Hierarchy of Chemical Bonding in the Synthesis of Fe-Phthalocyanine on Metal Surfaces: A Local Spectroscopy Approach

Abstract

Scanning tunneling spectroscopy (STS) has become a key tool for accessing properties of organometallic molecules adsorbed on surfaces. However, the rich variety of signatures makes it sometimes a difficult task to find out which feature is intrinsic to the molecule, i.e., relevant for a metal–ligand interaction or related to the interaction of the molecule with the substrate. Here we study the prototype covalent self-assembly of FePc and probe how electronic/magnetic properties at the local scale change as a function of temperature-induced step-by-step assembly, starting from TCNB (1,2,4,5- Tetracyanobenzene) molecular and Fe atomic precursors. Intermediate complexes with tetra-coordinated Fe atoms are then used both, as synthons for the FePc and as identifiers of specific features of the STS. As observed by STS and confirmed by spin-polarized DFT calculations, the occupied d_π states of Fe are present in both the FePc and Fe­(TCNB)₂ on Au(111). The main difference appears in the d_<i>z</i>² states, which play a key role in magnetism as confirmed by the presence/absence of the Kondo resonance. A comprehensive picture is obtained by following with STS the hybridization of the d_<i>z</i>² orbital of Fe to various substrates (Cu, Au and Co). Finally it is demonstrated that FePc units can be created by on-surface polymerization from the Fe­(TCNB)₂ network upon thermal annealing