Structure and electronic and charge-transfer properties of mercaptobenzoic acid and mercaptobenzoic acid-undecanethiol mixed monolayers on AU(111)

Abstract

The surface structure, molecular conductance, and charge-transfer properties of pure mercaptobenzoic acid (MBA) and mixed MBA?undecanethiol (UDT) self-assembled monolayers (SAMs) on Au(111) are studied by scanning tunneling microscopy, scanning tunneling spectroscopy, electrochemical techniques, and density functional theory calculations. MBA forms ordered diluted lattices without evidence of Au adatom complexes at terraces (absence of vacancy islands) at low MBA concentrations, whereas disordered lattices with possible formation of these complexes are found at high concentrations (presence of vacancy islands). In addition, some other interesting facts have been established. The stability of the MBA SAMs is improved in mixed MBA-UDT SAMs, revealing aromatic-aliphatic attractive interactions. The mixed SAMs at equivalent amounts of thiols hinder the vacancy island formation typical of the pure UDT SAMs. The MBA SAMs exhibit good molecular conductance with filled states from S and aromatic ring contributions near the Fermi level. Finally, it has been proven that MBA molecules efficiently wire the electron transfer through UDT SAMs.