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Resembling chicken wire on the nanoscale, graphene—single sheets of graphite—is only one atom thick, making it the world’s thinnest material. Two million graphene sheets stacked up would not be as thick as a credit card.

The tricky part physicists have yet to figure out is how to control the flow of electrons through the material, a necessary prerequisite for putting it to work in any type of electronic circuit.

Graphene behaves very differently than silicon, the material currently used in semiconductors.

Last year, researchers at the University of Arizona cleared the first hurdle by identifying boron nitride, a structurally identical but non-conducting material, as a suitable mounting surface for single-atom sheets of graphene. The team also showed that in addition to providing mechanical support, boron nitride improves the electronic properties of graphene by smoothing out fluctuations in the electronic charges.

Now the team finds boron nitride also influences how the electrons travel through the graphene. Published in the journal Nature Physics, the findings open up new ways of controlling the electron flow through graphene.

“If you want to make a transistor for example, you need to be able to stop the flow of electrons,” says Brian LeRoy, assistant professor of physics. “But in graphene, the electrons just keep going. It’s difficult to stop them.”

Relativistic quantum mechanical effects that come into play at atomic scales cause electrons to behave in ways that go against our everyday experiences of how objects should behave, LeRoy says. Take tennis balls, for example.

“Normally, when you throw a tennis ball against a wall, it bounces back. Now think of the electrons as tennis balls. With quantum mechanical effects, there is a chance the ball would go through and end up on the other side. In graphene, the ball goes through 100 percent of the time.”

This strange behavior makes it difficult to control where electrons are going in graphene. However, as LeRoy’s group has now discovered, mounting graphene on boron nitride prevents some of the electrons from passing to the other side, a first step toward a more controlled electron flow.

The group achieved this feat by placing graphene sheets onto boron nitride at certain angles, resulting in the hexagonal structures in both materials to overlap in such a way that secondary, larger hexagonal patterns are created. The researchers call this structure a superlattice. If the angle is just right, they found, a point is reached where almost no electrons go through.

“You could say we created holes in the wall,” LeRoy says, “and as soon as the wall has holes in it, we find that some of the tennis balls no longer go through. It’s the opposite of what you would expect. That shows you how weird this is. It’s all due to those relativistic quantum effects.”

The discovery puts the technology a bit closer to someday being able to actually control the flow of electrons through the graphene, the authors of the paper write. “The effect depends on the size of the hexagonal pattern resulting from the overlapping sheets,” explains the study’s lead author Matthew Yankowitz, a first-year graduate student in LeRoy’s lab.

The pattern, he explains, creates a periodic modulation of the potential—picture a ball rolling across an egg carton.

“It’s a purely electronic effect brought about by the structure of the two materials and how they sit on top of each other,” Yankowitz says. “It’s similar to the Moiré pattern you see when someone wears a striped shirt on TV.”

The researchers have yet to be able to control how the graphene and boron nitride end up oriented relative to each other when they combine the two materials. Therefore, they make many samples and check the structure of each one under an electron microscope.

“With our scanning tunneling microscope, we can get an image of each superlattice and measure its size,” Yankowitz says. “We take a picture and see what the pattern looks like. If the hexagonal pattern is too small, the samples are no good and we throw them out.”

About 10 to 20 percent of samples showed the desired effect. If it becomes possible to someday automate the process, graphene-based microelectronics might be well on their way to propel us from the silicon age to the graphene age.

The research study is a collaboration among LeRoy’s lab and researchers at Massachusetts Institute of Technology (MIT), the National Institute for Materials Science in Tsukuba, Japan, and the University of Geneva, Switzerland. The UA portion of the project was funded by grants from the U.S. Army Research Office and the National Science Foundation.

More news from the University of Arizona: http://uanews.org