Formation and Structure of Cyclopentanethiol Self-Assembled Monolayers on Au(111)

Abstract

Organic self-assembled monolayers (SAMs) prepared by the spontaneous adsorption of organosulfur compounds on gold have drawn much attention because they provide an excellent means to tailor the physical and chemical properties of surfaces for various technological applications in biosensing, nanopatterning, corrosion inhibition, and molecular electronic devices. To date, alkanethiol SAMs on gold surfaces have been the most thoroughly studied for understanding self-assembly phenomena of organic molecules. In addition, many studies have reported the structures and properties of aromatic thiol SAMs on gold surfaces, which are desirable due to their high electronic conductivity. It has been generally considered that the structures of the SAMs are determined by a balance of the chemical interactions between the sulfur headgroups and gold as well as van der Waals interactions between alkyl chains or aromatic groups. The contribution of van der Waals interactions plays a major role in determining the final SAM structure, and such interactions would be easily controlled by changing the alkyl groups or aromatic groups. As one of new classes in the SAM system, SAMs formed by thiols with spherical hydrocarbon cage or bulky alkyl groups on gold were prepared and characterized. Scanning tunneling microscopy (STM) studies reveals that sulfur compounds with bulky bicyclo[2.2.2]octane moieties can form unique ordered SAMs having a nearest neighbor spacing of 5.9 A, which is larger than that the ( × )R30 hexagonal structure for alkanethiol SAMs. From these STM studies, it is considered that van der Waals interactions between bulky alkyl groups is a dominant factor in the formation of ordered SAMs because alkanethiols with similar molecular lengths are rather difficult to form twodimensional (2D) ordered SAMs. The high-resolution STM study also revealed that cyclohexanethiol (CHT) containing a flexible six-membered aliphatic ring on Au(111) can form long-range 2D ordered SAMs, which can be described as a (5 × 2 )R48 superstructure. This unique superstructure can be ascribed to two geometrical isomers of the CHT molecule, the equatorial and axial chair isomers. In this study, the effect of the size of aliphatic cyclic ring on the structure of the SAMs formed by cyclopentanethiol (C5H9SH, CPT) with a five-membered aliphatic ring on the Au(111) surface was examined. To the best of my knowledge, there has been no study on the adsorption behavior of CPT molecules on gold surfaces thus far. To extend SAMbased applications, it is very important to understand the nature of the self-assembly processes of the molecules and the structure of SAMs. In this paper, the first molecularly resolved STM results of CPT SAMs with a ( × )R25 structure are reported.