Liquid crystals (LCs) from particles and molecular aggregates: Self-organized films from disk-like LCs

Abstract

Self-assembly is a powerful means of creation of molecular or particle organized structures without the intervention of external forces. Liquid crystals are an outstanding example of self-assembled systems exhibiting a large variety of assemblies and well known applications like flat panel displays. Liquid crystal is the phase that combines fluidity of isotropic liquids with macroscopic anisotropy in the properties like solid crystals due to the long-range orientational order. Materials forming liquid crystals phases are grouped into two main classes: thermotropic and lyotropic liquid crystals. Thermotropic liquid crystals are formed by single or multiple-component molecular systems that have liquid crystal phases within certain temperature range. Instead lyotropics can be formed by a variety of building blocks in aqueous solutions above a certain concentration threshold. For both classes, the building blocks are anisometric with typically rod- or disk-like shape and different nature, even formed by nanoparticles of graphene or cellulose. The self-assembly behavior of these systems is not trivial to understand but it is relevant for profiting from their properties, and find their use in various geometries with attractive optical or electrical properties. In the framework of exploring liquid crystals and nanoparticle systems, this work mainly deals with liquid crystal systems formed by disk-like building blocks, more precisely, hexapentyloxytriphenylene (HAT5) discotic liquid crystal molecules and graphene oxide or reduced graphene flakes. However, since the assembly of HAT5 goes through the formation of rod-like aggregates in solvents, also the self-assembly of elongated nanoparticles in solvent and during its evaporation has an attention here. Discotic liquid crystals (LCs) are formed by disk-like molecules that can form columnar hexagonal phases. They are attractive materials due to the 1-dimensional electrical conductivity along the columns as a result of π-π interaction between polycyclic aromatic cores. A key advantage of discotics is their self-organization into ordered molecular wires on macroscopic scale with the ability of annealing of defects by temperature. We investigate the formation of fiber-like structures of discotics deposited from solution, focusing on the factors that influence the wire assembly. In particular, the resulting molecular structures are strongly dependent on the evaporation rate of the solvent as well as the nature of the substrates. Moreover, the molecular structure of the solvent plays an important role in the structural formation influencing the morphology of the final assembly and presumably the molecular stacking. Aromatic solvents favor the formation of wires that in turn also self-organize assuming a common alignment direction. We argue that this process is driven by a lyonematic formation during the evaporation process. Our findings provide an attractive route for tailoring or changing the geometry of supramolecular assemblies not necessarily by changing the discotic molecular structure but by simply choosing a solvent with a desired structure. We use various deposition methods based on solution for the realization of molecular wire thin films and their morphology was investigated by atomic force microscopy (AFM) and polarized optical microscopy, for optically detectable films. Temperature can induce changes in of the structure even in very thin films, with thicknesses of few tents of nanometers, as monitored locally by AFM. Glazing incidence X-ray diffraction techniques confirmed that the alignment in our thin films is planar, which means that the discotic molecules have an edge-on position on the substrates, and columns with a lattice organization perpendicular to the substrates. Planar alignment was also confirmed by polarized Raman spectroscopy. We could also observe very strong anisotropic response, reflecting the much higher polarizability along the molecular wire axis compared to the perpendicular direction as a result of a good intra-columnar molecular overlap. The advantage of a structure formed by long and well aligned wires was confirmed by electrical measurements on isotropic versus macroscopically aligned samples, the latter showing a three time increase in electrical conductivity. The self-assembly ability and the promising conductive properties are attractive per se but also in view of the integration of these self-assembled structures with nanoparticles and nanowires in particular. The second liquid crystal system that was studied was based on graphene flakes in solvent. Graphene, a monolayer of graphite, is an attractive material for a variety of applications such as in electronic devices either as transparent and conductive electrodes or in sensors. Graphene can be produced with different methods such as mechanical exfoliation, chemical vapor deposition or chemical methods. The latter is interesting for the ease of processability and its versatility. This process goes through the formation of an oxidative form of graphene, called graphene oxide (GO). One of the greatest advantages of GO flakes is the dispersability in water due to the hydrophilic functional groups. In addition, GO can form spontaneously organized phases of discotic liquid crystal type, useful for creating long-range orientational order that is advantageous for optimizing charge paths in devices. The liquid crystal phase appears above a certain concentration of the GO flakes, and the threshold concentration is affected by several specific properties of graphene oxide sheets like their aspect ratio or flatness. Studying a series of GO suspensions from 1.0 mg/ml to 0.1 mg/ml, a clear nematic liquid crystal phase, phase exhibiting a certain degree of long-range orientational order but not positional, could be observed by polarized optical investigations at very low concentrations. The existence of a very low threshold, below 0.25 wt%, can be explained with the presence of ultra-large flakes of GO in our samples and to the relatively low amount of defects in the flakes which gives them flatness. We report the evaluation of GO concentration, especially the region of phase coexistence related to the size distribution of the flakes, using UV-visible spectroscopy, by taking advantage of the Lambert & Beer law. Unidirectional alignment of GO flakes can be induced by external electric fields from isotropic dispersions showing a very large Kerr coefficient. This behavior shows attraction to electro-optical switching devices such as a liquid crystal displays (LCDs). Since the electro-optic response is dependent on the particle polarizabiliy, the reduced GO having higher polarizability than GO is a very attractive system for improved electro-optic response. However, it is difficult to produce liquid crystalline phases with reduced GO (rGO) due to their poor dispersability in water and the consequent tendency to aggregate. Herein we induce suggest the LC phase in rGO by reducing directly GO suspension with L-ascorbic acid after pretreatment with surfactants. The samples were studied by dynamic light scattering and the birefringence of both GO and rGO suspensions was studied as a function of applied electric field.