A new carbon allotrope, phagraphene (standing for penta-hexa-hepta-graphene), has been discovered by an international team of physicists. It consists of penta-, hexa- and heptagonal carbon rings, and is a ‘patchwork’ analogue of graphene.
Structure of phagraphene. Image credit: Zhenhai Wang et al.
Two-dimensional materials have attracted attention of scientists for many years. The first of these materials, graphene, was discovered in 2004 by Andre Geim and Konstantin Novoselov from University of Manchester, who received the Nobel Prize in Physics for this in 2010.
Graphene is a material made of a single layer of carbon atoms arranged in a hexagonal lattice. Being a million times thinner than a human hair, it is the thinnest object ever created. Not only is graphene lightweight and flexible, it is also the world’s strongest material. It also conducts electricity faster than most other materials and if stacked in layers it forms graphite.
In graphene each carbon atom has three electrons that are bound to electrons in neighboring atoms, forming chemical bonds. The fourth electron is ‘delocalized’ 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. These are known as Dirac cones.
Due to this unique condition, electrons in graphene behave very 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. The velocity of electrons in graphene is about 10,000 km a second.
Phagraphene, as well as graphene, is a material where Dirac cones appear, and electrons behave similar to particles without mass.
“In phagraphene, due to the different number of atoms in the rings, the Dirac cones are inclined,” explained Dr Artyom Oganov of MIPT, senior author of a paper published in the journal Nano Letters.
“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 allows it to be considered an advanced material for flexible electronic devices, transistors, solar batteries, display units and many other things.”
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Zhenhai Wang et al. Phagraphene: A Low-Energy Graphene Allotrope Composed of 5–6–7 Carbon Rings with Distorted Dirac Cones. Nano Lett., published online August 11, 2015; doi: 10.1021/acs.nanolett.5b02512
