Nanoscopic Studies of Organic Self-Assembled Monolayers by Scanning Tunneling Microscopy

Abstract

Self-assembled monolayers (SAMs) prepared by organic molecules containing thiol moiety 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 structural order and disorder 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. For example, it is generally accepted that dialkyl disulfides adsorb on gold surface as alkanethiolates after S-S bond breaking during SAM formation. All studies on adsorption process of these molecules have been mainly performed with macroscopic tools. Therefore, we have tried for the first time to clarify such an unclear matter in the light of nanoscopic viewpoint using STM. We have attempted to observe striped phases of dialkyl disulfides formed at the initial growth stage of SAMs based on the following logic. It is well-known that the periods of striped phases formed on gold depend strongly on the overall length of the molecule used for the preparation of the SAMs. 11-Hydroxyundecyl octadecyl disulfide (HO(CH2)11SS(CH2)17CH3: HUOD) was used for target material in this study. Two kinds of striped phases with phase separation were observed at the initial SAM growth stage. The corrugation periods of striped phases are 1.8 and 2.53 nm corresponding to the lengths of HO(CH2)11S and CH3(CH2)17S molecules, respectively. These results imply that the S-S bond cleavage of disulfides has occurred when the HUOD adsorbs on the surface at room temperature, and the direct adsorption of original disulfide can not take place on the gold surface during the SAM formation.