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

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

On-Surface Reaction between Tetracarbonitrile-Functionalized Molecules and Copper Atoms

Self-assembly at surfaces has proven to be very efficient in creating ordered, atomically controlled organic nanostructures. On-surface synthesis has recently emerged as a promising strategy to create stable structures bound by strong and irreversible covalent bonds. Here we present on-surface reaction between pyrazino phenanthroquinoxaline-tetracarbonitrile (PPCN) molecules and copper atoms on a Au(111) substrate. The reaction is monitored in ultrahigh vacuum conditions by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). After a 475 K annealing, phthalocyanine cyclization occurs around a copper atom; the increase of annealing temperature to 540 and 675 K leads to the formation of 1D and 2D phthalocyanine polymers, respectively. This reaction is confirmed by the modification of the Cu 2p_3/2 and C 1s XPS spectra upon annealing

Self-Assembled Melamine Monolayer on Cu(111)

The self-assembled structure of melamine (1,3,5-triazine-2,4,6-triamine) deposited on Cu(111) was studied under ultra-high-vacuum conditions using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS). Molecular packing and conformation were investigated as a function of the annealing temperature. The molecules deposited at room temperature were found to be aligned with the molecular plane approximately perpendicular to the surface. At around 150 °C, the molecules became aligned in a row-type structure and were all coupled to the Cu(111) surface. At still higher annealing temperatures, a new two-dimensional (2D) network was formed via the polymerization of the adsorbed monolayer of melamine. A multistep model consistent with STM, XPS, and UPS results is proposed, starting with a self-organized vertically adsorbed melamine monolayer and ending with the formation of a 2D network lying on Cu(111) interpreted as a ring-opening polymerization of melamine. The reactive Cu­(111) surface is believed to be one of the keys in this multistep reaction