Chain Length Dependence of Adsorption Structure of COOH-terminated Alkanethiol SAMs on Au(111)

Abstract

A self-assembled monolayer (SAM) of alkanethiol (AT) molecule on a metal electrode has been attracted wide attention for fabricating a well-ordered functional organic monolayer and/or modification of surface properties of the electrode. The film morphology, adsorption structures, and optical/electric properties of the AT SAMs have been extensively investigated by various experimental methods such as scanning probe microscope and photoemission spectroscopy. We have investigated the growth process and the adsorption states of the COOH-terminated alkanethiol SAMs deposited on Au(111) surfaces. In this study, we elucidated the molecular orientation and adsorption states of 11-mercaptoundecanoic acid (MUA) and 6-mercapto- hexanoic acid (MHA) in the SAM by scanning tunneling microscopy (STM), contact angle measurements, angle-resolved X-ray photoelectron spectroscopy (ARXPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. We found that the film structure depends on the alkyl chain length, suggesting that the intermolecular interaction of the alkyl-chain spacer plays an important role for formation of the densely packed structure.