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Basics of multilayer graphene properties

Graphene is constructed of a layer of carbon just of atom thick and due to its 2D nature has unique electronic properties which are very useful in making ultra fast electronic devices. Researchers of US and UK have explained why different samples of multilayered graphene have very different electronic properties.

Dirac fermions

Graphene conducts electrons that travel near to the speed of light with zero mass called Dirac fermions. According to theory, graphene having multilayer of several atoms thick should not contain Dirac fermions because electron coupling between layers destroys its 2D nature. However, Dirac fermions have been spotted in some multilayer grown by depositing carbon atoms on surfaces, which has puzzled physicists.

Multilayer study

Eva Andrei and colleagues at Rutger's University, Massachusetts Institute of Technology and the University of Manchester have found that the relative angular orientation between successive layers plays a key role in whether or not a multilayer contains Dirac fermions. The team has created multilayer samples by depositing carbon on to a nickel surface and the graphene is then lifted off the surface using chemical. They studied the sample using a transmission electron microscope to calculate the relative angle between the 2D lattices of each layer. The presence of Dirac fermions was determined using Landau level spectroscopy, whereby a magnetic field is applied to the material. This causes the electrons in each layer to adopt quantized circular orbits or Landau levels. The energies of these levels are measured using scanning tunnelling spectroscopy and are distinct for Dirac fermions.

Twisted graphene

The team looked at samples where the orientation of graphene layers was close to the most common stacking scheme (Bernal), whereby successive layers are rotated by 60° to each other. They found that when successive layers were offset by about 22° from Bernal stacking, the electrons behaved just like Dirac fermions found in single layers. However, at much smaller rotation angles of about 4°, the velocity of the electrons had dropped to about 80% of that in a single layer. A sample studied with a rotation angle of about 1.2° had no evidence of Dirac fermions. Andre Geim of the University of Manchester speculates that the Dirac fermions are seen because the rotation breaks the spatial symmetry between the layers. This could reduce the coupling between layers, making each layer a 2D system. These transfer methods of giving a twist will open up numerous possibilities for graphene to be patterned with regions to create electronic devices.

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