NaNomaTerials Graphene rolls off the press 30 - inch graphene

The two-dimensional form of carbon, graphene, shows remarkable electronic properties1,2 that could prove useful in applications such as ultrafast energyefficient transistors3,4 for computers and mobile devices, and as flexible transparent conductors for macroelectronic devices such as solar cells, display panels and electronic newspapers. However, to realize the technological potential of graphene, we need to fabricate it on a large scale and with a quality that is reliably better than today’s best materials. Now, writing in Nature Nanotechnology, Byung Hee Hong, Jong-Hyun Ahn and co-workers report on a new milestone in this effort by growing rectangles of graphene that measure 30 inches in diagonal length5. Moreover, the sheets are predominantly made of singlelayer graphene — a sheet of paper would need to be 200 km long to have the same aspect ratio. And when used as a transparent conductor, the graphene sheets outperform indium tin oxide (ITO), which is at present the industry standard. Hong and co-workers — who are based at Sungkyunkwan University and other institutes in Korea, Singapore and Japan — grew their graphene by chemical vapour deposition (CVD) of carbon atoms (extracted from decomposing CH4 at high temperatures) onto copper foils, and used a roll-to-roll technique similar to a newspaper printing press to transfer the graphene between different substrates (Fig. 1). They also performed a comprehensive set of materials characterization steps to demonstrate the excellent quality of their graphene, particularly as a transparent conductor that is also ultrathin and highly flexible. CVD onto metal substrates has been around for several decades6 and represents probably the oldest known method of graphene synthesis, being much older than the ‘scotch tape’ method used to isolate the tiny flakes of graphene that kickstarted present interest in this material in 20041. Since last year, there has been spectacular progress in metal-based CVD to grow large-scale graphene that can be subsequently transferred (by etching off the metal) to insulating substrates for electronic applications. Although some initial experiments used nickel7, copper later emerged as the best substrate for growing large-area single-layer graphene8–11, and the areas produced rapidly increased from a few square centimetres8 to the sizes that have now been produced by Hong and co-workers5. Moreover, in principle there is no upper limit on the size of graphene that can be grown by CVD, aside from, perhaps, the size of copper foil that can be practically placed into the CVD furnace. In general, an optically transparent material (such as glass) does not have good electrical conductivity and a conductive material (such as copper) does not have good optical transmission. This is because light can easily be absorbed or reflected by exciting charge carriers in a good conductor. Applications such as liquid-crystal displays and solar cells require transparent conductors with both high optical transmission and low electrical resistance. ITO, as well as certain other oxides, can satisfy these competing requirements, which is why it has become the industry’s most popular transparent conductor. A good ITO thin film can transmit ~90% of light while having a sheet resistance of under ~100 Ω per square unit (so that any square-shaped area of the film has this resistance). However, steep increases in the cost of indium, combined with the various shortcomings of ITO, such as its fragility and rigidity, have spurred researchers to look for alternatives. Materials based on nanowires, carbon nanotubes and graphene have been previously studied, but the performance of ITO has proved hard to beat. By stacking four CVD-grown graphene layers together and applying chemical doping (and careful graphene fabrication and transfer), Hong and colleagues were able to achieve a sheet resistance as low as ~30 Ω per square unit at ~90% of light transmission — a performance level exceeding that of ITO. This technological milestone indicates NaNomaTerials