Anna L. Pinardi

Strain-driven Moire superstructures of epitaxial graphene on transition metal surfaces.

STM images of multidomain epitaxial graphene on Pt(111) have been combined with a geometrical model to investigate the origin of the coincidence Moiré superstructures. We show that there is a relation between the appearance of a particular Moiré periodicity and the minimization of the absolute value of the strain between the graphene and the substrate for the different orientations between both atomic lattices. This model predicts all the stable epitaxial graphene structures that can be grown on transition metal surfaces, and we have made use of it for reproducing previously published data from different authors. Its validity suggests that minimization of the strain within the coincident graphene unit-cell due to a strong local interaction is the driving force in the formation of Moiré superstructures.

Tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface (cyclo)dehydrogenation of heteroaromatics.

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH-substrate interaction rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest.

Sequential formation of N-doped nanohelicenes, nanographenes and nanodomes by surface-assisted chemical (cyclo)dehydrogenation of heteroaromatics

We report on the stepwise formation of N-doped nanohelicenes, nanographenes, nanodomes and graphenes from the same heteroaromatic precursor through subsequent dehydrogenations on Pt(111) upon thermal annealing. The combined experimental (UHV-STM) and computational (DFT) studies provide a full atomistic description of the intermediate reaction stages.

Sublattice Localized Electronic States in Atomically Resolved Graphene-Pt(111) Edge-Boundaries

Understanding the connection of graphene with metal surfaces is a necessary step for developing atomically precise graphene-based technology. Combining high-resolution STM experiments and DFT calculations, we have unambiguously unveiled the atomic structure of the boundary between a graphene zigzag edge and a Pt(111) step. The graphene edges minimize their strain by inducing a 3-fold edge-reconstruction on the metal side. We show the existence of an unoccupied electronic state that is mostly localized on the C-edge atoms of one particular graphene sublattice, which could have implications in the design of graphene based devices.

Vacancy formation on C60/Pt (111): unraveling the complex atomistic mechanism

The interaction of fullerenes with transition metal surfaces leads to the development of an atomic network of ordered vacancies on the metal. However, the structure and formation mechanism of this intricate surface reconstruction is not yet understood at an atomic level. We combine scanning tunneling microscopy, high resolution and temperature programmed-x-ray photoelectrons spectroscopy, and density functional theory calculations to show that the vacancy formation in C60/Pt(111) is a complex process in which fullerenes undergo two significant structural rearrangements upon thermal annealing. At first, the molecules are physisorbed on the surface; next, they chemisorb inducing the formation of an adatom-vacancy pair on the side of the fullerene. Finally, this metastable state relaxes when the adatom migrates away and the vacancy moves under the molecule. The evolution from a weakly-bound fullerene to a chemisorbed state with a vacancy underneath could be triggered by residual H atoms on the surface which prevent a strong surface-adsorbate bonding right after deposition. Upon annealing at about 440 K, when all H has desorbed, the C60 interacts with the Pt surface atoms forming the vacancy-adatom pair. This metastable state induces a small charge transfer and precedes the final adsorption structure.

Role of the Pinning Points in epitaxial Graphene Moiré Superstructures on the Pt(111) Surface.

The intrinsic atomic mechanisms responsible for electronic doping of epitaxial graphene Moirés on transition metal surfaces is still an open issue. To better understand this process we have carried out a first-principles full characterization of the most representative Moiré superstructures observed on the Gr/Pt(111) system and confronted the results with atomically resolved scanning tunneling microscopy experiments. We find that for all reported Moirés the system relaxes inducing a non-negligible atomic corrugation both, at the graphene and at the outermost platinum layer. Interestingly, a mirror “anti-Moiré” reconstruction appears at the substrate, giving rise to the appearance of pinning-points. We show that these points are responsible for the development of the superstructure, while charge from the Pt substrate is injected into the graphene, inducing a local n-doping, mostly localized at these specific pinning-point positions.

On-Surface (Cyclo-)Dehydrogenation Reactions: Role of Surface Diffusion

Creating or connecting together large organic molecules, as polycyclic aromatic hydrocarbons (PAH), readily on surfaces arises as a step of paramount importance towards a true advance in the field of nanotechnology, and particularly of molecular electronics. On-surface synthesis can be regarded as an efficient means to build new molecular species by using bottom-up strategies. In particular, temperature-driven surface-catalysed cyclodehydrogenation (CDH) processes have burgeoned in last years as a novel and powerful route to efficiently transform (hetero-)aromatic molecular precursors into a large variety of a la carte hierarchical nanostructures: from fullerenes and triazafullerenes, aromatic domes, or nanotubes, to polymeric nanonetworks and, depending on the specific precursor utilized, even to pristine and functionalized graphene. In the first section of this chapter, the foundations and main aspects of the on-surface synthesis methodology and CDH reactions are revised, as well as the current status in the field up to the date. In Sect. 2, the most advanced first-principles theoretical tools currently available for the characterization of CDH processes will be summarized and described, including the novel theoretical strategies to monitorize CDH reaction paths and to calculate STM images. The following two sections will report on a very recently paradigmatic example related to the tailored formation of N-doped nanoarchitectures by diffusion-controlled on-surface (cyclo-)dehydrogenation of heteroaromatics, where the strength of the PAH–substrate interaction dramatically rules the competitive reaction pathways (CDH versus dehydrogenative polymerization). On the basis of those findings, the stepwise formation of N-doped nanohelicenes, nanographenes, nanodomes, molecular networks and graphene from the same heteroaromatic precursor through subsequent dehydrogenations on Pt(111) upon thermal-annealing will be fully described. The combined experimental (in situ UHV-STM, XPS and NEXAFS) and detailed computational DFT-based studies provide a full atomistic and chemical description of the intermediate reaction stages along the dehydrogenation path.