The carbon nanostructure revolution refuses to cease. First, it was carbon nanotubes, followed by graphene. After these two “hot” materials, it may now be the dawning of another wonder material called ‘Graphyne’. Graphyne may surpass even graphene in its electrical properties. While graphyne has been researched for the last 30 years, it has suddenly become a hot material for condensed matter physics.
Graphene is known for its extremely high conductivity owing to a peculiar property of graphene electrons. In graphene, the so-called valence and conduction bands touch. Near the points where the two bands touch, called the Dirac point, the energy-momentum relation of the electrons is linear (graph, right), instead of quadratic as seen for other particles. This leads to the mass of the charge carriers (electrons or holes) inside the material being effectively zero. This allows them to travel at extremely high speeds, giving rise to very high mobilities and superior conduction properties. It was precisely this that led to the 2010 Nobel Prize being awarded to Andre Geim and his student Konstantin Novoselov.
Graphene and Graphyne
Graphyne doesn’t really exist; it has to be synthesized using special techniques. However, computer simulations show that its conduction properties can be better than graphene. Graphyne is a 2D lattice, just like graphene, but with double and triple bonds, rather than just single bonds as it is with graphene. The graphene lattice is strictly hexagonal, while graphyne lattice can take up an arbitrary shape due to the presence of the double and triple bonds. In particular, it can take up a rectangular lattice shape.
The key to good conduction is not only high mobility of the electrons, but also directionality. The electrons should be free to travel in a straight line. For graphene, the lattice has no preferred direction, but for graphyne, the lattice being rectangular, prefers conduction in one direction over the other. This means that it has gating properties depending on the direction of passage of current.
Simulations and predictions
A recent paper in Physical Review Letters, by Andreas Gorling and colleagues (link) presents simulations of electronic properties of graphyne. They discuss the so-called 6,6,12-graphyne (pic above) and simulate its properties. Density functional simulations predict the presence of Dirac cones in graphyne, which were thought to be unique to graphene. Moreover, the conduction turns out to be superior to graphene.
We should stress the fact that graphyne has not been made in the laboratory in significant quantities as yet; only trace amounts have been fabricated. Only proper experiments on real samples can verify the simulation results, but Mikhail Katsnelson, a big name in the field of graphene physics, expresses confidence in the density functional methods. The next step would be to prepare proper graphyne samples for study. Only then can all the fancy experimental tests be applied.
New material on the block and a lot of new physics to be known – it’s a mouth-watering prospect for physicists.