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 efficiency of the conduction of photocurrent in discotic liquid crystals is known to depend on the quality of the
columnar organization. Solvents have shown to be able to influence the formation of wire structures on substrates
promoting very long and ordered wired formations or bulkier structures depending on the affinity of the solvent with
parts of the molecular structure of discotics. Here we present a study on the effect of solvents when the liquid crystal is
confined between two substrates with the columns running perpendicular to them, geometry used in solar cells. We
focused on toluene and dodecane, solvents that have shown to promote on substrates the formation of aligned and long
nanowires and bulk large and isolated fibers, respectively. The phase transition behavior indicates that toluene does not
interfere with the columnar formation while dodecane strongly influence increasing the disorder in the structure.
Liquid crystals (LCs) are very attractive hosts for the organization of anisotropic nanoparticles such as carbon nanotubes
(CNTs) because of the macroscopic organization resulting in properties of nanoparticles manifest at a macroscopic scale.
Different types of LCs have demonstrated the ability to organize nanotubes, showing the generality of the approach, i.e.,
that the liquid crystallinity per se is the driving factor for the organization. Compared to standard nanotube composites
(e.g. with disordered polymer hosts) the introduction of carbon nanotubes into an LC allows not only the transfer of the
outstanding CNT properties to the macroscopic phase, providing strength and conductivity, but these properties also
become anisotropic, following the transfer of the orientational order from the LC to the CNTs. The LC molecular
structure plays an important even if ancillary role since it enters in the surface interactions, fulfilling a mediating action
between the particle and the bulk of the LC. Isolated nanotubes can be obtained by optimized dispersions at lower
concentrations and this process requires the use or development of tailored strategies like using solvents or even another
LC for pre-dispersing CNTs. Aggregates or networks can be observed in poor dispersions and at higher nanoparticle
concentrations. In those, due to surface interactions, the LC behaviour can be strongly affected with changes in phase
sequences or transition temperatures and the effect is expected to be more pronounced as the concentration of nanotubes
increases. We present preliminary investigations and observations on nanotube – LC systems based on a smectic LC
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.
We have visualized the internal structure of electrospun polymer fibers, having liquid crystals in the core, using focused ion beam milling. In this way we were able to correlate observed selective reflection and optical texture, in a specific fiber location, with the corresponding cavity dimensions and shape. It was found that cholesteric liquid crystals exhibit peculiar optical behavior, distinctively different from the one in bulk, when they are confined in sub-micrometer cavities. Because of the reduced dimensions, the pitch of the helix has to change even for tiny variations in cavity size, resulting in changes in the wavelength of the selective reflection. The ion beam milling is a destructive process and it is relevant to consider possible side effects and consequences on the polymer sheath and thus on the revealed cavities. We analyze the heating due to the ion beam exposure calculating the subsequent temperature increase in the polymer and at a polymer-liquid crystal interface. The derived increase of temperature is very small and is not expected to induce any notable change in the polymer cavities.
Lyotropic Liquid Crystals (LCs) are attractive materials as host systems for nanoparticles, in particular for carbon nanotubes (CNTs), due to the LC templating and dispersing action. Since carbon nanotubes have many remarkable properties their presence could also influence the aligning hosts and such effects need to be taken into account in CNTLC composites. CNTs can be dispersed efficiently in surfactant-based lyotropic hosts that can be removed after their templating action, being water based. However, residual surfactant has a detrimental effect on the nanotube properties and it becomes important to find ways to minimize its amount in CNT composites. In the present work we use, for CNT alignment, a lyotropic nematic LC host with a very low surfactant concentration, based on charge combination of cationic and anionic surfactant molecules. Small variations in the molar ratio of the two surfactants, still at a fixed total surfactant amount, yield a very different LC behavior. CNTs could be successfully dispersed in the host forming an overall low-surfactant composite. Interestingly, the presence of nanotubes strongly influences the behavior of the host, bringing a stabilization of the LC phase.
A guided light wave can be modulated by coupling it into a ferroelectric liquid crystal (FLC) waveguide. The FLC, which changes its optical properties under the application of an electric field, acts as active medium. With a carefully chosen aligned configuration, the large change in refractive index induced by the electric field, gives a good contrast ratio in the optical response. The optical and electro- optical properties of liquid crystals can be modified by the propagating light and these effects can be enhanced by the addition of a suitable absorbing dye in the liquid crystal. Thus changes in the guiding conditions of the light can be induced also by the guided light itself.