Effect of molecule-electrode contacts on single-molecule conductivity of μ-conjugated system measured by scanning tunneling microscopy under ultrahigh vacuum

A pi-conjugated compound with two thioacetate end groups was synthesized using a recently developed method. A self-assembled monolayer (SAM) of corresponding dithiol was prepared by clipping a Au(111)surface for 1 min in 112 mM dithiol toluene solution formed 30 min after the addition of a drop of 30% NH4OH in water to the dithioacetate toluene solution. We investigated electronic Conduction through isolated single molecules of the pi-conjugated compound by statistical analysis,of molecular junctions repeatedly formed between a gold tip and the SAM on Au(111) using scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV). The single-molecular conductivity of the pi-conjugated compound is described as G(m) = T-contact with tip center dot T pi-system center dot T-contact with substrate center dot G(0) according to the Landauer equation. Here, Ts are transmission functions and G(0) is the quantum conductance, i.e., 2e(2)/h. Because of observed high values of G(m) for the pi-conjugated compound, we could readily analyze the effect of molecule-electrode contacts on T-contact with tip center dot T-contact with substrate. Namely, we successfully observed the dependence of T-contact with tip center dot T-contact with substrate on binding sites of -S- on a Au(111) surface, i.e., the bridge (or the hollow) and the top sites.