Nontrivial central-atom dependence in the adsorption of M-TPP molecules (M = Co, Ni, Zn) on Fe(001)-p(1×1)O

Metal-tetraphenyl-porphyrin (M-TPP) molecules typically self-assemble forming square-like superlattices, as dictated by their shape. The dependence of the adsorption properties on the central atom is systematically studied for Co-, Ni-, and Zn-TPP adsorbed on oxygen passivated Fe(001), namely the Fe(001)-p(1×1)O surface. It is found by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) that despite the weak molecule–substrate interaction, preserving many features of quasi-free molecules, the self-assembled structure switches from the (5×5)R37° superlattice of Co-TPP and Ni-TPP to the plain (5×5) of Zn-TPP. Ab initio calculations based on density functional theory (DFT) are used to investigate the adsorption properties of the different molecules and the possible overlayers formed. Adsorption energies, structures, and electronic properties are reported, discussing the bonding mechanisms and the magnetic character. Only moderate energy differences are found, suggesting that subtle effects may steer the selection of the structure among overlayers with similar properties although differing substantially as for the LEED and STM experimental results.