Porphyrin central metal ion driven self-assembling in heterogeneous ZnTPP – CoTPP films grown on Fe(0 0 1)-p(1 × 1)O

Abstract

Metal tetraphenylporphyrins (MTPPs, with M = Zn, Co) deposited on an oxygen-passivated Fe substrate, namely Fe(001)-p(1 × 1)O, were studied to understand the mechanism of molecular self-assembling on ultrathin metal-oxide films. These were characterized by photoemission spectroscopy (PES) and low energy electron diffraction (LEED). Heterogeneous MTPP monolayers (ZnTPP + CoTPP) were grown at room temperature by depositing a sub-monolayer of ZnTPP on Fe(001)-p(1 × 1)O followed by a sub-monolayer of CoTPP or vice-versa, or by co-deposition.Homogeneous layers are characterized by markedly different molecular superstructures, as shown by LEED. Conversely, heterogeneous monolayers reflected an even more complex behavior: in particular, it was observed that the deposition of a small amount of CoTPP before or simultaneous to the dosing of ZnTPP molecules was able to stabilize a (5 × 5)R37° diffraction pattern, characteristic of homogeneous CoTPP, whereas homogeneous ZnTPP layers are associated to a straight (5 × 5) one. The observed differences in the ordering of the molecular self-assembling as a consequence of combining the two MTPPs highlights a significant influence of one MTPP over the other depending upon the central metal ion and not upon the molecular skeleton.