Tuning the deposition of molecular graphene nanoribbons by surface functionalization

"Clickable" graphene nanoribbons for biosensor interfaces

We report on the synthesis of "clickable" graphene nanoribbons (GNRs) and their application as a versatile interface for electrochemical biosensors. GNRs are successfully deposited on gold-coated working electrodes and serve as a platform for the covalent anchoring of a bioreceptor (i.e., a DNA aptamer), enabling selective and sensitive detection of Interleukin 6 (IL6). Moreover, when applied as the intermediate linker on reduced graphene oxide (rGO)-based field-effect transistors (FETs), the GNRs provide improved robustness compared to conventional aromatic bi-functional linker molecules. GNRs enable an orthogonal and covalent attachment of a recognition unit with a considerably higher probe density than previously established methods. Interestingly, we demonstrate that GNRs introduce photoluminescence (PL) when applied to rGO-based FETs, paving the way toward the simultaneous optical and electronic probing of the attached biointerface.

Biocompatible Graphene Oxide Nanosheets Densely Functionalized with Biologically Active Molecules for Biosensing Applications

Graphene oxide (GO) has immense potential for widespread use in diverse in vitro and in vivo biomedical applications owing to its thermal and chemical resistance, excellent electrical properties and solubility, and high surface-to-volume ratio. However, development of GO-based biological nanocomposites and biosensors has been hampered by its poor intrinsic biocompatibility and difficult covalent biofunctionalization across its lattice. Many studies exploit the strategy of chemically modifying GO by noncovalent and reversible attachment of (bio)molecules or sole covalent biofunctionalization of residual moieties at the lattice edges, resulting in a low coating coverage and a largely bioincompatible composite. Here, we address these problems and present a facile yet powerful method for the covalent biofunctionalization of GO using colamine (CA) and the poly(ethylene glycol) cross-linker that results in a vast improvement in the biomolecular coating density and heterogeneity across the entire GO lattice. We further demonstrate that our biofunctionalized GO with CA as the cross-linker provides superior nonspecific biomolecule adhesion suppression with increased biomarker detection sensitivity in a DNA-biosensing assay compared to the (3-aminopropyl)triethoxysilane cross-linker. Our optimized biofunctionalization method will aid the development of GO-based in situ applications including biosensors, tissue nanocomposites, and drug carriers.

Spiers Memorial Lecture. Carbon nanostructures by macromolecular design - from branched polyphenylenes to nanographenes and graphene nanoribbons

Nanographenes (NGs) and graphene nanoribbons (GNRs) are unique connectors between the domains of 1D-conjugated polymers and 2D-graphenes. They can be synthesized with high precision by oxidative flattening processes from dendritic or branched 3D-polyphenylene precursors. Their size, shape and edge type enable not only accurate control of classical (opto)electronic properties, but also access to unprecedented high-spin structures and exotic quantum states. NGs and GNRs serve as active components of devices such as field-effect transistors and as ideal objects for nanoscience. This field of research includes their synthesis after the deposition of suitable monomers on surfaces. An additional advantage of this novel concept is in situ monitoring of the reactions by scanning tunnelling microscopy and electronic characterization of the products by scanning tunnelling spectroscopy.

Graphene Nanoribbons: On-Surface Synthesis and Integration into Electronic Devices

Graphene nanoribbons (GNRs) are quasi-1D graphene strips, which have attracted attention as a novel class of semiconducting materials for various applications in electronics and optoelectronics. GNRs exhibit unique electronic and optical properties, which sensitively depend on their chemical structures, especially the width and edge configuration. Therefore, precision synthesis of GNRs with chemically defined structures is crucial for their fundamental studies as well as device applications. In contrast to top-down methods, bottom-up chemical synthesis using tailor-made molecular precursors can achieve atomically precise GNRs. Here, the synthesis of GNRs on metal surfaces under ultrahigh vacuum (UHV) and chemical vapor deposition (CVD) conditions is the main focus, and the recent progress in the field is summarized. The UHV method leads to successful unambiguous visualization of atomically precise structures of various GNRs with different edge configurations. The CVD protocol, in contrast, achieves simpler and industry-viable fabrication of GNRs, allowing for the scale up and efficient integration of the as-grown GNRs into devices. The recent updates in device studies are also addressed using GNRs synthesized by both the UHV method and CVD, mainly for transistor applications. Furthermore, views on the next steps and challenges in the field of on-surface synthesized GNRs are provided.

Atomistic Modelling of Size-Dependent Mechanical Properties and Fracture of Pristine and Defective Cove-Edged Graphene Nanoribbons

Cove-edged graphene nanoribbons (CGNR) are a class of nanoribbons with asymmetric edges composed of alternating hexagons and have remarkable electronic properties. Although CGNRs have attractive size-dependent electronic properties their mechanical properties have not been well understood. In practical applications, the mechanical properties such as tensile strength, ductility and fracture toughness play an important role, especially during device fabrication and operation. This work aims to fill a gap in the understanding of the mechanical behaviour of CGNRs by studying the edge and size effects on the mechanical response by using molecular dynamic simulations. Pristine graphene structures are rarely found in applications. Therefore, this study also examines the effects of topological defects on the mechanical behaviour of CGNR. Ductility and fracture patterns of CGNR with divacancy and topological defects are studied. The results reveal that the CGNR become stronger and slightly more ductile as the width increases in contrast to normal zigzag GNR. Furthermore, the mechanical response of defective CGNRs show complex dependency on the defect configuration and distribution, while the direction of the fracture propagation has a complex dependency on the defect configuration and position. The results also confirm the possibility of topological design of graphene to tailor properties through the manipulation of defect types, orientation, and density and defect networks.

From spin-labelled fused polyaromatic compounds to magnetically active graphene nanostructures

Molecular design of magnetically active graphene nanoscale structures is an emerging field of research. The key goal of this research is to produce graphene nanoribbons and graphene quantum dots with specified electronic, optical and magnetic properties. The review considers methods for the synthesis of spin-labelled polycyclic aromatic hydrocarbons, which are homologous precursors of graphene nanostructures, and discusses the advances and prospects of the design of magnetically active graphene materials.

The bibliography includes 134 references.

Backbone Chemical Composition and Monomer Sequence Effects on Phenylene Polymer Persistence Lengths

Despite a vast body of the literature devoted to the use of phenylene polymers in the fabrication of graphene nanoribbons, the study of the physical properties of these precursors still poses open questions whose answers will certainly contribute to the design of more efficient/precise synthesis protocols. Particularly, persistence length measurements combined with size exclusion chromatography techniques assign both semiflexible to semirigid structures depending on the molecular weight of the precursor (NaritaNat. Chem.2014, 6, 126-132). Peculiarly, these results suggest an apparent structural change upon increasing the length of the polymers. To address this puzzle, we use single-chain models to study the stiffness of polyphenylene precursors in a theta-like solvent as a function of chain composition and monomer sequence. Steric effects are isolated by considering random walk chains with segment length distributions and the position of monomers determined by the nature of the arene substitution along the backbone. Moreover, two homopolymer limiting cases are defined, that is, meta and para sequences, by associating two types of monomers to each possible substitution pattern. We consider, within these two limiting cases, chains with different compositions and monomer sequences. We compute persistence lengths, mean square end-to-end distances, and gyration and hydrodynamic radii. We find that distinct values of the persistence length for apparently the same chain chemistry are the result of different mixing ratios and the arrangement along the chain of the two positional isomers of the same monomer. Finally, we discuss the relation between two-dimensional density of the number of crossings and the length of polyphenylene segments as they would occur upon strong chain adsorption onto a substrate.

Spin polarization in graphene nanoribbons functionalized with nitroxide

Fine-tuning of magnetic states via an understanding of spin injection on the edge of graphene nanoribbons should allow for greater flexibility of the design of graphene-based spintronics. On the basis of calculations, we predict that coupling constants of the exchange interaction in the series of nitroxide-functionalized ribbon compounds are antiferromagnetic across the ribbons with values 0.2-0.4 cm^-1 and ferromagnetic along the ribbon with absolute values from 0.05 to 0.07 cm^-1. Such interacting nitroxide groups induce spin polarization of the edge states of stable graphene nanoribbons. Graphical abstract Exchange coupling constants inducing spin polarization in graphene nanoribbons functionalized with nitroxides.

Solution and on-surface synthesis of structurally defined graphene nanoribbons as a new family of semiconductors

Graphene nanoribbons (GNRs) are quasi-one-dimensional subunits of graphene and have open bandgaps in contrast to the zero-bandgap graphene. The high potential of GNRs as a new family of carbon-based semiconductors, e.g. for nanoelectronic and optoelectronic applications, has boosted the research attempts towards fabrication of GNRs. The predominant top-down methods such as lithographical patterning of graphene and unzipping of carbon nanotubes cannot prevent defect formation. In contrast, bottom-up chemical synthesis, starting from tailor-made molecular precursors, can achieve atomically precise GNRs. In this account, we summarize our recent research progress in the bottom-up synthesis of GNRs through three different methods, namely (1) in solution, (2) on-surface under ultrahigh vacuum (UHV) conditions, and (3) on-surface through chemical vapour deposition (CVD). The solution synthesis allows fabrication of long (>600 nm) and liquid-phase-processable GNRs that can also be functionalized at the edges. On the other hand, the on-surface synthesis under UHV enables formation of zigzag GNRs and in situ visualization of their chemical structures by atomic-resolution scanning probe microscopy. While the on-surface synthesis under UHV is typically costly and has limited scalability, the industrially viable CVD method can allow lower-cost production of large GNR films. We compare the three methods in terms of the affordable GNR structures and the resulting control of their electronic and optical properties together with post-processing for device integration. Further, we provide our views on future perspectives in the field of bottom-up GNRs.

Laterally extended atomically precise graphene nanoribbons with improved electrical conductivity for efficient gas sensing

Narrow atomically precise graphene nanoribbons hold great promise for electronic and optoelectronic applications, but the previously demonstrated nanoribbon-based devices typically suffer from low currents and mobilities. In this study, we explored the idea of lateral extension of graphene nanoribbons for improving their electrical conductivity. We started with a conventional chevron graphene nanoribbon, and designed its laterally extended variant. We synthesized these new graphene nanoribbons in solution and found that the lateral extension results in decrease of their electronic bandgap and improvement in the electrical conductivity of nanoribbon-based thin films. These films were employed in gas sensors and an electronic nose system, which showed improved responsivities to low molecular weight alcohols compared to similar sensors based on benchmark graphitic materials, such as graphene and reduced graphene oxide, and a reliable analyte recognition. This study shows the methodology for designing new atomically precise graphene nanoribbons with improved properties, their bottom-up synthesis, characterization, processing and implementation in electronic devices.

Atomically precise graphene nanoribbons are a promising platform for tailored electron transport, yet they suffer from low conductivity. Here, the authors devise a strategy to laterally extend conventional chevron nanoribbons, thus achieving increased electrical conductivity and improved chemical sensing capabilities.

Solution Synthesis of Atomically Precise Graphene Nanoribbons

Bottom-up fabrication of narrow strips of graphene, also known as graphene nanoribbons or GNRs, is an attractive way to open a bandgap in semimetallic graphene. In this chapter, we review recent progress in solution-based synthesis of GNRs with atomically precise structures. We discuss a variety of atomically precise GNRs and highlight theoretical and practical aspects of their structural design and solution synthesis. These GNRs are typically synthesized through a polymerization of rationally designed molecular precursors followed by a planarization through a cyclodehydrogenation reaction. We discuss various synthetic techniques for polymerization and planarization steps, possible approaches for chemical modification of GNRs, and compare the properties of GNRs that could be achieved by different synthetic methods. We also discuss the importance of the rational design of molecular precursors to avoid isomerization during the synthesis and achieve GNRs that have only one possible structure. Significant attention in this chapter is paid to the methods of material characterization of solution-synthesized GNRs. The chapter is concluded with the discussion of the most significant challenges in the field and the future outlook.

Interfacial Self-Assembly of Atomically Precise Graphene Nanoribbons into Uniform Thin Films for Electronics Applications

Because of their intriguing electronic and optical properties, atomically precise graphene nanoribbons (GNRs) are considered to be promising materials for electronics and photovoltaics. However, significant aggregation and low solubility of GNRs in conventional solvents result in their poor processability for materials characterization and device studies. In this paper, we demonstrate a new fabrication approach for large-scale uniform thin films of nonfunctionalized atomically precise chevron-type GNRs. The method is based on (1) the exceptional solubility of graphitic materials in chlorosulfonic acid and (2) the original interfacial self-assembly approach by which uniform films that are single-GNR (∼2 nm) thick can be routinely prepared. These films can be transferred to various substrates including Si/SiO₂ and used for the streamlined fabrication of arrays of GNR-based devices. The described self-assembly approach should be applicable to other types of solution-synthesized atomically precise GNRs as well as large polyaromatic hydrocarbon (PAH) molecules and therefore should facilitate and streamline their device characterization.

Uniform and perfectly linear current–voltage characteristics of nitrogen-doped armchair graphene nanoribbons for nanowires

Edge nitrogen-doping induces uniform and perfectly linearI–Vcharacteristics in AGNRs for nanowire applications in molectronics.

Raman Fingerprints of Atomically Precise Graphene Nanoribbons

Bottom-up approaches allow the production of ultranarrow and atomically precise graphene nanoribbons (GNRs) with electronic and optical properties controlled by the specific atomic structure. Combining Raman spectroscopy and ab initio simulations, we show that GNR width, edge geometry, and functional groups all influence their Raman spectra. The low-energy spectral region below 1000 cm(-1) is particularly sensitive to edge morphology and functionalization, while the D peak dispersion can be used to uniquely fingerprint the presence of GNRs and differentiates them from other sp(2) carbon nanostructures.

New advances in nanographene chemistry

This review discusses recent advancements in nanographene chemistry, focusing on the bottom-up synthesis of graphene molecules and graphene nanoribbons.