Recently, the transport properties of single-organic molecules have attracted much attention. One of the most prominent systems in this field is a dithiolate molecule between gold surfaces. Since the conductance of a benzene-(1,4)-dithiolate (BDT) sandwiched with two gold electrodes is experimentally measured, it has been intensively investigated in the theoretical field. However, it is still unclear how the conductance depends on the atomic configurations around the contact. Knowing how the contact geometry influences the conductance through the metal/molecule/metal sandwich structure requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different geometries with a fair accuracy. We investigated the transmission properties of a BDT molecule sandwiched between two gold electrodes by means of ab initio method, and examined the effect of the contact structures on it. The transmission coefficients are evaluated with the Lippmann-Schwinger equation. It is found that the peak energies and the peak widths in the transmission curves are sensitively dependent on the contact structures. Furthermore, the contributions of molecular orbitals (MOs) of the BDT molecule to the transmission coefficient also depend on the contact structures. Especially, only some of MOs contribute to the conductance at zero bias. It is also found that the interactions beyond first-nearest neighbors could have remarkable effect on the transport properties. These results suggest that the determination of the contact structure including the electrode system is essential to estimate the transport properties of molecular devices.

This work is supported by ACT-JST, FSIS, and the Special Coordination Funds of MEXT of the Japanese Government. The calculations were carried out using the Numerical Materials Simulator in NIMS.

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