Formation of π-conjugated molecular arrays on silicon (001) surfaces by heteroatomic Diels–Alder chemistry

Recent advances in molecular electronics and organic-based electronic materials are placing increased emphasis on developing new methods for attachment of π-conjugated organic molecules to surfaces. We have developed a new strategy for forming oriented arrays of π-conjugated organic molecules on silicon surfaces. The SiSi dimers of the reconstructed Si(001) surface can selectivity react with the dicarbonyl group of 9,10-phenanthrenequinone, leading to cyclic molecular structure between the attached molecule and underlying Si substrate. This reaction is formally analogous to a heteroatomic Diels–Alder reaction. X-ray photoemission spectroscopy data show that bonding occurs through the dicarbonyl functional groups, while surface infrared experiment data show that the π-conjugation is preserved. Ab initio calculations predict that this chemistry leads to aromatic rings that are rigidly attached to the surface, with the aromatic rings oriented perpendicular to the surface and parallel to the underlying SiSi dimers. This is confirmed by molecular-resolution scanning tunneling microscopy images, which clearly show the orientation of the individual molecules and are even able to resolve the two π lobes on adjacent faces of each vertically oriented molecule.