Each year, the Journal of Society for Information Display (JSID) recognizes a published student paper on the basis of originality, significance of results, organization, and clarity. The 2011 award went to Rafael S. Zola, Young-Cheol Yang, and Deng-Ke Yang, from the Liquid Crystal Institute and the Chemical Physics Interdisciplinary Program at Kent State University, for the paper "Limonene as a chiral dopant for liquid crystals: Characterization and potential applications."

by Rafael S. Zola

Rafael S. Zola

Young-Cheol Yang

Professor Deng-Ke Yang

It has been known for many years that adding a material to liquid crystals can change their physical properties in desirable ways, particularly in terms of display applications. Many liquid-crystal-display technologies take advantage of chirality to work. Chirality, the property that describes the helical (twisted) alignment of molecules, is one of the most intriguing subjects in nature. Over the years, it has attracted attention from chemists, physicists, biologists, and engineers.

In liquid crystals, chirality shows up in very elegant ways. From a research point of view, chirality is the difference between the ordinary nematic phase and the exciting Bragg-reflecting cholesteric phase, or the ferro-electric smectic C* and the regular non-ferroelectric smectic C. From an applications point of view, chiral phases can be used in many ways, including several quite successful modes: twisted nematic (TN), super-twisted nematic (STN), polymer-stabilized cholesteric textures (PSCT), and bistable reflective cholesteric displays.

Since chirality is so crucial for liquid-crystal displays (LCDs), every little step taken toward a better understanding and development on this matter is important. For many years, the scientific community has taken great efforts toward synthesizing new chiral materials, which is not a trivial task because a good chiral dopant must satisfy certain requirements, such as helical twisting power and solubility. However, while naturally occurring chiral materials exist, very few non-synthesized elements have been used as chiral dopants.

For my first thesis work, my advisor, Professor Deng-Ke Yang, suggested studying the chiral dopant effect on liquid crystals, aiming for physical parameter changes. After the initial experiments, our team realized that this addition, even if small, has a great impact on the host. Since the chiral additive is a "necessary evil," we decided to seek a material that would both induce twist and improve the host's properties. We started considering synthesized materials only, but after some research, we realized that scientists have leaned toward synthesized compounds over the years, almost forgetting the natural chiral materials, even though the first liquid crystals themselves were essentially biological systems.

I then noticed that while many textbooks use Limonene, a natural molecule easily extracted and purified from citrus fruits, as an example of chirality, none ever mentions any application in the liquid-crystal field. As the essence of science is experimenting with the new and avoiding bias, I decided to use Limonene as a chiral additive. As additional motivators for us, Limonene is a "green" material and is easily found at very low cost. Most importantly, Limonene is a low-molecular-weight compound, much lower and smaller than liquid-crystal molecules or commonly used chiral dopants.

We therefore used D-Limonene as a chiral dopant for TN displays and bistable cholesteric displays. The results: a lowering of the switching voltage and incredibly faster response times, achievements presented at the SID 2010 Symposium. This first set of results led us to an invitation for publishing more complete and detailed data in a JSID special edition on Nano-Technology in FPDs. This work was performed with much help from Dr. Young-Cheol Yang. Our findings were very interesting and confirmed what we observed before: We had a material that changed the physical parameters of a well-known nematic liquid crystal at different ratios, despite the small amount added. The most fascinating discovery was the large decrease in rotational viscosity, one of the most desirable achievements for fast displays. The elastic constants were also lowered, decreasing the switching voltage without compromising the improvement on response time induced by the great change of viscosity. These results suggest potential applications of liquid-crystal phases that were heretofore unreasonable to consider, such as for active-matrix bistable reflective cholesteric displays working at video rates.

With our paper published in the JSID, we hope to reach researchers with regard to the many possibilities encountered by "mixing" liquid-crystal compounds with other materials, an interdisciplinary field involving biology, physics, chemistry, and engineering. For us, it is surprising how many synthesized materials have been used in the industry as the liquid-crystal field moves forward, while naturally occurring molecules have not. D-Limonene, for example, can cost up to 250 times less in terms of price and helical twisting power (HTP) as compared to conventional chiral dopants such as (R)S811. This makes it a better choice for teaching purposes, for example, than more expensive synthesized chiral additives. And as a matter of practicality, the performance benefits we measured can be useful in engineering better displays.