Anomalously low dielectric constant of confined water
Authors: Laura Fumagalli, Ali Esfandiar, Rene Fabregas, Sheng Hu, Pablo Ares, Amritha Janardanan, Qian Yang, Boya Radha, Takashi Taniguchi, Kenji Watanabe, Gabriel Gomila, Kostya S Novoselov, Andre K Geim
Publication Date: 22 June, 2018
Department of: Physics and Astronomy
Water found to be electrically dead at surfaces
Water has many anomalous properties that make it a unique substance and the fundamental element for life. When it is near a surface, water has long been predicted to exhibit a reduced electric polarisability but the size of the effect remained unknown. Now, researchers at The University of Manchester and their international collaborators have finally succeeded in measuring this property by combining two recently developed technologies. First, they created special nanochannels of only a few nanometres in size and accommodated only few layers of water in them by assembling atomically flat crystals of graphite and hexagonal boron nitride. Second, they introduced a scanning probe microscopy technique capable of probing the dielectric constant of water inside the nanochannels. They found that the rotational polarisability of the trapped water is not only suppressed, but completely absent. In other words, the water is electrically dead. This is in contrast to bulk water, known for its anomalously high dielectric constant (around 80), which easily aligns along an electric field. The thickness of the dead layer was found to be less than one nanometre thick – which is only two to three molecules. This new anomaly has important implications in many scientific disciplines. In particular, it will improve our understanding of the structure and energetics of electrochemical systems and biological macromolecules such as proteins and nucleic acids.
- Electric polarisability is a physical property of matter measured by dielectric constant (or permittivity). It represents the ability of molecules and atoms to form electric dipoles and align them to an external electric field. For polar materials and substances like bulk water, it is the sum of three contributions: the dipolar (or rotational), atomic and electronic polarisability.
- Scanning probe microscopy is a family of microscopy techniques that are able to probe surface structure and other information such as mechanical, electrical and magnetic properties on the nanoscale. This is achieved by scanning a nanometrically sharp probe able to detect such properties of the surface. In this study, the probe detected the electrostatic force between the tip apex and the nanochannels, which allowed the researchers to determine the dielectric constant of water confined inside.
- Hexagonal boron nitride (hBN) is a van de Waals crystal with hexagonal crystal structure analogous to graphite but with different properties. Atomically flat layers of hBN can be isolated down to the single atomic layer, similarly to graphene. In this study, it was used for its electrically insulating character as top and lateral walls of the nanochannels on top of graphite crystals (the bottom layer). In this manner, the researchers created electrically-transparent nanochannels that allowed the electric field generated by the scanning probe to detect the water inside.