Bottom-Up Fabrication of Atomically Precise Graphene Nanoribbons

The effect of water on gold supported chiral graphene nanoribbons: rupture of conjugation by an alternating hydrogenation pattern

In the last few years we have observed a breakpoint in the development of graphene-derived technologies, such as liquid phase filtering and their application to electronics. In most of these cases, they imply exposure of the material to solvents and ambient moisture, either in the fabrication of the material or the final device. The present study demonstrates the sensitivity of graphene nanoribbon (GNR) zigzag edges to water, even in extremely low concentrations. We have addressed the unique reactivity of (3,1)-chiral GNR with moisture on Au(111). Water shows a reductive behaviour, hydrogenating the central carbon of the zigzag segments. By combining scanning tunnelling microscopy (STM) with simulations, we demonstrate how their reactivity reaches a thermodynamic limit when half of the unit cells are reduced, resulting in an alternating pattern of hydrogenated and pristine unit cells starting from the terminal segments. Once a quasi-perfect alternation is reached, the reaction stops regardless of the water concentration. The hydrogenated segments limit the electronic conjugation of the GNR, but the reduction can be reversed both by tip manipulation and annealing. Selective tip-induced dehydrogenation allowed the stabilization of radical states at the edges of the ribbons, while the annealing of the sample completely recovered the original, pristine GNR.

Detecting the spin-polarization of edge states in graphene nanoribbons

Low dimensional carbon-based materials can show intrinsic magnetism associated to p-electrons in open-shell π-conjugated systems. Chemical design provides atomically precise control of the π-electron cloud, which makes them promising for nanoscale magnetic devices. However, direct verification of their spatially resolved spin-moment remains elusive. Here, we report the spin-polarization of chiral graphene nanoribbons (one-dimensional strips of graphene with alternating zig-zag and arm-chair boundaries), obtained by means of spin-polarized scanning tunnelling microscopy. We extract the energy-dependent spin-moment distribution of spatially extended edge states with π-orbital character, thus beyond localized magnetic moments at radical or defective carbon sites. Guided by mean-field Hubbard calculations, we demonstrate that electron correlations are responsible for the spin-splitting of the electronic structure. Our versatile platform utilizes a ferromagnetic substrate that stabilizes the organic magnetic moments against thermal and quantum fluctuations, while being fully compatible with on-surface synthesis of the rapidly growing class of nanographenes.

Circumventing the stability problems of graphene nanoribbon zigzag edges

Carbon nanostructures with zigzag edges exhibit unique properties-such as localized electronic states and spins-with exciting potential applications. Such nanostructures however are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable integration into devices for practical purposes. Here we show two chemical protection/deprotection strategies, demonstrated on labile, air-sensitive chiral graphene nanoribbons. Upon hydrogenation, the chiral graphene nanoribbons survive exposure to air, after which they are easily converted back to their original structure by annealing. We also approach the problem from another angle by synthesizing a form of the chiral graphene nanoribbons that is functionalized with ketone side groups. This oxidized form is chemically stable and can be converted to the pristine hydrocarbon form by hydrogenation and annealing. In both cases, the deprotected chiral graphene nanoribbons regain electronic properties similar to those of the pristine nanoribbons. We believe both approaches may be extended to other graphene nanoribbons and carbon-based nanostructures.

On-Surface Synthesis of C144 Hexagonal Coronoid with Zigzag Edges

Coronoids as polycyclic aromatic macrocycles enclosing a cavity have attracted a lot of attention due to their distinctive molecular and electronic structures. They can be also regarded as nanoporous graphene molecules whose electronic properties are critically dependent on the size and topology of their outer and inner peripheries. However, because of their synthetic challenges, the extended hexagonal coronoids with zigzag outer edges have not been reported yet. Here, we report the on-surface synthesis of C144 hexagonal coronoid with outer zigzag edges on a designed precursor undergoing hierarchical Ullmann coupling and cyclodehydrogenation on the Au(111) surface. The molecular structure is unambiguously characterized by bond-resolved noncontact atomic force microscopy imaging. The electronic properties are further investigated by scanning tunneling spectroscopy measurements, in combination with the density functional theory calculations. Moreover, the values of the harmonic oscillator model of aromaticity are derived from calculations that suggest that the molecular structure is ideally represented by Clar's model. Our results provide approaches toward realizing a hexagonal coronoid with zigzag edges, potentially inspiring fabrication of hexagonal zigzag coronoids with multiple radical characters in the future.

Chemical Stability of (3,1)-Chiral Graphene Nanoribbons

Nanostructured graphene has been widely studied in recent years due to the tunability of its electronic properties and its associated interest for a variety of fields, such as nanoelectronics and spintronics. However, many of the graphene nanostructures of technological interest are synthesized under ultrahigh vacuum, and their limited stability as they are brought out of such an inert environment may compromise their applicability. In this study, a combination of bond-resolving scanning probe microscopy (BR-SPM), along with theoretical calculations, has been employed to study (3,1)-chiral graphene nanoribbons [(3,1)-chGNRs] that were synthesized on a Au(111) surface and then exposed to oxidizing environments. Exposure to the ambient atmosphere, along with the required annealing treatment to desorb a sufficiently large fraction of contaminants to allow for its postexposure analysis by BR-SPM, revealed a significant oxidation of the ribbons, with a dramatically disruptive effect on their electronic properties. More controlled experiments avoiding high temperatures and exposing the ribbons only to low pressures of pure oxygen show that also under these more gentle conditions the ribbons are oxidized. From these results, we obtain additional insights into the preferential reaction sites and the nature of the main defects that are caused by oxygen. We conclude that graphene nanoribbons with zigzag edge segments require forms of protection before they can be used in or transferred through ambient conditions.

Probing the Magnetism of Topological End States in 5-Armchair Graphene Nanoribbons

We extensively characterize the electronic structure of ultranarrow graphene nanoribbons (GNRs) with armchair edges and zigzag termini that have five carbon atoms across their width (5-AGNRs), as synthesized on Au(111). Scanning tunneling spectroscopy measurements on the ribbons, recorded on both the metallic substrate and a decoupling NaCl layer, show well-defined dispersive bands and in-gap states. In combination with theoretical calculations, we show how these in-gap states are topological in nature and localized at the zigzag termini of the nanoribbons. In addition to rationalizing the driving force behind the topological class selection of 5-AGNRs, we also uncover the length-dependent behavior of these end states which transition from singly occupied spin-split states to a closed-shell form as the ribbons become shorter. Finally, we demonstrate the magnetic character of the end states via transport experiments in a model two-terminal device structure in which the ribbons are suspended between the scanning probe and the substrate that both act as leads.

Hierarchy in the Halogen Activation During Surface-Promoted Ullmann Coupling

Within the collection of surface-supported reactions currently accessible for the production of extended molecular nanostructures under ultra-high vacuum, Ullmann coupling has been the most successful in the controlled formation of covalent single C-C bonds. Particularly advanced control of this synthetic tool has been obtained by means of hierarchical reactivity, commonly achieved by the use of different halogen atoms that consequently display distinct activation temperatures. Here we report on the site-selective reactivity of certain carbon-halogen bonds. We use precursor molecules halogenated with bromine atoms at two non-equivalent carbon atoms and found that the Ullmann coupling occurs on Au(111) with a remarkable predilection for one of the positions. Experimental evidence is provided by means of scanning tunneling microscopy and core level photoemission spectroscopy, and a rationalized understanding of the observed preference is obtained from density functional theory calculations.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Electronic Structure Tunability by Periodic meta-Ligand Spacing in One-Dimensional Organic Semiconductors

Designing molecular organic semiconductors with distinct frontier orbitals is key for the development of devices with desirable properties. Generating defined organic nanostructures with atomic precision can be accomplished by on-surface synthesis. We use this "dry" chemistry to introduce topological variations in a conjugated poly( para-phenylene) chain in the form of meta-junctions. As evidenced by STM and LEED, we produce a macroscopically ordered, monolayer thin zigzag chain film on a vicinal silver crystal. These cross-conjugated nanostructures are expected to display altered electronic properties, which are now unraveled by highly complementary experimental techniques (ARPES and STS) and theoretical calculations (DFT and EPWE). We find that meta-junctions dominate the weakly dispersive band structure, while the band gap is tunable by altering the linear segment's length. These periodic topology effects induce significant loss of the electronic coupling between neighboring linear segments leading to partial electron confinement in the form of weakly coupled quantum dots. Such periodic quantum interference effects determine the overall semiconducting character and functionality of the chains.

Switching from Reactant to Substrate Engineering in the Selective Synthesis of Graphene Nanoribbons

The challenge of synthesizing graphene nanoribbons (GNRs) with atomic precision is currently being pursued along a one-way road, based on the synthesis of adequate molecular precursors that react in predefined ways through self-assembly processes. The synthetic options for GNR generation would multiply by adding a new direction to this readily successful approach, especially if both of them can be combined. We show here how GNR synthesis can be guided by an adequately nanotemplated substrate instead of by the traditionally designed reactants. The structural atomic precision, unachievable to date through top-down methods, is preserved by the self-assembly process. This new strategy's proof-of-concept compares experiments using 4,4''-dibromo-para-terphenyl as a molecular precursor on flat Au(111) and stepped Au(322) substrates. As opposed to the former, the periodic steps of the latter drive the selective synthesis of 6 atom-wide armchair GNRs, whose electronic properties have been further characterized in detail by scanning tunneling spectroscopy, angle resolved photoemission, and density functional theory calculations.