Local structure and morphological evolution of ZnTPP molecules grown on Fe(001)-p(1 × 1)O studied by STM and NEXAFS

When used as substrates, thin metal-oxide (MO) layers can perturb the physical and chemical properties of molecules in contact with the surface. To study the molecule-MO layer interaction, we focused our investigation on a prototypical interface, namely zinc tetraphenylporphyrin (ZnTPP) film on Fe(001)-p(1 × 1)O. In a previous study, we found that no significant change of the electronic structure takes place at the monolayer (ML) coverage either in the core level photoemission spectra or in the highest occupied molecular orbitals (HOMOs). However, molecules showed the occurrence of a commensurate (5 × 5) diffraction pattern that indicates a certain degree of interaction with the substrate. In order to better understand the effective molecule/metal decoupling operated by the FeO layer, we performed a combined investigation based on a scanning tunneling microscopy (STM) study of the self-assembled ZnTPP molecular layer and on a near edge X-ray absorption fine structure spectroscopy (NEXAFS). Molecules are found to lie almost parallel to the substrate, even if the central macrocycle displays a characteristic small saddle-like distortion (<15°). The corresponding reduction of the molecular symmetry from D4h to D2h drives the azimuthal orientation with respect to the substrate and determines the co-existence of four equivalent (5 × 5) ZnTPP domains, following the substrate four-fold symmetry. The comparison with films of increasing thickness shows that, beyond the second layer, the molecules gradually tilt-off the surface (by at least 40°) ordering into 3D islands. The NEXAFS resonances of the lowest unoccupied orbitals (LUMOs) do not display significant changes from the monolayer to the multilayer thickness range, apart from minor modification of the LUMOs relative intensity. The latter variation may be associated with the change of spatial spread of the molecular orbitals in the contact layer due to the saddle-like distortion