Formation and Molecular Structure of Self-Assembled Monolayers by Adsorption of Cyclic Thiols on Gold

Abstract

Organic self-assembled monolayers (SAMs) prepared by self-assembly of sulfur-containing organic molecules on metal surfaces have been extensively studied for more than a decade because of the possibility of novel applications as biosensor, molecular recognition, nanolithography, nonlinear optics and corrosion inhibition. Especially, SAMs prepared from n-alkanethiols and -functionalized alkanethiols have been extensively studied to date, and these SAM systems are now relatively well characterized. A number of studies, on the other hand, have focused on various aromatic thiol SAMs to understand the packing structures, adsorption kinetics, and orientations of the molecules adsorbed on the gold surface. As a result, it was found that the packing arrangement and the ordering of aliphatic or aromatic thiol SAMs are strongly influenced by an interplay of various interactions, including the sulfur headgroup-substrate interaction, the interchain van der Waals interaction, and interchain end-group interactions. It was also revealed that the presence of a rigid and bulky aromatic moiety in the molecular backbone of aromatic thiols plays an important role in determining their final SAM structures. To our knowledge, however, there have been very few studies on the formation of cyclic thiol SAMs on gold surfaces. In order to extend the application of SAMs, it is essential to develop and characterize a new SAM system. It is also very important to understand the nature of the self-assembly process and the SAM structure. In this study, we investigated cyclic thiol SAMs on Au(111) using X-ray photoelectron spectroscopy and scanning tunneling microscopy, and we newly found that cyclic thiols with five or six-member ring at a molecular backbone would form ordered SAMs having a new, unique packing structure due to the presence of bulky rings. From this study, it is clearly revealed that the structural rigidity of the molecular backbone is an important parameter for determining the molecular packing structure of SAMs.