Creating a ‘crystal within a crystal’ for new electronic devices

Liquid crystals have enabled new technologies, like LCD screens, through their ability to reflect certain color wavelengths.

Researchers at the Pritzker School of Molecular Engineering (PME) at the University of Chicago have now developed a way to create a liquid “crystal within a crystal.” These new crystals could be used for next-generation display technologies or sensors that consume very little energy.

Juan de Pablo, the Liew Family Professor of Molecular Engineering, led the research with Paul Nealey, the Brady W. Dougan Professor of Molecular Engineering. It’s published in the Nov. 29 issue of Science Advances.

Use of blue phase crystals critical to research

Liquid crystals’ molecular orientation makes them useful for key aspects of many display technologies. They can also form “blue phase crystals,” in which molecules are organized in highly regular patterns that reflect visible light.

Blue phase crystals have the properties of both liquids and crystals, meaning they are able to flow and are pliable, while exhibiting highly regular features. They also have better optical properties and a faster response time than traditional liquid crystals, making them a good candidate for optical technologies.

Additionally, blue phase crystals have more distance between crystals – between 200 and 300 molecular diameters – than traditional crystals – which have a typical space of one atom between diffraction centers. The larger distance between blue phase crystals makes it easier to engineer interfaces between them, a notoriously difficult process. The borders between adjacent crystals provide ideal sites for chemical reactions and mechanical transformations.

Creating an interface between crystals

To engineer a blue phase crystal interface, de Pablo and Nealey developed technology that relies on lithography to chemically pattern surfaces on which liquid crystals are deposited, which manipulates their molecular orientation. That orientation is then amplified within the liquid crystal itself, allowing for a particular blue phase crystal to be sculpted within another blue phase crystal.

The process, a result of theoretical predictions and experimentation to determine the right design, allowed them to create specific shapes of crystals within the liquid crystals, which had never been done before.

Not only that, the new crystal could be manipulated with both temperature and an electric field to change from one blue phase into another type of blue phase, thereby changing color.

“That means the material changes its optical characteristics very precisely,” de Pablo said. “We now have a material that can reflect light at wavelengths for which we didn’t have good alternatives before.”

Useful for display technologies, sensors

Reflecting light in such wavelengths means this crystal-within-a-crystal could be used for better display technologies, and because these crystals can be manipulated with temperature or voltage, they could be used for sensing applications. Changes in temperature, for example, would result in color changes. Because such changes would require only slight temperature variations or small voltages, the devices would consume very little energy.

This ability to manipulate the crystals at such a small scale also allows researchers to use the them as templates for fabricating perfectly uniform structures at the nanoscale, Nealey said.

“We are already experimenting with growing other materials and experimenting with optical devices,” Nealey said. “We’re looking forward to using this method to create even more complex systems.”

Other authors of the paper include graduate students Kangho Park, Chun Zhoe and Yu Kambe, postdoctoral fellow Heyong Min Hin, and postdoctoral researchers Xiao Li and James Dolan of UChicago; Xuedan Ma of Argonne National Laboratory; and José A. Martínez-González of the Universidad Autónoma de San Luis Potosí.

Citation: Sculpted grain boundaries in soft crystals. X. Li, et al. Science Advances, Nov. 29, 2019, doi: 10.1126/sciadv.aax9112.

Funding: US Department of Energy