Graphene nanoribbons (GNRs) are excellent candidates for next-generation electronic materials. Unlike GNRs produced by "top-down" methods such as lithographical patterning of graphene and unzipping of carbon nanotubes that cannot reach structural perfection, the fabrication of structurally well-defined GNRs has been achieved by a "bottom-up" organic synthesis via solution-mediated or surface-assisted cyclodehydrogenation. Specifically, non-planar polyphenylene precursors were first "build up" from small molecules, and then "graphitized" and "planarized" to yield GNRs. However, fabrication of processable and longitudinally well-extended GNRs has remained a major challenge. Here we report a "bottom-up" solution synthesis of long (>200 nm), liquid-phase processable GNRs with well-defined structure and a large optical bandgap of 1.88 eV. Scanning probe microscopy demonstrates self-assembled monolayers of GNRs, and non-contact, time-resolved Terahertz conductivity measurements reveal excellent charge-carrier mobility within individual GNRs. Such structurally well-defined GNRs offer great opportunities for fundamental studies into graphene nanostructures, as well as development of GNR-based nanoelectronics.DOI: 10.1038/NCHEM.1819 http://www.nature.com/nchem/journal/v6/n2/abs/nchem.1819.html 2 Graphene nanoribbons (GNRs), defined as nanometre-wide strips of graphene, are attracting increasing attention as highly promising candidates for next generation semiconductor materials 1,2,3,4 . Quantum confinement effects impart GNRs with semiconducting properties, i.e. with a finite bandgap, which critically depends on the ribbon width and its edge structure 1,3 . Fabrication of GNRs has been primarily carried out by "top-down" approaches such as lithographical patterning of graphene 5,6 and unzipping of carbon nanotubes 7,8 , revealing their semiconducting nature and excellent transport properties 1 . However, these methods are generally limited by low yields and lack of structural precision, leading to GNRs with uncontrolled edge structures.In contrast, a "bottom-up" chemical synthetic approach based on solution-mediated 9,10,11,12,13 or surface-assisted 14 cyclodehydrogenation, namely "graphitization" and "planarization", of tailor-made three-dimensional polyphenylene precursors offers an appealing strategy for making structurally well-defined and homogeneous GNRs. The polyphenylene precursors are built up from small molecules, and thus their structures can be tailored within the capabilities of modern synthetic chemistry 15 . However, GNRs (>30 nm) produced by solution-mediated methods have been precluded from unambiguous structural characterization, i.e. microscopic visualization, due to their limited processability 9,12 . On the other hand, GNRs produced by the surface-assisted protocol have been characterized to be atomically precise using scanning tunnelling microscopy (STM) 14 . Nevertheless, this method can only provide a limited amount of GNR material, which is further bound to a metal surface, impeding wide...

The application of supramolecular chemistry on solid surfaces represents an exciting field of research that continues to develop in new and unexpected directions. This review highlights recent advances in the field which range from the fundamental aspects of the thermodynamics of self-assembly through to the development of new materials with potential application as new materials. The unique aspects of working on solid surfaces are highlighted and advances in the assembly of many component systems and highly complex fractal-like and quasicrystalline systems discussed. The unique features of working in the surface-based environment and the utilisation of scanning probe microscopies as a primary characterisation tool are highlighted.

Oxidative cyclodehydrogenation of laterally extended polyphenylene precursor allowed bottom-up synthesis of structurally defined graphene nanoribbons (GNRs) with unprecedented width. The efficiency of the cyclodehydrogenation was validated by means of MALDI-TOF MS, FT-IR, Raman, and UV-vis absorption spectroscopies as well as investigation of a representative model system. The produced GNRs demonstrated broad absorption extended to near-infrared region with the optical band gap of as low as 1.12 eV.

Structurally defined, long (>100 nm), and low-band-gap (∼1.2 eV) graphene nanoribbons (GNRs) were synthesized through a bottom-up approach, enabling GNRs with a broad absorption spanning into the near-infrared (NIR) region. The chemical identity of GNRs was validated by IR, Raman, solid-state NMR, and UV-vis-NIR absorption spectroscopy. Atomic force microscopy revealed well-ordered self-assembled monolayers of uniform GNRs on a graphite surface upon deposition from the liquid phase. The broad absorption of the low-band-gap GNRs enables their detailed characterization by Raman and time-resolved terahertz photoconductivity spectroscopy with excitation at multiple wavelengths, including the NIR region, which provides further insights into the fundamental physical properties of such graphene nanostructures.

The use of self-assembly to fabricate surface-confined adsorbed layers (adlayers) from molecular components provides a simple means of producing complex functional surfaces. The molecular self-assembly process relies on supramolecular interactions sustained by noncovalent forces such as van der Waals, electrostatic, dipole-dipole, and hydrogen bonding interactions. Researchers have exploited these noncovalent bonding motifs to construct well-defined two-dimensional (2D) architectures at the liquid-solid interface. Despite myriad examples of 2D molecular assembly, most of these early findings were serendipitous because the intermolecular interactions involved in the process are often numerous, subtle, cooperative, and multifaceted. As a consequence, the ability to tailor supramolecular patterns has evolved slowly. Insight gained from various studies over the years has contributed significantly to the knowledge of supramolecular interactions, and the stage is now set to systematically engineer the 2D supramolecular networks in a "preprogrammed" fashion. The control over 2D self-assembly of molecules has many important implications. Through appropriate manipulation of supramolecular interactions, one can "encode" the information at the molecular level via structural features such as functional groups, substitution patterns, and chiral centers which could then be retrieved, transferred, or amplified at the supramolecular level through well-defined molecular recognition processes. This ability allows for precise control over the nanoscale structure and function of patterned surfaces. A clearer understanding and effective use of these interactions could lead to the development of functional surfaces with potential applications in molecular electronics, chiral separations, sensors based on host-guest systems, and thin film materials for lubrication. In this Account, we portray our various attempts to achieve rational design of self-assembled adlayers by exploiting the aforementioned complex interactions at the liquid-solid interface. The liquid-solid interface presents a unique medium to construct flawless networks of surface confined molecules. The presence of substrate and solvent provides an additional handle for steering the self-assembly of molecules. Scanning tunneling microscopy (STM) was used for probing these molecular layers, a technique that serves not only as a visualization tool but could also be employed for active manipulation of molecules. The supramolecular systems described here are only weakly adsorbed on a substrate, which is typically highly oriented pyrolytic graphite (HOPG). Starting with fundamental studies of substrate and solvent influence on molecular self-assembly, this Account describes progressively complex aspects such as multicomponent self-assembly via 2D crystal engineering, emergence, and induction of chirality and stimulus responsive supramolecular systems.