For future optical technologies, blue phase liquid crystals offer tantalizing possibilities.
Because these crystals selectively reflect visible light, they could form the basis of new types of electronic displays or innovative camouflaging technologies. And because they respond to stimuli by changing color, blue phase crystals could be used as extremely sensitive colorimetric sensors for volatile organic compounds.
But to integrate these crystals into technology, scientists must make single crystals that are temperature stable. That has proven difficult, since blue phase crystals only exist in a small temperature range: in some cases, heating them up even 1 degree can destroy their properties.
In a new paper, a team from the Pritzker School of Molecular Engineering (PME) at the University of Chicago has developed a method to create blue phase single crystals that is applicable irrespective of the chemistry and complexity of the liquid crystal-forming material. They also developed a method of turning the liquid crystals into a gel, making them thermally stable. The results were published in the journal Advanced Functional Materials.
“This is another important step towards taking advantage of blue phase liquid crystals’ interesting optical properties and preparing them for use in new devices and sensors,” said Juan de Pablo, Liew Family Professor of Molecular Engineering and co-author of the research.
Turning liquid crystals into gel
The unique properties of blue phase liquid crystals stem from their structure, which results from millions of molecules self-assembling into lattices that are comparable in size to the wavelength of visible light. To use these crystals in future applications, the researchers needed to find a way to create thermally stable and macroscopically homogenous crystals.
The research team had previously shown that they could grow blue phase single crystals by growing them on top of striped chemical patterns. The chemical pattern directs the crystal growth in a certain direction, resulting in a macroscopic single crystal.
While previous blue phase crystals grown in this manner were made up of simple mixtures, here the team increased the chemical complexity of the material used to grow the crystals and found that the methodology still worked.
“We showed that this way of making single crystals with chemical patterns was much more generally applicable,” said Tadej Emeršič, a postdoctoral researcher on the team.
Once the crystals formed, the researchers shined UV light onto them, which “photopolymerized” the system. The liquid crystals became gel, but their crystalline structure was maintained. The gel crystals no longer had the temperature sensitivity that the liquid crystals had, taking a further step toward using them in novel applications.
“In doing this we created a material that combines the mechanical robustness of polymers with the unique structure of blue phase liquid crystals,” said Kushal Bagchi, a postdoctoral researcher on the team.
Understanding potential applications of blue phases
Next, the group plans to try to make larger patterns that create even bigger crystals, and they hope to expose these single crystals to different chemicals to better understand their potential responses as sensors.
“Here we show a generalizable and manufacturing-friendly strategy to create structures with a high degree of perfection over large areas in films of liquids that form blue phase liquid crystals, which will enable the development of optical devices such displays,” said Paul Nealey, Brady W. Dougan Professor of Molecular Engineering and co-author on the paper.
Other authors on the paper include José A. Martínez-González and Xiao Li.
Citation: “A Generalizable Approach to Direct the Self-Assembly of Functional Blue-Phase Liquid Crystals,” Emeršič et al, Advanced Functional Materials, July 8, 2022. DOI: 10.1002/adfm.202202721
Funding: Department of Energy, National Science Foundation