Molecular Precursors

Surface-assisted chemical reaction of single molecular units can be used to assemble more complex and ordered structures. The surface-induced polymerization of molecular monomers into extended chains has been used as the seed of graphene nanoribbon (GNR) formation, where nano-sized graphene has the advantage of a quantum-size derived energy gap. Valence-band photoemission, core-level spectroscopy in a fast XPS experiment as a function of temperature and scanning tunneling microscopy were able to follow the evolution of molecular states from single monomer 10,10′-dibromo-9,9′bianthracene (DBBA) precursors to polyanthryl polymers, and eventually to GNRs, as driven by the Au(110) surface.

The advantage of having a real 2D material, based on the graphene mechanical strength and properties, but with an energy gap, pushes to the integration of C with h-BN layers, towards the formation of a 2D BCN compound.  We recently exploited a direct and fast synthesis route to grow boron–carbon–nitrogen layers by microwave-assisted plasma enhanced chemical vapour deposition (PECVD), by using methylamine borane as a single source molecular precursor, in collaboration with Prof. F. Leardini (Universidad Autonoma de Madrid). We obtained controlled and reproducible B–C–N layers onto thin Cu foils; though disorder and segregation into C-rich and h-BN-rich domains have not been completely avoided, high doping levels have been reached, inducing strong modifications of the electronic, optical and transport properties of C-rich and h-BN-rich phases. This synthesis procedure can open new routes towards the achievement of homogeneous highly mixed ternary B–C–N phases.

Another class of organic molecules can be used for a metal-organic-framework (MOF) formation onto a surface, thanks to surface-asseisted reaction. We obtained a highly ordered array of copper tetrameric clusters, coordinated into a metal-organic network, by deposition of tetrahydroxyquinone molecules on the Cu(111) surface at room temperature, and subsequent thermal activation. This is a neat example of supramolecular organic ordered network, acting as a spacer for the highly ordered two-dimensional network of copper tetramers at the very surface.