In each case, an intrinsic property of graphene must be overcome for the material to be suitable for a transistor. As Tao explains, "a transistor is basically a switchyou turn it on or off. A graphene transistor is very fast but the on/off ratio is very tiny. " This is due to the fact that the space between the valence and conduction bands of the materialor band gap as it is knownis zero for graphene.
In order to enlarge the band gap and improve the on/off ratio of the material, larger sheets of graphene may be cut down to nanoscale sizes. This has the effect of opening the gap between valence and conductance bands and improving the on/off ratio, though such size reduction comes at a cost. The process is laborious and tends to introduce irregularities in shape and impurities in chemical composition, which somewhat degrade the electrical properties of the graphene. "This may not really be a viable solution for mass production," Tao observes.
Rather than a top down approach in which sheets of graphene are reduced to a suitable size to act as transistors, Tao's approach is bottom upbuilding up the graphene, molecular piece by piece. To do this, Tao relies on the chemical synthesis of benzene rings, hexagonal structures, each formed from 6 carbon atoms. "Benzene is usually an insulating material, " Tao says. But as more such rings are joined together, the material's behavior becomes more like a semiconductor.
Using this process, the group was able to synthesize a coronene molecule, consisting of 13 benzene rings arranged in a well defined shape. The molecule was then fitted on either side with linker groupschemical binders that allow the molecule to be attached to electrodes, form
|Contact: Richard Harth|
Arizona State University