DISCO · Substrate-induced phases of discotic liquid crystals

Substrate-induced phases of discotic liquid crystals

The general objective of the DISCO project is to characterize the structure and the thermodynamic parameters governing substrate-induced phases in discotic liquid crystals. The presence of substrate induced phases in thin films is an intriguing phenomenon with the physical and chemical factors responsible for its formation are not yet clearly understood. The knowledge of such interfacial structural information is of utmost interest as charge transport properties are governed by the molecular orientation and crystal packing near the interface. Liquid crystals with its unique properties like anchoring transitions, orientational wetting, and capillary condensation provide a favourable situation to study such phenomenon. Moreover, liquid crystals are extensively used as organic semiconductors because of their electronic properties and their ability to self-assemble into single domain thin films. In this context a model discotic liquid crystal, Pc (see http://www.tandfonline.com/doi/abs/10.1080/02678292.2013.809799) was used in our study. Although Substrate-induced phases are well known within organic thin films and generally observed for rod-shaped mesogens forming nematic and smectic mesophases, they are very rarely observed in case of discotics. The substrate-induced phases observed in the model discotic system exhibit a three-dimensional order whereas the bulk ones are liquid crystalline with a two-dimensional order. The structure and morphological changes associated with a substrate induced phase thin films of Pc have been extensively studied using specular X-ray Diffraction, Atomic Force Microscopy and Grazing Incidence X-ray Diffraction. The experiments were carried out utilizing in-house facilities and in large experimental centres like HASYLAB, Hamburg and ESRF, Grenoble. The structure of the substrate induced phase was established to be a 3-dimensional columnar tetragonal crystal plastic phase. The morphological changes associated with the appearance of the substrate-induced phase have been recorded using Polarized Optical Microscopy study. The behaviour of the substrate-induced phase was thoroughly studied with the evolution of time, temperature and changes at the interfaces. The results indicate that the substrate induced phase forms independent of the thickness of the films and the nature of the substrate, but it is dependent on the time for the Pc-films are aged. This structural phase transformation is attributed to the heterogeneous nucleation events initiated by the solid substrate. This is an unprecendented and unique result where the two-dimensional liquid-crystalline phase converts to a three-dimensional crystal plastic phase because of nucleation caused by the solid substrate over a time scale of a month or longer. The expected scientific outcome of this project is a fundamental understanding of the physical and chemical parameters that governs the formation of substrate-induced phases for discotic liquid crystals. The results indicates that the bulk liquid crystalline phase can be considered as intrinsically metastable with respect to the substrate induced phase, and the growth of the latter being kinetically slow with a nucleation process promoted by the presence of solid interfaces. The substrate induced phase can be viewed as a kinetically hindered phase, whose appearance and growth is a consequence of the geometrical effect imposed by the rigid and impenetrable flat substrate. The phase transition observed here is an intrinsic material property and no amount of surface engineering can affect its formation. The results establish a direct correlation between substrate induced phase and heterogeneous nucleation. This unprecedented result can have a broad scientific impact considering the role of heterogeneous nucleation on several industrial processes encompassing pharmaceutical compounds, pigments, food additives, organic electronics, etc. The long term objective will be to understand how substrate-induced phases influence: charge transport in transistors, liquid crystal alignment mechanism etc.

Data: CORDIS, © European Union