Elementary structural motifs in a random network of cytosine adsorbed on a gold(111) surface

Nonsymmetrical organic molecules adsorbed on solid surfaces may assemble into random networks, thereby providing model systems for organic glasses that can be directly observed by scanning tunneling microscopy ( STM). We investigated the structure of a disordered cytosine network on a gold( 111) surface created by thermal quenching, to temperatures below 150 K, of the two- dimensional fluid present on the surface at room temperature. Comparison of STM images to density functional theory calculations allowed us to identify three elementary structural motifs ( zigzag filaments and five- and six- membered rings) that underlie the whole supramolecular random network. The identification of elementary structural motifs may provide a new framework for understanding medium- range order in amorphous and glassy systems.