Mar 1, 2012
Could graphynes be better than graphene?
The "wonder material" graphene might be in for some competition from a new group of materials called graphynes – according to computer simulations done in Germany. Like graphene, a graphyne is a sheet of carbon just one atom thick. But while graphene can only exist with a honeycomb lattice structure, graphynes can assume several different 2D structures.
This latest work suggests that graphynes have unusual and potentially useful electronic properties characterized by "Dirac cones", which were once thought to be unique to graphene. Indeed, one type of graphyne with a rectangular lattice is particularly interesting because of the effect that its geometry has on the Dirac cones – something that could prove useful in developing new types of carbon-based electronic devices.
Ever since graphene was first created in 2004, its unique electronic and mechanical properties have amazed researchers. Indeed, many claim that it could be used in a host of device applications, even rivalling silicon as the electronics material of choice in the future.
Graphene's outstanding electronic properties come thanks to its peculiar band structure that features so-called Dirac cones. These are double-cone features in the band structure of the 2D material where the conduction and valence bands meet in a single point at the Fermi level. The bands approach this point in a linear fashion. As a result, the effective kinetic energies of the conduction electrons (and holes) are directly proportional to their momentum.
This unusual relationship is normally only seen for photons, which are massless, since the energies of electrons and other particles of matter at non-relativistic velocities usually show a quadratic relationship with momentum – that is, their energies depend on the square of their momenta. The result is that the electrons in graphene behave as though they are relativistic particles with no rest mass, and so can whiz through the material at extremely high speeds. This property could be exploited to make transistors that are faster than any that exist today.
Only in graphene?
Researchers had thought that Dirac cones could only exist in graphene, thanks to its hexagonal honeycomb structure. But now, Andreas Görling and colleagues at the University of Erlangen-Nürnberg have turned this idea on its head by calculating the electronic properties of a new group of materials called graphynes.
Graphynes are 2D carbon allotropes similar in structure to graphene but built from doubly- and triply-bonded carbon atoms instead of just double bonds as in graphene. The presence of triple bonds means that graphynes can exists in geometries other than the hexagonal lattice of graphene. In their work, Görling and colleagues studied the band structure of three graphynes – α-graphene, β-graphene and 6,6,12-graphyne (see image) – using density-functional-theory calculations. The first two graphynes have hexagonal structures and the third is rectangular, but the results clearly show that all three graphynes possess Dirac cones.
The 6,6,12-graphyne may be even more amazing than graphene for two reasons, says Görling. First, thanks to its rectangular symmetry, it should have electronic properties that are direction dependent – that is, they will be different along different directions in the plane of the material. For example, this might lead to a conductance that depends on the direction of the current. This property is not seen in graphene, which is (almost) isotropic in the plane of the material. This directional dependence could be put to good use in future nanoscale electronic devices.
Two cones are better than one
Second, graphyne appears to have two different Dirac cones lying slightly above and below the Fermi level. "This means that graphyne is 'self-doped',"Görling told nanotechweb.org, "and naturally contains conducting charge carriers (electrons and holes), without the need for external doping – unlike graphene." As a result, it could be used as a semiconductor in electronic devices.
The researchers hope that their calculations will encourage other researchers to create graphyne and investigate its properties in the lab. "In the long run, if materials other than graphene with Dirac cones can be produced, these may be used in carbon-based electronic nanodevices," adds Görling. So far, however, only extremely small samples of graphynes have been made in the lab.
For its part, the Erlangen-Nürnberg team is continuing its research into graphene alternatives and says it is already in touch with organic and physical chemists with the aim of making large quantities of graphynes and other new 2D carbon allotropes.
Andre Geim of the University of Manchester in the UK, who was awarded the 2010 Nobel Prize for Physics for discovering graphene, says that graphyne is "an extremely interesting material" and that "this report adds to the excitement".
The simulations are described in Physical Review Letters.
About the author
Belle Dumé is a contributing editor to nanotechweb.org