Contact-structure dependence of transport properties of a single organic molecule between Au electrodes

The contact-structure dependence of the transport properties are studied for the junction systems of a benzene dithiolate (BDT) molecule sandwiched between Au(111) atomic electrodes using the nonequilibrium Green's function method based on the density functional theory. The transport properties for the hollow, bridge, and on-top contact structures are calculated. We show clearly that molecular orbitals of the isolated BDT molecule are responsible for the transmission peaks near the Fermi level. The contribution of these molecular orbitals, observed as broadened and shifted peaks in the projected density of states, is determined by the strength of their interaction with the Au electrodes, and the strength of the molecule-electrode interaction is clearly dependent on the contact structures. In the case of hollow contact, which has the strongest molecule-electrode interaction, we observe a linear potential drop across the junction for the various bias voltages. In the on-top contact structure that has the weakest interaction, on the other hand, the nonlinear potential drop and a weak negative differential conductance are observed. We obtain the largest current for the bridge contact structure, which has a moderate interaction. In this way, the transport properties strongly depend on the strength of the molecule-electrode interaction caused by a change of the contact structure and the unusual behavior is obtained for the contact structure that has the weakest interaction.