Self-Assembly of ( l )-Cysteine Molecules at Ag(110): A Scanning Tunneling Microscopy and X-ray Photoemission Spectroscopy Study

Organic-inorganic interactions are at the basis of relevant applications, ranging from biocompatibility and nano(bio)technology to antifouling and hygiene issues. A fundamental understanding of the processes occurring at the interface between molecules of biological interest and inorganic surfaces is a key issue to optimize performance in this field. In the present paper, we report on the self-assembly and thermal evolution of the amino acid (l)-cysteine sublimated on Ag(110) under ultrahigh vacuum conditions. The morphology of the layer is investigated by scanning tunneling microscopy, and the chemistry of the system is determined by X-ray photoemission spectroscopy. The molecules adsorb on the surface at RT in the zwitterionic form, binding to the surface via dehydrogenation of the thiol group and, in part, of the carboxylate. When annealing the surface, they convert into the anionic form, and the morphology of the layer changes, indicating the presence of several local minima in the energy diagram of the Cys/Ag(110) system. Based on experimental evidence, empirical models of some self-assembled structures are proposed.