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Twist Solves Bilayer Graphene Mystery Print

Researchers have discovered a unique new twist to the story of graphene and, in the process, appear to have solved a mystery that has held back device development. Working at ALS Beamline 7.0.1, a research team applied angle-resolved photoelectron spectroscopy (ARPES) to bilayer graphene. Through direct band-structure measurements and calculations, they discovered that in the stacking of graphene monolayers, subtle misalignments arise, creating an almost imperceptible twist in the final bilayer graphene. Tiny as it is—as small as 0.1 degree—this twist can have surprisingly strong effects on the bilayer graphene's electronic properties.

The On/Off Problem

Electrons can race through graphene at nearly the speed of light—100 times faster than they move through silicon. In addition to being superthin and superfast when it comes to conducting electrons, graphene is also superstrong and superflexible, making it a potential superstar material in the electronics and photonics fields, the basis for a host of devices, starting with ultrafast transistors.

One big problem, however, has been that graphene's electron conduction can't be completely stopped, an essential requirement for on/off devices. The on/off problem stems from monolayers of graphene having no bandgaps—ranges of energy in which no electron states can exist. Without a bandgap, there is no way to control or modulate electron current and therefore no way to fully realize the enormous promise of graphene in electronic and photonic devices.

Previous research suggested that one way around this problem was to bring two layers of graphene together in a bilayer. The presence of a second layer changes the electronic structure just enough to allow the precise engineering of controlled bandgaps through the application of an external electric field. When tested in transistor-like geometries, however, the "off" state was not quite complete; a surprisingly high amount of current still passed through the device. In this work, Kim et al. demonstrate that a slight twisting of the layers explains why.

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 courtesy of Keun Su Kim.)

Graphene, an atomically thin carbon layer whose atoms are arranged in a honeycomb lattice, is an exciting new material with many unique properties. For example, its electrons have the highest room-temperature mobility of any material, making it a very attractive material for future semiconductor devices. A major drawback, however, is graphene's lack of a bandgap—the feature of the electronic structure that signifies the ability to control the flow of electrons.

Bandgap engineers went to work and found that when two layers of graphene are stacked, interactions between the layers allow the creation of a bandgap by chemical doping or the application of an external electric field perpendicular to the layers. In practical tests of actual devices, however, the gap did not live up to theoretical predictions. An unexpected amount of current would continue to flow even when the device should be switched "off." To explain these anomalous results, a number of theories involving strain-induced distortions, magnetism, or nematic symmetry breaking were proposed, but direct evidence for which class of model applied had been missing.

To tackle this mystery, researchers performed a series of ARPES experiments at ALS Beamline 7.0.1. ARPES is an ideal tool for addressing these issues because it is exquisitely sensitive to the symmetry of electronic states. The results showed that a new kind of lattice defect, in which the graphene layers have a vanishingly small relative twist, can explain the experimental data.

The researchers compared samples of bilayer graphene stacked in the normal, slightly offset, alignment (AB) with those in a direct-stacking alignment in which the atoms in each layer line up directly above and below one another (AA). Direct AA stacking is energetically unfavorable, but only by a very slight margin. Therefore, in response to local stress, bilayer graphene is more likely to induce a twist involving thousands of atoms rather than create an atomic-scale defect.

Model bilayer graphene electronic structures in an electric field for (a) standard AB stacking, (b) direct AA stacking, and (c) twisted AA stacking. Experimental data (d) shows the symmetry-broken spectral function pattern at the Dirac energy ED, characteristic of twisted bilayer graphene. The inset, which shows a constant-energy cut of the energy band structure, provides crucial evidence for the rotation between layers. The intensity is symmetric about the diagonal dashed line, which reflects a kind of symmetry breaking that is present neither in the photoemission measurement geometry, nor in AA and AB stacked graphenes, but is a consequence of the twist between graphene layers.

The introduction of the twist generates a completely new electronic structure that indicates the presence of both massive and massless Dirac fermions. The massless Dirac fermion branch of this new structure prevents bilayer graphene from becoming fully insulating even under a very strong electric field. Massless Dirac fermions, electrons that essentially behave as if they were photons, are not subject to the same bandgap constraints as conventional electrons. The twists that generate this massless Dirac fermion spectrum may be nearly inevitable in the making of bilayer graphene and can be introduced as a result of only ten atomic misfits in a square micron of bilayer graphene.

Now that scientists understand the problem, they can search for solutions. For example, they can try to develop fabrication techniques that minimize the twist effects, or reduce the size of the bilayer graphene to have a better chance of producing locally pure material. Beyond solving a bilayer graphene mystery, the discovery of the twist establishes a new framework on which various fundamental properties of bilayer graphene can be more accurately predicted. 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.



Research conducted by: Keun Su Kim (ALS and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany); Andrew L. Walter (Donostia International Physics Centre, Spain); Luca Moreschini, Eli Rotenberg, and Aaron Bostwick (ALS); Thomas Seyller (Technische Universität Chemnitz, Germany); and Karsten Horn (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany).

Research funding: U.S. Department of Energy (DOE), Office of Basic Energy Sciences (BES); the National Research Foundation of Korea; the Max Planck Society; the Swiss National Science Foundation; and the German Research Foundation. Operation of the ALS is supported by DOE BES.

Publication about this research: Keun Su Kim, Andrew L. Walter, Luca Moreschini, Thomas Seyller, Karsten Horn, Eli Rotenberg, and Aaron Bostwick, "Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene," Nature Materials 12, 887 (2013).

ALS Science Highlight #285


ALSNews Vol. 351