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The Dirac spectrum of bilayer graphene when the two layers are exactly aligned (left) shifts with a slight interlayer twist that breaks interlayer-coupling and potential symmetry, leading to a new spectrum with surprisingly strong signatures in ARPES data. Image Credit: Image courtesy of Keun Su Kim, Fritz Haber Institute.
It has long been thought that ‘wonder material’ graphene will eventually herald a manufacturing revolution in the electronics and photonics industries. Yet these hopes are currently still purely theoretical. Now a surprising twist may show why this revolution has yet to come to fruition.
As monolayers of graphene have no bandgaps, this means that electron conduction is all but
impossible to control – a necessity when trying to produce devices with on-off capabilities.
To try and counter this issue, researchers at Berkley have used an external electrical field to engineer controlled bandgaps in bilayer graphene, with the hope of better controlling the conduction of the material.
However, when these devices were put into practise, they did not act as hoped, with conduction in the bandgaps not fully brought to a halt.
Now, a Berkeley Lab team led by Aaron Bostwick believe they have found the reason for this – almost imperceivable misalignments that lead to twists in the final graphene structure.
It was found that these twists leads to the generation of massless Dirac fermions (in essence electrons that behave like photons), which do not adhere to the same bandgap constraints as normal electrons. This means that the graphene is prevented from becoming fully insulating.
Even though the twists are extremely small, sometimes as little as 0.1 degree difference, they can drastically alter the properties and usability of graphene.
Aaron Bostwick explains below:
"The introduction of the twist generates a completely new electronic structure in the bilayer graphene that produces massive and massless Dirac fermions," "The massless Dirac fermion branch produced by this new structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. This explains why bilayer graphene has not lived up to theoretical predictions in actual devices that were based on perfect or untwisted bilayer graphene."
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Aaron Bostwick at Berkeley Lab's Advanced Light Source led the discovery of a tiny twist in the formation of bilayer graphene that has a large impact on electronic properties. Image Credit: Photo by Roy Kaltschmidt, Berkeley Lab.
Eli Rotenberg, a co-author on a new paper outlining these findings published in Nature Materials, talks below about the experimental procedure utilised to attain their results.
"The combination of ARPES and Beamline 7.0.1 enabled us to easily identify the electronic spectrum from the twist in the bilayer graphene," "The spectrum we observed was very different from what has been assumed and contains extra branches consisting of massless Dirac fermions. These new massless Dirac fermions move in a completely unexpected way governed by the symmetry twisted layers."
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Eli Rotenberg oversees research at ALS Beamline 7.0.1, a premier facility for determining the electronic structure of metals, semiconductors, and insulators. Image Credit: Photo by Roy Kaltschmidt, Berkeley Lab
Here is a little reminder of why graphene could be such a beneficial material to electronics manufacturers: Electrons move 100 times faster through graphene than they move through silicon, it is the thinnest material known to man and yet also one of the strongest, and it is extremely dense, but almost transparent.
This new research furthers the understanding of graphene as a whole, but also provides another step towards the commercialization of graphene.
Keun Su Kim, lead author of the recent paper, sums up the key benefits of this research:
"Now that we understand the problem, we can search for solutions”. "For example, we can try to develop fabrication techniques that minimize the twist effects, or reduce the size of the bilayer graphene we make so that we have a better chance of producing locally pure material." "A lesson learned here is that even such a tiny structural distortion of atomic-scale materials should not be dismissed in describing the electronic properties of these materials fully and accurately."
Original source: DOE/Lawrence Berkeley National Laboratory
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