Carlo A. Pignedoli

On-surface synthesis of graphene nanoribbons with zigzag edge topology.

Graphene-based nanostructures exhibit electronic properties that are not present in extended graphene. For example, quantum confinement in carbon nanotubes and armchair graphene nanoribbons leads to the opening of substantial electronic bandgaps that are directly linked to their structural boundary conditions. Nanostructures with zigzag edges are expected to host spin-polarized electronic edge states and can thus serve as key elements for graphene-based spintronics. The edge states of zigzag graphene nanoribbons (ZGNRs) are predicted to couple ferromagnetically along the edge and antiferromagnetically between the edges, but direct observation of spin-polarized edge states for zigzag edge topologies—including ZGNRs—has not yet been achieved owing to the limited precision of current top-down approaches. Here we describe the bottom-up synthesis of ZGNRs through surface-assisted polymerization and cyclodehydrogenation of specifically designed precursor monomers to yield atomically precise zigzag edges. Using scanning tunnelling spectroscopy we show the existence of edge-localized states with large energy splittings. We expect that the availability of ZGNRs will enable the characterization of their predicted spin-related properties, such as spin confinement and filtering, and will ultimately add the spin degree of freedom to graphene-based circuitry.

Surface-assisted cyclodehydrogenation provides a synthetic route towards easily processable and chemically tailored nanographenes

Atomically thin sheets of sp(2)-hybridized carbon--graphene--have enormous potential for applications in future electronic devices. Particularly promising are nanostructured (sub)units of graphene, the electronic properties of which can be tuned by changing the spatial extent or the specific edge termination of the carbon network. Processability and precise tailoring of graphene-derived structures are, however, still major obstacles in developing applications; both bottom-up and top-down routes are presently under investigation in attempts to overcome this limitation. Here, we propose a surface chemical route that allows for the atomically precise fabrication of tailored nanographenes from polyphenylene precursors. The cyclodehydrogenation of a prototypical polyphenylene on Cu(111) is studied using scanning tunnelling microscopy and density functional theory. We find that the thermally induced cyclodehydrogenation proceeds via several intermediate steps, two of which can be stabilized on the surface, yielding unprecedented insight into a dehydrogenative intramolecular aryl-aryl coupling reaction.

Graphene nanoribbon heterojunctions

Despite graphenes remarkable electronic properties, the lack of an electronic bandgap severely limits its potential for applications in digital electronics. In contrast to extended films, narrow strips of graphene (called graphene nanoribbons) are semiconductors through quantum confinement, with a bandgap that can be tuned as a function of the nanoribbon width and edge structure. Atomically precise graphene nanoribbons can be obtained via a bottom-up approach based on the surface-assisted assembly of molecular precursors. Here we report the fabrication of graphene nanoribbon heterojunctions and heterostructures by combining pristine hydrocarbon precursors with their nitrogen-substituted equivalents. Using scanning probe methods, we show that the resulting heterostructures consist of seamlessly assembled segments of pristine (undoped) graphene nanoribbons (p-GNRs) and deterministically nitrogen-doped graphene nanoribbons (N-GNRs), and behave similarly to traditional p-n junctions. With a band shift of 0.5 eV and an electric field of 2 × 10(8) V m(-1) at the heterojunction, these materials bear a high potential for applications in photovoltaics and electronics.

Electronic structure of atomically precise graphene nanoribbons.

Some of the most intriguing properties of graphene are predicted for specifically designed nanostructures such as nanoribbons. Functionalities far beyond those known from extended graphene systems include electronic band gap variations related to quantum confinement and edge effects, as well as localized spin-polarized edge states for specific edge geometries. The inability to produce graphene nanostructures with the needed precision, however, has so far hampered the verification of the predicted electronic properties. Here, we report on the electronic band gap and dispersion of the occupied electronic bands of atomically precise graphene nanoribbons fabricated via on-surface synthesis. Angle-resolved photoelectron spectroscopy and scanning tunneling spectroscopy data from armchair graphene nanoribbons of width N = 7 supported on Au(111) reveal a band gap of 2.3 eV, an effective mass of 0.21 m(0) at the top of the valence band, and an energy-dependent charge carrier velocity reaching 8.2 × 10(5) m/s in the linear part of the valence band. These results are in quantitative agreement with theoretical predictions that include image charge corrections accounting for screening by the metal substrate and confirm the importance of electron-electron interactions in graphene nanoribbons.

Porous Graphene as an Atmospheric Nanofilter

The fabrication of nanoscale membranes exhibiting high selectivity is an emerging field of research. The possibility to use bottom-up approaches to fabricate a filter with porous graphene and analyze its functionality with first principle calculations is investigated. Here, the porous network is produced by self-assembly of the hexaiodo-substituted macrocycle cyclohexa-m-phenylene (CHP). The resulting porous network exhibits an extremely high selectivity in favor of H(2) and He among other atmospheric gases, such as Ne, O(2), N(2), CO, CO(2), NH(3), and Ar. The presented membrane is superior to traditional filters using polymers or silica and could have great potential for further technological applications such as gas sensors or fuel cells.

Surface-supported 2D heterotriangulene polymers

We report on the assembly of tribromo-substituted dimethylmethylene-bridged triphenylamine (heterotriangulene) on Ag(111). Depending on activation temperature, two-dimensional porous metal-coordination or covalent networks are obtained.

Termini of Bottom-Up Fabricated Graphene Nanoribbons

Atomically precise graphene nanoribbons (GNRs) can be obtained via thermally induced polymerization of suitable precursor molecules on a metal surface. This communication discusses the atomic structure found at the termini of armchair GNRs obtained via this bottom-up approach. The short zigzag edge at the termini of the GNRs under study gives rise to a localized midgap state with a characteristic signature in scanning tunneling microscopy (STM). By combining STM experiments with large-scale density functional theory calculations, we demonstrate that the termini are passivated by hydrogen. Our results suggest that the length of nanoribbons grown by this protocol may be limited by hydrogen passivation during the polymerization step.

Intraribbon Heterojunction Formation in Ultranarrow Graphene Nanoribbons

Graphene nanoribbons-semiconducting quasi-one-dimensional graphene structures-have great potential for the realization of novel electronic devices. Recently, graphene nanoribbon heterojunctions-interfaces between nanoribbons with unequal band gaps-have been realized with lithographic etching techniques and via chemical routes to exploit quantum transport phenomena. However, standard fabrication techniques are not suitable for ribbons narrower than ~5 nm and do not allow to control the width and edge structure of a specific device with atomic precision. Here, we report the realization of graphene nanoribbon heterojunctions with lateral dimensions below 2 nm via controllable dehydrogenation of polyanthrylene oligomers self-assembled on a Au(111) surface from molecular precursors. Atomistic simulations reveal the microscopic mechanisms responsible for intraribbon heterojunction formation. We demonstrate the capability to selectively modify the heterojunctions by activating the dehydrogenation reaction on single units of the nanoribbons by electron injection from the tip of a scanning tunneling microscope.

Molecules–Oligomers–Nanowires–Graphene Nanoribbons: A Bottom-Up Stepwise On-Surface Covalent Synthesis Preserving Long-Range Order

We report on a stepwise on-surface polymerization reaction leading to oriented graphene nanoribbons on Au(111) as the final product. Starting from the precursor 4,4″-dibromo-p-terphenyl and using the Ullmann coupling reaction followed by dehydrogenation and C-C coupling, we have developed a fine-tuned, annealing-triggered on-surface polymerization that allows us to obtain an oriented nanomesh of graphene nanoribbons via two well-defined intermediate products, namely, p-phenylene oligomers with reduced length dispersion and ordered submicrometric molecular wires of poly(p-phenylene). A fine balance involving gold catalytic activity in the Ullmann coupling, appropriate on-surface molecular mobility, and favorable topochemical conditions provided by the used precursor leads to a high degree of long-range order that characterizes each step of the synthesis and is rarely observed for surface organic frameworks obtained via Ullmann coupling.

Superlubricity of graphene nanoribbons on gold surfaces.

A golden opportunity for graphene Reducing friction can limit wear and improve the energy efficiency of mechanical devices. Graphene is a promising lubricant because the friction between sheets is minuscule under certain circumstances. Kawai et al. show that the same ultra-low frictional properties extend to other surfaces. They find ultralow friction when dragging graphene nanoribbons across a gold surface using an atomic force microscope. This discovery sets up the potential for developing nanographene frictionless coatings. Science, this issue p. 957 Experiments reveal ultralow friction when graphene nanoribbons slide across an oriented gold surface. The state of vanishing friction known as superlubricity has important applications for energy saving and increasing the lifetime of devices. Superlubricity, as detected with atomic force microscopy, appears when sliding large graphite flakes or gold nanoclusters across surfaces, for example. However, the origin of the behavior is poorly understood because of the lack of a controllable nanocontact. We demonstrated the superlubricity of graphene nanoribbons when sliding on gold with a joint experimental and computational approach. The atomically well-defined contact allows us to trace the origin of superlubricity, unraveling the role played by ribbon size and elasticity, as well as by surface reconstruction. Our results pave the way to the scale-up of superlubricity and thus to the realization of frictionless coatings.