New Method Manipulates Electrical Conductivity of Graphene with Pressure

An international team of researchers led by Columbia University has developed a method to exploit the electrical conductivity of graphene with compression, taking the material a step closer to being a workable semiconductor for application in present-day electronic devices.

Graphene is the best electrical conductor that we know of on Earth. The problem is that it’s too good at conducting electricity, and we don’t know how to stop it effectively. Our work establishes for the first time a route to realizing a technologically relevant band gap in graphene without compromising its quality. Additionally, if applied to other interesting combinations of 2D materials, the technique we used may lead to new emergent phenomena, such as magnetism, superconductivity, and more.

Matthew Yankowitz, Postdoctoral Research Scientist in Columbia’s Physics Department & First Author

The research, funded by the National Science Foundation and the David and Lucille Packard Foundation, has been published in the May 17 edition of Nature.

The unique electronic properties of graphene, a two-dimensional (2D) material made up of hexagonally-bonded carbon atoms, have excited the physics community since its discovery over ten years ago. Graphene is said to be the strongest and thinnest material in existence. It also happens to be an excellent conductor of electricity - the distinctive atomic arrangement of the carbon atoms in graphene allows its electrons to effortlessly travel at very high velocity without the significant chance of scattering, thus saving valuable energy usually lost in other conductors.

But switching off the transmission of electrons through the material without changing or sacrificing the promising qualities of graphene has proven futile thus far.

“One of the grand goals in graphene research is to figure out a way to keep all the good things about graphene but also create a band gap - an electrical on-off switch,” said Cory Dean, assistant professor of physics at Columbia University and the study’s chief investigator.

He explained that previous efforts to alter graphene to produce such a band gap have degraded the inherently good properties of graphene, rendering it much less valuable. One superstructure does show potential, however. When graphene is placed between layers of boron nitride (BN), an atomically-thin electrical insulator, and the two materials are rotationally aligned, the BN has been observed to alter the electronic structure of the graphene, producing a band gap that allows the material to act as a semiconductor - that is, both as an insulator and an electrical conductor. The band gap formed by this layering alone, however, is not sufficiently large to be useful in the operation of electrical transistor devices at room temperature.

In an attempt to improve this band gap, Yankowitz, Dean, and their colleagues at the National High Magnetic Field Laboratory, the University of Seoul in Korea, and the National University of Singapore, compressed the layers of the BN-graphene structure and discovered that applying pressure considerably increased the size of the band gap, more effectively obstructing the flow of electricity through the graphene.

As we squeeze and apply pressure, the band gap grows. It’s still not a big enough gap - a strong enough switch - to be used in transistor devices at room temperature, but we have gained a fundamentally better understanding of why this band gap exists in the first place, how it can be tuned, and how we may target it in the future. Transistors are ubiquitous in our modern electronic devices, so if we can find a way to use graphene as a transistor it would have widespread applications.

Matthew Yankowitz, Postdoctoral Research Scientist in Columbia’s Physics Department & First Author

Yankowitz added that researchers have been performing experiments at high pressures in conventional three-dimensional materials for many years, but so far no one had discovered a way to do them with 2D materials. Now, researchers will be able to test how applying different degrees of pressure alters the properties of a huge range of combinations of stacked 2D materials.

“Any emergent property that results from the combination of 2D materials should grow stronger as the materials are compressed,” Yankowitz said. “We can take any of these arbitrary structures now and squeeze them and the strength of the resulting effect is tunable. We’ve added a new experimental tool to the toolbox we use to manipulate 2D materials and that tool opens boundless possibilities for creating devices with designer properties.”