Effect of Molecule–Electrode Contacts on Single-Molecule Conductivity of $\pi$-Conjugated System Measured by Scanning Tunneling Microscopy under Ultrahigh Vacuum

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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 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% 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 $G_{\text{m}}=T_{\text{contact with tip}}\cdot T_{\text{$\pi$-system}}\cdot T_{\text{contact with substrate}}\cdot G_{0}$ according to the Landauer equation. Here, $T$s are transmission functions and $G_{0}$ is the quantum conductance, i.e., $2e^{2}/h$. Because of observed high values of $G_{\text{m}}$ for the $\pi$-conjugated compound, we could readily analyze the effect of molecule–electrode contacts on $T_{\text{contact with tip}}\cdot T_{\text{contact with substrate}}$. Namely, we successfully observed the dependence of $T_{\text{contact with tip}}\cdot T_{\text{contact with substrate}}$ on binding sites of –S- on a Au(111) surface, i.e., the bridge (or the hollow) and the top sites.
Published by the Japan Society of Applied Physics through the Institute of Pure and Applied Physicsの論文
2006-03-30

Effect of Molecule–Electrode Contacts on Single-Molecule Conductivity of $\pi$-Conjugated System Measured by Scanning Tunneling Microscopy under Ultrahigh Vacuum

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