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Searching for topological states in tungsten ditelluride

Researchers at the University of Pennsylvania have become among the first to produce a single, three-atom-thick layer of a unique two-dimensional (2D) material called tungsten ditelluride. This should now allow scientists to test whether, as predicted, tungsten ditelluride has topological electronic states, meaning it can possess many different electronic properties rather than just one. The researchers report their findings in a paper in 2-D Materials. 

Graphene is the best-known example of a 2D material, comprising a one-atom thick sheet of carbon with many potentially useful properties. These include being a zero bandgap semiconductor, able to behave as both a metal and a semiconductor.

But other 2D materials can have many other properties. Some can be insulating, others can emit light and still others can be spintronic, meaning they possess magnetic properties.

"Graphene is just graphene," said A.T. Charlie Johnson, a physics professor at the University of Pennsylvania. "It just does what graphene does. If you want to have functioning systems that are based on 2D materials, then you want 2D materials that have all of the different physical properties that we know about."

The ability of 2D materials to possess topological electronic states is a phenomenon that was pioneered by Charles Kane, also a physics professor at the University of Pennsylvania. In this new research, Johnson, physics professor James Kikkawa, and graduate students Carl Naylor and William Parkin were able to produce and measure the properties of a single layer of tungsten ditelluride.

"Because tungsten ditelluride is three atoms thick, the atoms can be arranged in different ways," Johnson explained. "These three atoms can take on slightly different configurations with respect to each other. One configuration is predicted to give these topological properties."

"It's very much a Penn product," Johnson added. "We're collaborating with multiple other faculty members who investigate the material in their own ways, and we brought it all together to put a paper out there. Everybody comes along for the ride." These other members of the research team include: Marija Drndic, another professor of physics; Andrew Rappe, a professor of chemistry and a professor of materials science and engineering; and Robert Carpick, chair of the Department of Mechanical Engineering and Applied Mechanics.

The researchers produced tungsten ditelluride using a process called chemical vapor deposition. Using a hot-tube furnace, they heated a chip containing tungsten to the right temperature and then introduced a tellurium-based vapor.

"Through good fortune and finding exactly the right conditions, these elements will chemically react and combine to form a monolayer, or three-atom-thick regions of this material," Johnson said.

Although tungsten ditelluride degrades extremely rapidly in air, Naylor figured out ways to protect the material so that it could be studied before it was destroyed. One thing the researchers found is that the material grows as little rectangular crystallites, rather than the triangles seen with other materials.

"This reflects the rectangular symmetry in the material," Johnson said. "They have a different structure so they tend to grow in different shapes."

Although this research is still at an early stage and the researchers haven't yet been able to produce a continuous film of tungsten ditelluride, they hope to conduct experiments to confirm that it possesses the predicted topological electronic properties.

One of these properties is that any current traveling through tungsten ditelluride would only be carried at the edges; no current would flow through the center of the material. If researchers were able to produce single-layer-thick materials with this property, it could offer a way to route an electrical signal to different locations.

The ability of this material to have multiple properties could also have implications in quantum computing, which taps into the power of atoms and subatomic particles to perform calculations significantly faster than current computers. These 2D materials might allow for an intrinsically error-tolerant form of quantum computing called topologically-protected quantum computing, which requires both semiconducting and superconducting materials.

"With these 2D materials, you want to realize as many physical properties as possible," Johnson said. "Topological electronic states are interesting and they're new and so a lot of people have been trying to realize them in a 2D material. We created the material where these are predicted to occur, so in that sense we've moved towards this very big goal in the field."


2017-02-13