Coupling of carbon nanotubes to metallic contacts

The modeling of carbon nanotube-metal contacts is important from both basic and applied viewpoints. For many applications, it is important to design contacts such that the transmission is dictated by intrinsic properties of the nanotube rather than by details of the contact. In this paper, we calculate the electron transmission probability from a nanotube to a free-electron metal, which is side contacted. If the metal-nanotube interface is sufficiently ordered, we find that k-vector conservation plays an important role in determining the coupling, with the physics depending on the area of contact, tube diameter, and chirality. The main results of this paper are (i) conductance scales with contact length, a phenomena that has been observed in experiments; (ii) in the case of uniform coupling between metal and nanotube, the threshold value of the metal Fermi wave vector (below which coupling is insignificant) depends on chirality; and (iii) an armchair tube couples better than a zigzag tube to ordered Au/Ag contacts. Disorder and small phase coherence length relax the need for k-vector conservation, thereby making the coupling stronger.