Two-dimensional phase behavior of a bimolecular porphyrin system at the solid-vacuum interface.

The "in vacuo" self-assembly of a two-component porphyrin system on a metal surface is studied by means of scanning tunneling microscopy in the sub-monolayer regime. The observed self-assemblies are systematically analyzed by their dependence on the total coverage and on the ratio of the two components resulting in a two-dimensional phase diagram. In a small region within the parameter space, a mixed surface layer is observed. The results are discussed consistently on the basis of molecule-surface and molecule-molecule interactions as well as thermodynamic aspects, leading to a qualitative comprehension of the phase behavior of the two-dimensional bimolecular system.