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

In situ scanning tunneling microscopy (STM) and cyclic voltammetry were emp loyed to investigate self-assembled monolayers of 3-mercaptopropionic acid (MPA) on Au(111) in perchloric acid solutions. In situ STM revealed that th e MPA monolayer was composed of translationally and rotationally ordered MP A domains, and some disordered portions also coexisted on the surface, part icularly near the etch pits. The ordered monolayer exhibited a molecularly ordered (3 x 3) structure, where three MPA molecules in triangular position s stand fairly close together due to intermolecular hydrogen bonding. Two d istinctly different phase boundaries were found between the ordered (3 x 3) domains, and high-resolution STM images disclosed the molecular arrangemen t of the MPA monolayers. The MPA monolayer desorbed at negative potentials, causing transformation of the surface into the Au(111)-(root3 x 23) recons truction. The in situ STM successfully elucidated the interfacial structure and phase boundary of the MPA monolayers on Au(111) under electrochemical conditions.