Ferroelectric materials have a spontaneous electric polarization that can be reversed by the application of an external electric field. Whether water can show ferroelectric ordering has been a subject of debate among researchers. So far, there had been no unambiguous evidence of water ice with the necessary proton ordering. The small energy differences between ordered and disordered forms of ice make this challenging. Confining ice to a low-dimensional structure, e.g., between two graphene layers, could influence the ordering of water and potentially allow for ferroelectric properties.
Ya-Ping Hsieh, Institute for Atomic and Molecular Science, Academia Sinica, Taipei, Taiwan, and colleagues have observed ferroelectric properties of water ice in a two-dimensional phase. The team confined water to a monolayer that is sandwiched between two graphene layers. They used a nanoelectromechanical actuation approach in which they applied a voltage between two separated graphene layers to bring them closer together by electrostatic attraction. This approach allowed them to adjust the distance between the graphene layers and the interlayer pressure, which could be used to expel excess water and create the desired monolayer.
After the electric field was removed, the researchers found evidence for the ferroelectric nature of the confined water ice. They observed a sizable and stable dipole that originates from the monolayer. The dipole strength and the temperature stability agree with simulation results for a 2D phase with an ice XI configuration. When ice multilayers are present instead of a monolayer, the ferroelectric properties vanish. This indicates that the two-dimensional structure of the monolayer is important for its unique properties.
- Ferroelectric 2D ice under graphene confinement,
Hao-Ting Chin, Jiri Klimes, I-Fan Hu, Ding-Rui Chen, Hai-Thai Nguyen, Ting-Wei Chen, Shao-Wei Ma, Mario Hofmann, Chi-Te Liang, Ya-Ping Hsieh,
Nat. Commun. 2021.