Positioning of single Co atoms steered by a self-assembled organic molecular template

The bonding and organization of cobalt atoms on a self-assembled organic molecular template are investigated by low-temperature scanning tunneling microscopy. In a first step, N,N′-diphenyl oxalic amide is deposited on the Ag(111) surface with submonolayer and monolayer coverage, leading to the formation of supramolecular nanogratings and a dense-packed layer, respectively. These templates are exposed to evaporated cobalt at different substrate temperatures in the range of 110 to 240 K. We find that Co always binds on top of the phenyl rings, and thus the realization of Co-phenyl complexes is preferred over metal cluster growth on the bare Ag(111) surface. In the case of the dense-packed template, a large fraction of the provided Co is engaged in the formation of well-defined, uniform monomeric Co-half-sandwich structures. At optimal temperatures in the 180-200 K range, the fraction of monomeric Co species on the template exceeds 80% of the total amount of Co deposited. The temperature-dependent adsorption behavior and monomer fraction are compared with calculations, simulating the site-selective positioning in the diffusionless limit by a hit-and-stick adsorption model. This analysis indicates that the organic template suppresses the clustering tendency inherent to diffusing Co atoms and allows the production of a monomer fraction as high as that for statistical growth in the low-coverage regime.