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Thiol molecules adsorbed on gold became a model system for molecular self-assembly on metal substrates long ago. In most cases the strong molecule-gold interaction is able to restructure the substrate, resulting in vacancy islands and steps. Today it is widely accepted that gold adatoms produced by this process form stable thiol-adatom complexes, usually termed "staples" (RS-Au-ad-SR or RS-Au-ad-SR-Au-ad-SR), which are the basic units of the self-assembled monolayers. Here we report on a different scenario for 6-mercaptopurine (6MP), a heterocyclic aromatic thiol, namely its adsorption on the Au(001)-(5 x 20) reconstructed surface. Our results show that 6MP lifts the reconstruction upon adsorption, thus ejecting a large excess of gold adatoms. Surprisingly, 6MP molecules prefer to arrange in highly ordered adatom-free domains in the bridging configuration, while the ejected adatoms form gold islands. Our investigation reveals that the formation of thiol-gold adatom complexes is not always a thermodynamically favored process but rather depends on the nature of the thiol molecule.
Raffaella Buonsanti, Philippe Benjamin Green, Alexander Nicolas Chen, Victoria Lapointe
Mihai Adrian Ionescu, Paul Muralt, Daesung Park, Kanghyun Chu