Zinc(II) tetraphenylporphyrin adsorption on Au(111): An interplay between molecular self-assembly and surface stress

The interaction between zinc(II) tetraphenylporphyrin (ZnTPP) molecules and a Au(111) surface is investigated using scanning tunnel microscopy, from initial adsorption sites to monolayer self-assembly, with a particular emphasis on its relation to the surface atomic structure reorganization. At low coverage, no ZnTPP molecules are observed on terraces, and adsorbates appear to only decorate step edges. At intermediate coverage, ZnTPPs adsorb into self-organized islands of flat-lying macrocycles in quasi-registry with the underlying surface reconstruction, in areas delimited by herringbone reconstruction domain walls. At monolayer coverage, the adsorption geometry of the self-organized molecular layer can be fully characterized with respect to the atomic structure of Au(111) surface atoms. Moreover alteration of the Au(111) surface reconstruction domain size is observed, caused by an adsorbate-induced reduction of the Au(111) surface stress anisotropy. This behavior is not ubiquitous to ZnTPP monolayers as a monolayer prepared from the desorption of a ZnTPP multilayer does not alter the domain size of the Au(111) surface. In this case, the additional thermal energy leads to a complete rearrangement of the self-assembled structure, and the surface stress anisotropy returns to its value for the clean surface.