Electronic tailoring of graphene nanostructures via on-surface synthesis

Resumen

This thesis presents a thorough study on the electronic properties of several types of graphene nanoribbons (GNRs) produced by on-surface synthesis strategies on gold substrates. The research has been performed mainly by Low Temperature Scanning Tunneling Microscopy and Spectroscopy (LTSTM/STS) in ultra high vacuum conditions. These results were obtained in collaboration with the group of Prof. Daniel Sánchez-Portal who performed first principle Density Functional Theory calculations. The novel molecular precursors used in this thesis were synthesized by the group of Prof. Diego Peña and the group of Yamaguchi-Sensei. GNRs are a new class of material combining many of the superlative qualities of graphene with unique electronic properties arising from their nanometric dimensions. The presence of a tunable electronic bandgap or low dimensional magnetism is intrinsically related to the width and edge structure of the ribbon down to the atomic scale. In order to achieve atomic precision in the structure of the ribbons, we grow our ribbons using specially designed molecular precursors as building blocks. The reaction of the precursors is catalyzed by the metal surface and leads to ribbons with very low density of defects. This on-surface approach produced model systems that facilitate the study of the electronic properties of 7 armchair grapheme nanoribbons (7-AGNRs), two chemically modified 7-AGNR species and hybrid ribbons combining pristine and boron modified 7-AGNR sections. Two strategies were followed to chemically modify 7-AGNRs: using substitutional nitrile functional groups at the edges and using substitutional boron atoms within the backbone of the ribbon. The 7-AGNR species modified with edge cyano (CN) groups presented a case study of the use of functional groups to modify the electronic properties of GNRs. The strong electron withdrawing character of CN groups induced a charge redistribution within the ribbon backbone, leading to a very efficient n-like doping of the ribbon. The substitution of boron atoms within the backbone of 7-AGNRs backbone highlighted additional consequences of doping ribbons with other chemical species. On the one hand, the electronic structure was heavily modified by the appearance of a new acceptor band. On the other hand, the boron atoms lead to a higher ribbon-substrate interaction producing a buckling of the ribbon structure and a hibridization of the new acceptor band with gold’s surface bands. Another adopted strategy was the combination of pristine and chemically modified 7-AGNRs precursors to construct hybrid GNRs and GNR heterostructures. By combining boron substituted and pristine 7-AGNRs we embedded quantum wells within the pristine sections of the hybrid ribbons. The boron substitutes acted as very efficient scatterers of the pristine 7-AGNR valence band, selectively confining this band but leaving unaffected the band below. Finally, we show the on-surface synthesis of other graphene nanostructures. Particularly, we demonstrated a new on-surface route to create benzoazulene moieties within the structure of a [18]annulene based polycyclic aromatic hydrocarbon. The benzoazulenes, which play a key role in the planarization of the molecule, resulted from two conjoined bay regions in the reacted molecular precursors. Moreover, the [18]annulene core was found to host peculiar pore localized states, which we associate to super atom molecular orbitals.