Metallic carbon nanotubes with metal contacts: electronic structure and transport

We study quasi-ballistic electron transport in metallic $(6,0)$ carbon nanotubes (CNTs) of variable length in contact with Al, Cu, Pd, Pt, Ag, and Au electrodes by using the non-equilibrium Green's function formalism in combination with either density functional theory or self-consistent extended Hückel theory. We find good agreement between both. Visualizing the local device density of states of the systems gives a descriptive link between electronic structure and transport properties. In comparison with bare finite and infinite tubes, we show that the electronic structure of short metallic CNTs is strongly modified by the presence of the metallic electrodes, which leads to pronounced size effects in the conductance. The mean conductances and linear response currents allow a ranking of the metals regarding their ability to form low-Ohmic contacts with the nanotube: ${\rm Ag}\lesssim {\rm Au}\lt {\rm Cu}\ll {\rm Pt}\approx {\rm Pd}\ll {\rm Al}$. These findings are contrasted with similar trends in contact distance, binding energy, calculated work function of the metal surfaces, and various results from literature.