Narrow Single-walled Carbon Nanotubes Research Articles

Effects of KI Encapsulation in Single-Walled Carbon Nanotubes by Raman and Optical Absorption Spectroscopy

The effect of KI encapsulation in narrow (HiPCO) single-walled carbon nanotubes is studied via Raman spectroscopy and optical absorption. The analysis of the data explores the interplay between strain and structural modifications, bond-length changes, charge transfer, and electronic density of states. KI encapsulation appears to be consistent with both charge transfer and strain that shrink both the C-C bonds and the overall nanotube along the axial direction. The charge transfer in larger semiconducting nanotubes is low and comparable with some cases of electrochemical doping, while optical transitions between pairs of singularities of the density of states are quenched for narrow metallic nanotubes. Stronger changes in the density of states occur in some energy ranges and are attributed to polarization van der Waals interactions caused by the ionic encapsulate. Unlike doping with other species, such as atoms and small molecules, encapsulation of inorganic compounds via the molten-phase route provides stable effects due to maximal occupation of the nanotube inner space.

Density-functional study of the mechanical and electronic properties of narrow carbon nanotubes under axial stress

The behavior of some narrow single-wall carbon nanotubes under uniaxial stress was studied. The first principles local density-functional calculations were carried out using helical repeat units and periodic boundary conditions for a single infinite chain. The mechanical response of the tube, the geometrical deformations, and the axial elastic modulus are calculated. The mechanical deformations are reflected also in the electronic properties. The change of the band gaps of the chiral tubules under axial stress was investigated. The indirect gaps of $\ensuremath{\sim}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ can be closed applying small $(l5%)$ deformations which results in semiconductor-metal transitions.

Phonons in narrow carbon nanotubes

Accurate calculations of the phonon dispersion relations, phonon density of states, and phonon eigenvectors of the narrow single-wall carbon nanotubes in optimized geometry are carried out. The method applied is based on the force constants for graphene which reflect the long-range character of the dynamical matrix. Further, the relaxation and symmetry imposed modifications of the force constants are performed and the calculations are carried out by means of the fully symmetry implemented POLSym code. Shortcomings of the widely used frozen phonon model are overcome. The results obtained are compared to the Raman scattering measurements on the zeolite-grown nanotubes.

Diameter control and growth mechanism of single-walled carbon nanotubes

Single-walled carbon nanotubes (SWNTs) have been synthesized using the alcohol chemical vapor deposition technique. On comparing the SWNTs synthesized using different catalyst compositions, an obvious fact has been clarified – the diameter of SWNTs is strongly dependent on the sizes of the catalyst particles and metal species. Iron atoms exhibit a strong interaction with carbon atoms and keep their diameters narrow. Cobalt atoms exhibit a poorer interaction than the iron atoms, but work to decompose ethanol. As a result, narrow SWNTs are generated on the iron-rich catalyst. In this Letter, we propose the most probable growth mechanism of the SWNTs.

Theoretical study of the electronic properties of narrow single-walled carbon nanotubes: Beyond the local density approximation

In this work we present a systematic density functional theory study of the electronic properties of single-walled carbon nanotubes (SWNT) with diameters ranging from 3 to 5 A. In this work meta-generalized-gradient approximation, hybrid, and screened exchange hybrid functionals are utilized to compute energy band gaps in these narrow SWNT. Our calculations using hybrid functionals show that the only true exceptions to the zone folding predictions are the (4,0) and (5,0) SWNT. The remaining chiral SWNT are semiconducting with band gaps that can be as large as 1.7 eV. However, the calculated energy band gaps are significantly smaller than those predicted by the zone folding scheme. This difference is primarily attributed to the sigma-pi hybridization present in such narrow SWNT.

Localized States of a Narrow Single-Walled Carbon Nanotube by Using Su-Schrieffer-Heeger Model Hamiltonian

As a continuation of our previous work [N. Sunel and O. Ozsoy: Int. J. Quant. Chem., Vol. 99 (2004)] on deriving the most general matrix representations, we investigate localized states of a carbon sheet and those of a single-walled carbon nanotube and of a nanotorus within the extended Su-Schrieffer-Heeger model.

Density and thermodynamics of hydrogen adsorbed inside narrow carbon nanotubes

A model is proposed for calculating the thermodynamic functions and the equilibrium density of a one-dimensional chain of molecules (atoms) adsorbed inside a narrow nanotube. The model considers both the interaction between introduced atoms (molecules) and their interaction with the nanotube walls. The quantum-mechanical effects resulting in discrete energy levels of a particle and in its smeared position between neighbors are taken into account. In calculating the free energy at a nonzero temperature, the phonon contribution and the particle transitions to excited levels are considered. The model is applied to calculate the thermodynamic parameters of adsorbed hydrogen molecules inside extremely narrow single-wall carbon nanotubes of the (3,3) and (6,0) type. It is shown that external pressure gives rise to a sequence of first-order phase transitions, which change the density of adsorbed hydrogen molecules.

Distribution of the superfluid density of 4He inside a carbon nanotube

We report results of the iteration calculations for the superfluid density 4 He absorbed in a narrow single walled carbon nanotube around a point of the λ-transition. The calculations were made for different values of the diameter d of a carbon nanotube and of an inoculating interaction V 0 (the depth of a potential well).

Influence of Temperature and Species in the Feedstock on theGrowth of Single-Wall Carbon Nanotubes

A molecular dynamics simulation method isused to study the growth of narrow single-wall carbon nanotubes. It isfound that the growth temperature and the species of small carbon clustersin the feedstock are important for the quality of the nanotubes grown.There is a temperature range of 1000-1500 K in which the narrowarmchair single-wall carbon nanotubes can grow rapidly via adduction ofC2 dimers, even without the existence of catalysts. The narrow zigzagtubes cannot keep open-ended growth. If the feedstock consists of variousspecies of carbon clusters, the tubes cannot keep open-ended growth. Athigher temperatures, the narrow nanotubes close rapidly in thenoncatalytic environment, and the products grown are fullerene-likecapsules. PACS: 61. 46. +w, 36. 40. -c

Quasi-one-dimensional4Heinside carbon nanotubes

We report results of diffusion Monte Carlo calculations for both $^4$He absorbed in a narrow single walled carbon nanotube (R = 3.42 \AA) and strictly one dimensional $^4$He. Inside the tube, the binding energy of liquid $^4$He is approximately three times larger than on planar graphite. At low linear densities, $^4$He in a nanotube is an experimental realization of a one-dimensional quantum fluid. However, when the density increases the structural and energetic properties of both systems differ. At high density, a quasi-continuous liquid-solid phase transition is observed in both cases.

The Crystallography of Metal Halides formed within Single Walled Carbon Nanotubes

AbstractThe crystal growth behaviour and crystallography of a variety of metal halides incorporated within single walled carbon nanotubes (SWNTs) as determined by high resolution electron microscopy (HRTEM) is described. Simple packed structures, such as the alkali halides, form related structures within SWNTs that are found to be integral atomic layers in terms of their thickness as a function of the encapsulating SWNT diameter. An enhanced HRTEM image restoration technique reveals precise data concerning lattice distortions present in these crystals. More complex structures, such as those derived from 3D complex, layered and chain halides form related crystal structures within SWNTs. In narrow SWNTs (i.e. with diameters less than ca. 1.6 nm), structures consisting of individual 1D polyhedral chains (1D-PHCs) were obtained that were derived from the corresponding bulk halides structures. In the case of infinite 3D network and layered halides, the 1D polyhedral chains form with lower co-ordinations than in the bulk. Molecular halides also intercalate into SWNTs but these do not readily form organised structures within SWNTs.

Self-assembly growth of single-wall carbon nanotubes

The growth of single-wall carbon nanotubes through adduction of small carbon clusters is studied using a molecular-dynamics simulation method. The perfection of the tubes grown depends on the collisional direction, the growth temperature, and the cluster species involved. Narrow single-wall carbon nanotubes can grow without existence of catalysts under controlled conditions. C 2, colliding with the side wall of the tubes, can be assembled into the network forming localized defects during annealing.