Carbon tailoring

Even if there was surprise at the rapidity with which graphene garnered a Nobel prize, it has undoubtedly galvanized condensed-matter physics, not least because of its versatile electronic properties. Commercial applications are in the pipeline, and carbon seems sure to feature in future generations of microelectronics¹. One of the most promising features of graphene — single sheets of graphitelike carbon one atom thick — is the sensitivity of the electronic band structure to the sheet topology.

One of the easiest distortions to introduce, in principle, to graphene is the fold, because the bending energy at the folded edge can be compensated by the favourable interaction of the layered plane portions of the fold. Stimulated by carbon-nanotube nanotechnology, the microscopic folding of graphene sheets has a long history². Now Kim et al. have considered how the formation of folds, hems and pleats in a graphene sheet might introduce specific electronic defects and modifications, so that arrays and circuits of electronic devices can be made literally by a form of micro-tailoring of carbon³. They call such a folded graphene sheet 'grafold'.

Because graphene is not isotropic in plane, a fold has different energetic and electronic properties depending on the angle of the folded edge relative to the hexagonal symmetry of the underlying carbon lattice. Kim et al. import the language of textile tailoring to describe such folded structures as the tapered tuck (a wedge-shaped 'back and forward' double fold that introduce a relative rotation between the lattices of the upper and lower parts of the sheet) and the pleat (a parallel double fold that creates an extended trilayer strip).

Kim et al. calculate that a simple pleat has an altered electronic band structure, relative to the flat sheet. Both are semi-metals (with zero bandgap), but in different ways. In graphene there is in theory zero density of states where the conduction and valence bands meet, whereas in a pleat they overlap indirectly (that is, with a change in the electrons' momentum) and the occupied states are localized in the fold. This differentiates the electronic behaviour of folds from that of sheets. And folds offer further opportunities to manipulate the material properties: the multilayered structure can act as the host for intercalated substances such as ions, and the curved edges become regions of enhanced chemical reactivity, susceptible to functionalization.

It may in fact be hard to avoid such folds appearing spontaneously in graphene, especially when it is made by chemical vapour deposition — the strain introduced as the sheets cool from the high-temperature growth process can induce wrinkles that will collapse to stable folds such as pleats. Kim et al. show that a free-standing graphene sheet made this way is laced with such defects.

But can such folds be created systematically, rather than just by accident? Kim et al. show this can be done by depositing a graphene sheet on a corrugated metal substrate, then etching away the substrate and thereby allowing the graphene corrugations to collapse. Such ingenuity could usher in an unlikely alliance between microelectronics and textile drapery.