Structural study of monolayer cobalt phthalocyanine adsorbed on graphite

We present microscopic investigations on the two-dimensional arrangement of cobalt phthalocyanine molecules on a graphite (HOPG) substrate in the low coverage regime. The initial growth and ordering of molecular layers is revealed in high resolution scanning tunneling microscopy (STM). On low coverages single molecules orient mostly along one of the substrate lattice directions, while they form chains at slightly higher coverage. Structures with two different unit cells can be found from the first monolayer on. A theoretical model based on potential energy calculations is presented, which relates the two phases to the driving ordering forces.

► CoPc molecules grown in submonolayer coverage on HOPG show three ordering regimes. ► Single molecules align with substrate and formation of chains in low coverage. ► Beginning from the first filled monolayer certain structures can be observed. ► The epitaxy of the two dominant structures is investigated. ► Molecular mechanic simulations on a simplified system affirm the found structures.