by Brian Clegg
It’s likely that there had been a similar building pressure to recognise the discovery of graphene over several years leading up to 2010 – but we won’t know for certain until 2060. We do know, though, that nomination forms would have been sent out around September 2009 to approximately 3,000 professors located all over the world and that from these forms, around 300 people may have been nominated by the February 2010 deadline. After consultation on a number of potential candidates, the Nobel committee would have put forward a report with a shortlist during the summer, and the winners would have been selected from the final candidates by majority vote in October 2010.
It’s typical of Andre Geim that, when asked about the benefits of winning the Nobel Prize, he didn’t mention the considerable cash sum or the academic kudos. Instead he said that when the vice-chancellor at Manchester had asked the new superstar what he would like as a reward, Geim requested a better parking place. ‘It was a fifteen-minute round trip to my car. But it changed that afternoon!’
It’s not often that something with an immediate practical application wins a Nobel Prize in Physics. Perhaps the nearest equivalent to the graphene award was the 1956 prize, won by William Shockley, John Bardeen and Walter Brattain for their researches on semiconductors and ‘discovery of the transistor effect’ – what the rest of us would call the invention of the transistor. In the case of graphene there was certainly a practical innovation, but there were also remarkable new physical properties, arising from its thinness. Because it’s thin on a scale that’s hard to get your head around.
Plenty of room at the bottom
Before exploring the specifics that make graphene so special, it’s worth taking a brief detour to a talk that the great American quantum physicist (and Nobel Prize winner) Richard Feynman made way back in 1959. Feynman’s talk ‘There’s plenty of room at the bottom’ was subtitled ‘an invitation to enter a new field of physics.’ Let’s take a look at the opening of Feynman’s talk:
I imagine experimental physicists must often look with envy at men like Kamerlingh Onnes, * who discovered a field like low temperature, which seems to be bottomless and in which one can go down and down. Such a man is then a leader and has some temporary monopoly in a scientific adventure. Percy Bridgman, † in designing a way to obtain higher pressures, opened up another new field and was able to move into it and to lead us all along. The development of ever higher vacuum was a continuing development of the same kind.
I would like to describe a field, in which little has been done, but in which an enormous amount can be done in principle. This field is not quite the same as the others in that it will not tell us much of fundamental physics (in the sense of, ‘What are the strange particles?’) but it is more like solid-state physics in the sense that it might tell us much of great interest about the strange phenomena that occur in complex situations. Furthermore, a point that is most important is that it would have an enormous number of technical applications.
What I want to talk about is the problem of manipulating and controlling things on a small scale.
Feynman was primarily talking about making physical objects on a small scale, suggesting eventually we could be looking at controlling individual atoms using an army of manipulators. However, what he was describing in that opening section so perfectly describes why Geim and Novolselov’s apparently small breakthrough has such significance – they opened up an entirely new field, and though it was taking a different approach to smallness than that envisaged by Feynman, two-dimensional materials such as graphene are very much exploiting the potential of controlling things on a small scale. Because the definitive property of graphene is how incredibly thin it is.
It’s thin
When Geim and Novoselov first peeled away a sheet of graphene on their Scotch tape, ‡ one obvious characteristic was that it was thin. Really, really thin. So thin that it couldn’t be seen sideways on and was transparent from above. Specifically, and crucially, the graphene was a slice of matter exactly one atom thick, making it the thinnest known material in the universe. This skimpy substance was a layer of carbon in a hexagonal lattice that was no deeper than the size of the carbon atoms themselves.
How deep is that? A sheet of graphene is around 0.3 nanometres from top to bottom, where a nanometre is a billionth of a metre. To put that into context, the thickness of graphene is about 60 times smaller than the tiniest of viruses, 3,000 times less porky than a typical bacterium and 300,000 thinner than a typical sheet of paper. That is thin. Although graphene is very strong, such a thin material generally needs supporting – the material’s strength comes in when it is pulled in the same plane as the sheet, but it is very floppy because of its lack of structure out of the two-dimensional plane. To date, samples of graphene have been produced ranging from micrometres (thousandths of a millimetre) to nearly a metre in length.
You’re never alone with a substrate
To cope with this floppiness, graphene is always used on a ‘substrate’ – a piece of material that supports it but doesn’t interfere with its properties. The substrates for most of graphene’s uses tend to be rigid solids, though they can also be flexible materials like sheets of plastic polymer. In fact, this ability of graphene to twist and shape can be a positive benefit beyond working on a malleable substrate, because a sheet of graphene moulds itself to a surface, although this typically results in the graphene forming creases and folds (which drop away when it is removed). This moulding to the surface and creasing is due to van der Waals forces (see below).
The substrates Geim and Novoselov used in the first exploration of graphene were oxidised silicon wafers, which are readily available from the first stage of silicon chip device production. The starting point for an integrated circuit is a silicon wafer, sliced off an ingot of silicon produced by melting the element. The whole wafer is not oxidised – that would essentially turn the silicon back to silica (also known as sand). It’s just the surface that is oxidised to help the layers to cohere.
In practice, it has been discovered that sitting on a silicon dioxide layer degrades the performance of the graphene, because the silicon dioxide surface is relatively uneven, resulting in graphene that is a little tangled in its structure. This produces patchy distribution of areas with a perfect zero band gap and makes graphene a less effective conductor than it usually is. Substrates with an intermediate single layer of boron nitride (of which more later) on top of the silicon dioxide for the graphene to rest on work significantly better. The reason that the nature of the substrate surface has such an effect is because of the forces between the graphene layer and the substrate.
These are called ‘van der Waals forces’, named after Dutch scientist Johannes van der Waals, and are attractive or repulsive forces that we don’t usually notice on the scale of objects we can see. At the atomic and molecular level, though, the action of these forces can be very significant. Attraction is caused by slight changes in charge distribution in adjacent atoms, where the statistically determined position of the electrons happens to put more charge briefly on one side of an atom than another, and also due to other quantum effects.
Although the van der Waals forces are tiny for any particular atom or molecule, they can add up to a powerful effect. Many of the gecko families of lizards are able to run up a vertical wall or even a sheet of glass, because their feet have vast numbers of tiny hair-like structures, each of which generates a small van der Waals attraction to the surface. One of Geim’s previous side projects was to produce a prototype of ‘Gecko tape’, an adhesive tape where the stickiness comes not from a glue but from van der Waals forces generated by a gecko-foot-like array of structures on the tape’s surface. Prototypes of the tape have proved surprisingly effective, bearing in mind there is no adhesive involved. As far as graphene goes, the van der Waals attraction is nowhere near as strong as that produced by Gecko tape, but it’s enough to make the graphene shape itself to an uneven surface.
In practice, the technique of using Sco
tch tape to remove a layer from a block of graphite proved an effective combination with the van der Waals force between graphene and a substrate. As Geim and Novoselov discovered when they raided their colleagues’ bins, the tape usually picks up multiple layers of graphene. But when that tape is subsequently pressed onto a substrate, it leaves behind flakes of graphite, due to the van der Waals attraction. These are even thinner than the layer that was initially removed, as some of the graphene sheets will remain on the tape. If necessary, the process can be repeated, but even on a first application some of the flakes on the substrate will be single layers of graphene.
It might seem an impossible task to separate out which of the flakes are truly two-dimensional, as even several layers are still transparent, but it turned out that when using an oxidised silicon substrate, there was a clear visual difference between single layered graphene and multiple layers. The way the light reflects back through the layers produces different coloured effects depending on how many layers are present, making it possible to isolate the pure flakes of two-dimensional graphene.
As we’ve seen (image on page 16 ), if you could zoom in with a magical ultra-microscope to see the atoms and bonds in graphene, it would look a little like chicken wire with a repeating hexagonal pattern. At each of the six corners of each hexagon sits a carbon atom. Like the material itself, these hexagons are small. Each side is around 142 picometres in length. A picometre is a thousandth of a nanometre, so the sides of the hexagons are 0.142 nanometres long – around half the thickness of the sheet.
Earlier we discussed the ‘two-dimensional’ claim made of graphene, and concluded that although it may not be two-dimensional in a pure mathematical sense, graphene really does share some properties with theoretical two-dimensional objects. It’s useful to think of this two-dimensional environment from a simple visual viewpoint, imagining we have a giant sheet of graphene in space that we can fly around and observe. Its two-dimensional nature means, for example, that we will never see the bonds between the atoms crossing each other. And that means you can’t have a knot-like structure, with one bit of the ‘string’ passing over another, as you can’t make a knot in two dimensions. Similarly, two-dimensional graphene can never form structures that require any venture into the third dimension, limiting the way that particles within the space formed by the two-dimensional object can interact and go past each other. What may at first seem like a trivial difference in conformation would result in remarkable capabilities being discovered as graphene was further examined, as we’ll see.
The Eureka moment
Just producing graphene was a remarkable breakthrough in itself, but what would change the Manchester discovery from something merely interesting into a whole new potential field of applications came when the team began to try out the substance’s physical capabilities. Geim describes his Eureka moment with graphene coming when they had got to work on its electrical properties. This was easier said than done. It’s one thing to have an ultrathin slice of graphite on a piece of sticky tape or oxidised silicon substrate; quite another to be able to test its reaction to electricity.
Novoselov and Geim used tweezers to transfer one of their thinnest flakes to a pristine substrate, then applied tiny spots of silver paint to make electrical contacts to the material. This was an impressive bit of micro-manipulation. The graphene crystal was only about the width of a human hair, making it around 20 nanometres across, and was not as yet a distinct single layer, with several sheets of graphene still together. Lacking any more appropriate technology, the pair applied the silver paint using a toothpick and steady hands. It took a good number of attempts to get it right. But when they did, the result was impressive.
Not only did the material prove to be highly conductive, graphene’s resistance – the ease with which an electrical current flows through it – was modified when it was brought near to an electric field. We’ve already come across the concept of fields a number of times – for example, when Geim levitated a frog using a magnetic field – but it would be useful to clarify exactly what they are before going any further, as electric fields become extremely important when dealing with small-scale electronics.
Working in the field
The concept dates back to Michael Faraday. Before Faraday became involved in electromagnetism in the 1820s, while it was known that electrically charged materials could attract each other, it was described scientifically in the same kind of way that gravity was thought of – as an attraction at a distance. The mathematical approaches taken by Newton to calculate the force of gravity worked in a similar fashion in dealing with electrical attraction. However, Faraday was no mathematician, but rather a superb intuitive scientist.
In thinking about electricity and magnetism, Faraday imagined lines of influence, stretching out from the electrical charge or from a magnetic pole. As an impressive experimenter, Faraday was well aware of the way that iron filings lined up to run from pole to pole of a magnet like a series of contours. These lines, Faraday thought, were a measure of the strength of the force field that the magnet produced.
This idea held that a ‘field’ was something that spread throughout space and had a value at every location. When something else moved through the field, if it was the right kind of material, it would interact with the field. So, for example, moving a conducting wire through a magnetic field, the wire would repeatedly cut the lines of force in the field and this cutting, Faraday suggested, produced the electrical current that would flow through the wire.
When the Scottish physicist James Clerk Maxwell formalised the mathematics of electromagnetism some four decades after Faraday’s first work on the subject, he took Faraday’s descriptive science and turned it into a clean, clear mathematical representation of the way that electric and magnetic fields behaved, how they interacted and how a particular type of interaction between them could produce a wave that travelled at the speed of light. Once Maxwell had done this it was no longer necessary to consider mysterious forces at a distance to deal mathematically with the forces of electricity and magnetism. Maxwell’s formulation made fields central to the consideration of electromagnetism, as they remain to this day.
One essential aspect of the electric field is that it will influence the flow of electricity in a nearby body – a process known as a field effect. This is the basis of many of the transistors in chips used in vast numbers of electronic devices. These are usually ‘MOSFET’ devices: metal oxide semiconductor field effect transistor. The part of the transistor that controls the flow through it, the gate, is used to produce an electric field. It is separated by an insulator from the rest of the transistor, which stops current flowing from it, but the field influences the current flowing between the other two terminals of the transistor, reaching it through the insulator.
Despite being such a crude, hand-constructed device, the graphene crystal that Geim and Novoselov were testing changed electrical resistance by several percentage points when an electrical field was applied from a separate source. As Geim put it: ‘If those ugly devices made by hand from relatively big and thick platelets already showed some field effect, what could happen, I thought, if we were to use our thinnest crystallites and apply the full arsenal of microfabrication facilities?’
For months, the Manchester team worked on reducing the thickness of their samples until they were able to repeatedly produce true single layers of graphene. They also expanded the size they could make, from the width of a hair up to around a millimetre across, producing over 50 trial electronic devices. Their paper on producing graphene and a first exploration of its properties would be accepted in September 2004 by the prestigious journal Science , after it had first been rejected by the rival Nature . The editors at Nature felt that there was not enough originality in the work. It’s likely that they later regretted this decision.
Not the way it should be
One of the biggest surprises in the development of graphene was that it could be made at all. It had been assumed by p
hysicists for decades that it was physically impossible to make such a thin material. § Attempts to make atom-thin layers of metals by evaporation and deposition resulted in unsatisfactory patchy blobs, rather than a continuous layer. And when attention turned to carbon, theory predicted that for sheets containing up to around 24,000 atoms, the lattice would be unstable and would tend to curl up to form three-dimensional lumps. What’s more, as we’ve already seen, it was assumed that at room temperature, thermal vibration would rip the graphene apart. In practice, the largest two-dimensional carbon molecule ever synthesised (as opposed to removed from a three-dimensional block as Geim and Novoselov did) was made up of just 222 atoms. Larger sheets were also predicted to be unstable over and above the thermal vibration damage, as it was assumed that inter-atomic forces would result in the sheets curling up to form tubular whiskers.
To make matters worse, the traditional mechanisms for growing crystals would require very high temperatures to make graphene, making the impact of thermal vibrations even greater. And then there was the whole aspect of interaction with the environment. This is one of the ways that a two-dimensional material is so different from a conventional crystal. If you think of a typical sheet of graphene in a block of graphite, it has other graphene sheets on either side of it, protecting it from reaction. In a sheet of material that is a single atom thick, every atom in the crystal is directly exposed to its surroundings. Any air molecules, moisture, reactive compound and general contamination will hit it full on, and from not one but two directions, as the same individual atoms are exposed on both surfaces of the material.
