Self-assmebly of molecules on nanostructured graphene

• Autores: Flavio Pendolino
• Directores de la Tesis: Amadeo L. Vázquez de Parga (dir. tes.)
• Lectura: En la Universidad Autónoma de Madrid ( España ) en 2014
• Idioma: inglés
• Tribunal Calificador de la Tesis: José Ángel Martín Gago (presid.), Pilar Segovia Cabrero (secret.), Fabrizio Lo Celso (voc.), Josemaria Gallego (voc.), Enrico Gnecco (voc.)
• Materias:
  • Física
    • Física del estado sólido
      • Superficies
• Enlaces
  • Tesis en acceso abierto en: Biblos-e Archivo
• Resumen

  • In this thesis the growth and morphological characterisation for the self-assembly of various electron acceptor molecules were investigated by mean of Scanning Tun- nelling Microscope (STM) in Ultra High Vacuum. Molecules, from the quinone and xanthine classes, such as 11,11, 12,12 - tetracyano-9,10-anthraquino dimethane (TCAQ), 13,13,14,14 - tetracyano-5,12 -tetracenequino dimethane (TCTQ), 15,15, 16,16 -tetracyano -6,13-pentacenequino dimethane (TCPQ) and 1,3,7-Trimethyl xan- thine (caffeine), were chosen. Two different substrate, graphene/Ru(0001) and Cu (111), were used to study the influence of the substrate molecule interaction in the self-assembly processes. The metallic substrate allows the hybridisation of the molecular states and the metallic DOS. On the other hand, the graphene decouples the molecules from the metal providing a good playground to study the molecule- molecule interactions. These molecules aggregate and eventually self-assembly on the surface and the aggregation process is influenced by the number of the rings in the backbone of the molecule, the interaction with the substrate and the cov- erage of the surface. Basically, the TCxQ molecules show on Cu(111) a very lim- ited aggregation character with the formation of small clusters, such as chains or dimers, and eventually, disordered structures. A qualitative explanation for the cluster formation can be given using the molecular potential maps, where the neg- ative potential, associated with the cyano groups, interacts with the positive po- tential, located on the hydrogens in the molecular backbone, forming a hydrogen bond network. Analysing the apparent size in the STM images on the energetics in theoretical calculations it can be concluded that all TCxQ molecules adopt the so-called X-configuration upon adsorption on Cu(111). Unlike, the deposition of those molecules on graphene/Ru(0001) brings to a very different structure which is characterized by a formation of an extensive hydrogen bond network and a very weak interaction with the substrate. Each of the quinone derived molecules self- assemblies in a different way depending on the number of the aromatic rings in the backbone of the molecule and the coverage of the surface. At low coverage, only symmetric molecules stay on the surface and occupy the low area of the graphene moiré with unordered clusters. By increasing the coverage TCAQ molecules self- assembly into representative structure covering low and top area of the moiré . By contrast, TCTQ and TCPQ molecules formed a porous network filling the low area only. When the monolayer conditions are reached, a compact self-assembly struc- ture is obtained only for the symmetric molecules. To investigate the influence of the rigid molecular structure on the surface, caffeine molecules were deposited on Cu(111). Caffeine is basically adsorbed flat-lying on Cu(111) and forms dimers ar- ranged into parallel rows. The molecules are easy to move indicating the absence of a preferential adsorption position and the important of the intermolecular interac- tions in the formation of the self-assembled structure.