The Trials of new carbon

Το άρθρο μας έστειλε ο Γιώργος Μπανίλας, Κυριακή 9 Ιανουαρίου 2011

Researchers have spent 25 years exploring the remarkable properties of fullerenes, carbon nanotubes and graphene. But commercializing them is neither quick nor easy.

By Richard Van Noorden

It has been just six years since Nobel laureates Andre Geim and Kostya Novoselov at the University of Manchester, UK, first reported using sticky tape to peel atomically thin layers of graphene from lumps of graphite. But the material — essentially just an unrolled nanotube — has turned out to have properties just shy of miraculous: a single layer of graphene is simultaneously the world’s thinnest, strongest and stiffest material, as well as being an excellent conductor of both heat and electricity.

Graphene has been showered with media attention as companies vie to bring those attributes to market. Last year, graphene was the subject of around 3,000 research papers and more than 400 patent applications. In fairy tales, third place is often the best it’s usually the third casket that contains the treasure, and the third child who finds fame and fortune. And so it may be for graphene, the third and most recently discovered form of ‘new carbon’. The footballshaped fullerenes1, discovered in 1985, and the hollow cylindrical carbon nanotubes2, first characterized in 1991, have so far had a limited impact on industry. But now graphene, a one-atom-thick flat sheet of carbon, seems to be surrounded by favourable omens — not the least of which is the speed with which groundbreaking experiments on its properties were rewarded with the 2010 Nobel Prize in Physics.

South Korea is planning a US$300-million investment to commercialize the material, and companies ranging from IBM to Samsung are  testing graphene electronics — ultra-small, ultra-fast devices that might one day replace the silicon chip. The hype over graphene has reached such a pitch that a casual follower might wonder why it hasn’t conquered the technological world already.

The reality is not such a fairy tale. Graphene’s carbon forebears were once hyped in much the same way. Yet fullerenes have found hardly any practical applications. And although nanotubes have done better, they are costly to produce and difficult to control. Their subdued industrial impact is a lesson in just how hard commercialization of a new material can be.

Yet the story of nanotubes has some encouraging features. High-tech electronics applications are still years in the future, but a more low-tech application — nanotube-based conducting films for energy storage or touch screens — is much closer to commercialization. Another, comparatively straightforward use — nanotube- reinforced composite materials for aeroplanes and automobiles — is now reaching the market. Anticipating growing demand, nanotube manufacturers have scaled up production to many hundreds of tonnes a year. For that very reason, the graphene manufacturers following in their wake may have hit on the right moment to start mass-producing the sheets.

Graphene is being considered for the  same types of application as nanotubes, but it has some key advantages in ease of production and handling, and should benefit from two decades of research with nanotubes. That hindsight also means that graphene manufacturers have a better idea of which applications are worth chasing, and of how to avoid the false starts that nanotubes made in their first decade.

The remarkable properties shared by nanotubes and graphene arise from their common structure: an atomically thin mesh of carbon atoms arranged in a honeycomb pattern. Immensely strong carbon–carbon bonds produce an exceptionally high strength-to-weight ratio. Such is the strength of graphene, for example, that according to the Nobel prize committee, a hypothetical 1-metre-square hammock of perfect graphene could support a 4-kilogram cat.

The hammock would weigh 0.77 milligrams — less than the weight of a cat’s whisker — and would be virtually invisible.

The symmetry with which carbon atoms are arranged on the hexagonal lattice also allows both forms of nano-carbon to conduct electricity far more easily than the silicon used in computer chips. This means that they have much lower electrical resistance and generate much less heat — an increasingly useful property as chip manufacturers try to pack features ever more densely onto circuits.

Furthermore, even small variations in carbon structure can create a multitude of new properties. In graphene, for example, electronic  behaviour depends on the size of a given sheet, the presence or absence of defects in the sheet’s lattice and whether it is lying on a conductive surface. In nano tubes, likewise, a given structure can be made semiconducting or metallic just by changing its diameter, length or ‘twist’ (the angle between the lines of hexagons and the direction of the tube). And there are differences between single tubes and those in which several cylinders are nested inside each other — called multi-walled nanotubes. These properties have long sparked hopes of game-changing electronics applications.

And researchers have made great progress — in the laboratory. In 1998, for example, physicists demonstrated a transistor made from a single, semiconducting nanotube. And in 2007, researchers reported the synthesis of a carbonnanotube- based transistor radio. But for industrial-scale mass production of such circuits, the great variability of nanotubes is a curse. They are most commonly produced in a reactor, in which catalysts guide formation of the tubes from a carbon-rich vapour. This typically leaves a jumble of multi-walled, single- walled, semiconducting and metallic tubes of various lengths and diameters, all with different electronic properties.

“Diversity is great until you have too diverse a population: then it becomes a real headache,” says John Rogers, a physical chemist at the University of Illinois in Urbana-Champaign. Only in the past five years have researchers worked out how to sort nanotubes into semiconducting and metallic types

But there are further difficulties in assembling selected nanotubes in predetermined places on a chip and connecting these separate tubes together without compromising performance, so most physicists have come to believe that it is impractical for carbon nanotubes to replace silicon. “An integrated circuit would have to involve billions of identical carbon-nanotube transistors, all switching at exactly the same voltage,” says Phaedon Avouris, who works on nanoscale electronics at IBM’s Thomas J. Watson Research Center in Yorktown Heights, New York.