Impact of junction formation processes on single molecular conductance

We have investigated the electric conductance and atomic structure of single molecular junctions of pyrazine (Py), 4,4'-bipyridine (BiPy), fullerene (C₆₀), and 1,4-diaminobutane (DAB). The single molecular junctions were fabricated by breaking Au contacts between an Au tip and the Au electrode surface in the presence of the target molecules (breaking process) or approaching the Au tip to the Au electrode surface covered by the molecules (making process). In the making process, no major conductance state was observed in the conductance histogram for DAB, while single conductance states were observed for the π-conjugated molecules of Py (10 mG₀, G₀∼ 77.45 μΩ^-1), BiPy (3 mG₀), and C₆₀ (5 mG₀). In contrast to the making process, two major conductance states were observed for Py (1 mG₀ and 0.3 mG₀), BiPy (0.6 mG₀ and 0.2 mG₀), and C₆₀ (30 mG₀ and 3 mG₀) in the breaking process. The observed conductance behavior could be explained by the characters of the anchoring unit of molecules. In the making process, anchoring of molecules cannot be established by the point contact between the Au electrode and the localized lone pair of the N atom of DAB, Py, and BiPY, while molecules anchor on the Au electrode by using the plane-like contact between the Au electrode and the delocalized π electrons in the aromatic ring of Py, BiPY, and C₆₀, which explains the difference in the observed conductance behavior between DAB and the π conjugated molecules of Py, BiPY, and C₆₀ in the making process. In the breaking process, the delocalized π-electrons of Py, BiPY, and C₆₀ can make a contact with the Au electrode surface and the lone pair on the N atom of DAB as well as Py and BiPY can bind to Au atoms, and therefore, single molecular junctions were formed for all systems. The present results indicate that the π-plane is important for making metal-molecule contacts and for the formation of the single molecular junction in the making process.