Researchers Provide First Definitive Evidence for Twist-Bend Phase of Liquid Crystals

Discovered more than 125 years ago, liquid crystals play a significant role in screens of computer monitors and TV, watches, clocks and a variety of other electronics displays. Scientists are still attempting to improve the molecular makeup of these liquid crystals.

Researchers examined the spiral “twist-bend” structure (right) formed by boomerang-shaped liquid crystal molecules (left and center) measuring 3 nanometers in length, using a pioneering X-ray technique at Berkeley Lab’s Advanced Light Source. A better understanding of this spiral form, discovered in 2013, could lead to new applications for liquid crystals and improved liquid-crystal display screens. (Credit: Zosia Rostomian/Berkeley Lab; Physical Review Letters, DOI: 10.1103/PhysRevLett.116.147803; Journal of Materials Chemistry C, DOI: 10.1039/C4TC01927J)

Liquid crystals are a unique state of matter, flowing like a fluid, in which it is possible to orient the molecules in a crystal-like manner. At the microscopic scale, liquid crystals are available in varied configurations, including a molecular arrangement discovered in 2013, which naturally twists and bends. This arrangement has resulted in a burst of new research.

A new X-ray technique, invented at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) was used by a team of researchers to record the first direct measurements that confimed the existence of a firmly wound spiral molecular arrangement. This arrangement could reveal the secret of its formation and enhance the performance of liquid-crystal displays (LCDs), including the rate of switching on or off the light in very small screen areas.

Molecules are capable of exhibiting a wide variety of properties based on their left- or right-handedness (chirality), which is of immense interest in chemistry, materials science, and biology. The findings describe how the “chiral” structure can develop organic molecules that do not display handedness.

This newly discovered ‘twist-bend’ phase of liquid crystals is one of the hottest topics in liquid crystal research. Now, we have provided the first definitive evidence for the twist-bend structure. The determination of this structure will without question advance our understanding of its properties, such as its response to temperature and to stress, which may help improve how we operate the current generation of LCDs.

Chenhui Zhu, Research Scientist, Berkeley Lab

Zhu was the chief author on a related research paper published in the April 7 issue of Physical Review Letters.

Despite the existence of many competing screen technologies to the conventional LCDs, the standard LCD market still continues to be large and represents over one-third of the revenue in the electronic display market. This year the revenue of the total display market is expected to reach $150 billion.

The Berkeley Lab determined the individual molecules present in the structure. These molecules are formed like flexible, nanoscale boomerangs that measure only a few nanometers, or billionths of a meter, and comprise of flexible middles and rigid ends. The molecules formed the spiraling structure in the twist-bend phase, and this structure looks very much like a bunch of snakes all lined up and then twisted around an invisible pole.

Low-energy or “soft” X-rays were tuned by Zhu at the ALS in order to analyze carbon atoms present in the liquid crystal molecules, which supplied details regarding the molecular orientation of their chemical bonds and the structure formed by them. Zhu used the soft X-ray scattering for his study. The standard X-ray scattering techniques are incapable of detecting the helical, spiraling molecular arrangement of the liquid crystal samples.

The measurements highlight that the liquid crystals finish a 360-degree twist-bend covering a distance of only 8 nm under room temperature, which according to Zhu is an “amazingly short” distance based on the fact that every single molecule is 3 nm in length, and a firmly coiled structure like this is not very common.

Zhu pointed out that the reason for the development of the firm spiral in the twist-bend arrangement still remains unclear, and the structure displays unique optical properties that warrant further study.

The researchers discovered that the spiral “pitch” or width of a single spiral to becomes a bit longer under increased temperature. They also found that the spiral suddenly disappears at high temperature, as a totally varied configuration is adopted by the material.

Currently, this experiment can’t be done anywhere else. We are the first team to use this soft X-ray scattering technique to study this liquid-crystal phase.

Chenhui Zhu, Research Scientist, Berkeley Lab

Conventional LCDs often use nematic liquid crystals, which refer to a phase of liquid crystals that naturally position themselves in the same direction, much like a group of compass needles arranged parallel to each other, pointing in the same direction.

In the conventional LCD devices, rod-like liquid crystal molecules are placed between selectively treated glass plates that make the molecules to lie down instead of pointing towards the glass. The glass is specially treated to produce a 90-degree twist in the molecular arrangement, resulting in the molecules extremely adjacent to one glass plate to be at right angles to the molecules extremely adjacent to the other glass plate.

It is just like a group of compass needles facing north at the top, effortlessly reorienting to the northeast in the middle, and pointing towards the east at the bottom. This molecularly twisted state is then electrically twisted to permit the polarized light to travel at different brightness levels, or for blocking light by completely straightening the twist.

Zhu stated that experiments in the future would aim at examining how the spirals rely on molecular shape and react to differences in temperatures, ultraviolet light, stress, and electric field.

He also aims to explore similar spiraling structures like a liquid crystal phase referred to as the helical nanofilament, which guarantees scope for solar energy applications. The role played by handedness in self-assembling organic molecules can be explained by DNA studies, synthetic proteins, amyloid fibrils such as those related to Alzheimer’s disease.

Zhu further stated that it may be possible to get information on how the spiraling twist-bend structure develops and changes in real time in materials, by using more laser-like and brighter X-ray sources and extremely fast X-ray detectors.

I am hoping our ongoing experiments can provide unique information to benefit other theories and experiments in this field.

Chenhui Zhu, Research Scientist, Berkeley Lab

Other team members include Anthony Young, Cheng Wang, and Alexander Hexemer at Berkeley Lab, and Michael Tuchband, Min Shuai, Alyssa Scarbrough, David Walba, Joseph Maclennan, and Noel Clark at the University of Colorado Boulder.

Soft X-ray scattering measurements were carried out at Beamline 11.0.1 at the Advanced Light Source, a DOE Office of Science User Facility at Berkeley Lab. The National Science Foundation and the DOE Office of Basic Energy Sciences supported the research work.

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