Twisting graphene to make electrons flow

 By Eniday Staff

Italian borrows, and often butchers, a lot of words from English for actions and habits, often to do with computer processes. These words describe gestures that would otherwise be difficult to define as precisely and quickly…

Let’s start with two examples: in Italian, the verb “flaggare” can mean drawing a moustache on yourself with a pen or ticking a box on a computer screen with a mouse. The discipline that combines industrial expertise in the fields of mechanics, electronics and IT to create equipment that can carry out complex operations is called “meccatronica”. So, what would the Italians christen a discipline that took different layers of material and removed the molecules within them to create a system that behaved very differently? The answer is twistronica, from “twist” (in the sense of turning and bending, not the dance Chubby Checker urged us all to do way back in 1960).

Twistronics and graphene

And twistronics, as we call them, are a big deal, especially since 30 October last year, when an article appeared in Nature about the recent studies done on the behaviour of graphene by a team of international researchers led by Dmitri Efetov, a professor at the Institute of Photonic Sciences (ICFO), a Spanish research centre of excellence working on the science and technology of light. To sum it up, we can do a lot more with graphene than previously thought. It is a material made up of a layer of carbon, with a hexagonal grill structure (sort of like a chicken coop) and is infinitely thin, the width of just one atom, that is to say about half a millionth of a millimetre. Graphene was invented in in 2004 by Andrej Gejm and Konstantin Novoselov, at the laboratories of Manchester University, and won the pair the Nobel Prize in Physics in 2010. It is a quite extraordinary material, light, more resistant than steel, an excellent thermal and electric conductor and almost transparent. When it came onto the scene, it looked like an ideal material, one that would revolutionise technological research in various fields, from electronics to mechanics to optics. But it revealed itself to be tricky to exploit on an industrial scale. ICFO’s discovery filled the lacuna between theoretical and practical use. So far, the hexagonal grids had been perfectly superimposed when each atomic sheet of graphene was placed on top of the other. This was done to create a structure suited to easy use, for example for filtration systems. Different configurations were tried, and the Massachusetts Institute of Technology found that when they layered two sheets of graphene on top of each other with a tiny difference in orientation between the hexagons, 1.1 degrees to be precise, they changed the behaviour of the material’s electrons. It is a little like when light hits a photograph with a moiré, or rippling, effect and produces an image that seems 3D despite not being so.

A graphic representation of two graphene sheets superimposed with twistronics (Kai Fu, Yazdani Lab, Princeton University)

When modified like this, graphene turns out to be a truly perfect conductor, so much so that when subjected to a temperature close to absolute zero, namely -270°, it becomes a superconductor, the equal of many other materials, from aluminium to zinc, always retaining its plasticity and resistance. (Just to recap, superconductors are ones that offer little or no resistance to currents. They can be used to create very powerful electromagnets, for example in magnetic resonance machines.)

Endless new possibilities

Now, as the Nature article explains, they have created graphene elements far larger than before. Playing around with the spatial orientation of the hexagonal grids in layers of graphene prompts different behaviour. The sheets of graphene are placed between two conductive panels, increasing or decreasing the flow of electrons depending on the tension applied to them. This is done until the graphene is emptied or filled and takes on the form of a superconductor or an insulator, going through a range of intermediate states with different levels of magnetism in the process. Simply rotating the hexagons in relation to each other, you produce infinite configurations. And perhaps this will clear the way for new strains of research, into superconductor systems that operate at ever higher temperatures and are therefore more manageable technologically speaking.

READ MORE: Eni’s art of chemistry by Nicholas Newman

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Eniday Staff