Origin of high- and low-conductance traces in alkanediisothiocyanate single-molecule contacts

Abstract

Zero-bias transport properties of alkanediisothiocyanate $[\text{-SCN-}{({\text{CH}}_{2})}_{n}\text{-NCS-},\text{ }n=4,6,8]$ molecules anchored to various quasi-one-dimensional Au(111) electrodes are studied by an ab initio method within a standard two-probe methodology based on the density-functional theory and the nonequilibrium Green's functions formalism. We have shown that the high-/low-conductance traces observed in STM break-junction experiments on this molecular species can be clearly related to the difference in the electronic structures between the molecular contacts with different geometric configurations of the near-interface planes of the electrodes. The difference in transport properties of alkanediisothiocyanate single-molecule junctions can be understood in terms of the distribution of the surface states, emerging in the energy gap of the alkanes due to the electrode environment. A general feature of the high-conductance junctions is a number of closely located surface states around the Fermi energy, well coupled to the scattering states of the junctions. These well-coupled states broaden the transmission peaks responsible for conductance, increasing their amplitudes at the Fermi energy, while the sparser distribution of the surface states gives sharper peaks with smaller amplitudes at the Fermi energy, naturally representing the low-conductance junctions. The effects of changing the end groups, junction distance, and electrode cross-section are also investigated and analyzed in terms of changes in the electronic structure of the molecular junctions. A comparison with the available experimental data is also provided.