Researchers Characterize Graphene’s 'Disappearing Act'

FAYETTEVILLE, Ark. – University of Arkansas researchers have characterized an unusual property of graphene, a sheet of carbon atoms first isolated in 2005 and touted as a possible replacement for silicon in the electronics industry. Unlike other substances, graphene’s structure proves impossible to detect using conventional methods. Their insights into the reason for this provide insight into some of graphene’s other unique properties, such as being able to carry high currents and dissipate heat.

The researchers reported their findings in a rapid communications report published in Physical Review B.

“When we tried to map graphene using the scanning tunneling microscope, we found something unusual,” said Paul Thibado, professor of physics. “Normally we operate the mapping process at low currents, because high currents can damage a sample. But we couldn’t see the honeycomb structure that is characteristic in graphene using a low current.”

The researchers amped up the current and found that they could “see” the honeycomb structure at about 300 times the normal level of current used.

“We went back and said, ‘Why is this?’” he said. They took images of the graphene using high, medium and low currents. Then a group of theoretical physicists, led by University of Arkansas professor Laurent Bellaiche, simulated the images.

They found that a characteristic called density of states offered an explanation. Density of states is the probability of an electron’s location around a carbon atom To understand density of states, imagine a marble as an electron and holes in a piece of wood as different states. With a high density of states, the holes are close together, and the marble will not have to travel far to find one of the holes. With a low density of states, the holes are farther apart, and the marble will have to travel further to find a hole. 

Because the electron remains closer to the surface at the high density of states, the tip of the scanning tunneling microscope can "see" the honeycomb shape at the high current. At low current and low density of states, the electrons can travel farther away from the carbon atom, filling the space around the atoms and stretching above and below the surface of the single layer. “This explains why the tip of the microscope pulls back at low currents,” Thibado said, because the surface is actually expanding toward the tip.

This characteristic, along with others, disappears if graphene is converted into the multiple layers of carbon known as graphite. In that case, the charge density gets spread into the other layers and is not forced to expand.

“This gives us insight into why graphene has this large thermal capacity and carrying capacity,” Thibado said.