Scientists use ferroelectric materials to create futuristic motor

Rice University materials scientist Boris Yakobson and his team have stumbled upon a property of ferroelectric 2D materials that can be controlled to act as a nanoscale switch or even a motor, according to a press release published on Monday.

Single-layer or 2D materials have physical, electrical, chemical, and optical properties, which make them useful in applications ranging from consumer electronics to medical and industrial technologies.

“2D materials are very thin and very flexible,” Yakobson said. “In single-layer ferroelectrics, this produces an unexpected spontaneous, active bending behavior.”

“The novelty we found in this study is that there is a connection or coupling between the ferroelectric state and the bending or flexing of the material. This work combines the discovery or prediction of a fundamental property of a class of 2D materials with a practical application angle.”

Ferroelectrics boast ions that segregate based on their electrical charge.

“The interesting thing is that the atoms are not identical,” explains Jun-Jie Zhang, a Rice postdoctoral research associate and lead author of the study. “Some of them are larger, and some are smaller, so the layer symmetry is broken.”

Bending in a ferroelectric state

Polarization results in an asymmetrical distribution of the atoms or ions that further causes the material surface to bend in a ferroelectric state.

“So instead of remaining flat, in ferroelectric state the material will bend,” Yakobson said. “If you switch the polarization – and you can switch it by applying electrical voltage – you can control the direction in which it will bend. This controllable behavior means you have an actuator.

“An actuator is any device that translates a signal – in many cases an electrical signal, but it can be a different kind of signal – into mechanical displacement or, in other words, movement or work.”

The research took into account 2D indium phosphide.

“This new property or flexing behavior has to be tested in a laboratory for specific substances,” Yakobson said. “Its most likely use will be as a type of switch. This behavior is very fast, very sensitive, which means that with a very tiny local signal you can maybe switch on a turbine or electrical engine, or control adaptive-optics telescopes’ mirrors. That’s basically the essence of these actuators."

“When you drive your car, you have a lot of knobs and switches, and it makes everything really easy. You don’t have to crank open your car window anymore, you can just turn on a switch,” said the scientist in a statement.

The research is published in the journal ACS Nano.

Study abstract:

Well recognized mechanical flexibility of two-dimensional (2D) materials is shown to bring about unexpected behaviors to the recently discovered monolayer ferroelectrics, especially those displaying normal, off-plane polarization. A “ferro-flexo” coupling term is introduced into the energy expression, to account for the connection of ferroelectricity and bending (strain gradient) of the layer, to predict and quantify its spontaneous curvature and how it affects the phase transitions. With InP as a chemically specific representative example, the first-principles calculations indeed reveal strong coupling ∼P·ϰ between the ferroelectric polarization (P) and the curvature of the layer (ϰ ≡ 1/r), having profound consequences for both mechanics and ferroelectricity of the material. Due to flexural relaxation, the spontaneous polarization and the transition barrier rise significantly, leading to large changes in the Curie temperature, coercive field, and domain wall width and energy, based on Monte Carlo simulations. On the other hand, the polarization switching, characteristic to ferroelectrics, does induce an overall layer bending, enabling a conversion of electrical signal to movement as an actuator; its possible work-cycles and maximum work-efficiency are briefly discussed.