Graphene nanoribbons are the class of next-generation carbon materials that are attracting many researchers in various research fields. Their unique properties, such as band gap, conductivity, carrier mobility, thermal conductivity, spin polarization, and on−off behavior, heavily depend on structural factors such as edge structure, width, and length. Therefore, the synthesis of graphene nanoribbons with control over these structural factors with atomic precision is crucially important. Among various synthetic approaches, bottom-up synthetic methods such as on-surface polymerizations and solution-phase polymerizations represent promising ways to control the width, length, and edge structure of graphene nanoribbons. In this Perspective, we introduce the recently reported bottom-up synthetic methods for graphene nanoribbons and the theoretical and experimental research on those physical properties and applications. Reviewing these researches along with highlighting advantages and limitations, we emphasize how structurally controlled synthesis is important and provides future outlooks in graphene nanoribbon science.

No abstract

Charged-impurity scattering is a serious problem that hinders the electrical properties of graphene. Toward large-scale and/or flexible graphene-based electronics, there is a strong demand for a high-κ dielectric layer which reduces charged-impurity density and screens the impurity scattering that passivates the graphene. We herein demonstrate that the structurally precise and soluble graphene nanoribbons (GNRs) act as excellent dielectric passivation layers. The wide-gap GNRs, synthesized through annulative π-extension polymerization, were selectively and stably fixed onto graphene via a simple drop-casting method. The carrier mobility of ∼30 nm thick GNR-adlayer-deposited graphene was approximately twice that in its original state. Electrical transport and Raman spectroscopic measurements revealed that the deposition of the GNR dielectric passivation layers reduced the charge puddles. These results suggest that the GNR adlayers prevent graphene from the oxygen/water redox couple adsorption and lift the graphene up from the underlying SiO2 substrate via strong π–π and CH−π interactions. Additionally, the relatively high dielectric constant (∼5.2) of the GNRs contributes to the increased screening effect. All these effects lead to a reduced impurity scattering, which increases carrier mobility.

Graphene nanoribbons (GNRs), nanometer-wide strips of graphene, are attracting significant attention in materials science as candidates for next-generation carbon materials. As their physical properties mainly depend on their structures, the precise synthesis of structurally well-defined GNRs is highly desirable to control their properties. Herein, we report a step-growth annulative π-extension polymerization that allows for the rapid and modular synthesis of cove-type GNRs with pyrene and/or coronene diimide repeating units. The structures and photophysical properties of the separated GNRs were confirmed by various spectroscopic analyses. In addition, gas-blow-assisted uniform on-surface self-assembly of the GNRs was accomplished.

The synthesis, X-ray crystal structure, and photophysical properties of unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene are described. Unlike the previously reported tert-butyl-substituted analogues, unsubstituted hexabenzo[a,c,fg,j,l,op]tetracene showed a helically twisted conformation in the solid state. Density functional theory calculations on the possible conformers were also studied.