X-Ray View on Liquid Crystals

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 (Source: Z. Rostomian, LBNL)

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 (Source: Z. Rostomian, LBNL)

Liquid crystals, discovered more than 125 years ago, are at work behind the screens of TV and computer monitors, clocks, watches and most other electronics displays, and scientists are still disco­vering new twists—and bends—in their molecular makeup. Liquid crystals are an exotic state of matter that flows like a fluid but in which the molecules may be oriented in a crystal-like way. At the microscopic scale, liquid crystals come in several different confi­gurations, including a naturally spiraling “twist-bend” molecular arrangement, discovered in 2013, that has excited a flurry of new research.

Now, using a pioneering X-ray technique developed at the Lawrence Berkeley National Labo­ratory, a research team has recorded the first direct measurements confirming a tightly wound spiral molecular arrangement that could help unravel the mysteries of its formation and possibly improve liquid-crystal display per­formance, such as the speed at which they selectively switch light on or off in tiny screen areas. The findings could also help explain how chiral structure or chirality can form from organic molecules that do not exhibit such handedness.

“This newly discovered twist-bend phase of liquid crystals is one of the hottest topics in liquid crystal research,” said Chenhui Zhu, a research scientist at Berkeley Lab’s Advanced Light Source, where the X-ray studies were performed. “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.”

The individual molecules in the structure determined at Berkeley Lab are constructed like flexible, nanoscale boomerangs, just a few nanometers, or billionths of a meter, in length and with rigid ends and flexible middles. In the twist-bend phase, the spiraling structure they form resembles a bunch of snakes lined up and then wound snugly around the length of an invisible pole.

Zhu tuned low-energy X-rays at the ALS to examine carbon atoms in the liquid crystal molecules, which provided details about the molecular orientation of their chemical bonds and the structure they formed. The measurements show that the liquid crystals complete a 360-degree twist-bend over a distance of just eight nanometers at room temperature, which Zhu said is an amazingly short distance given that each molecule is three nanometers long, and such a strongly coiled structure is very rare.

The driving force for the formation of the tight spiral in the twist-bend arrange­ment is still unclear, and the structure exhibits unusual optical properties that also warrant further study, Zhu said. He found that the spiral pitch becomes a little longer with increasing temperature, and the spiral abruptly disappears at sufficiently high temperature as the material adopts a different confi­guration. “Currently, this experiment can’t be done anywhere else,” Zhu said. “We are the first team to use this soft X-ray scattering technique to study this liquid-crystal phase.”

Future experiments will explore how the spirals depend on molecular shape and respond to variations in tempe­rature, electric field, ultra­violet light, and stress, Zhu added. He also hopes to explore similar spiraling structures, such as a liquid crystal phase known as the helical nanofilament, which shows promise for solar energy applications. Studies of DNA, synthetic proteins, and amyloid fibrils such as those associated with Alzheimer’s disease, might help explain the role of handedness in how organic molecules self-assemble.

With brighter, more laser-like X-ray sources and faster detectors, it may be possible to see details in how the spiraling twist-bend structure forms and fluctuates in real time in materials, Zhu also said. “I am hoping our ongoing expe­riments can provide unique infor­mation to benefit other theories and expe­riments in this field,” he noted. (Source: LBNL)

Reference: C. Zhu et al.: Resonant Carbon K-Edge Soft X-Ray Scattering from Lattice-Free Heliconical Molecular Ordering: Soft Dilative Elasticity of the Twist-Bend Liquid Crystal Phase, Phys. Rev. Let. 116, 147803, DOI: 10.1103/PhysRevLett.116.147803

Link: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA

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