Structural Order-Disorder of Self-Assembled Monolayers Studied by Scanning Tunneling Microscopy

Abstract

Self-assembled monolayers (SAMs) derived from organic molecules containing thiol moiety on metal surfaces have attracted much attention because of their fundamental interests in surface science as well as the high possibility of their practical applications[1-3]. In order to design nano-devices with high-efficiency based on thiol-modified gold surface, it is essential to understand molecular packing structure, self-assembly process, and structural order and disorder of organic SAMs formed on a metal surface For this purpose, we have examined organic self-assembled monolayers (SAMs) formed on Au(111) by organosulfur compounds such as aromatic thiols and aliphatic cyclic thiols using various surface analysis techniques, especially, using scanning tunneling microscopy. In this presentation, we have demonstrated new fundamental aspects of organic SAMs on Au(111), such as, interactions, orientation mechanism, and packing structure. For instance, The molecularly resolved STM image clearly revealed that cyclohexanethiol (CHT) SAMs have an oblique unit cell consisting three adsorbed molecules, which can be described as the (5 2 10)R48 superstructure. The superstructure observed for CHT SAMs on Au(111) is comparable to that observed for the conventional alkanethiol or aromatic thiol SAMs. The formation of such a unique superstructure is attributed to the adsorption geometry with the stable chair conformations, i.e., the equatorial and axial chair conformations, for the flexible aliphatic ring in the CHT molecules. In this study, we revealed that the structural rigidity of the molecular backbone is an important parameter for determining the molecular packing structure of SAMs.