Shape of large single- and multiple-shell fullerenes

Thermal effect on DWCNTs as rotational bearings

We have investigated the rotational motion and dynamic friction in a molecular bearing composed of double-walled carbon nanotubes (DWCNTs) using molecular dynamics simulations. The main study was on thermal effects due to the rotational friction. The diameters of the bearings varied between 6 and 16 Å for the inner shafts, and between 12 and 20 Å for the outer sleeves. The rotation velocity varied from 0.05 rotations ps(-1) to 0.25 rotations ps(-1). The simulations show that the energy dissipation, and hence the temperature of the system, increases linearly with rotation time. The value of energy dissipation is around 0.59 meV/atom per rotation at ω = 0.05 rotations ps(-1) for a (15, 0)@(23, 0) bearing. Correspondingly, the average friction force is around 1.75 × 10(-5) nN/atom. The dependence of the energy dissipation on the rotation velocity, the interwall distance, and the contact area of the DWCNT is also discussed. It was observed that the energy dissipation becomes lowest when the interwall distance of the DWCNT bearing reaches about 0.34 nm, the equilibrium distance of the Lennard-Jones (L-J) potential. This low energy dissipation suggests that the DWCNT can be a good candidate for a wearless rotational bearing, which supports the previous studies.

Rotational dynamics and friction in double-walled carbon nanotubes

We report a study of the rotational dynamics in double-walled nanotubes using molecular dynamics simulations and a simple analytical model that reproduces the observations very well. We show that the dynamic friction is linear in the angular velocity for a wide range of values. The molecular dynamics simulations show that for large enough systems the relaxation time takes a constant value depending only on the interlayer spacing and temperature. Moreover, the friction force increases linearly with contact area and the relaxation time decreases with the temperature with a power law of exponent -1.53+/-0.04.

The degenerate Fermi gas of π electrons in fullerenes and the σ surface instabilities

The departure from perfect spherical symmetry in the case of fullerenes (C60 being the sole exception) induces instabilities due to the stresses generated by the pentagonal protrusions in the σ-bonded surfaces. By assuming σ-π separability and treating π electrons as a degenerate Fermi gas in the two shells around the central σ structure, the resulting degeneracy pressures can further enhance the σ-surface initiated instabilities for non-icosahedral structures (especially for those <C60) as well as the icosahedral fullerenes (>C60) with large protrusions. Under certain circumstances the net degeneracy pressure across the σ surface may have a structure stabilizing effect. The role of the π-electron degeneracy in a broad range of fullerenes from C20 to C1500 and its effects on fullerene stability are investigated.

A theoretical model of the photoabsorption spectra of carbon buckyonions

A theoretical model has been developed to predict the photoabsorption spectra of spherical carbon buckyonions in the region dominated by the pi-plasmon feature. This model makes use of the microscopic electronic structure of the system, which is provided by an effective Huckel one-electron model. The important screening effects are treated within the random phase approximation, whose form is an extension to the dynamic case of the one derived in a previous work for the static polarizabilities of these species. A systematic analysis as a function of the buckyonion size is performed. We compare the spectra obtained in this way with those derived from a different representation of the electron motion, namely a two-dimensional spherical electron gas, and from a classical dielectric model.

Thermal physics in carbon nanotube growth kinetics

The growth of single wall carbon nanotubes (SWNTs) mediated by metal nanoparticles is considered within (i) the surface diffusion growth kinetics model coupled with (ii) a thermal model taking into account heat release of carbon adsorption-desorption on nanotube surface and carbon incorporation into the nanotube wall and (iii) carbon nanotube-inert gas collisional heat exchange. Numerical simulations performed together with analytical estimates reveal various temperature regimes occurring during SWNT growth. During the initial stage, which is characterized by SWNT lengths that are shorter than the surface diffusion length of carbon atoms adsorbed on the SWNT wall, the SWNT temperature remains constant and is significantly higher than that of the ambient gas. After this stage the SWNT temperature decreases towards that of gas and becomes nonuniformly distributed over the length of the SWNT. The rate of SWNT cooling depends on the SWNT-gas collisional energy transfer that, from molecular dynamics simulations, is seen to be efficient only in the SWNT radial direction. The decreasing SWNT temperature may lead to solidification of the catalytic metal nanoparticle terminating SWNT growth or triggering nucleation of a new carbon layer and growth of multiwall carbon nanotubes.

Unusual high degree of unperturbed environment in the interior of single-wall carbon nanotubes

Double wall carbon nanotubes were prepared by vacuum annealing of single wall carbon nanotubes filled with C60. Strong evidence is provided for a highly defect free and unperturbed environment in the interior of the tubes. This is concluded from unusual narrow Raman lines for the radial breathing mode of the inner tubes. Lorentzian linewidths scale down to 0.35 cm(-1) which is almost 10 times smaller than linewidths reported so far for this mode. A splitting is observed for the majority of the Raman lines. It is considered to originate from tube-tube interaction between one inner tube and several different outer tubes. The highest RBM frequency detected is 484 cm(-1) corresponding to a tube diameter of only 0.50 nm. Labeling of the Raman lines with the folding vector is provided for all inner tubes. This labeling is supported by density functional calculations.

High-Rate, Gas-Phase Growth of MoS2 Nested Inorganic Fullerenes and Nanotubes

The gas-phase reaction between MoO3-x and H(2)S in a reducing atmosphere at elevated temperatures (800 degrees to 950 degrees C) has been used to synthesize large quantities of an almost pure nested inorganic fullerene (IF) phase of MoS(2). A uniform IF phase with a relatively narrow size distribution was obtained. The synthesis of IFs appears to require, in addition to careful control over the growth conditions, a specific turbulent flow regime. The x-ray spectra of the different samples show that, as the average size of the IF decreases, the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large quantities of quite uniform nanotubes were obtained under modified preparation conditions.