this effect being more pronounced for longer tubes. Semi-conducting zigzag (10, 0) tube was found to be a better OH radical scavenger than the metallic armchair (5, 5) and zigzag (9, 0) tubes. Stone–Wales and single vacancy defected armchair tubes were found to be less and more reactive respectively towards an OH radical than the corresponding pristine tubes.

Theoretical Exploration of the Structural, Electronic, and Magnetic Properties of ZnO Nanotubes with Vacancies, Antisites, and Nitrogen Substitutional Defects

We investigated vacancies, antisites, and nitrogen substitutional defects in ZnO single-walled zigzag and armchair nanotubes using spin-polarized density-functional calculations. We found that all defects introduced defect levels in the band gap. Among the investigated defects, oxygen vacancy had the lowest formation energy under zinc-rich conditions, but induced no magnetism in the tubes. A Zn vacancy induced a magnetic moment of 2.0 μB/cell in the tubes, resulting from the oxygen dangling-bond states. On the other hand, while a ZnO antisite defect induced a magnetic moment of 2.0 μB/cell in the zigzag tube, we found no magnetism in the armchair tube. An important prediction is that antisite defects, with high formation energies in bulk, could have relatively low formation energies in ZnO tubes. Finally, we report on the nitrogen-doped ZnO tubes. Most interestingly, we found a magnetic moment of 1.0 μB/cell for the N substitution on both the oxygen and zinc sites.

MD investigation of the collective carbon atom behavior of a (17, 0) zigzag single wall carbon nanotube under axial tensile strain

The collective dynamic behavior of carbon atoms of a (17, 0) zigzag single wall carbon nanotube is investigated under tensile strains by molecular dynamics (MD) simulations. The “slip vector” parameter is used to study the collective motion of a group of atoms and the deformation behavior in three different directions (axial, radial, and tangential) of a (17, 0) carbon nanotube. The variations of radial slip vectors indicate almost all carbon atoms of the (17, 0) carbon nanotube will stay on the cylindrical surface before the yielding of the single wall carbon nanotube (SWNT). Furthermore, the tangential vectors show kinking deformation for the (17, 0) zigzag tube only rarely appears when the crack occurs. Non-symmetrical deformation around a carbon atom along the axial direction also can be found. The variations in the slip vector values of each atom display a symmetrical crack along the horizontal direction and normal to the tube axis. Chain-like structures with 3–4 atoms can be observed, with the number of chain-like structures decreasing before the breakage of the SWNT. The mechanical properties and dynamic behavior of a (17, 0) zigzag SWNT under tensile strain are also compared with that of a (10, 10) armchair tube in our previous study (Weng et al. 2009).

First-principles study of alkali-atom doping in a series of zigzag and armchair carbon nanotubes

First-principles calculations are performed to study the Li doping in a series of carbon nanotubes with different diameters and chiralities. It is found that the Li–Li interaction inside or outside zigzag tubes is repulsive but strongly screened. Moreover, small diameter zigzag tubes are energetically more favorable than larger ones for Li doping. In contrast, almost all the armchair tubes have the same Li binding energy, especially for the outside doping. Our theoretical results suggest that small diameter zigzag tubes could be plausible candidates for Li-ion battery application. In addition, the doping of other alkali atoms in zigzag tubes is also investigated and the optimal binding distance between them are determined.

Molecular dynamics simulations of adsorption and diffusion of gases in silicon-carbide nanotubes

Silicon carbide nanotubes (SiCNTs) are new materials with excellent properties, such as high thermal stability and mechanical strength, which are much improved over those of their carboneous counterparts, namely, carbon nanotubes (CNTs). Gas separation processes at high temperatures and pressures may be improved by developing mixed-matrix membranes that contain SiCNTs. Such nanotubes are also of interest in other important processes, such as hydrogen production and its storage, as well as separation by supercritical adsorption. The structural parameters of the nanotubes, i.e., their diameter, curvature, and chirality, as well as the interaction strength between the gases and the nanotubes' walls, play a fundamental role in efficient use of the SiCNTs in such processes. We employ molecular dynamics simulations in order to examine the adsorption and diffusion of N(2), H(2), CO(2), CH(4), and n-C(4)H(10) in the SiCNTs, as a function of the pressure and the type of the nanotubes, namely, the zigzag, armchair, and chiral tubes. The simulations indicate the strong effect of the nanotubes' chirality and curvature on the pressure dependence of the adsorption isotherms and the self-diffusivities. Detailed comparison is made between the results and those for the CNTs. In particular, we find that the adsorption capacity of the SiCNTs for hydrogen is higher than the CNTs' under the conditions that we have studied.

Orientation-selective unzipping of carbon nanotubes

We carried out first-principles calculations to explore the oxidative longitudinal unzipping of single-walled carbon nanotubes (SWCNTs) of different diameters and chiralities. We found that the initial attack leading to nanotube unzipping prefers to occur in the middle region for armchair tubes, and at the tube ends for zigzag tubes. Once the initial attack has taken place, by overcoming an energy barrier whose value decreases with increasing tube diameter, the subsequent breakage of C-C bonds parallel to the ones broken in the former process is barrierless. The energetically preferred unzipping path is parallel to the tube axis for armchair tubes, resulting in straight zigzag-edged graphene nanoribbons. For zigzag tubes, there are two energetically equivalent unzipping directions corresponding to the opening of two types of C-C bonds tilted towards the tube axis, giving rise to helical unzipping paths. This is disadvantageous for the production of straight graphene ribbons. A local curvature modulation procedure is proposed to efficiently control the location of the initial attack and thus the shape of the produced graphene nanoribbons.

Characterizing single-walled carbon nanotubes by pressure probe

We compare the behavior of bond lengths, cross sectional shape and bulk modulus at ambient conditions and under hydrostatic pressure of all the three kinds of long uncapped single-walled carbon nanotubes. Results show that two bond lengths completely describe the structure of armchair and zigzag tubes (non-chiral), whereas only one bond length is required to define structure of chiral tubes. In armchair tubes, one bond length is found to be larger and the other smaller than the value for graphene, while in zigzag tubes one of the bond lengths remains equal to and the other larger than graphene bond length. In chiral tubes, three bond lengths are found to be equal to each other and their value depends on the chirality. At some critical pressure values depending on radius, both bond lengths become equal to each other in non-chiral tubes. Above these critical pressure values, the cross section shape changes from circular to oval and the pattern of bond lengths becomes different and dependent on the chirality of the tubes.

The influence of tube length, radius and chirality on the buckling behavior of single-walledcarbon nanotubes filled with copper atoms

The buckling behavior of single-walled carbon nanotubes completely filled with copperatoms under uniaxial compression is investigated using molecular dynamics simulations andcompared with that modeled by continuum mechanics. The effects of geometricalcharacteristics, i.e. tube length, radius and chirality, on buckling deformations are exploredseparately. Results show that the behavior of encapsulated tubes is more complicated thanthat of empty ones due to the accommodation of the internal metal atoms. There areboth similarities and differences between the results obtained by the moleculardynamics method and continuum mechanics. For a group of completely filled (10, 10)tubes with different length, the dependence of the critical strain on the tubelength can be roughly divided into four different linear stages and is accompaniedby a transition of the buckling mode from local to global. It is the competitionbetween the evolution of the structure of metal atoms and the variation of the tubelength that determines the critical strain. There exists a rather wide range oftube radii within which the critical strain has a weak dependence on tube radius,which differs from the observation for empty tubes. As compared with a zigzagtube of the same length and radius, an armchair tube has a lower critical strainbut can be easily strengthened with the incorporation of internal metal atoms.

Electronic and optical properties of single-walled carbon nanotubes under a uniform transverse electric field: A first-principles study

A systematic study on the effect of a transverse electric field on the optical properties of carbon nanotubes has been performed using density-functional calculations. The band gaps of the zigzag tubes decrease significantly as the electric field is increased. In the case of armchair tubes, the metallic band energy dispersion relation develops multivalleys at the Fermi level at high fields. The electric-field-induced changes in band dispersions lead to changes in optical properties as manifested in the dielectric functions. Some of the changes are not obvious. For example, a significant field-induced change in band gap can lead to small changes in dielectric functions in some special cases. Our results show that the armchair carbon nanotubes can be a promising material for electro-optical modulation device applications.