Zigzag Carbon Nanotubes Research Articles

Properties of Carbon Nanotubes: An ab Initio Study Using Large Gaussian Basis Sets and Various DFT Functionals

The structural, electronic, dielectric, and elastic properties of zigzag and armchair single-walled carbon nanotubes are investigated at different DFT levels (LDA, GGA, hybrids) with Gaussian type basis sets of increasing size (from 3-21G to 6-1111G(2d,f)). The longitudinal and transverse polarizabilities are evaluated by using the Coupled Perturbed Hartree–Fock and Kohn–Sham computational schemes, which take into account the orbital relaxation through a self-consistent scheme. It is shown that the difference between the frequently adopted SOS (sum over states, uncoupled) and the fully coupled results is far from being negligible and varies as a function of the tensor component and the adopted functional. Helical symmetry is fully exploited. This allows simulation of tubes larger (up to 140 atoms in the unit cell) than in previous studies by using extended basis sets and severe computational conditions. All the 12 functionals considered here provide similar results for the structural and the elastic properties and for the relative stability among nanotubes and with respect to graphene. On the contrary, the stability with respect to diamond, which has a quite different density than that of nanotubes, sensitively depends on the adopted functional. The band gap and the longitudinal polarizability are strongly dependent on the level of approximation: hybrid functionals provide the least deviation from experimental data. In general, data obtained for (n, n), (3n, 0), (3n + 1, 0), and (3n + 2, 0) rolling directions approach the slab limit for large radii following four distinct trends.

Dimer-covering resonating-valence-bond treatment of single-walled zigzag carbon nanotubes

Single-walled zigzag carbon nanotubes with h hexagons around the carbon nanotube, h ranging from 3 to 19, have been investigated from a resonating-valence-bond point of view. The energies calculated for the undoped h = 3n–1 zigzag carbon nanotubes, n integer, suggest that the two lowest-lying phases are degenerate. Therefore, de-confined low-energy topological spin defects would occur. Then, these carbon nanotubes should be conductors, in analogy to polyacethylene. In clear contrast, no such degeneracy is obtained for either, h = 3n+1 or h = 3n, so bound pairs of topological spin defects are expected to occur in these cases. Our findings provide further insights into electron correlation and exchange effects in carbon nanotubes.

Electronic and magnetic properties of nitrogen-doped finite-size and open-ended zigzag carbon nanotubes

Density functional calculations have been used to investigate the effect of the N doping on the electronic and magnetic properties of finite-size and open-ended zigzag carbon nanotubes (CNT). The carbon atoms at the edge site are more easily replaced by N atoms energetically, compared to other sites. The single N atom substitution can reduce the magnetic moment of edge carbon atoms near the doping site and result in the suppression of spin polarization. Such spin suppression is independent of tube diameter and it can be ascribed into perturbation of the π/π * state localized at the doped edge. At low N-doping concentrations, the finite-length zigzag CNTs can maintain an antiferromagnetic (AFM) ground state, but the conversion from AFM to a nonmagnetic state can occur at high impurity concentrations. More interestingly, the tunable electronic properties from the half-semiconducting to semiconducting state can be realized in these N-doped finite-size and open-ended CNTs if subjected to a variably external electric field along the tube axis.

Conductance switching in diarylethenes bridging carbon nanotubes

The recently reported photoswitching of diarylethene derivative molecules bridging carbon nanotube (CNT) contacts is theoretically analyzed. The short lifetime of the lowest unoccupied molecular orbital (LUMO) indicates that neither the open nor closed form of the molecule can be photoexcited into a charge-neutral excited state for any appreciable length of time preventing photochromic ring opening. Analysis of the highest occupied molecular orbital (HOMO) and LUMO lifetimes also suggests that photoexcitation results in oxidation of the molecules. This either reduces the quantum yield of photochromic ring closing, or it gives rise to the possibility of oxidative ring closing. Analysis of the resistance values and energy levels indicates that the HOMO energy levels of the closed isomers relevant for transport must lie within a few k(B)T of the CNT Fermi level. For armchair contacts, the change in resistance with isomer or substituent group is the result of shifts in the energy level of the molecular HOMO. The coupling of the molecular HOMO to the CNT contacts is insensitive to the isomer type or substituent group. For zigzag CNTs, the conductance is dominated by surface states at the Fermi level on the cut ends of the CNTs so that the conductance is relatively insensitive to the isomer type, and the conductance switching ratio is low. Multiple bridging molecules can interact coherently, resulting in energy splitting, shifting, and interference that cause a nonlinear change in conductance with increasing numbers of molecules. Instead of a factor of 3 increase in conductance expected for three independent channels, a factor of 10(3) increase in conductance is obtained for three bridging molecules.

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.

Electronic energy band structures of carbon nanotubeswith spin-orbit coupling interaction

Based on the symmetry adapted tight-binding model, the electronic energy band structures of single wall carbon nanotubes are calculated by considering the spin-orbit coupling interaction. The energy gaps at the Dirac point for the armchair nanotubes are formed due to the spin-orbit coupling interaction and the curvature effect. For the zigzag and chiral carbon nanotubes,the energy band splittings for the lowest unoccupied states and the highest occupied states are also formed by the spin-orbit coupling interaction. The energy splittings are not only dependedent on the diameter and the chiral angle of the carbon nanotubes, but also a symmetric with respect to the Fermi energy level. According to the chiral index (n, m), different tube behaviors are grouped into three families. The numeral results are in good agreement with the experimental results.

Molecular Dynamics Simulation Studies of Structural and Mechanical Properties of Single-Walled Carbon Nanotubes

Structural and mechanical properties of armchair, zig-zag and chiral single-walled carbon nanotubes are computed by employing Molecular Dynamics simulation technique using Discover code with Compass force field via Materials Studio program developed by the Accelrys. Consistent with the literature, we find that the armchair SWCNT is energetically favored over zig-zag and chiral nanotubes. Predicted structural parameters agree well with experimental observations. Observed radial distribution functions show that the single-walled carbon nanotubes remain crystalline after exposing them to 300 K. The predicted Young's and the Shear moduli were in reasonable agreement with other reports. Our predictions show that the Young's modulus of the tubes increases as the diameter of the tube decreases.

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.

The electronic spectra of double-wall zig-zag carbon nanotube affected by the magnetic field

We study the electronic spectra of (9,0)–(18,0) double walled carbon nanotubes influenced by the external magnetic field. We choose the orientation of the magnetic field parallel to the axis of nanotube. We found the strong change of the electronic spectra due to external magnetic field. That means gap between valence and conductive bands in DWN will be changed.

The stabilization of magnetic solitons in carbon nanotubes by a constant electric field at low temperatures

The propagation of a variable electromagnetic field in a bundle of zigzag carbon nanotubes in the presence of a constant electric field at low temperatures is considered. The electronic system of nanotubes is treated microscopically ignoring interactions with the phonon subsystem, because the electromagnetic field pulse is extremely short. An effective equation for the amplitude of the vector-potential of a variable electromagnetic field was obtained. Solutions similar to solitons that corresponded to isolated waves in the case of the cosine dispersion law for the electronic subsystem were revealed. The dependences of the obtained nonlinear solutions on problem parameters and an applied constant electric field were analyzed.

Theoretical phonon dispersion of armchair and metallic zigzag carbon nanotubes beyond the adiabatic approximation

AbstractThe dynamic corrections to the phonon dispersion of armchair and metallic zigzag carbon nanotubes are studied systematically within a non‐orthogonal tight‐binding model. These corrections were found to be important in the vicinity of the Kohn anomaly of the phonon dispersion at two points of the Brillouin zone. The first point is the zone‐centre and the dynamic corrections are large for the longitudinal optical phonons, which give rise to the G−‐band in the Raman spectra. The obtained frequencies of these phonons for a large number of metallic nanotubes are in good agreement with previous estimations and available experimental data. The second point is inside the zone for armchair nanotubes and at the zone centre for zigzag nanotubes. In this case, the dynamic effects are also studied and discussed in view of their importance for modelling of double‐resonance scattering processes.

Alternating field-induced phase transition in zigzag carbon nanotubes

We predict the possibility of spontaneous emergence of a constant electric field in carbon nanotubes if a high-frequency electric field is applied to them. The effect can be related to the nonequilibrium state of the electron subsystem of carbon nanotubes. The characteristics of the spontaneous field with respect to the parameters of the problem under consideration are revealed.

Optical transitions in semiconducting zigzag carbon nanotubes with small diameters: A first-principles broad-range study

We study all possible band-to-band transitions between 12 valence and 16 conduction bands of (8,0), (10,0), and (7,0) nanotubes and calculate the corresponding dipole moments using first-principles methods in a wide ultraviolet-visible-infrared range of photon energies. The goal is to investigate the optical transitions and the selection rules for nanotubes with small diameters in a broad range of energies, taking into account the curvature and rehybridization effects on the dipole moment. Our calculations show the conservation of a modified quantum number, $\stackrel{\fontencoding{LECO}\selectfont\char177{}}{m}$, for all the transitions and reversal of the horizontal parity. Besides the $\ensuremath{\pi}\text{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{\ast}}$ transitions, we observe the possibility of $\ensuremath{\pi}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{\ast}}$, $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\pi}}^{\ensuremath{\ast}}$, and $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{\ast}}$ transitions. In fact, the maximum dipole moment for the three nanotubes corresponds to the transition between valence and conduction bands that are both of $\ensuremath{\sigma}$ nature. The maximum transition rate for an (8,0) nanotube happens at approximately 1.42 eV, corresponding to the transition between the first valence and fourth conduction bands and mapping to the infrared region. The maximum of transition rate for (10,0) and (7,0) nanotubes happens at 0.79 eV and 3.03 eV, respectively. There exist high absorption probabilities in the infrared and visible region in all three nanotubes. Also, high absorption probability in the low ultraviolet region is demonstrated, mostly as a result of $\ensuremath{\sigma}\text{\ensuremath{-}}{\ensuremath{\sigma}}^{\ensuremath{\ast}}$ transitions.

Multiple localized states and magnetic orderings in partially open zigzag carbon nanotube superlattices: An ab initio study

Using ab initio calculations, we examine the electronic and magnetic properties of partially open (unzipped) zigzag carbon nanotube (CNT) superlattices. It is found that depending on their opening degree, these superlattices can exhibit multiple localized states around the Fermi energy. More importantly, some electronic states confined in some parts of the structure even have special magnetic orderings. We demonstrate that, as a proof of principle, some partially open zigzag CNT superlattices are by themselves giant (100%) magnetoresistive devices. Furthermore, the localized (and spin-polarized) states as well as the band gaps of the superlattices could be further modulated by external electric fields perpendicular to the tube axis. We believe that these results will open the way to the production of novel nanoscale electronic and spintronic devices.

Electronic structures and transport properties of sulfurized carbon nanotubes

The electronic and transport properties of side-walled sulfurized (8, 0) zigzag carbon nanotube were investigated by using density functional theory coupled with a non-equilibrium Green function approach. It is found that the adsorption of the sulfur chains largely reduces the bandgap of the semiconducting (8, 0) carbon nanotube, even changing it into a metallic one. More importantly, the transmission eigenstates around the Fermi level are contributed by not only the sulfur chains but also the complex system made of the sulfur chains and the single-walled carbon nanotube. Our results provide a method to improve the conductivity and utilization rate of the surface in the electrodes of supercapacitor which are made of the carbon nanotubes.

Selective Oxidation of Carbon Nanotubes into Zigzag Graphene Nanoribbons

Fabrication of graphene nanoribbons of desired edge and width is a critical challenge for its practical applications. Our first-principles calculations and molecular dynamics simulations show that oxygen atoms are favorable to adsorb on the high curvature sidewalls of radially deformed armchair single-walled carbon nanotubes (SWNTs) and form unzipped C−O−C epoxy lines. With further oxidation, the unzipped epoxy lines are broken into carbonyl pairs and the armchair SWNTs transform into bilayer graphene nanoribbons of zigzag edges and approximately identical widths. Under the same radial deformation and oxidation, the formation of normal epoxy groups on the high curvature regions of zigzag SWNTs are more favorable because of different bond arrangement, and the zigzag carbon nanotubes could not be broken into zigzag graphene nanoribbons by the same procedure. This result provides a possible way to selectively produce bilayer graphene nanoribbons of controlled structure from carbon nanotubes by mechanical and chemical coupling actions.

Low-energy electronic structures of nanotube–nanoribbon hybrid systems

The electronic properties of a nanotube–nanoribbon hybrid system are investigated by the first-principles calculations. This hybrid system is constructed by a zigzag carbon nanotube and an armchair graphene nanoribbon. Its electronic structures strongly depend on the nanotube location and stacking configuration. The interactions between the two subsystems would break the state degeneracy, open subband spacings, and induce more band-edge states. The predicted results could be measured directly by the scanning tunneling spectroscopy.

FREQUENCY-DEPENDENT NONLINEAR CURRENT OSCILLATION AND CHAOTIC DYNAMICS IN SEMICONDUCTING CARBON NANOTUBES

We have analyzed the spatio-temporal patterns of current self-oscillation under DC bias and terahertz frequency-dependent nonlinear dynamics in semiconducting single-walled zigzag carbon nanotubes (CNTs). It is found that different transport states, including periodic and chaotic, appear with the influence of an external AC signal. The global transitions between periodic and chaotic states are clearly resolved from Poincaré bifurcation diagram. When the driving frequency is fixed at inverse golden mean ratio times natural frequency, the nonlinear CNT system exhibits a more complex transition behavior than at other driving frequencies.

Nerve agents interacting with single wall carbon nanotubes: Density functional calculations

First-principles calculations based on density functional theory (DFT) method are used to investigate the adsorption properties of nerve agent DMMP on typical zigzag (semiconducting) and armchair (metallic) single wall carbon nanotubes (SWCNTs). The adsorption energies for DMMP molecule on different adsorption sites on SWCNTs are obtained. The results indicate that DMMP is weakly bound to the outer surface of both the considered SWCNTs and the obtained adsorption energy values and binding distances are typical for the physisorption. We find that DMMP adsorptive capability of metallic CNTs is about twofold that of semiconducting one. The adsorption of DMMP on the higher chiral angle nanotubes was also investigated and the results indicate that nanotube’s chirality increases the adsorption capability of the tube but however the adsorption characteristic is typical for the physisorption. Furthermore, co-adsorption of two DMMP molecules on the SWCNTs as a single-layer/bi-layer of adsorbed molecules as well as the adsorption of one DMMP molecule on the CNT bundles consisting of three SWCNTs has also been examined. The obtained results reveal that for both the considered systems the binding energy was increased for the DMMP adsorption but it’s still typical for the physisorption, consistent with the recent experimental result. The study of the electronic structures and charge analysis indicate that no significant hybridization between the respective orbital takes place and the small interaction obtained quantitatively in terms of binding energies.

Influence of High Atomic Hydrogenation on the Electronic Structure of Zigzag Carbon Nanotubes: A First-Principles Study

Using density functional theory, we study high hydrogenated zigzag single-walled carbon nanotubes from (7,0) to (11,0). Two structure transitions are classified: type A is a metallic transition and type B is a “semiconductive transition" according to the energy band structure. The charge density transforms only at the C-C bonds without hydrogenated sites. The sp3 hybridization is mainly enhanced for all the C-C bonds in the vertical axial direction for type-A configurations, and the sp3 hybridization mainly increases for all C-C bonds along the axial direction for the type-B case.