'Straintronics' Debuts in Graphene
(PhysicsWorld.com) - Just when you thought that graphene could do no more, physicists in the US have found yet another new and unique property of the wonder material. The team has shown that when graphene – a sheet of carbon just one atom thick – is stretched in a particular way, its electrons behave as though they are in strong magnetic field. The phenomenon, which had been predicted in theory, could be used to modify the electronic properties of graphene to make strain gauges or even transistors and optoelectronics components.
Since graphene was first created in 2004, its unique electronic and mechanical properties have amazed researchers, who have been eyeing up the material for a host of practical applications. In particular, it could be used to make ultrafast transistors because the electrons in graphene whizz through the material at extremely high speeds.
This latest property of graphene was discovered by Michael Crommie and colleagues at the University of California at Berkeley and the Lawrence Berkeley National Labs, who were trying to grow a graphene layer on the surface of a platinum crystal. It turns out that carbon atoms in graphene – which are organized in a hexagonal lattice – do not line up perfectly with the triangular crystal structure of the platinum. The resulting strain in the graphene causes tiny triangular structures to push out of the surface.
Moving in circles These graphene "nanobubbles" are about 4–10 nm long and about 2 nm high. Because the structures are very small, the electrons within each nanobubble inhabit discrete energy levels. This is unlike electrons in unstrained graphene, which occupy continuous energy bands. These discrete energy levels, or "Landau levels", are the same as those an electron would occupy if it were moving in circles in an extremely high magnetic field of around 300 Tesla.
The researchers observed the new electronic behaviour using a scanning tunnelling microscope (STM), which is able to determine the electron energy states in very small regions of the graphene's surface.
A likely explanation for why the electrons orbit in circles, according to Crommie, lies in how the distance between the carbon atoms changes with applied strain. If graphene is strained in just the right way then these differences combine to cause an electron's motion to bend just as if a magnetic field were present.
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