Coaxial Carbon Nanotubes Research Articles

Electrostatics of Partially Gated Carbon Nanotube FETs

The finite-element method is used to solve Poisson's equation, under equilibrium conditions, for coaxial carbon nanotube field-effect transistors in which the gate electrode does not entirely cover the nanotube channel between the source- and drain-end contacts. A conformal transformation is applied to overcome the problems that arise in this open structure of specifying boundary conditions and of terminating the model space. The effect on the potential distribution within the transistor of changing various geometrical properties of the device is investigated, and some special conditions under which appropriate boundary conditions may be defined a priori are identified. The effects on the potential energy profile along the nanotube of varying the work function of the end contacts, and of introducing charge into the gate dielectric, are also investigated. The latter is shown to be effective in suppressing the otherwise dominant role that the end contacts play in determining the barrier to charge flow in the nanotube, thereby allowing bulk control to occur.

Energy dissipation mechanisms in carbon nanotube oscillators.

Energy transfer from the translational degrees of freedom to phonon modes is studied for isolated systems of two coaxial carbon nanotubes, which may serve as a nearly frictionless nano-oscillator. It is found that for oscillators with short nanotubes (less than 30 A) a rocking motion, occurring when the inner tube is pulled about 1/3 out of the outer tube, is responsible for significant phonon energy acquisitions. For oscillators with long nanotubes translational energies are mainly dissipated via a wavy deformation in the outer tube undergoing radial vibrations. Frictional forces between 10(-17) and 10(-14) N per atom are found for various dissipative mechanisms.

Collective excitations of multishell carbon microstructures: Multishell fullerenes and coaxial nanotubes.

\ensuremath{\sigma} and \ensuremath{\pi} plasmons in coaxial carbon nanotubes and multishell fullerenes are modeled in analogy with coupled collective excitations in finite, layered, two-dimensional-electron-gas, planar semiconductor superlattices. The curvature of the surface of these complex carbon clusters plays an important role in shaping the dimensionality (one dimensional, two dimensional, or three dimensional) of the plasmons. Direct crossover from a one-dimensional to a three-dimensional regime is found under readily fulfilled conditions for carbon nanotubes in the case of small finite longitudinal momentum transfer \ensuremath{\Elzxh}q, while for q=0 bulk graphitic plasmons fail to develop. For large q, a two-dimensional behavior is found. The case of multishell fullerenes resembles in all instances the q=0 behavior of carbon nanotubes. Such behavior correlates with the observed systematic redshift of the strong interstellar absorption band as compared to the \ensuremath{\pi} plasmon of bulk oriented graphite (i.e., the 5.7 eV position of the former compared to the 6.2 eV energy of the latter). Furthermore, in the case of \ensuremath{\pi} plasmons in carbon nanotubes, a special surface mode can develop for large q, due to the difference in the values of the dielectric constants between the graphitic structures and the surrounding medium. \textcopyright{} 1996 The American Physical Society.

Carbon nanotubes—II. Cohesion and formation energy of cylindrical nanotubes

A (6, 12) Lennard-Jones formulation is used to evaluate the enthalpy of formation of multilayered coaxial or scroll-like carbon nanotubes. The role of various parameters—such as interlayer spacing, value of the innermost radius, number of layers—is examined. Most of the nanotubes examined turn out to be metastable, with a small but positive heat of formation.

Dimensionality crossovers of the sigma plasmon in coaxial carbon nanotubes.

Dimensionality crossovers of plasmons in carbon nanotubes, modeled as curved layered electron-gas superstructures, are investigated. For small wave vectors, the \ensuremath{\sigma} plasmons exhibit a transition from a one- to a three-dimensional character as the number of graphitic shells increases, in correspondence with recent experiments. For large wave vectors, plasmons of two-dimensional character are predicted.