The carbon-nanotube  (CNT) conjugated-molecule interface has received relatively little attention compared to metal-molecule and metal-semiconductor interfaces. The CNT contacts are a π-bond surface and, as such, they are both chemically and geometrically different from metal contacts or sp³ semiconductor contacts. CNTs connected by conjugated molecules have been experimentally demonstrated and their current-voltage response has been measured [1].  To understand electron transport through such systems, we consider a model system consisting of two metallic (12,0) CNTs connected by a trans-polyacetylene chain (CH)_n [n =  10, 20, and 40]. The system is relaxed using the ONIOM model in Gaussian03, and the electron transmission through the structure is calculated with our own density functional theory (DFT) non-equilibrium Green function (NEGF) software built upon the FIREBALL DFT code. The structures relax such that the dihedral angle between the plane of the polyacetylene and the plane of the CNT at the point of contact is between 17 and 30 degrees depending on the structure which provides good overlap between the π orbitals of the polyacetylene and the π orbitals of the CNTs.  For these structures, the polyacetylene provides good transmission between the CNT contacts. For comparison, we took the same 3 structures and forced the polyacetylene to be perpendicular to the CNTs, and the transmission was reduced approximately 3 orders of magnitude. The effect of chain length on conductance is found to be the opposite of that observed for alkanes. The conductance increases with chain length. This same trend has also been observed experimentally for oligothiophenes [2]. This is attributed to the length dependence of the HOMO-LUMO gap and the closer position of the HOMO to the Fermi level for the longer chains.

[1] Guo, X et al., Science, 311, 356 (2006).

[2] Xu, B. et al., Nano Lett., 5, 1491 (2005).