Nanoparticles: a very versatile nanocapsule.

nature nanotechnology | VOL 2 | APRIL 2007 | www.nature.com/naturenanotechnology 201 density of charge carriers is zero. Moreover, all the superconducting transistors made by the Delft group can support a supercurrent, making the ability to sustain a Josephson eff ect yet another robust feature of graphene. Th e development of commercial graphene-based nanoelectronics will encounter a number of technological challenges. First and foremost, is it possible to produce high-quality graphene wafers in suffi cient quantities? Options include dissolving graphite and depositing it as a monolayer on a substrate in a bottom-up approach, or burning off silicon in the top layer of silicon carbide crystals in a top-down approach. Another problem in the transistors made so far is that there is a noticeable leakage current, even when the device is meant to switched off . Th is is related to the unstoppable nature of the electrons in graphene (see Box 1) and can only be remedied by creating a bandgap. One way to solve this problem is to use bilayer graphene, because, according to theory, a voltage diff erence between the two layers will generate a bandgap. However, making bilayer graphene on a large scale is probably even more diffi cult than making single-layer graphene. SETs could circumvent the problems with leakage currents, but again the large-scale manufacture of such devices with reliable characteristics will be a challenge. Th e use of graphene ‘nanoribbons’ could be another solution because the electronic properties of the ribbon depend on the type of edge it has: simple theoretical considerations suggest that all ribbons with so-called zigzag edges are metallic, whereas those with ‘armchair’ structures can be semiconductors (with a bandgap) or metals, depending on their width. However, some theorists have recently suggested that all graphene ribbons are semiconducting7. At present the edges are naturally passivated by hydrogen atoms, but they could be functionalized by attaching diff erent chemical groups, providing potential for sensor applications. Preliminary reports on graphene ribbons are appearing8,9, but the challenge of routinely cutting and pasting graphene with the required level of precision will require the very best tools from nanotechnology. Although graphene research is still in its infancy, the number of experimental breakthroughs in the past three years has been stunning: the observation of the quantum Hall eff ect at room temperature, the fi rst graphene-based SET, and the bipolar superconducting transistor are just a few of many highlights. Moreover, the development of graphene-based nanoelectronics looks possible, if diffi cult. Th e main obstacles concern large-scale manufacturing and the challenge of patterning graphene at the atomic level. However, it took 50 years for silicon technology to reach maturity, so the parents of today would not be irresponsible if they told their kids that when they grow up, they might well fi nd themselves using devices made from carbon pancakes just one atom thick.