Encapsulation of methane molecules into carbon nanotubes

Methane gas (CH₄) is a chemical compound comprising a carbon atom surrounded by four hydrogen atoms, and carbon nanotubes have been proposed as possible molecular containers for the storage of such gases. In this paper, we investigate the interaction energy between a CH₄ molecule and a carbon nanotube using two different models for the CH₄ molecule, the first discrete and the second continuous. In the first model, we consider the total interaction as the sum of the individual interactions between each atom of the molecule and the nanotube. We first determine the interaction energy by assuming that the carbon atom and one of the hydrogen atoms lie on the axis of the tube with the other three hydrogen atoms offset from the axis. Symmetry is assumed with regard to the arrangement of the three hydrogen atoms surrounding the carbon atom on the axis. We then rotate the atomic position into 100 discrete orientations and determine the average interaction energy from all orientations. In the second model, we approximate the CH₄ molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the center of the sphere. The total interaction energy between the CH₄ molecule and the carbon nanotube for this model is calculated as the sum of the individual interaction energies between both the carbon atom and the spherical surface and the carbon nanotube. These models are analyzed to determine the dimensions of the particular nanotubes which will readily suck-up CH₄ molecules. Our results determine the minimum and maximum interaction energies required for CH₄ encapsulation in different tube sizes, and establish the second model of the CH₄ molecule as a simple and elegant model which might be exploited for other problems.     

Structure and electronic properties of the double-wall nanotubes constructed from SiO2 nanotubes encapsulated inside zigzag carbon nanotubes

This paper presents ab initio self-consistent field crystal orbital calculations on the structures, stabilities, elastic and electronic properties of the double-wall nanotubes made of SiO(2) nanotubes encapsulated inside zigzag carbon nanotubes based on density functional theory. It is found that formation of the combined systems is energetically favorable when the nearest distance between the two constituents is in the area of the van der Waals effect. The obtained band structures show that all the combined systems are semiconductors with nonzero energy gaps. Based on the deformation potential theory and effective mass approximation, the mobilities of charge carriers are calculated to be in the range of 10(2)-10(4) cm(2) V(-1) s(-1), the same order of magnitude as those of the corresponding zigzag carbon nanotubes. The Young's moduli are also calculated for the combined systems.     

Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

Oscillation of nested fullerenes (carbon onions) in carbon nanotubes

Nested spherical fullerenes, which are sometimes referred to as carbon onions, of I _h symmetries which have N(n) carbon atoms in the nth shell given by N(n) = 60n ² are studied in this paper. The continuum approximation together with the Lennard-Jones potential is utilized to determine the resultant potential energy. High frequency nanoscale oscillators or gigahertz oscillators created from fullerenes and both single- and multi-walled carbon nanotubes have attracted much attention for a number of proposed applications, such as ultra-fast optical filters and ultra-sensitive nano-antennae that might impact on the development of computing and signalling nano-devices. Further, it is only at the nanoscale where such gigahertz frequencies can be achieved. This paper focuses on the interaction of nested fullerenes and the mechanics of such molecules oscillating in carbon nanotubes. Here we investigate such issues as the acceptance condition for nested fullerenes into carbon nanotubes, the total force and energy of the nested fullerenes, and the velocity and gigahertz frequency of the oscillating molecule. In particular, optimum nanotube radii are determined for which nested fullerenes oscillate at maximum velocity and frequency, which will be of considerable benefit for the design of future nano-oscillating devices.     

Precise positioning of carbon nanotubes by ac dielectrophoresis using floating posts

Single-walled carbon nanotubes (SWNTs) have been precisely aligned and positioned in device architectures using ac dielectrophoresis by patterning floating metal posts or strips within the electrode gaps. These structures perturb the electric field, causing local enhancements in the field intensity, as seen in simulation, that guide the nanotubes along a predictable path in given directions, in zigzag patterns, or as single or a sequence of tubes along a series of posts. This method enables the assembly of SWNTs in complex multi-electrode geometries, when specifying the electrode voltages is not sufficient to direct the desired assembly. The device characteristics of the dielectrophoretically-aligned SWNTs are discussed. PACS 61.46.Fg; 73.63.-b; 87.15.Tt     

Oxide ion conduction in Nd9.33(SiO4)6O2 and Sr2Nd8(SiO4)6O2 single crystals grown by floating zone method

Single crystals of Nd_9.33(SiO₄)₆O₂ and Sr₂Nd₈(SiO₄)₆O₂ oxide ion conductors with the oxyapatite structure were grown without any macroscopic defect by the floating zone method. The oxide ion conductivity of the Sr₂Nd₈(SiO₄)₆O₂ single crystal varied with the distance from its seed crystal along the growth direction, because of its changing deficiency in Sr content. Its stoichiometric portion had a lower electrical conductivity by about five orders of magnitude at 600 °C than the value for Nd_9.33(SiO₄)₆O₂ single crystal. Structural refinement of neutron diffraction data for powdered Nd_9.33(SiO₄)₆O₂ single crystal showed that the previous structure analysis was misleading. Cation vacancies were present only at 4f site and its channel oxygen site was fully occupied in space group of P6₃/m. The oxygen had an anisotropic displacement along the channel in the refined oxyapatite structure.     

SU(4) Kondo effect in carbon nanotubes

We investigate theoretically the nonequilibrium transport properties of carbon nanotube quantum dots. Owing to the two-dimensional band structure of graphene, a double orbital degeneracy plays the role of a pseudospin, which is entangled with the spin. Quantum fluctuations between these 4 degrees of freedom result in an SU(4) Kondo effect at low temperatures. This exotic Kondo effect manifests as a four-peak splitting in the nonlinear conductance when an axial magnetic field is applied.     

Lattice dynamics and electron-phonon interaction in (3,3) carbon nanotubes

We present a detailed study of the lattice dynamics and electron-phonon coupling for a (3,3) carbon nanotube which belongs to the class of small diameter based nanotubes which have recently been claimed to be superconducting. We treat the electronic and phononic degrees of freedom completely by modern ab initio methods without involving approximations beyond the local density approximation. Using density functional perturbation theory we find a mean-field Peierls transition temperature of approximately 240 K which is an order of magnitude larger than the calculated superconducting transition temperature. Thus in (3,3) tubes the Peierls transition might compete with superconductivity. The Peierls instability is related to the special 2k(F) nesting feature of the Fermi surface. Because of the special topology of the (n,n) tubes we also find a phonon softening at the Gamma point.     

Radical functionalization of single-walled carbon nanotubes with azo(bisisobutyronitrile)

The isobutyronitryl groups from thermal decomposition of azobisisobutyronitrile in a 1,2-dichlorobenzene solution were successfully attached to the sidewalls of single-walled carbon nanotubes; thermogravimetic analysis shows a weight loss of 20% (compared to raw SWNTs), which was calculated to be ca. 1 isobutyronitryl group in 23 carbon atoms.     

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.     

Connection of silicon nanocrystals (Si-nc) with multi-walled carbon nanotubes

During the last decade silicon nanocrystals (Si-nc) have received widespread interest because of their high quantum efficiency for light emission at room temperature. However, the challenge still ahead is to study and apply these to single Si-ncoptoelectronics, i.e., solving problems linked with connection and manipulation. In this letter we report on connecting (wiring) single Si-nc with conducting multi-walled carbon nanotubes (MWNTs). We have been able to establish a strong mechanical connection by direct growth of MWNTs on Si-nc used as support of iron nanoparticles, by catalytic chemical vapor deposition (CCVD). To monitor the initial stage of the MWNTs growth process, we used a tapered element oscillating microbalance (TEOM). We compared the growth process on Si-nc coated by iron (Fe/Si-nc) to the standard process of growing MWNTs on alumina as support for iron (Fe/Al). The results showed that in the case of Fe/Si-nc catalyst, we obtained three times larger diameter of multi-walled CNTs compared to Fe/Al. This was mainly due to the Si-nc size. The diameter of the CNTs only depended on the size of the Si-nc particles that rested stuck on the tip of the MWNTs. The connected Si-nc kept their photoluminescence properties at room temperature. The present findings open new opportunities in the development of nanodevices for the optoelectronic application field. PACS 81.07.Bc; 81.07.Lk; 81.07.De     

Long lived photo excitations in (6, 5) carbon nanotubes

We study the photo-induced transmission change (ΔT/T) upon quasi-continuum laser excitation, for different modulation frequency and excitation intensity in films of (6, 5) enriched CoMoCat single wall nanotubes (SWNTs) embedded in a polymeric matrix. This technique probes long-lived excited species, with lifetime in the 1 ms–1 s domain. The observed line-shape of ΔT/T spectra that can be assigned to a superposition of ground state bleaching and absorption modulation. We speculate that photo-excitation creates charges which get trapped in the sample, acquiring ms lifetime. This causes bleaching of the ground state absorption due to state filling and possibly a tube diameter expansion, with consequent blue shift of the optical transition.     

Formation of sp(3) bonding in nanoindented carbon nanotubes and graphite

Nanoindentation-induced interlayer bond switching and phase transformation in carbon nanotubes (CNTs) and graphite are simulated by molecular dynamics. Both graphite and CNTs experience a soft-to-hard phase transformation at room temperature at compressive stresses of 12 and 16 GPa, respectively. Further penetration leads to the formation of interlayer sp(3) bonds, which are reversible upon unloading if the compressive stress is under about 70 GPa, beyond which permanent interlayer sp(3) bonds form. During nanoindentation, the maximum nanohardness of graphite can reach 109 GPa, and CNTs 120 GPa, which is comparable to that of diamond.     

A new purification method for single-wall carbon nanotubes (SWNTs)

A new purification procedure is introduced, which uses the advantages of column chromatography and vacuum filtration. Potassium polyacrylate is used as stationary phase. This method is based on the idea that the size of the existing cavities in the polymer increases during a swelling process in distilled water. The cavities are big enough to entrap the nanoparticles, but allow for a free movement of nanotubes and bundles. Received: 6 March 2000 / Accepted: 7 March 2000 / Published online: 5 April 2000     

Oxidation of carbon nanotubes by singlet O2

Chemisorption of singlet (1)Delta(g) O2 on single-walled carbon nanotubes is reexamined by first principles calculations, and the reaction barrier is substantially lower than previously reported when the spin on O2 is correctly treated. The process is initiated by the cycloaddition of a singlet O2 on top of a C-C bond and ended with an epoxy structure with each of the two oxygen atoms occupying a bridge position. The overall process is exothermic, with an activation barrier as low as 0.61 eV for the (8, 0) tube. Our results raise the possibility that carbon nanotubes with small diameters could be degraded after exposure to air and sunlight, similar to the degradation of natural rubber and synthetic plastics.     

Growth of aligned single-walled carbon nanotubes under ac electric fields through floating catalyst chemical vapour deposition

Through floating catalyst chemical vapour deposition(CVD) method,well-aligned isolated single-walled carbon nanotubes (SWCNTs) and their bundles were deposited on the metal electrodes patterned on the SiO₂/Si surface under ac electric fields at relatively low temperature(280℃). It was indicated that SWCNTs were effectively aligned under ac electric fields after they had just grown in the furnace.The time for a SWCNT to be aligned in the electric field and the effect of gas flow were estimated. Polarized Raman scattering was performed to characterize the aligned structure of SWCNTs. This method would be very useful for the controlled fabrication and preparation of SWCNTs in practical applications.     

Diameter-dependent thermal-oxidative stability of single-walled carbon nanotubes synthesized by a floating catalytic chemical vapor deposition method

In this paper, purified single-walled carbon naotubes (SWCNTs) with three different diameters were synthesized using a floating catalytic chemical vapor deposition method with ethanol as carbon feedstock, ferrocene as catalyst, and thiophene as growth promoter. The thermal-oxidative stability of different-diameter SWCNTs was studied by using thermal analysis (TG, DTA), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analysis. The results indicate that small diameter SWCNTs (∼1 nm) are less stable and burn at lower temperature (610 °C), however, the larger diameter SWCNTs (∼5 nm) survive after burning at higher temperature (685 °C), the oxidation rate varies inversely with the tube diameter of SWCNTs, which may be concluded that the higher oxidation-resistant temperature of larger diameter SWCNTs can be attributed to the lower curvature-induced strain by rolling the planar graphene sheet for the larger diameter, so small tubes will become thermodynamically unstable.     

Covalent attachment of poly(ethylene glycol) on multi-walled carbon nanotubes

A new 'graft-onto' method to attach poly(ethylene glycol) (PEG) onto multi-walled carbon nanotubes (MWNTs) has been developed. The method is based on the coupling reaction of radicals formed at the chain end of PEG onto the surface of MWNTs. The polymeric radicals are generated by atom (halogen) transfer reaction between chloroacetyl-terminated PEG and transition metal catalysts. The method allows direct covalent attachment of PEG to pristine MWNTs without pretreatment that could alter their original structure. The resulting PEG-grafted MWNTs showed improved dispersion stability in isopropanol and methanol.     

Vertically aligned peapod formation of position-controlled multi-walled carbon nanotubes (MWNTs)

We have succeeded in making peapod structures with vertically aligned multi-walled carbon nanotubes (MWNTs), grown on a silicon substrate using hot filament chemical vapor deposition. Opening the tips of the MWNTs was performed at 600 C using oxygen and then C₆₀ molecules were deposited on them to fill their inner spaces. We confirmed that the MWNTs were open at the tips and filled with C₆₀ molecules using transmission electron microscopy. In addition, it has been determined that MWNTs can be filled with other materials, such as molybdenum oxide.