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Farjand
Farjand • Jul 16, 2011

Graphene Reveals More Properties - Berkeley Lab

Graphene is an important material in today's technologies starting from batteries and metamaterials and ending at still undiscovered techniques. Now scientists, taking an important step in uncovering the properties of this curious material, have stumbled upon some facts as to how undoped Graphene acts near "Dirac point" or the valance band.

The research group led by David Siegel points, a graduate student in Berkeley Labs, used a specially prepared Graphene sample called ALS beamline 12.0.1 and experimented it with Angle Resolved Photoemission spectroscopy (ARPES). The material gave an insight to the team of how Graphene reacts at the Dirac point. Speaking of Graphene, Siegel said that it is a special material with curious properties. However it is not a metal rather it is a semiconductor that too with different properties than that of usual semiconductors. Unlike other semiconductors, Graphene does not have any gap in between two valance bands. So in Graphene the two valance bands are represented by the two crossing lines (X shaped). The point at the crossing of these two lines is called the Dirac point.

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Dirac cones of graphene are often drawn with straight sides (left) indicating a smooth increase in energy, ARPES spectrum near the Dirac point of undoped graphene (sketched in red at right)

Students, using ARPES, measured the angle of electron movement when the Graphene was excited with X-rays. They were also able to plot Kinetic energy of the moving electrons. Researchers observed a spectrum in the shape of a cone, as the electrons struck the detector screen.

The major problem faced by researchers was, any accurate measurement to be done, is possible only if the sheet of Graphene is suspended in a medium. However, there are many such experiments which cannot be performed unless the sheet is resting on a solid substrate. The solid surface almost always tends to alter the properties of sheet there by affecting the accuracy of experiments. The team found a potential solution in "Quasi Freestanding Graphene" for which the used silicon carbide as a substrate. During experiments, the substrate is heated thereby driving out the silicon in it. The carbon left in the compound gathers on surface as a thick layer of Graphite and the Graphene in the Graphite sample is rotated with respect to each other. This makes the Graphene layer a single isolated layer still supported by Substrate.

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long-range-interactions

After a thorough research on the Graphene layer so obtained, researchers found that Graphene's properties differ from the ordinary Fermi liquid of most of the material. This was evident from the fact that there is long-range interaction among electrons in semi-metallic graphene. In case of normal metals these interactions would be screened. The team was successful in confirming the presence of unscreened, long-range interactions capable of altering the quasi particle of Graphene in a fundamental way. The research would prove to be beneficial in exploiting Graphene's properties in future. The paper in this regard is published in Proceedings of the National Academy of Sciences. 

Source: Berkeley Lab

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