Some electron-donating (red) and electron-accepting (blue) compounds form hydrogen-bonded mixed stacks that yield crystals (about 100 µm wide) with ferroelectric properties.
New organic ferroelectric materials that operate at room temperature made their debut at the American Chemical Society national meeting in Philadelphia last week. Scientists have in the past synthesized charge-transfer ferroelectrics—mixed stacks of electron-donating and electron-accepting small molecules. But the materials worked only at cryogenic temperatures, which are impractical for real-world use.
Ferroelectrics, found in ultrasound imaging equipment and computer memory, are prized for their ability to flip polarization when exposed to an external electric field. Because the materials are currently made of inorganic compounds such as barium titanate, researchers have been seeking alternatives that are cheaper, lighter, and easier to fabricate.
Synthesized by J. Fraser Stoddart and Samuel I. Stupp of Northwestern University and colleagues, the new materials fit the bill, Stupp said. “They are easy to produce” because they self-assemble, he explained. “You take two molecules, mix them together, grow a crystal, and you’re done.”
The researchers demonstrated that mixtures of an electron acceptor—a pyromellitic diimide-based molecule—and an electron donor such as a naphthalene, pyrene, or tetrathiafulvalene derivative alternately stack to form ferroelectric crystals (Nature, DOI: 10.1038/nature11395). When an electric field is applied to the crystals at room temperature, electrons from the donors transfer to the acceptors, switching the material’s polarization.
The team thinks that its ferroelectrics work at room temperature because of hydrogen bonds between the electron-accepting and electron-donating compounds. “The hydrogen bonds contribute to the stability of the entire supramolecular network,” Stupp said.
The organic ferroelectrics fabricated by Stoddart, Stupp, and coworkers overcome the drawbacks of previous materials of this type, namely cryogenic operating temperatures and low electrical resistivities, said Yoshinori Tokura, a physicist at the University of Tokyo. With the production of these and other versatile organic ferroelectrics, he added, there may be a future for this class of materials in electronics.