Minute though it may be, the carbon atom is single-handedly responsible for an entire branch of chemistry, and lies at the base of most of biology. But the remarkable properties also extend to its allotropes – materials consisting of pure carbon. These include diamond, fullerenes, nanotubes, and graphene, names that will ring familiar to many.
Graphene consists of carbon atoms surrounded by three neighbors, arranged along a honeycomb lattice. Because this arrangement is entirely planar, it is effectively a two-dimensional crystal, forming extended sheets with single-atom thickness. Its electronic structure and regularity convey some fascinating properties, such as an exceptional electric conductance, a tunable bandgap, and high flexibility, while serving as a model system for relativistic physics.
Natural graphene occurs only in graphite, which consists of tiny crystals containing stacks of weakly-bound sheets. Because the stacking causes many of the interesting properties to disappear, the isolation or preparation of graphene sheets has become the focus of intense research. Now Korean researchers have reported a novel breakthrough, presenting details on a technique that allows the preparation and processing of individual graphene sheets of high quality [Kim et al, Nature, 10.1038/nature07719].
Their approach is based on epitaxial growth on a nickel substrate. After heating to 1000 °C, exposure to a carbon source, and cooling, one or a few sheets of graphene are formed on the surface, covering an area of over a square centimeter.
Probing the electronic properties of the films revealed very little defects, demonstrating that the carbon atoms are continuously connected throughout the film. At the same time the high flexibility of the sheets allows convenient processing by stamping or chemical etching, followed by transfer to a substrate of choice. In fact, the researchers believe that these excellent properties make it one of the best materials around for stretchable and transparent electronics, possibly replacing indium tin oxide (ITO) in the future.
During their study the researchers found that a thickness of a few hundred nanometers for the nickel substrate was essential, as using a thicker substrate resulted in the uptake of a large amount of carbon atoms, ultimately leading to the formation of several graphene layers. Carefully controlling the cooling rate was also instrumental, as cooling too fast reduces the mobility of the carbon atoms, while cooling too slowly leads to the absorption of the carbon in the nickel substrate.