Effect of Molecule-Electrode Contacts on Single-Molecule Conductivity of π-Conjugated System Measured by Scanning Tunneling Microscopy under Ultrahigh Vacuum

摘要

A π-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 dipping a Au(111) surface for 1 min in a 2 mM dithiol toluene solution formed 30 min after the addition of a drop of 30% NH_4OH in water to the dithioacetate toluene solution. We investigated electronic conduction through isolated single molecules of the π-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 π-conjugated compound is described as G_m = T_(contact with tip)· T_(π-system) · T_(contact with substrate)· 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 π-conjugated compound, we could readily analyze the effect of molecule-electrode contacts on T_(contact with tip) · T_(contact with substrate)· Namely, we successfully observed the dependence of T_(contact with tip) · 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.