Graphene, a monoatomically thick film made by carbon atoms arranged in honeycomb lattice, for its exceptional electrical, thermal and mechanical properties is one of the most attractive materials to be incorporated in electronic devices and in composites. New interest has been recently arisen from water suspensions of flakes of graphene oxide (GO), obtained from chemical exfoliation of graphite, since they form liquid crystal (LC) phases, for the easiness of handling graphene, otherwise forming aggregates, and their high yield. Interestingly, GO LC suspensions are responsive to electric fields with an extremely high Kerr coefficient resulting in an induced birefringence at macroscopic scale, achieved with very low electric fields. The LC phase formation and its responsiveness to electric fields are dependent on suspension characteristics such as flake average dimension, aqueous matrix and flake properties. In particular, bare graphene flakes have larger response to electric fields, due to their higher polarizability, than GO flakes. As it will be described, this results in improved electro-optic performance of reduced-graphene compared to GO LC with remarkably higher optical transmission for the same field strength thanks to a more efficient flake reorientation enabling a larger optical modulation.
The self-organization of discotic liquid crystal molecules in columns has enormous interest for soft nanoelectronic applications. A great advantage of discotic liquid crystal is that defects can be self-annealed in contrast to typical organic materials. Through the overlap of molecular orbitals, the aromatic cores assemble into long range ordered one-dimensional structures. Very thin structured films can be obtained by spin-coating from solution and the resulting morphologies are strongly dependent on the interaction between discotics and solvent molecules. Toluene produces films formed by very long nanowires, spontaneously aligned along a common direction and over fairly large areas. These nanostructured films are a result of the interplay between liquid crystal self-organization and solvent driven assembly. The ordered nanowire structures exhibit improvement in the electrical properties compared to misaligned structures and even to pristine HAT5, deposited without the aid of solvent. In this study we show that the toluene-based deposition of discotic liquid crystals is advantageous because it allows a uniform coverage of the substrate, unlike pristine HAT5 but also thanks to the type of induced structures exhibiting one order of magnitude higher conductivity, in the aligned nanowire films, compared to bare HAT5 ones.
Discotic liquid crystal (LC) can arrange in columnar structures along which electrical conduction occurs via π-π interaction between adjacent molecular cores. The efficiency of the conductivity is strongly dependent on the overlap of the orbitals of neighbor molecules and, in general, on the structural arrangements. The understanding of the factors that influence the organization is crucial for the optimization of the final conductive properties of the self-assembled columns. In this paper we present a study on the self-organization into molecular wires of a discotic LC using a solution based method. In particular, we focus on the effect of solvents used for preparing the LC solution. The resulting morphologies were investigated by atomic force microscopy (AFM) and optical microscopy, showing that diverse structures result from different solvents. With suitable conditions, we were able to induce very long fibers, with several tents of micrometer in length that, in turn, self-organize assuming a common orientation on a macroscopic scale.