Are 4-Mercaptobenzoic Acid Self Assembled Monolayers on Au(111) a Suitable System to Test Adatom Models?

Abstract

The adsorption of 4-mercaptobenzoic acid (MBA) has been studied on Au(111) by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), electrochemical techniques, and density functional theory (DFT) calculations. Results show that MBA molecules adsorb on the Au surface via a thiolate bond, arranged in a (√3×4) lattice with coverage θRS = 0.25. DFT data show the strong effect of aromatic ring interactions in stabilizing the adsorbed molecules. The total absence of vacancy islands upon thiol adsorption and the surface coverage of gold islands after desorption are difficult to conciliate with the usual gold-adatom models for thiol adsorption. This is an important issue, as the strongest evidence for the existence of gold-adatom complexes arises from MBA-capped Au nanoparticles. Our results raise an interesting point as regards the validity of making a straightforward parallel between the thiol–Au interface in single crystal surfaces and in monolayer-protected clusters.