Crystals are typically thought of as being brittle, but new research done by the Naumov Group at NYU Abu Dhabi has gained insight into a special class of crystals that exhibit an odd property — they can bend like plastic. The research was published in Nature Chemistry.
These flexible crystals are intriguing because they bridge the gap between single crystals, which are known to be brittle, and plastics, which are malleable, said Panče Naumov, associate professor of Chemistry at NYU Abu Dhabi and director of the Naumov Group. "The material we studied is something between the two, and this is a very important property that results from inter-molecular interactions in the crystal," Naumov said. He also noted that these materials, despite having a very simple structure, mimic similar mechanical phenomenon that occur with some plants that shrivel in response to touch or heat, the mimosa being an example.
When pressure is applied, the crystal the team studied can bend up to 360 degrees. The researchers had known that this particular crystal was malleable before they conducted the experiment, but they didn't understand exactly how it happened. "It is an important class of materials because with externally applied mechanical force, the crystal can adapt itself and change," said Manas Panda, research scientist at NYUAD and lead author on the paper.
In the lab, the researchers carefully applied pressure to the crystal using forceps, making sure that they didn't press too hard and break it. They found that molecules in it are layered over each other in a particular direction and are joined with halogen-halogen bonds. But once pressure is applied, an interaction occurs that breaks the bonds and allows one molecular layer to slide over another. "The molecules swap their interlayer partner," Panda said, "and that's how the crystal can maintain its flexibility without cracking." Moving forward, Naumov and his team will try to understand how this crystal "self-heals" — the way bonds are broken and reformed — after pressure is removed.
Crystals are interesting subjects because their molecular makeup can be precisely analyzed by using diffraction of X-rays. Researchers can gain detailed information about distance and angles of the inner structure of the molecules in crystals that is not possible with other materials like polymers, Naumov said. "That's why we are interested in single crystals, and in particular we are interested in crystals that have these exotic properties." Naumov's group has also studied a fascinating class of crystals that explode when exposed to UV light.
Though the research is in its early stages, "we can think about applications for these crystals in organic electronics," Naumov said. "We are now shifting to flexible displays and these kinds of materials are very important for the development of electronics that don't contain metals." Flexible displays could be bent according to the angle of the user, or could even adopt the shape of the object on which they are placed.