Ferroelectricity and the nematic liquid crystal phase

The nematic phase (N) is the least ordered liquid crystal phase, and in which the long axes of the rod-like molecules are more or less aligned in the same direction, known as the director, whereas their centres of mass are randomly distributed. This phase is easily replicated by throwing a handful of matches into a box and shaking it. Providing there are enough matches, then, for packing reasons, they will all line-up in the same direction and effectively you have a nematic phase. Providing the matches were thrown into the box randomly, there will be an equal number of matches pointing with their heads in one direction as in the other. This is exactly the case for the conventional nematic phase, and the molecules are equally likely to be pointing in either direction along the director, and the phase is described as being non-polar. The conventional N phase underpins liquid crystal display technology which has a market value predicted to grow to almost $200 billion by 2025. Over 100 years ago, it was first suggested that a nematic phase could exist in which all the molecules could align in the same direction. This is the molecular equivalent of taking the matches, throwing them into the box, shaking it, and discovering that all the matches now lay with their heads pointing in the same direction. This is known as polar ordering and the phase is called the ferroelectric nematic (NF) phase. Very recently a new nematic phase was discovered having remarkable properties, and it has been suggested that this is the long sought after NF phase. This has the potential to be a hugely significant discovery from both fundamental and technological viewpoints. The polar ordering in the NF phase makes it vastly more sensitive to an electric field than the conventional N phase, and this will dramatically improve the performance of liquid crystal display devices in terms of both speed and power consumption. In addition, the study of this new phase has the potential of generating transformative new fundamental chemistry, physics and biology. For example, it was predicted over forty years ago that the NF phase, in order to reduce electrostatic energy, will twist giving a polar cholesteric phase, the spontaneous chirality being controlled through steric and electrostatic interactions between achiral molecules. Such an observation could have huge significance in understanding the origins of chirality. It has been proposed that on cooling the conventional N phase into the NF phase, the molecular dipoles will align spontaneously in a single direction. At this point there is a strong tendency towards crystallisation. If this can be suppressed, however, equal numbers of domains having opposite polarisations should form, separated by domain walls. The application of an electric field will remove this degeneracy and domains having favourable polarity will grow and unfavourable will shrink. The aim of this programme is to begin to understand what molecular features are required to observe the NF phase. Some time ago computer simulations suggested that an asymmetric or tapered shape combined with a longitudinal dipole moment promote polar order, and the very early experimental data available support this view. To achieve our aim, we will need to enhance our understanding of how to manipulate liquid crystallinity though molecular electrostatic and steric interactions. This programme has the very real potential to deliver materials that will lead to transformative new fundamental chemistry, physics and biology, and new technologies including the next generation of display devices.