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.
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.
Drops or shells of a planar-aligned short-pitch cholesteric liquid crystal exhibit unique optical properties due to the combination of Bragg reflection in the cholesteric helix and a radial orientation of the helix axis. If such a droplet is illuminated from above, light is reflected into a continuous set of cones, the opening angles of which depend on where on the droplet the light hits its surface. For the wavelength that fulfills the Bragg condition the reflection is dramatically enhanced, yielding the light cones colored. A photonic cross communication scheme arises for certain angles, reflecting light back to the observer from a different droplet than the one originally illuminated. This gives rise to an intricate pattern of colored and circularly polarized spots. A number of interesting applications may be developed based on this pattern, e.g. in identification and authentication devices. We have carried out a detailed spectrophotometric analysis of the patterns, localized to individual spot maxima. A quantitative comparison between the measured spectra and the reflection wavelength expected from a model for the pattern generation allows us to conclude that the droplets are in fact not spherical but slightly ellipsoidal.
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.
Biaxial nematic liquid crystals have attracted much attention from both fundamental and application points of view,
because the fast response based on the rotation of the minor director is expected. So far, different molecular designs have
been proposed for the emergence of the biaxial nematic phase. Among that, we have been interested in applying "preorganization"
concept on generating the biaxiality. Dimeric liquid crystal compounds have been prepared in line with
this concept in which two mesogenic parts are linked by the biphenyl connecting group. The pre-organized dimmer
shows an anomalous textural change, for vertically-aligned and free-standing film samples, at the smectic C (SmC)-
nematic (N) phase transition, in which the Schlieren texture of the SmC changes into the other Schlieren texture of the N
phase. There are two possible explanations for this textural change, i.e., the occurrence of the director change at the
SmC-N phase transition or the emergence of biaxiality in the N phase. The electric-field-induced birefringence has also
been measured in detail for investigating the biaxial nature of the sample.