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Scientists predict a relative of graphene: 'phagraphene'

03 September 2015

Scientists from Russia, the US and China use computer simulation to predict the existence of a 'patchwork' analogue of graphene, dubbed 'phagraphene'.

Phagraphene structure (illustration courtesy of Artyom Oganov, Moscow Institute of Physics and Technology)

"Unlike graphene, a hexagonal honeycomb structure with atoms of carbon at its junctions, phagraphene consists of penta-, hexa- and heptagonal carbon rings," says lead researcher, Artyom Oganov from the Moscow Institute of Physics and Technology. "Its name comes from a contraction of Penta-Hexa-heptA-graphene."

Most materials can transmit electric current when unbound electrons have an energy that corresponds to the conduction band of the material. When there is a gap between the range of possible electron energies, the valence band, and the range of conductivity (the so-called forbidden zone), the material acts as an insulator. When the valence band and conduction band overlap, it acts as a conductor, and electrons can move under the influence of an electric field.

In graphene each carbon atom has three electrons that are bound to electrons in neighbouring atoms, forming chemical bonds. The fourth electron of each atom is 'de-localised' throughout the whole graphene sheet, which allows it to conduct electrical current. At the same time, the forbidden zone in the graphene has zero width. If you plot the electron energy and their location in graph form, you get a figure resembling an hour glass - two cones connected by vertices (Dirac cones).

It is this that makes electrons in graphene behave rather strangely: all of them have one and the same velocity (which is comparable to the velocity of light), and they possess no inertia; they appear to have no mass. And, according to the theory of relativity, particles travelling at the velocity of light must behave in this manner. The velocity of electrons in graphene is about ten thousand kilometres a second (electron velocities in a typical conductor vary from centimetres up to hundreds of metres per second).

Phagraphene, discovered by Oganov and his colleagues through the use of the USPEX algorithm, as well as graphene, is a material where Dirac cones appear, and electrons behave in a similar way to particles without mass.

"In phagraphene, due to the different number of atoms in the rings, the Dirac cones are 'inclined'," Oganov explains. "That is why the velocity of electrons in it depends on the direction. This is not the case in graphene. It would be very interesting for future practical use to see where it will be useful to vary the electron velocity."

Phagraphene possesses all the other properties of graphene that allow it to be considered an advanced material for flexible electronic devices, transistors, solar batteries, display units and many other things.

The team's work is described in the journal Nano Letters.

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