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Stacked graphene sheets can insulate

Posted: 27 Jan 2012     Print Version  Bookmark and Share

Keywords:graphene  sheets  insulator  insulation 

Researchers at the University of California, Riverside report that bilayer graphene (BLG)—two graphene sheets stacked in a certain way—demonstrate an insulating property.

Graphene is nature's thinnest elastic material. It is a one-atom thick sheet of carbon atoms arranged in a planar hexagonal lattice with a chicken wire-like structure that makes the material easily stacked.

Like graphene, BLG has high current-carrying capacity or electron conductivity. The high current-carrying capacity results from the extremely high velocities that electrons can acquire in a graphene sheet.

However, researchers found that when the number of electrons on the BLG sheet is close to 0, the material becomes insulating, resisting the flow of electrical current. The finding has implications for the use of graphene as an electronic material in the semiconductor and electronics industries.

"BLG becomes insulating because its electrons spontaneously organise themselves when their number is small," said Chun Ning Lau, an associate professor of physics and astronomy and the lead author of the research paper. "Instead of moving around randomly, the electrons move in an orderly fashion. This is called 'spontaneous symmetry breaking' in physics, and is a very important concept since it is the same principle that 'endows' mass for particles in high energy physics."

An important finding of the research team is that the intrinsic "energy gap" in BLG grows with increasing magnetic field.

Single layer graphene (SLG) is gapless, however, and cannot be completely turned off because regardless of the number of electrons on SLG, it always remains metallic and a conductor.

graphene sheet

A scanning electron microscope image of a graphene sheet (red) suspended between two electrodes. The length of the graphene sheet shown is about 1/100 of the width of a human hair. Image credit: Lau lab, UC Riverside..

Novel materials
"This is terribly disadvantageous from an electronics point of view," said Lau. "BLG, on the other hand, can in fact be turned off. Our research is in the initial phase, and, presently, the band gap is still too small for practical applications. What is tremendously exciting though is that this work suggests a promising route—trilayer graphene and tetralayer graphene, which are likely to have much larger energy gaps that can be used for digital and IR technologies. We already have begun working with these materials."

Allan MacDonald, the Sid W. Richardson Foundation Regents Chair in the Department of Physics at The University of Texas at Austin and a co-author of the research paper, noted that the team has measured the mass of a new type of massive quantum particle that can be found only inside BLG crystals.

"The physics which gives these particles their mass is closely analogous to the physics which makes the mass of a proton inside an atomic nucleus very much larger than the mass of the quarks from which it is formed," he said. "Our team's particle is made of electrons, however, not quarks."

MacDonald explained that the experiment the research team conducted was motivated by theoretical work which anticipated that new particles would emerge from the electron sea of a BLG crystal.

The physicists reported their experimental results in Nature Nanotechnology.

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