Structural changes at interfaces

The liquid crystal phase is a distinct state of matter between solid (crystalline) and liquid states. All liquids exhibit anisotropy at interfaces such as surfaces where the molecules take on a positional and orientational ordering not normally present. With liquid crystals, this is even more pronounced. Thus, liquid crystals are ideal systems with which to study surface or substrate effects. Further, liquid crystals are extensively used as organic semiconductors due to their electrical properties and ability to self-assemble into thin films.

Discotic liquid crystals, crystals of disc-shaped molecules, are an interesting sub-class discovered about 30 years ago. Phases generated in discotic liquid crystal thin films in the vicinity of solid surfaces exhibit a three-dimensional (3D) order. Perturbation of their precarious thermodynamic equilibrium leads to a liquid-crystalline bulk phase with two-dimensional (2D) order.

The EU-funded project ‘Substrate-induced phases of discotic liquid crystals’ (DISCO) was designed to characterise the structure and thermodynamic properties governing these phases. The researchers chose a model discotic liquid crystal system given that substrate-induced phases are rarely observed experimentally in discotic liquid crystal systems.

Project scientists studied structure and structural changes associated with substrate-induced phases of discotic liquid crystal thin films using X-ray diffraction and atomic force microscopy. They identified a 3D columnar tetragonal crystal plastic phase and showed that its formation and morphology is independent of the thickness of the films but dependent on time. In unprecedented results, the 2D liquid-crystalline phase converted to a 3D crystal plastic phase because of heterogeneous nucleation events initiated by the solid substrate over a time scale of a month or longer.

Heterogeneous nucleation involving the phase transformation between any two phases at sites such as phase boundaries or surfaces is much more common than homogeneous nucleation. It plays a role in production of many industrially relevant materials including pharmaceuticals, food additives, metal alloys and organic electronics. DISCO outcomes and the team's continued research into the effects of substrate-induced phases on materials' properties will thus have far-reaching impact on design and production of novel compounds.