CARBON – From the element of life to advanced technology _ PART III (HOW carbon has made nanotechnology possible?)

After the discovery of Lonsdaleite in 1967, a hexagonal structure of diamond which is almost 60% stronger that the classical diamond (with cubic structure), soon, another synthetic material joined the list of carbon forms, thanks to the aircraft industry. Early aircraft were made of wood because it is light and stiff. Indeed, one of the fastest aircraft in the Second World War was a wooden aeroplane called the Mosquito. Making airframes out of wood is problematic though because it is hard to join into a defect-free structure. So as the scale of aircraft engineers’ ambitions increased they turned instead to a light metal called aluminium. But even aluminium is not super-light, and nagging away at the back of many engineers’ minds was the hope that there might somehow be a material that was stronger and lighter even than aluminium. It didn’t seem to exist, so in 1963 engineers from the Royal Aircraft Establishment in Farnborough decided to invent one.

Carbon fiber, as they named it, was made by spinning graphite into a fiber. By rolling sheets of this material up, with the fibers running length-ways, they could take advantage of the huge strength and stiffness within the sheets. The weakness, as with pure graphite, still lay in the material’s structural dependence on van der Waals forces, but this was overcome by encasing the fibers in an epoxy glue. A new material was born: Carbon Fibers Composite.

Although this material would, in the end, displace aluminum in the building of aircraft (the recent Boeing Dreamliner is 70% carbon fiber composite), it took a long time for the material to prove itself worthy of the aircraft industry. Sports equipment manufacturers, however, took a liking to the material immediately. It transformed the performance of racquet sports so quickly that those who stuck to traditional materials such as wood and aluminum were quickly outclassed.

Soon the material was changing any and every sport that required low-weight and high-power materials – in other words, pretty much all of them, bicycle racing was transformed in the 1990s when engineers started to produce bikes with even more aerodynamic shapes using carbon fiber structures. The development of these bikes probably reached its zenith in Chris Boardman’s classic sporting rivalry with Graeme Obree to beat ‘The Hour’ record: the competition that seeks to determine the furthest a human being can travel in one hour under their own power. In the 1990s both cyclists were able to smash the world record and then each other’s records repeatedly with the help of ever more sophisticated carbon fiber bicycles. In 1996 Chris Boardman rode 56.375 km in one hour and provoked an outcry from the Cyclist International Union. They promptly banned the use of these new carbon fiber-inspired designs, so worried were they at how radically it would change the nature of the sport.

Formula One took the opposite approach to the innovation offered by carbon fiber, with constant changes in the rules forcing further innovation in materials design. Indeed, mastery of technology is integral to the sport and success is achieved as much through engineering advance as it is through the skill of driver. Meanwhile, carbon fiber plays a role even in the sport of running. More and more disabled athletes are using carbon fiber trastibial artificial limbs. In 2008 the International Association of Athletics Federations tried to prevent such athletes from performing against able-bodied athletes on the grounds that the carbon fiber legs gave them an unfair advantage. However this ruling was overturned by the Court for Arbitration of Sport and in 2011 the athlete Oscar Pistorius competed as part of the able-bodied South African World Championship 4x400m Relay team, which won a silver medal. Carbon fiber is set to become a big part of athletics unless the athletics federations take same approach as the cycling federations. The huge success of carbon fiber composite inspired engineers to imagine its use on the grandest possible scale: was this material strong enough to achieve a longstanding dream, that of building an elevator into space?

The Space Elevator, also known by its aliases Sky Hook, Heavenly Ladder or Cosmic Funicular, would be a structure linking a point on the equator to a satellite in geostationary orbit directly above it. If a space elevator could be constructed, it would democratize space travel at a stroke, allowing people and cargo to be transported into space with ease and with an almost negligible energy cost. The concept, which was developed in 1960by a Russian engineer, Yuri Artsutanov, would require the construction of a 36,000-km-long cable connecting a satellite to a ship floating in the ocean at the Earth’s equator. All studies indicate that the idea is mechanically feasible but requires the cable to be made from a material with an extraordinarily high strength-to-weight ratio. The reason why weight comes into it, as with any cable structure, is that it must first be able to hold its own weight without snapping. At 36,000 km long, you would need a material so strong that a single thread of it could be used to lift an elephant. In practice even the best carbon fiber thread could only lift a cat. But this is because it is full of defects. Theoretical calculations make clear that if a completely pure carbon fiber could be engineered then its strength would be much higher, exceeding the strength of diamond. The search was on to find a way to make such a material.

A clue to how this might be done came with the discovery of a 4th carbon structure, one that was found in the most unlikely of places: the flame of a candle. In 1985 Professor Harry Kroto and his team discovered that inside the flame of a candle carbon atoms were miraculously self-assembling in groups of exactly 60 atoms to form super-molecules of carbon. The molecules looked like giant footballs and were nicknamed ‘buckyballs’ after the architect Buckminster Fuller, who had designed geodesic domes with the same hexagonal structure. Kroto’s team received the 1996 Nobel Prize for Chemistry for this discovery, and also woke everyone up to the fact that the microscopic world might contain a whole zoo of other carbon structures that had never been seen before.

The Buckminsterfullerene C60 is the smallest fullerene in which no 2 pentagons share an edge. It is also the most common in terms of natural occurence, as it can often be found in soot. The structure of C60 is a truncated T= 3 icosahedron, which resembles a football of the type made of hexagons and pentagons, with a carbon atom at the corners of each hexagon and a bond along each edge.

From AIDS medicined to superconductors to flat-screen TVs a wide range of medical and industrial uses are envisioned for the buckminsterfullerene, an incredibly strong football-shaped molecule that is the 4th form of carbon after lonsdaleite, diamond and graphite.

Almost overnight carbon became one of the sexiest topics in materials science, and soon another type of carbon emerged, a carbon that could form tubes that are only a few nanometers wide: THE Carbon nanotubes.

Despite the complexity of their molecular architecture, these carbon nanotubes had a peculiar property: they could self-assemble. They needed no outside help in order to form these complex shapes, nor did they need high-tech equipment. They could do it in the smoke of a candle. It was a moment on a par with the discovery of microscopic bacteria; the world suddenly seemed a much more complex and extraordinary place than we had imagined. It wasn’t just living organisms that could self-assemble into complex structures, the non-living world could do it too. An obsession with the production and examination of nanoscale molecules gripped the world, and nanotechnology became fashionable.

Single-wall carbon nanotubes are a new form of carbon made by rolling up a single graphite sheet to a narrow but long tube closed at both sides by fullerene-like end caps. However their attraction lies not only in the beauty of their molecular structures but through intentional alteration of their physical and chemical properties fullerenes exhibit an extremelly wide range of interesting and potentially useful properties, too.

Carbon nanotubes are like miniature carbon fibers except that they have no weak van der Waals bonding. They were found to have the highest strength-to-weight ratio of any material on the planet, which meant that potentially they might be strong enough to build a space elevator. Problem solved? Well, not quite. Carbon nanotubes are, at most, a few hundred nanometres in length, but they would need to be meters in length to be of use. Currently there are hundreds of nanotechnology research teams around the world working to solve this problem.

So the simple question is: if all of these new forms of carbon were based on the hexagonal structure of graphite, and graphite was full of these layers of hexagonal carbon, then why wasn’t graphite a wonder material too?

Answer: because the sheets slip over one another too easily, so the material is very weak.

Ok, but then what if there were only one sheet of hexagonal carbon? What would that material be like? Well… here comes the Graphene.

The Graphene

I was mentioning in the Part 1 of this Carbon Story that the reputed professor Andre Geim one of the world´s foremost carbon expert from Manchester University and his team have received in 2010 the Nobel Prize for Physics fir his groundbreaking work on graphene, a 2-dimensional version of graphite and a marvel of the material world. He had managed to make a single sheet of hexagonal carbon in this way:

He took a piece of sticky tape and stuck it on to the lump of graphite. When he removed it a thin wafer of brightly metallic graphite was still stuck to the tape. He then took another piece of the tape and stuck that to the thin wafer, and then peeled it back. Now the wafer had been split into two parts. Doing this 4 or 5 times created yet thinner wafers of graphite. Finally he announced that he had made some graphite that was one atom thick which at this scale is transparent but under a microscope the atomic layers or graphite can be easily seen. Andre’s team didn’t get the Nobel Prize for making a single layer of graphite. They got the Nobel Prize for demonstrating that these single layers of graphite had properties that were extraordinary even by nanotechnology standards – so extraordinary that they merited their own name as a new material: graphene.

Just for starters, graphene is the thinnest, strongest and stiffest material in the world; it conducts heat faster than any other known material; it can carry more electricity, faster and with less resistance, than any other material; it allows Klein tunnelling, an exotic quantum effect in which electrons within the material can tunnel through barriers as if they were not there.

All this means that the material has the potential to be an electronic powerhouse, possibly replacing silicon chips at the heart of all computation and communication. Its extreme thinness, transparency, strength and electronic properties mean also that it may end up being the material of choice for touch interfaces of the future, not just the touch screens we are used to but perhaps bringing touch sensitivity to whole objects and even buildings. But perhaps its most intriguing claim to fame is that it is 2-dimensional material. This doesn’t mean it has no thickness, but rather that it cannot be made any thicker or thinner and be the same material. This is what Andre’ s team showed: add an extra layer of carbon to graphene and it goes back to being graphite, take a layer away and the material does not exist at all.

So that being said it is clearly proven that graphite is a superior form of carbon to diamond, it´s all obvious in the atomic structure of graphite. Graphene is the atomically thin building block of graphite. It is what you sometimes deposit on your paper as you use a pencil. It can be used solely as an expressive artistic material. But it is much more than that: this material and its rolled-up version in the form of nanotubes are going to be an important part of our future world, from the smallest scale to the very largest, from electronics, to cars, to aeroplanes, rockets and even – who knows? – to space elevators.

So has graphite, by giving birth to graphene, finally outshone diamond? Is it finally the unexpected winner of this age-old rivalry? It’s a bit too early to tell, but it seems doubtful to me. Although it appears likely that graphene will usher in a new age of engineering, and indeed scientists and engineers are in love with this material already, this may not give it high status in the world at large. Diamonds may not be the hardest, strongest material any more, and we know that they will not last for ever, but they still represent those qualities to most people. They are still the rock that romantically binds lovers everywhere. The association of diamond with true love may originally have been thanks to a PR campaign, but it is no less real to us now. Graphene, on the other hand, may be functionally better than diamond but it doesn’t sparkle, is virtually invisible, is extremely thin and is 2-dimensional – not the kinds of qualities that anyone wants to associate their love with. My guess is that until the PR companies discover graphene, the cubic crystal structure of carbon will continue to be a girl’s best friend.