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...

We shine light on the covalent modification of graphite and graphene substrates using diazonium chemistry under ambient conditions. We report on the nature of the chemical modification of these graphitic substrates, the relation between molecular structure and film morphology, and the impact of the covalent modification on the properties of the substrates, as revealed by local microscopy and spectroscopy techniques and electrochemistry. By careful selection of the reagents and optimizing reaction conditions, a high density of covalently grafted molecules is obtained, a result that is demonstrated in an unprecedented way by scanning tunneling microscopy (STM) under ambient conditions. With nanomanipulation, i.e., nanoshaving using STM, surface structuring and functionalization at the nanoscale is achieved. This manipulation leads to the removal of the covalently anchored molecules, regenerating pristine sp(2) hybridized graphene or graphite patches, as proven by space-resolved Raman microscopy and molecular self-assembly studies.

The surface-mediated synthesis of epitaxially aligned and separated polyphenylene lines on Cu(110) by exploiting the Ullmann dehalogenation reaction is reported. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) show that the C-I bonds of 1,4-diiodobenzene and 1,3-diiodobenzene (C(6)H(4)I(2)) are catalytically cleaved when dosed onto the surface. Subsequent annealing transforms the copper-bound phenylene intermediates into covalent conjugated structures: linear chains of poly(p-phenylene) for 1,4-diiodobenzene and zigzag chains of poly(m-phenylene) as well as macrocyclic oligomers in the case of 1,3-diiodobenzene. The chains are strongly bound to the surface (likely through C--Cu bonds at the chain-ends) while the macrocycles are very mobile and can only be imaged by STM at low temperature. The detached halogens adsorb on the surface and separate the polymer chains from each other.

We describe a surprising cooperative adsorption process observed by scanning tunneling microscopy (STM) at the liquid-solid interface. The process involves the association of a threefold hydrogen-bonding unit, trimesic acid (TMA), with straight-chain aliphatic alcohols of varying length (from C 7 to C 30 ), which coadsorb on highly oriented pyrolytic graphite (HOPG) to form linear patterns. In certain cases, the known TMA "flower pattern" can coexist temporarily with the linear TMA-alcohol patterns, but it eventually disappears. Time-lapsed STM imaging shows that the evolution of the flower pattern is a classical ripening phenomenon. The periodicity of the linear TMA-alcohol patterns can be modulated by choosing alcohols with appropriate chain lengths, and the precise structure of the patterns depends on the parity of the carbon count in the alkyl chain. Interactions that lead to this odd-even effect are analyzed in detail. The molecular components of the patterns are achiral, yet their association by hydrogen bonding leads to the formation of enantiomeric domains on the surface. The interrelation of these domains and the observation of superperiodic structures (moire ṕatterns) are rationalized by considering interactions with the underlying graphite surface and within the two-dimensional crystal of the adsorbed molecules. Comparison of the observed two-dimensional structures with the three-dimensional crystal structures of TMA-alcohol complexes determined by X-ray crystallography helps reveal the mechanism of molecular association in these two-component systems.

Scanning tunneling microscopy (STM) of monolayers comprising oligothiophene and fullerene molecular semiconductors reveals details of their molecular-scale phase separation and ordering with potential implications for the design of organic electronic devices, in particular future bulk heterojunction solar cells. Prochiral terthienobenzenetricarboxylic acid (TTBTA) self-assembles at the solution/graphite interface into either a porous chicken wire network linked by dimeric hydrogen bonding associations of COOH groups (R(2)(2) (8)) or a close-packed network linked in a novel hexameric hydrogen bonding motif (R(6)(6) (24)). Analysis of high-resolution STM images shows that the chicken wire phase is racemically mixed, whereas the close-packed phase is enantiomerically pure. The cavities of the chicken wire structure can efficiently host C60 molecules, which form ordered domains with either one, two, or three fullerenes per cavity. The observed monodisperse filling and long-range co-alignment of fullerenes is described in terms of a combination of an electrostatic effect and the commensurability between the graphite and molecular network, which leads to differentiation of otherwise identical adsorption sites in the pores.

We demonstrate a surprising cooperative adsorption process at the liquid-solid interface, involving self-assembly in which a three-fold hydrogen-bonding unit (trimesic acid, TMA) is forced into a linear pattern by noncovalent interaction with an alcohol. Our work shows that the unexpected linear pattern formed by coadsorption of TMA and alcohols can be modulated in size by choosing alcohols with different chain lengths.

A dominant theme within the research on two-dimensional chirality is the sergeant-soldiers principle, wherein a small fraction of chiral molecules (sergeants) is used to skew the handedness of achiral molecules (soldiers) to generate a homochiral surface. Here, we have combined the sergeant-soldiers principle with temperature-dependent molecular self-assembly to unravel a peculiar chiral amplification mechanism at the solution-solid interface in which, depending on the concentration of a sergeant-soldiers solution, the majority handedness of the system can either be amplified or entirely reversed after an annealing step, furnishing a homochiral surface. Two discrete pathways that affect different stages of two-dimensional crystal growth are invoked for rationalizing this phenomenon and we present a set of experiments where the access to each pathway can be precisely controlled. These results demonstrate that a detailed understanding of subtle intermolecular and interfacial interactions can be used to induce drastic changes in the handedness of a supramolecular network.

Hydrogen bonding is one of the most important non-covalent interactions in both biological (DNA, peptides, saccharides etc.) and artificial systems (various soft materials, host-guest architectures, molecular networks, etc.). Carboxylic acids are some of the most simple yet powerful hydrogen-bonding building blocks, that possess a particularly rich supramolecular chemistry. This tutorial review focuses on the structural diversity of supramolecular architectures accessible via hydrogen bonding of carboxylic acids, as observed both in single crystals using X-ray analysis and in monolayers on surfaces using scanning probe techniques. It provides a concise overview of the key concepts and principles of modern supramolecular design and is given in the form of case studies of finely selected literature examples, covering formation of macrocycles, chains, ladders, rotaxanes, catenanes, various 2D and 3D nets, host-guest systems and some applications thereof.