CHAPTER 8. Aromatic Molecules on Metallic Surfaces: Structure and Reactivity

Abstract

In this chapter, a decade-long series of investigations about aromatic molecules on metallic surfaces has been reviewed. The most relevant studies regarding both structural investigation and chemical reactivity of aromatic systems on metallic surface is described. A major emphasis has been placed on the investigation techniques that allow for a direct visualization of the structural and electronic properties of both isolated and extended aromatic systems on surface (e.g. Scanning Tunnelling Microscopy, non contact-Atomic Force Microscopy and Kelvin Probe Force Microscopy). The synthesis, imaging and characterization of structure such as extended polyaromatic hydrocarbon, 1-D assembly, 2-D network and Graphene Nanoribbons are discussed. Among the different types of reactions, surface mediated reactions, such as acetylene homocoupling, cyclodehydrogenation, cycloaddition and metal-coordination, have been described. a feedback loop in order to maintain a constant user-defined tunnelling current. The final STM image is then reconstructed from the feedback signal. From the time of its invention, Scanning Tunnelling Microscopy has been used to investigate the structure and properties of a wide range of molecules and materials such as isolated organic and bio-molecules, supramolecular systems and 2-D inorganic crystals. 29-33 However, although atomic resolution has been obtained for a wide range of crystalline surfaces, atomic resolution on molecules adsorbed on metallic surface has only been achieved very recently. Two SPM techniques able to achieve such kind of spatial resolution are Scanning Tunnelling Hydrogen Microscopy (STHM) and Non-Contact Atomic Force Microscopy (nc-AFM). 33 In both cases, the key to enable the direct visualization of individual bonds and atoms in individual molecules is the functionalization of the probe with diatomic molecules, such as dihydrogen or carbon monoxide for STHM and nc-AFM, respectively. Figure X.2. Schematic representation (A) of the STHM setup. Molecular structure (B) of the PTDCA molecule (Grey: Carbon, White: Hydrogen, Red: Oxygen). STHM image of PTCDA molecule (C,D) on Au(111) in its herringbone phase (image size 2.5 nm × 2.5 nm) (Reproduced from ref. 35 with permission from The American Chemical Society). Molecular model (F) of pentacene (Grey: Carbon, White: Hydrogen). Constant current mode STM (G) and CO tip functionalized nc-AFM images (H-I) of pentacene on Cu(111) (Reproduced from ref. 38 with permission from the American Association for the Advancement of Science). The first technique, STHM, was developed by Tamirov and co-workers. 34, 35 Atomic resolution on isolated flat aromatic molecules was obtained when the STM chamber was flooded with molecular dihydrogen or dideuterium (Figure X.2A). High sensitivity of the STM tunnelling junction with molecular hydrogen was achieved by leaking ultrapure hydrogen (99.9999 %) into the LT-STM (low temperature STM) chamber at a pressure varying between 10-9 and 10-7 mbar. Using this technique Tamirov et al. were able to obtain subm...