The Emergence of Covalent On-Surface Polymerization

The covalent linking of molecular building blocks directly in the two-dimensional confinement of a surface, the so-called on-surface polymerization, has developed rapidly in the last years since it represents a reliable strategy to grow functional molecular nanostructures in a controlled fashion. Here, we review the growth of such structures via on-surface Ullmann coupling and highlight the major chemical and physical aspects. These systems are typically studied by scanning tunneling microscopy that allows exploration of the initial monomer species, intermediate products and final nanostructures with sub-molecular spatial resolution. In this way, the chemical structures of the ex situ synthesized molecular building blocks are directly correlated with the outcome of the chemical reaction. We also present examples with different monomer species in view of growing heterogeneous molecular structures as well as the importance of the molecular interaction with the template surface as a further key parameter to control the molecular diffusion and tune the final molecular architecture.