A new twist for graphene: Trying to solve the bandgap problem

Although it’s like a sheet of paper (if paper could be only one atom thick), but “Do not bend, fold, mutilate or spindle” does not apply to sheets of graphene. Scientists all over the world are doing all of the above and a lot more to graphene in search of its many surprising properties. From time to time, it’s instructive to dip into this research to see how the saga of graphene is developing since its ‘rediscovery’ in 2004.

As a sheet of pure carbon, graphene has some very tantalizing electronic properties. For one thing, electrons can zip through it at almost the speed of light – about 100 times faster than they can through the usual semiconductor material, silicon. It’s thin, strong, flexible – all the things an electrical engineer would want in an electronics material. Except for one thing, there is no way to stop the flow of electrons through graphene. In effect, there is no on or off switch.

Graphene lacks a bandgap, a range of energy (something like layers) in which various states of the electron flow can exist (from no flow to full speed). A big bandgap is typical of an insulating material – no electron flow. A medium bandgap is a semiconductor, where depending on conditions there may be more or less electron flow (or on/off). No bandgap is typical of a conductive material, which of course, applies to graphene. Without a bandgap – the essential property of semiconductors such as silicon – there is no easy way to control or modulate electron current. It’s kind of a showstopper for a potential titan of electronic materials.

That problem doesn’t stop scientists. It challenges them (especially when fame and fortune might attend a solution). This means that research on simulating, stimulating and otherwise jury-rigging bandgap properties in graphene has proliferated. One such attempt, which seems rather obvious, is to put two layers of graphene together (bilayer) leaving a small distance between them – et voila! Bandgap. Only it didn’t work, and nobody knew why.

This created a new challenges for scientists, which were picked up by researchers at the U.S. Department of Energy’s Berkeley Laboratory (California, USA). Researcher Aaron Bostwick discovered that in the process of stacking (layering) graphene, subtle misalignments were introduced – a twist, if you will, of as little as 0.1 degree. Surprisingly, he discovered these tiny twists strongly changed the electronic properties.

With this discovery in hand, Bostwick and colleagues trotted down to the lab’s angle-resolved photoemission spectrograph (ARPES), which throws a beam of X-ray photons at a material’s surface to study how electrons in the material behave under energy pressure. As the electrons sputter off the material, their angle and energy level is recorded, which provides an electronic spectrum. In this case, the spectrum was not what was expected. It showed that the bilayer graphene emitted a large quantity of massless Dirac fermions.

Massless Dirac fermions are close cousins of photons and they have a habit (property) of ignoring bandgaps – the flow of Dirac fermions across the graphene “bilayer bandgap” explained why, from the electronics component point of view, the graphene bilayer failed. The Berkeley Lab Advanced Light Source team published the finding in Nature Materials [28 July 2013, paywalled, Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene].

Mystery solved, but not of course, the end of research. First thing, now that they know what the problem is, there may be a way to produce bilayer graphene that removes or significantly reduces the “twist” that produces the fermions. Perhaps more importantly in the long run, this discovery opens a path to learning more about the electronic properties of bilayer graphene. From such knowledge, science often (but not necessarily) finds new ways of using the material.

Research Spectrum

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  1. Marceli Firlej
    Posted August 15, 2013 at 3:38 am | Permalink

    Dirac Fermions are not massless but massive, referring to:

  2. Posted August 15, 2013 at 5:08 am | Permalink

    By standard definition Dirac fermions are massive, but graphene under the conditions of a bilayer bandgap produces massless Dirac fermions, which behave more like photons than the protons or neutrons normally constructed by massive fermions. See: https://wiki.physics.udel.edu/phys824/Band_structure_of_graphene,_massless_Dirac_fermions_as_low-energy_quasiparticles,_Berry_phase,_and_all_that

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