Ordered structures of self-assembled monolayers of 3-mercaptopropionic acid on Au(111): in situ scanning tunneling microscopy study

Abstract

In situ scanning tunneling microscopy (STM) and cyclic voltammetry were employed to investigate self-assembled monolayers of 3-mercaptopropionic acid (MPA) on Au(111) in perchloric acid solutions. In situ STM revealed that the MPA monolayer was composed of translationally and rotationally ordered MPA domains, and some disordered portions also coexisted on the surface, particularly near the etch pits. The ordered monolayer exhibited a molecularly ordered (3 × 3) structure, where three MPA molecules in triangular positions stand fairly close together due to intermolecular hydrogen bonding. Two distinctly different phase boundaries were found between the ordered (3 × 3) domains, and high-resolution STM images disclosed the molecular arrangement of the MPA monolayers. The MPA monolayer desorbed at negative potentials, causing transformation of the surface into the Au(111)-(√3 × 23) reconstruction. The in situ STM successfully elucidated the interfacial structure and phase boundary of the MPA monolayers on Au(111) under electrochemical conditions.