Substituted coronenes for molecular electronics: from supramolecular structures to single molecules

The ongoing miniaturization of silicon integrated circuits makes the understanding of the electronic properties of nanoscale structures and the exploration of novel materials and device concepts more and more important. One promising approach to construct future electronic systems is the usage of organic molecules and utilizing their ability to self-assemble and/or taking advantage of the possibility to achieve various electronic functions just by modifying their chemical structures. This thesis explores a highly conjugated molecular system, namely dodecakis(arylthio)- coronenes (DATCs), with a view to potential applications as molecular electronic building blocks. The techniques of scanning tunneling microscopy and spectroscopy are applied to characterize the structural and the electronic properties of monolayers of these molecules on metal surfaces. Variations of the substituents allowed to specifically affect the self-assembly of the molecules. Supramolecular structures with different orientations of the molecules relative to the substrate and with different intermolecular interactions are obtained. The growth of highly ordered supramolecular chains is observed in the case of the basic molecular building block dodecakis(phenylthio)coronene (Cor-H) on Au(111) surfaces. The formation of delocalized electronic states along the chains suggests the potential of this system as a basis for novel organic materials with anisotropic charge transport properties. Substituents with varying electron-accepting or electron-donating ability are used to modify Cor-H and enhance or prevent the molecular stacking. Assemblies of molecules with molecular quantum dot behavior can also be obtained in this way. The tailored functionalization allows a decoupling of the aromatic system of the molecules from the substrate states, which in turn leads to the occurrence of single electron tunneling effects. Different substitutions of the DATC system can thus be used to create desired electronic functions. Furthermore, several fabrication routs for nanoscale electrode structures were worked out to “wire up” single molecules in a device-like configuration and to investigate their electrical properties. In particular, the technique of electron-beam lithography in conjunction with unconventional nanofabrication methods like electromigration were utilized to fabricate nanometer-spaced metal electrodes. Such nanopatterns additionally allowed to characterize the charge transport through embedded single molecules