Plastic crystals open up possibilities for novel materials in colour monitor screens based on electronic ink

Plastic crystals, which constitute a special type of matter, can be made with the help of rod-shaped particles – a discovery made by researchers of the Dutch FOM Foundation. The crystals could be used, for example, in colour monitor screens based on electronic ink. In the article, FOM PhD Thijs Besseling and FOM postdoc Dr. Bing Liu together with their colleagues from Utrecht University show how they produced plastic crystals with the help of rod-shaped particles. Liu and Besseling carried out their research under the supervision of FOM workgroup leader prof. Dr. Alfons van Blaaderen and Dr. Arnout Imhof. Plastic crystals are related to the better-known liquid crystals. In plastic crystals elongated molecules are arranged in a regular three-dimensional lattice, as is the case in a solid substance. However, unlike solid particles, these particles are free to rotate just like in a liquid. That is because the particles are electrically charged as a result of which they repel each other. That creates just enough freedom of movement to rotate. The outcome is a special aggregative state that lies in between the liquid and solid phases. The substance is a solid but at the same time so motile that some plastic crystals collapse under their own weight. The FOM researchers have now created such a phase for the first time using colloids, particles with a size of between one and a thousand nanometres (a nanometre is one billionth of a metre). Their work makes it possible to quantitatively investigate the special phase of the plastic crystals at the particle level. Their research led to the creation of a plastic glass phase. This is a phase in which the particles can still rotate but are not arranged in a regular lattice. Nevertheless, the material behaves as a solid substance. The researchers discovered that an external electric field can be used to convert this phase into a 3D crystal in which a strict order does exist. First of all the electric field ensures that the free rotations disappear. An unexpected consequence of this is that the rods assume a regular 3D lattice as well. This process is reversible: as soon as the electrical field disappears the plastic glass phase returns. The discovery opens up new possibilities for investigating the glass transition. That could provide interesting new insights, as the glass phase is still poorly understood. The Utrecht group has already started on the first steps of a study that must yield a better understanding of this phase.


A two-dimensional confocal microscopy image of a three-dimensional plastic crystal. On the left, a snapshot of the particles can be seen. The right image is averaged over a longer period of time. This clearly shows that the particles are positioned on a 3D lattice whereas the rods can freely rotate.	The rotations of the rods can be stopped by applying an external electric field. The matter forms a three-dimensional crystal.

New and improved energy efficient digital screens as well as improved TV images could be some of the benefits of a new discovery in the field of liquid crystals, by a team from University of Aberdeen. Liquid crystals are a technology that impacts on almost everybody, used as they are in devices such as mobile phones, computer screens and televisions. The vast majority of these devices use nematic liquid crystals.
Nematics are similar to conventional liquids, in which molecules are randomly oriented, but with a crucial difference. The molecules are preferentially ordered in one direction but when electrical fields are applied, they can be switched to another direction and act as shutters which can either let light through, or not. A large international research effort to understand the structure of the new nematic phase saw an interdisciplinary team of researchers assembled involving chemists at the Universities of Aberdeen and Hull, physicists at the Liquid Crystal Institute at Kent State University, USA, and electrical engineers at Trinity College in Dublin.
The team found that the molecules are arranged in a 'twist-bend' structure with a periodicity of ~ 8nm - the length of two to three molecules, about 10 000 times smaller than the thickness of a human hair. A technique called transmission electron microscopy was crucial to identify the new structure. The studies showed arch-like structures and periodic arrays, which are not observed for conventional nematics, but are typical for this new liquid crystal phase. The applications could be anything from really impressive fast switching display devices such as improved colour TV screens, biological sensors, etc.