Bilayer graphene works as an insulator

January 24, 2012, University of California - Riverside
The image shows a bilayer graphene schematic. The blue beads represent carbon atoms. Credit: Lau lab, UC Riverside

A research team led by physicists at the University of California, Riverside has identified a property of "bilayer graphene" (BLG) that the researchers say is analogous to finding the Higgs boson in particle physics.

Graphene, nature's thinnest , is a one-atom thick sheet of arranged in a . Because of graphene's planar and chicken wire-like structure, sheets of it lend themselves well to stacking.

BLG is formed when two graphene sheets are stacked in a special manner. Like graphene, BLG has high current-carrying capacity, also known as high electron . The high current-carrying capacity results from the extremely high velocities that electrons can acquire in a .

The report online Jan. 22 in Nature that in investigating BLG's properties they found that when the number of electrons on the BLG sheet is close to 0, the material becomes insulating (that is, it resists flow of electrical current) – a finding that has implications for the use of graphene as an electronic material in the semiconductor and electronics industries.

"BLG becomes insulating because its electrons spontaneously organize themselves when their number is small," said Chun Ning (Jeanie) 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."

Lau explained that a typical conductor has a huge number of electrons, which move around randomly, rather like a party with ten thousand guests with no assigned seats at dining tables. If the party only has four guests, however, then the guests will have to interact with each other and sit down at a table. Similarly, when BLG has only a few electrons the interactions cause the electrons to behave in an orderly manner.

New quantum particle

Allan MacDonald, the Sid W. Richardson Foundation Regents Chair in the Department of Physics at The University of Texas at Austin and a coauthor on the research paper, noted that 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."

Photo shows 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. Credit: Lau lab, UC Riverside

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.

"Now that the eagerly anticipated particles have been found, future experiments will help settle an ongoing theoretical debate on their properties," he said.

Practical applications

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

In solid state physics, an energy gap (or band gap) refers to an energy range in a solid where no electron states can exist. Generally, the size of the energy gap of a material determines whether it is a metal (no gap), semiconductor (small gap) or insulator (large gap). The presence of an in silicon is critical to the semiconductor industry since, for digital applications, engineers need to turn the device 'on' or conductive, and 'off' or insulating.

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

"This is terribly disadvantageous from an electronics point of view," said Lau, a member of UC Riverside's Center for Nanoscale Science and Engineering. "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 , which are likely to have much larger energy gaps that can be used for digital and infrared technologies. We already have begun working with these materials."

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More information: … /nnano.2011.251.html

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not rated yet Jan 24, 2012
The band gap measured was 2 meV. A similar level of voltage / electric field can remove the gap (Ie go to 0ev). This suggests to me that very low energy operation of devices built with bilayer graphene may be possible, if it is not disturbed by temperature or impurities.
not rated yet Jan 25, 2012
"Superinsulation" used to preserve the contents of liquid helium tanks consists of alternating layers of good heat conductors (incidentally also good reflectors) with layers of good insulators (and a vacuum is a great insulator).
Graphene layers, perhaps spaced apart by SF6 molecules or some such, would make an excellent ultra-thin superinsulator.
not rated yet Jan 28, 2012
Does anyone now the best method for making graphene for commercial applications ? Scotch-tape, chemical exfoliation, chemical vapor deposition, induced growth, graphite oxide reduction or another I'm unaware of? Which one is most likely to become the standard for graphene production and why?
not rated yet Jan 28, 2012
It depends on the usage planned. The low quality graphene for touch screens is produced here with copper sheet method http://www.techno...3/page1/
1 / 5 (1) Feb 11, 2012
"[The] electrons spontaneously organize themselves when their number is small." This article illustrates, once again, why physicists should study Art Winfree's theory of coupled oscillators, circa 1968. Art proved that limit cycle oscillators have a tendency to self-organize. That's spontaneous symmetry breaking. Happens all the time in physics. Electrons are limit cycle oscillators. With graphene, the atoms are more orderly than in other materials: a sheet one atom thick, regular hexagonal arrangement. And the sheets stack well. So the atoms are nicely ordered. From that good starting point, the electrons are likely to self-organize their oscillations.

My thesis is that Art Winfree's "law" of coupled oscillators applies broadly to physics. Examples: phase transitions, superconductivity low-temp and high, and countless other examples, including this graphene result. All of physics is quantum oscillations and derivations thereof, and that is the stuff of Art's law.
1 / 5 (1) Feb 12, 2012
Continuing with my prior post, Art Winfree's theory has stood the test of time. Mathematicians have confirmed and extended his law of coupled oscillators (Kuramoto, Steve Strogatz, Ian Stewart, Mirollo), and biologists have found numerous biological applications. Physicists have ignored his work.

Turning to the graphene article above, portions of the article miss the mark. The electrons self-organize not essentially because their "numbers are small." They self-organize because they are part of atoms that are themselves organized, in graphene, in a simple, repetitive, two dimensional system. When two graphene sheets are stacked, they stack in an exceptionally orderly fashion. That makes it easy for their electrons to self-organize--position, proximity, and orientation are all just right.

The article omits an important detail. What exactly is the pattern of organization? Winfree's law identifies the exact patterns that will arise.
1 / 5 (1) Feb 12, 2012
Looking ahead, the scientists envision three and four layer sheets of graphene. Again, an important detail will be the exact patterns in which the electrons self-organize. I predict that any and all patterns will be ones that were specified by Winfree, 44 years ago.

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