Patching and tearing single-wall carbon-nanotube ropes into multiwall carbon nanotubes

Bundles of single-wall carbon nanotubes (SWCNTs) coalesce forming multiwall carbon nanotubes (MWCNTs), containing from two to six nested tubes, under thermal treatment at high temperatures [(2200-2400) degrees C]. This structural transformation is confirmed by extensive molecular dynamics (MD) simulations. The simulations suggest a "patching-and-tearing" mechanism for the single-wall-to-multiwall transformation underlying the "concerted" coalescence of the tubes that begins with their polymerization. Tubes of different sizes and chiralities are considered.     

Nonlinear structural mechanics based modeling of carbon nanotube deformation

A nonlinear structural mechanics based approach for modeling the structure and the deformation of single-wall and multiwall carbon nanotubes (CNTs) is presented. Individual tubes are modeled using shell finite elements, where a specific pairing of elastic properties and mechanical thickness of the tube wall is identified to enable successful modeling with shell theory. The effects of van der Waals forces are simulated with special interaction elements. This new CNT modeling approach is verified by comparison with molecular dynamics simulations and high-resolution micrographs available in the literature. The mechanics of wrinkling of multiwall CNTs are studied, demonstrating the role of the multiwalled shell structure and interwall van der Waals interactions in governing buckling and postbuckling behavior.     

Carbon nanostructures in the industrial production of alkali metals by electrolysis

The carboniferous component of slimes in the electrolytic production of alkali metals (in our case, lithium) is studied for the first time with the aim to find nanostructures in it. The content of nanostructures in this component, which accounts for 10% of the slimes and can be separated by mild chemical treatment, is shown to be 2–4%. These structures are multiwall nanotubes or bundles of multiwall nanotubes, most of which are open. No twisted or single-wall nanotubes have been detected.     

Addition energies of fullerenes and carbon nanotubes as quantum dots: the role of symmetry

Using density-functional theory calculations, we investigate the addition energy (AE) of quantum dots formed of fullerenes or closed single-wall carbon nanotubes. We focus on the connection between symmetry and oscillations in the AE spectrum. In the highly symmetric fullerenes the oscillation period is large because of the large level degeneracy and Hund's rule. For long nanotubes, the AE oscillation is fourfold. Adding defects destroys the spatial symmetry of the tubes, leaving only spin degeneracy; correspondingly, the fourfold behavior is destroyed, leaving an even/odd behavior which is quite robust. We use our symmetry results to explain recent experiments.     

Mechanical detection of carbon nanotube resonator vibrations

Bending-mode vibrations of carbon nanotube resonators were mechanically detected in air at atmospheric pressure by means of a novel scanning force microscopy method. The fundamental and higher order bending eigenmodes were imaged at up to 3.1 GHz with subnanometer resolution in vibration amplitude. The resonance frequency and the eigenmode shape of multiwall nanotubes are consistent with the elastic beam theory for a doubly clamped beam. For single-wall nanotubes, however, resonance frequencies are significantly shifted, which is attributed to fabrication generating, for example, slack. The effect of slack is studied by pulling down the tube with the tip, which drastically reduces the resonance frequency.     

单壁碳纳米管杂化轨道计算

根据轨道杂化理论以及碳纳米管的几何结构,计算了(n,0),(n,n)和(n,m)三种单壁碳纳米管的杂化轨道,给出了杂化轨道s轨道成分和p轨道成分的解析式.对于管径较小的纳米管,锯齿型(n<40),扶手椅型(n<20),手性型(n<30,m相似文献   

Static dielectric properties of carbon nanotubes from first principles

We characterize the response of isolated single-wall (SWNT) and multiwall (MWNT) carbon nanotubes and nanotube bundles to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs, the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. The dielectric response of nonchiral SWNTs in both directions remains linear up to very high values of applied field.     

Ultrafast carrier dynamics in single-wall carbon nanotubes

Time-resolved carrier dynamics in single-wall carbon nanotubes is investigated by means of two-color pump-probe experiments. The recombination dynamics is monitored by probing the transient photobleaching observed on the interband transitions of the semiconducting tubes. This dynamics takes place on a 1 ps time scale which is 1 order of magnitude slower than in graphite. Transient photoinduced absorption is observed for nonresonant probing and is interpreted as a global redshift of the pi-plasmon resonance. We show that the opening of the band gap in semiconducting carbon nanotubes determines the nonlinear response dynamics over the whole visible and near-infrared spectrum.     

On the limiting physical adsorption of hydrogen in carbon materials

The specific features of hydrogen adsorption (and adsorption of other gases) at supercritical temperatures (specifically, the absence of capillary condensation and polymolecular adsorption and the appearance of a maximum in the adsorption isotherm in the pressure range 1–10 MPa) are discussed. Hydrogen adsorption decreases by an order of magnitude as the temperature increases from the critical temperature to the room value. The experimental adsorption isotherms in the supercritical range found in the literature are used to deduce a criterion of limiting hydrogen adsorption at various temperatures. Carbon adsorbents of different types (individual single-wall nanotubes, bundles of such nanotubes, multiwall nanotubes, and carbon fibers) are considered. A model of single graphite plane shows that the limiting hydrogen adsorption is 5 wt % at 77 K and 1 wt % at 293 K. These values can only be approached by adsorption in a material made of individual single-wall nanotubes. Methods to increase the adsorption are proposed.     

Towards processing of carbon nanotubes for technical applications

Production methods for carbon nanotubes are now well established and allow their synthesis on a scale of grams per day. For many potential applications of this unique material, its purification still remains a crucial problem. In this article various purification methods for single- and multi-wall carbon nanotubes are reviewed. These methods are compared in terms of their capacity, efficiency, and effects on the tubes. In addition, the use of Raman spectroscopy for monitoring the chromatographic purification of single-wall nanotubes is described. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999     

Electronic structure of collapsed C,BN, and BC3 nanotubes

The electrical properties of single-wall C, BN, and BC₃ nanotubes in ideally rolled-up forms show a wide spectrum from truly metals to large band gap semiconductors. In the presence of radial deformations that collapse tubes, the electrical properties are severely modified such that metals turn into semiconductors and vice versa. Based on first-principles pseudopotential calculations, we find that metallic C nanotubes have a finite band gap if radial deformations break all mirror symmetries of the tubes, and that original finite gaps (∼0.5 eV) of semiconducting C and BC₃ tubes are closed by collapsing deformations. In BN tubes, band gaps can be tuned in the range 2–5 eV. On the other hand, the band gaps of armchair BN and zigzag BC₃ nanotubes are found to be insensitive to radial deformations. These new findings can be applied to design new types of nanotube-based functional devices using radial deformations.     

Far-infrared to visible optical conductivity of single-wall carbon nanotubes

The reflectance of unoriented single-wall carbon nanotube films has been measured over a wide wavelength range (far-IR–UV). The results are consistent with the film being a mixture of conducting (armchair), small bandgap (n≡m, mod 3) and semiconducting nanotubes. The optical conductivity shows peaks corresponding to transitions between density-of-states peaks of these tubes, at energy locations consistent with 1.4 nm diameter tubes. In addition optical absorption spectroscopy of aligned single-wall carbon nanotubes shows that the optical transitions are well-aligned along the tube axis. This behavior is consistent with polarized resonant Raman and electronic structure calculations.     

Fabrication of a carbon nanotube device using a patterned electrode and a local electric field

Suspensions of carbon nanotubes (CNTs) and organic solvent were dropped onto a substrate which had patterned electrodes while applying a DC voltage between the electrodes. Both multiwall and single-wall (SW) CNTs were purified from the mixture of CNTs and the undesirable particles of carbon when the solvent dichloromethane was used at high temperature. It is found that a SW CNT bridges the gap of the electrodes. This enables us to fabricate CNT devices at a controlled position.     

Superconductor-insulator transition in a two-dimensional array of resistively shunted small josephson junctions

Using a scattering technique based on a parametrized linear combination of atomic orbitals Hamiltonian, we calculate the ballistic quantum conductance of multiwall carbon nanotubes. We find that interwall interactions not only block some of the quantum conductance channels, but also redistribute the current nonuniformly over individual tubes across the structure. Our results provide a natural explanation for the unexpected integer and noninteger conductance values reported for multiwall nanotubes by Stefan Frank et al. [Stefan Frank et al., Science 280, 1744 (1998)].     

On ballistic transport in carbon nanotubes

We investigate theoretically the ballistic regime exhibited by conduction electrons in multiwalled carbon nanotubes in relation to the conductance quantization in these tubes. Starting from the fact that electron drift mobility is quantized in multiwall tubes, essential aspects related to both ballistic and diffusive regimes are discussed.     

Chemically active substitutional nitrogen impurity in carbon nanotubes

We investigate the nitrogen substitutional impurity in semiconducting zigzag and metallic armchair single-wall carbon nanotubes using ab initio density functional theory. At low concentrations (less than 1 at. %), the defect state in a semiconducting tube becomes spatially localized and develops a flat energy level in the band gap. Such a localized state makes the impurity site chemically and electronically active. We find that if two neighboring tubes have their impurities facing one another, an intertube covalent bond forms. This finding opens an intriguing possibility for tunnel junctions, as well as the functionalization of suitably doped carbon nanotubes by selectively forming chemical bonds with ligands at the impurity site. If the intertube bond density is high enough, a highly packed bundle of interlinked single-wall nanotubes can form.     

Electron spin resonance signal of Luttinger liquids and single-wall carbon nanotubes

A comprehensive theory of electron spin resonance (ESR) for a Luttinger liquid state of correlated metals is presented. The ESR measurables such as the signal intensity and the linewidth are calculated in the framework of Luttinger liquid theory with broken spin rotational symmetry as a function of magnetic field and temperature. We obtain a significant temperature dependent homogeneous line broadening which is related to the spin-symmetry breaking and the electron-electron interaction. The result crosses over smoothly to the ESR of itinerant electrons in the noninteracting limit. These findings explain the absence of the long-sought ESR signal of itinerant electrons in single-wall carbon nanotubes when considering realistic experimental conditions.     

Intertube coupling in ropes of single-wall carbon nanotubes

We investigate the coupling between individual tubes in a rope of single-wall carbon nanotubes using four probe resistance measurements. By introducing defects through the controlled sputtering of the rope we generate a strong nonmonotonic temperature dependence of the four terminal resistance. This behavior reflects the interplay between localization in the intentionally damaged tubes and coupling to undamaged tubes in the same rope. Using a simple model we obtain the coherence length and the coupling resistance. The coupling mechanism is argued to involve direct tunneling between tubes.     

High-energy phonon branches of an individual metallic carbon nanotube

We present excitation-energy dependent Raman measurements between 2.05 and 2.41 eV on the same individual carbon nanotube. We find a change in the Raman frequencies of both the D mode (63 cm(-1)/eV) and the high-energy modes. The observed frequencies of the modes at approximately 1600 cm(-1) as a function of laser-energy map the phonon dispersion relation of a metallic tube near the Gamma point of the Brillouin zone. Our results prove the entire first-order Raman spectrum in single-wall carbon nanotubes to originate from double-resonant scattering. Moreover, we confirm experimentally the phonon softening in metallic tubes by a Peierls-like mechanism.