Understanding and tuning the epitaxy of large aromatic adsorbates by molecular design

If the rich functionality of organic molecules is to be exploited in devices such as light-emitting diodes or field-effect transistors1,2,3,4,5,6,7, interface properties of organic materials with various (metallic and insulating) substrates must be tailored carefully7,8,9,10. In many cases, this calls for well-ordered interfaces. Organic epitaxy11,12,13—that is, the growth of molecular films with a commensurate structural relationship to their crystalline substrates—relies on successful recognition of preferred epitaxial sites. For some large π-conjugated molecules (‘molecular platelets’) this works surprisingly well14,15, even if the substrate exhibits no template structure into which the molecules can lock13,15,16. Here we present an explanation for site recognition in non-templated organic epitaxy, and thus resolve a long-standing puzzle11. We propose that this form of site recognition relies on the existence of a local molecular reaction centre in the extended π-electron system of the molecule. Its activity can be controlled by appropriate side groups and—in a certain regime—may also be probed by molecularly sensitized scanning tunnelling microscopy. Our results open the possibility of engineering epitaxial interfaces, as well as other interfacial nanostructures for which specific site recognition is essential.