We describe the first coronoid nanographene C216-molecule. As an extended polycyclic aromatic hydrocarbon containing a defined cavity, our molecule can be seen as a model system to study the influence of holes on the physical and chemical properties of graphene. Along the pathway of an eight-step synthesis including Yamamoto-type cyclization followed by 6-fold Diels-Alder cycloaddition, C216 was obtained by oxidative cyclodehydrogenation in the final step. The defined molecular structure with a cavity was unambiguously validated by MALDI-TOF mass spectrometry and FTIR, Raman, and UV-vis absorption spectroscopy coupled with DFT simulations.

Structurally well-defined graphene nanoribbons (GNRs) have attracted great interest as next-generation semiconductor materials. The functionalization of GNRs with polymeric side chains, which can widely broaden GNR-related studies on physiochemical properties and potential applications, has remained unexplored. Here, we demonstrate the bottom-up solution synthesis of defect-free GNRs grafted with flexible poly(ethylene oxide) (PEO) chains. The GNR backbones possess an armchair edge structure with a width of 1.0-1.7 nm and mean lengths of 15-60 nm, enabling near-infrared absorption and a low bandgap of 1.3 eV. Remarkably, the PEO grafting renders the GNRs superb dispersibility in common organic solvents, with a record concentration of ∼1 mg mL(-1) (for GNR backbone) that is much higher than that (<0.01 mg mL(-1)) of reported GNRs. Moreover, the PEO-functionalized GNRs can be readily dispersed in water, accompanying with supramolecular helical nanowire formation. Scanning probe microscopy reveals raft-like self-assembled monolayers of uniform GNRs on graphite substrates. Thin-film-based field-effect transistors (FETs) of the GNRs exhibit a high carrier mobility of ∼0.3 cm(2) V(-1) s(-1), manifesting promising application of the polymer-functionalized GNRs in electronic devices.

On-surface synthesis is a successful approach to the creation of carbon-based nanostructures that cannot be obtained via standard solution chemistry. In this framework, we have established a novel synthetic pathway to one-dimensional conjugated polymers composed of indenofluorene units. Our concept is based on the use of ortho-methyl groups on a poly( para-phenylene) backbone. In this situation, surface-assisted oxidative ring closure between a methyl and the neighboring aryl moiety gives rise to a five-membered ring. The atomically precise structures and electronic properties of the obtained indenofluorene polymers have been unambiguously characterized by STM, nc-AFM, and STS, supported by theoretical calculations. This unprecedented synthetic protocol can potentially be extended to other polyphenylenes and eventually graphene nanoribbons, to incorporate five-membered rings at desired positions for the fine-tuning of electronic properties.

We present a novel type of "rod-coil" graft copolymer containing a polyphenylene backbone linked with poly(ethylene oxide) (PEO) side chains. Such graft copolymers manifest unprecedented temperature-dependent one-dimensional (1D) and two-dimensional (2D) self-assembly in solution. At 20 °C, which is higher than the crystallization temperature (Tc) of the PEO chains, the achiral graft copolymers self-organize into nanoribbons that twist into ∼30 μm ultralong helices with controlled pitch depending on the grafting ratio of the PEO chains. At 10 °C, which is lower than the Tc, quadrangular multilayer sheets of over 10 μm in lateral size are obtained. To our knowledge, this work presents the first example of controlled self-assembly of graft polymers into 1D helix and 2D sheet superstructures.

The effect of edge engineering of graphene nanoribbons (GNRs) on their ultrafast photoconductivity is investigated. Three different GNRs were fabricated by bottom-up synthesis in the liquid phase, where structure, width, and edge planarity could be controlled chemically at the atomic level. The charge carrier transport in the fabricated GNRs was studied on the ultrafast, sub-picosecond time scale using time-resolved terahertz spectroscopy, giving access to the elementary parameters of carrier conduction. While the variation of the side chains does not alter the photoconductive properties of GNRs, the edge structure has a strong impact on the carrier mobility in GNRs by affecting the carrier momentum scattering rate. Calculations of the ribbon electronic structure and theoretical transport studies show that phonon scattering plays a significant role in microscopic conduction in GNRs with different edge structures. A comparison between theory and experiment indicates that the mean free path of charge carriers in the nanoribbons amounts to typically ∼20 nm.

Graphene nanoribbons synthesized by the bottom-up approach with optical energy gaps in the visible are investigated by means of optical spectroscopy. The optical absorption and fluorescence spectra of two graphene nanoribbons with different structures are reported as well as the life-time of the excited states. The possibility of the formation of excimer states in stacks of individual graphene nanoribbons is discussed in order to interpret the broad and highly Stokes-shifted luminescence lines observed on both structures. Finally, combined atomic force microscopy and confocal fluorescence measurements have been performed on small aggregates, showing the ability of graphene nanoribbons to emit light in the solid state. These observations open interesting perspectives for the use of graphene nanoribbons as near-infrared emitters.

Here we present a combined experimental and theoretical study of graphene nanoribbons (GNRs), where detailed multi-wavelength Raman measurements are integrated by accurate ab initio simulations. Our study covers several ultra-narrow GNRs, obtained by means of solution-based bottom-up synthetic approach, allowing to rationalize the effect of edge morphology, position and type of functional groups as well as the length on the GNR Raman spectrum. We show that the low-energy region, especially in presence of bulky functional groups is populated by several modes, and a single radial breathing-like mode cannot be identified. In the Raman optical region, we find that, except for the fully-brominated case, all GNRs functionalized at the edges with different side groups show a characteristic dispersion of the D peak (8-22 cm −1 /eV). This has been attributed to the internal degrees of freedom of these functional groups, which act as dispersion-activating defects. The G peak shows small to negligible dispersion in most of the cases, with larger values only in presence of poor control of the edges functionalization, exceeding the values reported for highly defected graphene. In conclusion, we have shown that the characteristic dispersion of the G and D peaks offer further insight on the GNR structure and functionalization, by making Raman spectroscopy an important tool for the characterization of GNRs. PACS numbers: 73.22.Pr, 78.30.Ly, 71.15.Mb

We report a detailed investigation by Raman spectroscopy of the holey nanographene C216, a hexagon-shaped disk with armchair edges (1.4 nm long), from which the central aromatic ring is missing. Density functional theory (DFT) calculations allow to assign the main features of the Raman spectra that have been recorded with several excitation wavelengths. In the Raman spectra, we observe signatures of the hole in the structure, several G and D modes, as well as their overtones and combinations-up to third order.