Nanoelectronics: graphene gets a better gap.

atomic scale, their results can be compared to the predictions of density functional theory. Not surprisingly, these calculations suggest that the clusters deform when the tip makes contact with them, and that certain arrangements of the metal clusters are unstable — in agreement with the experimental observations. In particular, the researchers find that the contact conductance of the tip–molecule–metal structure gets larger as the number of copper atoms in the cluster increases from one to five. Whereas this result is expected, they also show that the normalized conductance (the conductance divided by the number of atoms in the cluster) reaches a maximum for clusters that contain five atoms. The same trend is found in quantum transport calculations and is illustrated by the extent of the electron wave function across the interface between the cluster and the C 60 molecule (see Fig. 4 of ref. 5). Hence a cluster of fewer than five copper atoms is a 'bad' contact: that is, the contact resistance depends on the number of atoms in the cluster and the measured contact conductance is a mix of the conductance of the contact and the molecule. However, for clusters containing more than five atoms, the measured conductance is more intrinsic to the molecule because it has a weak dependence on the number of atoms in the cluster. Schull and co-workers refer to such a contact as a 'good' contact. There are several aspects that make this work truly noteworthy beyond the demonstration of this utmost precision in the contact formation. First, when the cluster contains five or so copper atoms, the conductance through the entire device is comparable to the quantum of conductance (G 0 = 2e 2 /h where e is the charge on the electron and h is Planck's constant), which suggests that a C 60 molecule is indeed a good electrical conductor owing to its delocalized electron system. Second, it has been suggested that C 60 molecules could be used to connect metal electrodes to the large organic molecules that are of interest for applications in molecular electronics 6. Finally, extending the distinction between 'good' and 'bad' to the atomic scale might prove to be the most salient feature of this work. D espite its impressive electronic properties, large-area graphene is not well suited to controlling electric currents because it is a semimetal, rather than a semiconductor. Therefore there is no energy gap separating …