The world of materials science is getting flatter — two-dimensional, in fact. Two-D layered nanomaterials are all the rage at the moment, and Kristie Koski, assistant professor of chemistry, is right at the cutting edge.
Interest in 2-D nanomaterials started about a decade ago with the discovery of graphene, one-atom-thick sheets of carbon that have remarkable strength and electrical properties. Since then, the field has expanded to include other materials — molybdenum disulfide, vanadium oxide, layered chalcogenides, and others. Koski, who has a Ph.D. in chemistry from the University of California–Berkeley, became interested in this next generation of 2-D materials during a recently completed postdoctoral position at Stanford University.
From batteries and gas storage devices to optical electronics and next-generation transistors, the possibilities for 2-D nanomaterials are practically endless. But before any of these materials makes the jump from the lab bench to revolutionary new technology, there’s much basic science to be done. That is the kind of work Koski plans to do in her lab.
“All of these materials are layered with a gap between successive layers,” Koski said. “You can place materials in that gap and radically alter the 2-D material properties. So we’ll study how material properties change as we chemically tailor them, and take the next step toward application.”
Koski will use a variety of methods to probe and image these 2-D nanomaterials, including electron microscopy and X-ray diffraction. She’ll also use an imaging technique that she herself helped to develop as an undergraduate at the University of Wyoming called Brillouin imaging.
The technique builds on an overlooked form of spectroscopy theorized in the early 20th century by the French physicist Leon Brillouin (who taught at Brown in 1942 and 1943). The idea is that when a light hits an object, it creates a sound wave that in turn scatters some of the light back. The spectrum of the scattered light varies depending upon certain properties of the object. Whereas other forms of spectroscopy help scientists detect chemical compositions, the Brillouin technique detects physical properties like strength and elasticity.
“There are only a small number of people in the world that do Brillouin spectroscopy, but there are a couple different ways to do it,” Koski said. “What I developed as an undergrad was a simple, cheap method, where you use two mirrors coupled with a charge-coupled device to get a spectrum from a sample.”
Koski recently used her technique to probe properties of a material long renowned for its pound-for-pound strength: spider silk. Her technique enabled her to look at silk as nobody else ever had.
“The way people usually measure mechanical properties is extract the silk, clamp it, and pull it. The problem is that it’s so small that it’s difficult to clamp correctly. It’s also viscoelastic; as soon as you start to pull it, the silk properties begin to change, affecting the mechanical properties.”
Because Brillouin imaging uses nothing more than light, Koski could examine the strength of an intact web without touching it. Among her findings was that the strength of a web isn’t uniform, but varies at web intersections and glue spots. The study was also able to show how strength and elasticity vary with the amount of moisture present in the silk strands.
Koski plans to set up that same imaging technique at Brown to look at 2-D materials that are now the focus of her research. Eventually, she’d like to merge those two lines of research.
“The reason why spider silk is so strong is it contains these things called 2-D nanocrystalline beta sheets that look a lot like a 2-D layered nanomaterial,” she said. “But the problem with silk is that it’s organic; it degrades with time. The spider itself eats it after 24 hours. Ultimately, what I’d like to do is develop a composite material from some of these novel 2-D nanomaterials towards developing an inorganic version of spider silk.”
Synthetic spider silk. Add that to the growing list of potential applications for 2-D materials.