Reactive Dynamics Simulation of Monolayer and Multilayer Adsorption of Glycine on Cu(110)

The process of mono- and multilayer adsorption of glycine on copper surface Cu(110) and the preferred binding modes of the molecule were studied theoretically by means of classical reactive (ReaxFF) molecular dynamics simulations. Starting from glycines in gas phase in the neutral nonzwitterionic form, the most stably adsorbed structures are found to be the molecules which release their carboxyl protons (anionic form) to molecules in the second layer and place both the carboxyl oxygens and the nitrogen atom on top of copper sites, at an average distance of about 2 angstrom. The surface binding mechanism consists of different phases during which major conformational rearrangements and several intermediate adsorption configurations are observed. The overall stability of the glycine adlayers is essentially due to the combination of different intermolecular forces, namely chemical bonds with the copper top layer and intermolecular hydrogen bonds within the adsorbed molecular units. At low coverage the molecules are prevalently attached to the substrate in a bidentate fashion, i.e., through the nitrogen atom and one oxygen atom of the carboxyl group, whereas at higher coverage the molecules tend to diffuse on the surface and pack in long-range ordered heterochiral domains where tridentate geometries are most likely observed. The picture that emerges from the present reactive dynamics simulations satisfactorily agrees with the experimental data and theoretical results, based on geometry optimization, reported to date.