Effect of scattering and contacts on current and electrostatics in carbon nanotubes

We computationally study the electrostatic potential profile and current carrying capacity of carbon nanotubes as a function of length and diameter. Our study is based on solving the nonequilibrium Green’s function and Poisson equations self-consistently, including the effect of electron-phonon scattering. A transition from the ballistic to diffusive regime of electron transport with an increase of applied bias is manifested by qualitative changes in the potential profiles, differential conductance, and electric field in a nanotube. In the low-bias ballistic limit, most of the applied voltage drop occurs near the contacts. In addition, the electric field at the tube center increases proportionally with diameter. In contrast, at high biases, most of the applied voltage drops across the nanotube, and the electric field at the tube center decreases with an increase in diameter. We find that the differential conductance can increase or decrease with bias as a result of an interplay of nanotube length, diameter, and a quality factor of the contacts. From an application viewpoint, we find that the current carrying capacity of nanotubes increases with an increase in diameter. Finally, we investigate the role of inner tubes in affecting the current carried by the outermost tube of a multiwalled nanotube.