MOLECULAR SELF-ASSEMBLY AND ADSORPTION STRUCTURE OF ORGANOSULFUR MOLECULES ON AU(111)

Abstract

Organic self-assembled monolayers (SAMs) on metal surfaces have drawn much attention because of their fundamental interests in surface science as well as the high possibility of their practical applications [1-4]. 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 control of structural ordering of organic SAMs formed on a metal surface. One of main purposes in this study is to reveal such problems from the nanoscopic viewpoint using scanning tunneling microscopy (STM). In this talk, we will provide clear answers with molecular precision for a long-standing open question regarding organic SAMs and a new method to control two-dimensional ordered of SAMs. For instance, it was revealed that the adsorption of benzenethiols (BT) on Au(111) usually leads to the formation of disordered phases. However, we have clearly demonstrated that the displacement of preadsorbed cyclohexanethiol (CHT) self-assembled monolayers (SAMs) on Au(111) by BT molecules can be a successful approach to obtain two-dimensional BT SAMs with long-range ordered domains. After displacement of CHT SAMs (88.3 ?2/molecule) by BT molecules (67.08 ?2/molecule), the molecular adsorption density increased, resulting that the interactions between the sulfur and gold surface can be largely increased. This can be a main driving force for the facile displacement of CHT by BT [1]. We also found that THT molecules, containing endo-sulfur aliphatic cyclic rings, can form chemisorbed SAMs, in contrast with the formation of physisorbed SAMs by dialkyl monosulfide, suggesting that the adsorption ability of monosulfide compounds on gold strongly depends on the structure of tail groups, such as an aliphatic cyclic ring or two alkyl groups attached to sulfur headgroups. In addition, high-resolution STM imaging revealed, for the first time, that the adsorption of THT molecules on Au(111) results in long-range, two-dimensional, ordered SAMs, having a (3 2 3) superlattice with many unique structural defects and a few vacancy islands [3]. It is suggested that the unique surface structures of THT SAMs on Au(111) are mainly due to the weak van der Waals interactions between THT cyclic rings, as well as a dynamic structural variation of the THT ring in the SAMs. Our results from this study will be very useful in understanding SAM formation and the structures of organosulfur molecules containing an endo-sulfur cyclic ring.