Nanoscale assembly into extended and continuous structures and hybrid materials

Harnessing synthetic power to create novel materials with unique assembly properties is of fundamental interest for pushing the limits of molecular and macromolecular assembly, while further holding the potential for improving electronic and medical device performance. Controlling the assembly of aromatic molecules into nanostructures provides routes towards continuous and extended structures with performance that is improved markedly over that of their individual molecular components, opening possibilities for the formation of composites that have tailored interactions with other materials and thus improved applications. This review covers recent advances in the synthesis of conjugated materials, their controlled assembly into nanoscale architectures and implementation into devices. Specifically, we discuss the solution-based assembly of polythiophene into nanowire fibrils, the formation of columnar stacks of hexabenzocoronene liquid crystals and the preparation of functionalized solution processible graphene for nanocomposite formation. When polymers are mentioned, it is ‘plastics’ that typically spring to mind. Yet the assembly of conjugated polymers or aromatic molecules into hierarchical and hybrid structures also brings about materials that are attractive for electronic and medical devices. Emily Pentzer and Todd Emrick from the University of Massachusetts review recent advances into the preparation of well-defined molecular building blocks, their assembly into complex architectures, and the properties emerging from these arrangements. This approach is illustrated by the variety of assemblies featuring conjugated polythiophene chains, such as nanowires, fibrils that can further aggregate into ribbons, or composites with quantum dots. These higher-order structures display a variety of electronic properties, useful in semiconductor and photovoltaic applications among others. Another example of the potential of this approach is the construction of carbon nanotubes or graphene flakes from aromatic molecules. Pentzer and Emrick also highlight the remaining challenges that relate to controlling the structures obtained, their stability, and their integration into devices. Poly(3-hexylthiophene) nanowires, microns in length and nanometers in diameter, are stably suspended in solution, and crystallize by adjusting solvent strength. Cocrystalization of P3HT with P3HT-covered CdSe nanorods gave hybrid nanostructures in which the p-type P3HT fibrils were flanked by the n-type CdSe nanorods. The close proximity and arrangement of the two materials provides direct, continuous pathways suitable for charge transport in photovoltaic devices.