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Dielectric constant of nanoconfined water measured

By Lauren Borja July 12, 2018
(a) Schematic illustration of the experimental device. The top layer and side walls made of hexagonal boron nitride (hBN) are shown in light blue; graphite serving as the ground electrode is in black. The three-layer assembly covers an opening in a silicon nitride membrane (light brown). The channels are filled with water from the back. The atomic force microscope tip served as the top electrode and was kept in a dry nitrogen atmosphere. (b-d) Microscopy images for three different channel thicknesses. The change in dielectric constant can be seen in the change in contrast of the images. Size markers are 500 nm. Credit: Science

A research collaboration led by Laura Fumagalli and Andre Geim of the University of Manchester has measured the dielectric constant of water confined to the nanoscale. Published recently in Science, this work could help scientists better understand a plethora of phenomena ranging from protein folding and function or interactions at the surfaces of electrodes in batteries.

Water’s molecular geometry and ability to form hydrogen bonds influence many of its interactions and properties in the bulk. Among these are water’s dielectric properties, described by the dielectric constant, that allow water to effectively stabilize charged ions. This stability is crucial for many biological and electrochemical processes. How these physical characteristics change going from bulk to nanoscale water at its surface or interfaces, remains an open scientific question.

Many previous experimental and theoretical work focused on understanding these properties, but quantification of the dielectric constant remained elusive. “People have been arguing over the dielectric properties of [interfacial] water for quite some time,” says Sergei Kalinin, director of Oak Ridge National Laboratory’s Institute for Functional Imaging of Materials, “and it’s extremely difficult to measure the properties of a 1 nm layer of water.” Kalinin was not associated with the work published in Science.

The research team succeeded by combining two recently developed technologies. A group led by Geim built nanochannel devices made of hexagonal boron nitride (hBN), a two-dimensional material. “Because the devices are made of two-dimensional crystals, the channels are atomically flat,” Fumagalli says. The flatness of hBN allowed Geim and his colleagues to create uniform channels with a precise thickness down to a single nanometer. Fumagalli’s team then measured the dielectric constant of nanoconfined water inside the channels of the devices using scanning probe microscopy (SPM).

Surprisingly, the researchers measured a dielectric constant of ~2 for the interfacial water confined in the device nanochannels. This means that the dielectric properties of interfacial water lie much closer to those of air (dielectric constant of 1) than those of bulk water (dielectric constant ~80). “[Bulk] water was famous for its anomalously high dielectric constant,” Geim says, “but the dielectric constant of interfacial water is anomalously low.”

The group built a series of devices, systematically varying the height of the hBN nanochannels from 1 nm to 300 nm. The change in dielectric constant across the different nanochannels confirmed the presence of interfacial water at the surfaces of the device, even if most of the water contained in the larger channels exhibited bulk properties. From these measurements, Fumagalli and her colleagues estimated the height of the interfacial layer to be 7.5 Å or 2-3 water molecules.  

“Multiple scientific communities are interested in water’s properties on the nanoscale because they play a role in virtually everything—from biological systems to energy technologies to corrosion,” Kalinin says. This work confirms that ions or charged particles experience a very different environment in interfacial water than in a bulk solution. Water coats proteins and DNA; the dielectric constant determined in this study could help scientists better understand how such molecules fold and function in the body. The dielectric properties of water on the surface of electrodes could also influence their interactions with ions inside an electrochemical cell, which is important for developing new devices for energy storage.   

But more generally, these results open a new direction for scanning probe microscopy, which senses forces on the atomic scale. However, these forces are often the product of some intrinsic property of the material, such as the dielectric constant, and the geometry of the microscope and sample. Careful design and engineering of the experimental geometry, as was done by Fumagalli and her colleagues, could allow researchers to extract quantitative information in addition to images. Using these or similar methods, “scanning probe microscopy becomes a totally different creature,” Kalinin says, “It’s not just an image—your microscope becomes a measurement device.”

Read the abstract in Science