Simulation and microwave measurement of the conductivity of carbon nanotubes

Abstract

Recently, excellent properties have been realised from structures formed by carbon nanotubes. This propelled their use as nanoscale electronic devices in the information technology industry. The discovery of carbon nanotubes has stimulated interest in carbonbased electronics. Metal-Oxide-Semiconductor systems (MOS) are used to model charge transport within these carbon structures. Schrodinger‟s equation is solved self-consistently with Poisson‟s equation. The Poisson equation, which defines the potential distribution on the surface of the nanotube, is computed using a two-dimensional finite difference algorithm exploiting the azimuthal symmetry. A solution to the Schrodinger‟s equation is required to obtain the wavefunctions within the nanotube model. This is implemented with the scattering matrix method. The resulting wavefunctions defined on the nanotube surface are normalised to the flux computed by the Landauer equation. A novel implementation of the Schrodinger- Poisson solver for providing a solution to a three dimensional nanoscale system is described. To avoid convergence problems, an adaptive Simpson‟s method is employed in the model devices. Another main contribution to this field is the highlighting of the differences in the output characteristics of carbon nanotube- and graphene-based devices. In addition, the source and drain contacts that give an optimum device performance are identified. The limitation of this model is that quantised conductance appears on making contact to the nanotube ends. Electron transport in carbon nanotubes can be studied using non-contacting means. A new approach is to induce current in the nanotubes using microwave energy. A resonator-based measurement method is used to examine the electrical properties of the nanotubes. Remarkably, the nanotubes appear to have the smallest sheet resistance of any non-superconducting material. The possibility of a ferromagnetic carbon nanotube is investigated due to the remarkable screening properties observed. Measurements of the magnetisation as a function of the applied magnetic field are conducted using a vector vibrating sample magnetometer. The morphology and microstructure of the nanotubes are observed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. Carbon nanotubes can be contaminated with metal particulates during growth. These impurities can modify charge transport in these carbon structures