Zigzag Single-walled Carbon Nanotubes Research Articles

Exciton states in metallic zigzag single-walled carbon nanotubes under uniaxial strain

The exciton energy spectrum and its binding energy under the uniaxial strain have been theoretically studied by using the tight-binding model in the metallic zigzag single-walled carbon nanotubes (SWNTs). It is found that the energy of M11- and M22- excitons and their binding energies increase with the increase of uniaxial strain, but the energy of M11+ and M22+ excitons and their binding energies decrease as the uniaxial strain increase. So, we can deduce that the splitting of M11 and M22 exciton will disappear as the uniaxial strain increases up to some degree, which is expected to be detected by the future experiment. On the other hand, it is interesting to calculate the exciton energy spectrum and binding energies of the bands that nearest to the Fermi level, with a narrow gap under the uniaxial strain. The obtained results show that they increase with increasing the uniaxial strain, based on which a supplemented tool is offered to detect the deformation degree of a metallic SWNT under uniaxial strain. In addition, we expect the results obtained here can offer some useful information for the future THz applications.

Thermal buckling properties of zigzag single-walled carbon nanotubes using a refined nonlocal model

In the present article, the thermal buckling of zigzag single-walled carbon nanotubes (SWCNTs) is studied using a nonlocal refined shear deformation beam theory and Von-Karman geometric nonlinearity. The model developed simulates both small scale effects and higher-order variation of transverse shear strain through the depth of the nanobeam. Furthermore the present formulation also accommodates stress-free boundary conditions on the top and bottom surfaces of the nanobeam. A shear correction factor, therefore, is not required. The equivalent Young's modulus and shear modulus for zigzag SWCNTs are derived using an energy-equivalent model. The present study illustrates that the thermal buckling properties of SWCNTs are strongly dependent on the scale effect and additionally on the chirality of zigzag carbon nanotube. Some illustrative examples are also presented to verify the present formulation and solutions. Good agreement is observed.

Influence of chirality on the thermal conductivity of single-walled carbon nanotubes

The influence of chirality on the thermal conductivity of single-walled carbon nanotubes (SWNTs) is discussed in this paper, using a non-equilibrium molecular dynamics (NEMD) method. The tube lengths of the SWNTs studied here are 20, 50, and 100 nm, respectively, and at each length the relationship between chiral angle and thermal conductivity of a SWNT is revealed. We find that if the tube length is relatively short, the influence of chirality on the thermal conductivity of a SWNT is more obvious and that a SWNT with a larger chiral angle has a greater thermal conductivity. Moreover, the thermal conductivity of a zigzag SWNT is smaller than that of an armchair one. As the tube length becomes longer, the thermal conductivity increases while the influence of chirality on the thermal conductivity decreases.

Spontaneous formation of one-dimensional hydrogen gas hydrate in carbon nanotubes.

We present molecular dynamics simulation evidence of spontaneous formation of quasi-one-dimensional (Q1D) hydrogen gas hydrates within single-walled carbon nanotubes (SW-CNTs) of nanometer-sized diameter (1-1.3 nm) near ambient temperature. Contrary to conventional 3D gas hydrates in which the guest molecules are typically contained in individual and isolated cages in the host lattice, the guest H2 molecules in the Q1D gas hydrates are contained within a 1D nanochannel in which the H2 molecules form a molecule wire. In particular, we show that in the (15,0) zigzag SW-CNT, the hexagonal H2 hydrate tends to form, with one H2 molecule per hexagonal prism, while in the (16,0) zigzag SW-CNT, the heptagonal H2 hydrate tends to form, with one H2 molecule per heptagonal prism. In contrast, in the (17,0) zigzag SW-CNT, the octagonal H2 hydrate can form, with either one H2 or two H2 molecules per pentagonal prism (single or double occupancy). Interestingly, in the hexagonal or heptagonal ice nanotube, the H2 wire is solid-like as the axial diffusion constant is very low (<5 × 10(-10) cm(2)/s), whereas in the octagonal ice nanotube, the H2 wire is liquid-like as its axial diffusion constant is comparable to 10(-5) cm(2)/s.

On the wrapping of poly(phenylacetylene), polystyrene sulfonate and polyvinyl pyrrolidone polymer chains around single-walled carbon nanotubes using molecular dynamics simulations

The adsorption of poly(phenylacetylene), polystyrene sulfonate and polyvinyl pyrrolidone on the surface of the armchair and zigzag single-walled carbon nanotubes is studied by using molecular dynamics simulations. The wrapping process of polymer chains around the nanotubes is pursued graphically. It is shown that at 20ps, the polymer chains are adsorbed on the nanotube surface. The interaction energy between nanotubes and polymer chains is computed. The effect of nanotube diameter and temperature on the interaction energy is investigated. It is shown that increasing the nanotube diameter results in increasing the energy. However, the effect of temperature on the interaction energy is negligible. The radius of gyration of polymer chains is also computed. It is shown that nanotube diameter has an insignificant role in the radius of gyration of polymer chains.

Immunosuppressive agent leflunomide: A SWNTs-immobilized dihydroortate dehydrogenase inhibitory effect and computational study of its adsorption properties on zigzag single walled (6, 0) carbon and boron nitride nanotubes as controlled drug delivery devices

Leflunomide [HWA 486 or RS-34821, 5-methyl-N-(4trifluoromethylphenyl)-4-isoxazole carboximide] is an immunosuppressive agent effective in the treatment of rheumatoid arthritis. Dihydroortate dehydrogenase (DHODH, EC 1.3.3.1) immobilization on the nanotubes was carried out and biochemical characterization of free and immobilized enzyme was determined. In comparison with free enzyme, the immobilized DHODH showed improved stability and reusability for investigation of inhibition pattern of drugs such as leflunomide. The experimental data showed that, DHODH was inhibited by the active metabolite of leflunomide (RS-61980) with a Ki and KI of 0.82 and 0.06mM, respectively. Results exhibited mixed-type inhibition kinetics towards dihydroorotate as a substrate in the free and immobilized enzyme. Furthermore, the behavior of anticancer drug leflunomide adsorbed on the external surface of zigzag single walled (6,0) carbon and boron nitride nanotubes (SWCNT and SWBNNT) was studied by means of DFT calculations at the B3LYP/6-31G* level of theory. The larger adsorption energies and charges transfer showed that the adsorption of leflunomide onto SWBNNT is more stable than that the adsorption of leflunomide onto SWCNT. Frontier molecular orbitals (HOMO and LUMO) suggest that adsorption of leflunomide onto SWBNNT behave as charge transfer compounds with leflunomide as an electron donor and SWBNNT as an electron acceptor. Thus, nanotubes (NTs) have been proposed and actively explored as multipurpose innovative carriers for drug delivery and diagnostic application. The AIM theory has been also applied to analyze the properties of the bond critical points: their electron densities and their laplacians. Also, the natural bond orbital (NBO) calculations were performed to derive natural atomic orbital occupancies, and partial charges of the interacting atoms in the equilibrium tube-molecule distance.

Strain effects on the band gap and work function of zigzag single-walled carbon nanotubes and graphene: A comparative study

First-principles local density functional calculation of the electronic structure and work function of zigzag single-wall carbon nanotubes (SWCNTs) and graphene under strain are presented. We found that there is a small circumference strain even for unstrained zigzag SWCNTs that release the curvature energy of the tube. Therefore, we propose that there are two effects contributing to the opening of the band gap of unstrained (3q, 0) SWCNTs: the electron transfer enhancement effect and the circumference strain effect. We show that the band gap and work function of strained zigzag SWCNTs can be successfully estimated from the band structure and work function of strained graphene based on the zone folding method with circumference strain effect. The circumference strain effect is crucial to obtain a correct result. The rate of change of the band gap with respect to the strain for zigzag SWCNTs is almost independent of the value of the strain and the radius of the tube.

Monomer-dimer problem on random planar honeycomb lattice

We consider the monomer-dimer (MD) problem on a random planar honeycomb lattice model, namely, the random multiple chain. This is a lattice system with non-periodic boundary condition, whose generating process is inspired by the growth of single walled zigzag carbon nanotubes. By applying algebraic and combinatorial techniques we establish a calculating expression of the MD partition function for bipartite graphs, which corresponds to the permanent of a matrix. Further, by using the transfer matrix argument we show that the computing problem of the permanent of high order matrix can be converted into some lower order matrices for this family of lattices, based on which we derive an explicit recurrence formula for evaluating the MD partition function of multiple chains and random multiple chains. Finally, we analyze the expectation of the number of monomer-dimer arrangements on a random multiple chain and the asymptotic behavior of the annealed MD entropy when the multiple chain becomes infinite in width and length, respectively.

BUCKLING OF DEFECTIVE CARBON NANOTUBES UNDER AXIAL AND TRANSVERSE LOADS

Elastic buckling of single walled carbon nanotubes (SWCNTs) with di-, triple- and pinhole vacancy defects under the transverse and axial compression loading is investigated based on molecular structural mechanics. In this research, the effects of length, radius, loading ratio, and the position of vacancy defect on the buckling behavior of armchair and zigzag single-walled carbon nanotubes are studied. It is found that the position of pinhole-vacancy has a significant effect on the percent of the reduction of the critical buckling force. It is also seen, that the effect of loading kind on the critical buckling forces loses its importance if the length of carbon nanotube (CNT) increases.

Effects of B–N co-doping into the ultra-small diameter zigzag single-walled carbon nanotubes: A density functional theory study

The boron(B)–nitrogen(N) co-doping ultra-small diameter (5,0) and (4,0) single-walled carbon nanotubes (SWCNTs) have been investigated by using the first-principles pseudopotential plane wave method. The results show that B–N pairs are easy to dope into ultra-small diameter zigzag nanotubes, which have numbers of differences with other large diameter nanotubes, and the B–N co-doping configurations are the energetically stable structures. Furthermore, by introducing B–N pairs, the (5,0) and (4,0) SWCNTs both transform from metal to semiconductor. These ultra-small diameter nanotubes have potential to be applied in semiconductor integrated circuit of extremely small size.

Numerical Simulation of the Vibration Behavior of Curved Carbon Nanotubes

Several zigzag and armchair single-walled carbon nanotubes (CNTs) were modeled by a commercial finite element package and their vibrational behavior was studied. Numerous computational tests with different boundary conditions and different bending angles were performed. Both computational and analytical results were compared. It was shown that the computational results are in good agreement with the analytical calculations in the case of straight tubes. In addition, it was concluded that the natural frequency of straight armchair and zigzag CNTs increases by increasing the chiral number of both armchair and zigzag CNTs. It was also revealed that the natural frequency of CNTs with higher chirality decreases by introducing bending angles. Nevertheless, the influence of increasing bending angle on the natural frequency of armchair and zigzag CNTs with lower chiral number is almost negligible.

Induced chirality in single walled carbon nanotube based self-assembly

Chirality is an important determinant of the optical and electronic properties of single walled carbon nanotubes (SWNTs). Pristine zigzag SWNTs have been functionalized with lipids by a solid state functionalization process. Neither the lipids nor the SWNTs show a circular dichroism (CD) signal prior to functionalization. Upon functionalization, a change in the joint density of electronic states occurs which is reflected by a three-peak G′ band profile and the emergence of a CD signal. A molecular mechanism for the emergent CD signal associated with this lipid-specific self-assembly is described.

On the interfacial characteristics of polyethylene/single-walled carbon nanotubes using molecular dynamics simulations

Molecular dynamics simulations are used to study the interactions between polyethylene and single-walled carbon nanotubes. The effect of initial angle of polyethylene chain and nanotube axis is investigated. To study the influence of nanotube geometry on the interfacial properties of polyethylene/nanotube system, a range of nanotubes with different radii are considered. Besides, the influences of temperature and polyethylene chain length on the final conformation of polyethylene chain adsorbed on the nanotube surface are studied. It is shown that the polymer chain structures adsorbed on the armchair and zigzag single-walled carbon nanotubes are almost similar. Finally, systems containing a nanotube placed at the center of multiple polyethylene chains are studied. Long parallel patterns are observed in the final morphologies of multiple chains placed beside the single-walled carbon nanotube.

Effects of Topological Defects and Diatom Vacancies on Characteristic Vibration Modes and Raman Intensities of Zigzag Single-Walled Carbon Nanotubes

Defects are ubiquitous in carbon nanotubes (CNTs), despite their large formation energies, and have astounding effects on their physicochemical properties. In this study, we employ density-functional theory (DFT) calculations to study systematically the atomic structure, stability, and characteristic vibrations of pristine and defected zigzag CNTs, where the defects are of the form of Stone-Wales (SW) and diatom vacancies (DV). The DFT optimized structures and the phonon modes are subsequently used in conjunction with a semiempirical bond-polarization model to study the nonresonant Raman spectra. For each defect type, we find two CNT structures with defects parallel or oblique to the tube axis. For the SW defects, the two structures have similar formation energies, whereas for the DV defect, only defects parallel to the tube axis are likely to exist. The results show that the defects induce a blue shift in the radial breathing mode (RBM) of metallic CNTs, whereas this mode is not shifted for semiconducting CNTs. However, the RBM shift or its Raman profile is not sensitive to the defect type. The G-band showed more sensitivity to the defects in the form of a red/blue shift in the frequency, or a partial/complete defragmentation of the G bands.

Ab initio systematic study of chirality effects on phonon spectra, mechanical and thermal properties of narrow single walled carbon nanotubes

Pseudo-potential method under frame work of density functional perturbation theory (DFPT) has been utilized to calculate the phonon spectrum, phonon DOS, specific heat capacity and mechanical properties of narrow armchair and zigzag single walled carbon nanotubes (SWCNTs). Our calculations show that although their Young modulus are about 1TPa, but armchair SWCNTs have greater compressive modulus than the zigzag tubes while the situation would be vice versa for tensile modulus. On the other hand, it was found out that specific heat capacity of narrow tubes shows chirality independence and decreases by increasing the radius of nanotube whereas, this is not the case for the wider ones. In addition, chirality dependence of the radial breadth mode (RBM) frequency of narrow SWCNTs has been shown.

Density functional study of Al/N co-doped (10,0) zigzag single-walled carbon nanotubes as CO sensor

Al/N co-doped (10,0) SWCNT was investigated through density functional theory (DFT) calculations to find out its sensing properties for the toxic CO gas. The results were compared with pristine, aluminum and nitrogen doped (10,0) SWCNT. The CO molecule was adsorbed to different sites of the modified nanotubes and their geometric structures and electronic properties before and after CO adsorption were investigated. The results show that the adsorption of CO molecule, does not change the properties of pristine (10,0) SWCNT, and has little effect on band structure and DOS of N or Al doped SWCNT. However, adsorbing the CO molecule to Al/N co-doped SWCNT reduces the band gap and changes its electronic properties considerably. So Al/N SWCNT is potentially a good CO sensor.

THE ELECTRONIC TRANSPORT PROPERTIES IN NAPHTHOPYRAN-BASED OPTICAL MOLECULAR SWITCH WITH CARBON NANOTUBE ELECTRODES

By applying nonequilibrium Green's function (NEGF) formalism combined first-principles density functional theory (DFT), we investigate the electronic transport properties of optical molecular switch based on the naphthopyran molecule with two different single-walled carbon nanotube (SWCNT) electrodes. The molecule that comprises the switch can convert between the closed and open forms upon photoexcitation. Theoretical results show that these two forms exhibit very different conductance properties both in armchair and zigzag junction, which can realize the on and off states of the molecular switch. Meantime, the chirality of the SWCNT electrodes strongly affects the switching characteristics of the molecular junctions. The maximum value of on–off ratio can reach 292 at 1.6 V for the switch with zigzag SWCNT electrodes.

Prediction of Elastic Mechanical Behavior and Stability of Single-Walled Carbon Nanotubes Using Bar Elements

A computationally effective structural mechanics approach based on the exclusive use of bar elements is developed in order to test its efficiency to predict the elastic longitudinal, torsional, and radial mechanical properties of zigzag, chiral, and armchair single-walled carbon nanotubes (SWCNTs). In addition, the proposed method is used for the investigation of the stability of SWCNTs under compressive and radial loads. The proposed model uses three-dimensional, two-noded, linear bar finite elements of three degrees-of-freedom per node to represent the force field appearing between carbon atoms due to the basic interatomic interactions. The numerical results, which are compared with corresponding data given in the open literature, demonstrate the convergence and stability of the method in predicting the elastic properties and critical forces that cause the instability of the tubes. The effect of chirality, length, and radius of carbon nanotubes on numerical predictions is thoroughly and clearly demonstrated.

New insights in the adsorption of oxygen molecules on single walled carbon nanotubes

We study the adsorption of molecular oxygen, both singlet and triplet forms on single-walled arm chair and zig-zag carbon nanotubes (CNTs) using density functional calculations. It is found that singlet oxygen has more potential of chemisorption than the ground state oxygen on the walls of the pristine CNTs and that the adsorption behaviour is different for semiconducting and metallic nanotubes. Molecular oxygen in its singlet state is found to adsorb strongly on semiconducting pristine nanosurfaces with appreciable binding energies and is found at bonding distances, ∼1.47Å from the side walls. Ground state oxygen is found to be only weakly physisorbed with distances in the range of 3.3–3.5Å from the tube confirming the previous investigations. The effect of substitutional doping on the adsorption behaviour is studied and the adsorption is found to be enhanced with ground state oxygen coming closer at distances of ∼1.5Å, with appreciable charge transfer, on semiconducting nanotubes doped with boron, in contrast to the weak physisorption of triplet oxygen on the intrinsic tubes. This is an interesting finding, and as substitutional doping offers a permanent solution for possible use as sensor, CNTs doped with boron can find application as a potential molecular oxygen sensor. On doping the metallic nanotubes with boron, the adsorption of singlet oxygen is found to be enhanced with appreciable charge transfer compared to adsorption on the pristine form. We also find that singlet oxygen molecule dissociate near the semiconducting nanotube walls and chemically adsorb onto the surface to form a four membered dioxetane like ring. All the cases of relevant adsorption are accompanied by significant amount of charge transfer from the nanotube to the molecule.

The structural and electronic properties of (10,0) zigzag Single-Wall Carbon Nanotubes modified by thiophene groups

In this letter, modifications of (10,0) functionalized zigzag Single-Wall Carbon Nanotubes in different sites by thiophene groups were studied using density functional theory. Geometric structures and electronic properties were investigated. After modification a significant change was observed in band structure and density of states and metallic properties were obtained. The best result for application in organic photovoltaic cells was achieved when two organic groups were placed on opposite sides of the wall of the SWCNT symmetrically.