On-Surface Synthesis on the (10.4) Surface of the Bulk Insulator Calcite

In the last decade, impressive progress has been made, demonstrating covalent linkage of precursor molecules on various metal surfaces kept in ultrahigh vacuum. However, especially when having applications in mind, it is desirable to transfer the synthesis strategies from metal surfaces to other substrates. When aiming for using the created structures in future molecular electronic devices, bulk insulator surfaces are favored because of their potential to largely preserve the adsorbate’s electronic structure. On bulk insulator surfaces, however, on-surface synthesis is still in its infancy. This is partly due to the fact that insulators are difficult to access with standard surface science techniques such as scanning tunneling microscopy. More importantly, insulator surfaces pose fundamental challenges when aiming for on-surface reactions. A severe obstacle is the comparably weak interaction of precursor molecules with prototypical insulator surfaces. This weak interaction frequently leads to desorption of the precursor molecules before the reaction temperature is reached when initiating the desired reaction by heating the substrate. Moreover, many reactions—such as Ullmann coupling—are known to be catalyzed by the presence of metals. Despite these challenges, few examples exist that demonstrate covalent linking of precursor molecules on a bulk insulator surface. For these studies, calcite, being an insulator with a band gap of around 6 eV, has been proven to constitute a very suitable substrate. On this substrate, C–C coupling of halogen-substituted precursors has been presented even in the absence of a metal catalyst. Moreover, exploiting the different binding energies of chlorine, bromine, and iodine toward the phenyl group has enabled sequential and site-specific linkage as a promising approach toward hierarchical design. To avoid the above-mentioned obstacle of desorption upon thermal initiation, also photochemical initiation has been demonstrated by inducing a [2 + 2] cycloaddition of C 60 molecules. Current efforts are now directed toward exploring further classical reactions such as Glaser coupling or diacetylene polymerization for on-surface synthesis on bulk insulator substrates.