Supramolecular assemblies of molecules on surfaces have attracted considerable attention in recent years, with various possible applications in the field of nanoelectronics. One-dimensional (1D) chains and two-dimensional (2D) patterns have been grown on surfaces by both self-assembly and mechanisms involving directed growth. Various intermolecular forces have been utilized to drive the formation of these structures, for example hydrogen bonding, π-π stacking and van der Waals forces.
Here we present the first formation of covalently bonded networks of molecules on a surface. Density functional theory (DFT) calculations and scanning tunnelling microscopy (STM) have been used to show that covalently bonded networks of tetrakis(4-bromophenyl)porphyrin (TBrPP) molecules can be formed on the Au(111) surface with various 1D and 2D surface structures.
A protrusion in STM images of the molecular network at high bias voltage is shown to be due to the presence of a covalent bond. DFT calculations are used to investigate the electronic states responsible for the increased local density of states at this bias and show that they are indeed due to covalent bonding between the phenyl groups of two neighbouring molecules.
We also present results showing that these porphyrin-based molecules are adsorbed on the Au(111) surface in two different conformations which can be distinguished using scanning tunnelling spectroscopy. These two conformations are investigated using DFT calculations.