by Brian Clegg
Bringing to life a zombie project was not Geim’s intention when he asked a Manchester PhD student, Da Jiang, to take a chunk of graphite and to make from it the thinnest sheet that he could achieve. Despite newspaper articles to the contrary, the graphite used was not a piece of pencil lead, but a type known as highly oriented pyrolytic – a carefully produced block of near-perfect carbon layers produced by heating carbon sources in high temperatures and pressures, §§ costing around £300 a block. At least, the intention was to give Da a piece of highly oriented pyrolytic graphite. Geim admitted in his Nobel lecture that he unintentionally gave Da a block of high-density graphite instead, which makes it far harder to produce thin, uniform layers. Geim hoped that Da could produce a sample so thin that it would act more like a two-dimensional atomic lattice, which was predicted to have quite different properties to ordinary graphite.
Da approached this task by grinding away layers until he had produced the thinnest slice of graphite that the equipment he was using could leave behind – but the sliver of carbon was not near enough to the atom-scale thinness that Geim knew was required to achieve the special properties he hoped to explore. This was a typical example of Geim pushing the boundaries: many theorists at the time thought that a two-dimensional crystal, such as the theoretical atom-thin sheet of graphite that was graphene, would be inherently unstable if separated from a block and would disintegrate to dust.
It seems that after failing, Da asked for a second piece of graphite to have another go – something that went beyond the practically non-existent budget of what Geim had by then christened his Friday night experiments, spare-time ventures into new directions, inspired by the success of his levitation work. As Geim drily put it, when Da asked for another block of graphite, ‘You can imagine how excited I was.’ But rather than send more good money after bad, Geim followed up an idea suggested by his post-doctoral student, Oleg Shklyarevskii.
It seems that Shklyarevskii overheard Geim moaning about Da’s disappearing graphite block, which Geim compared to polishing an expensive mountain to get a grain of sand. Shklyarevskii pointed out that scientists who worked with blocks of graphite needed to clean them first, to make sure the surfaces were even. Shklyarevskii was an expert in scanning tunnelling microscopy and in that field, blocks of graphite were (and are) used as a standard reference sample to set up the microscope.
To make a pristine sample for the microscope’s calibration, the scientists applied strips of Scotch tape to the surface of the block and peeled them back off, taking away a thin layer of the carbon and leaving behind a smooth, clean surface. The pieces of tape were then binned. Geim observed: ‘What these guys did not realise was that throwing away the Scotch tape they were throwing away the Nobel Prize as well.’
In true Friday night experiment style, Geim and Shklyarevskii looked through their colleagues’ waste baskets and found plenty of pieces of tape which had graphite layers left behind on them. (Thankfully, solid state physics lab bins tend to be less hazardous than those in biology or chemistry labs.) Though the layers of graphite on the tape strips still weren’t thin enough to have the potentially remarkable properties of graphene, under the microscope, the samples proved to be thinner than anything Da had achieved using the grinding method. Some of the deposits of graphite on the tape were so thin that they were transparent – which meant that there could only be a small number of graphene layers present.
‘We did not invent graphene,’ Geim says, ‘we only saw what was laid up for five hundred years under our noses.’ ¶¶
By this time, Konstantin Novoselov was fully committed to the project with Geim, and together the pair worked on the separation of thin sheets of graphite, repeatedly peeling them off with adhesive tapes, then pressing the tapes on to an oxidised silicon wafer. The closest layer of graphene tended to stick to the silicon oxide as a result of a weak inter-atomic force called a van der Waals force (of which much more later). When the tape was carefully peeled away from the silicon wafer, a much thinner layer would be left behind. After a year of experimenting with different approaches, the pair managed to reach their ultimate goal – a layer of carbon a single atom deep, forming a neat lattice of atoms like an interlocked set of hexagonal benzene rings but with only carbon present, which would prove to be remarkably versatile.
Graphene forms a lattice of hexagonal carbon rings.
The mere fact that this near-two-dimensional sheet existed at all hinted at its unusual nature. All atoms are constantly on the move, and in normal circumstances it is usually the larger structure of a body of matter that holds a thin layer together. Graphite is fine, as each layer supports the next, but you might expect a single layer of atoms to pull itself apart from the thermal activity of the atoms alone. However, this new material was strong enough to resist those rippling forces. Graphene is both stronger than steel by a factor of 100, and 100 times more conductive than copper.
Geim and Novoselov’s work has enabled them in just a handful of years since their discovery to open up possibilities from super-strong materials and flexible atom-thin electronics to molecular sieves to purify seawater. But before we can understand graphene and its amazing promise, we need to start with the ultimate Lego bricks – the atoms that make it up – and how those atoms interact.
* To be fair, the primary role of the International Space Station is not scientific experimentation but to provide a test bed for approaches to space exploration, so arguably it shouldn’t be considered a science project. It is, however, often misleadingly portrayed as a scientific endeavour.
† Although not the same building – the 1900 building is now an administration block.
‡ Geim is the first person ever to win both an Ig Nobel and a Nobel prize.
§ The Ig Nobel was started by Marc Abrahams – see www.improbable.com for more details.
¶ Gravity may seem powerful, but compare the gravitational influence of the whole Earth trying to pull a fridge magnet off the fridge and the tiny bit of magnet that is holding it in place. The magnet wins. Gravity is around a trillion, trillion, trillion times weaker than electromagnetism.
|| Diamagnetic materials are substances that are repelled by magnetic fields despite not being themselves magnetic materials.
** Magnetic susceptibility is a measure of how much a substance is attracted or repelled by a magnetic field.
†† Tisha was not the only animal in history to co-author a scientific paper. For example, American biologist Polly Matzinger co-authored with her dog Galadriel Mirkwood, while the American physicist Jack Hetherington not only co-authored with his cat, Chester (operating under the name F.D.C. Willard), Chester even has a byline for writing a solo article for a French popular science magazine.
‡‡ Hence the location of the UK’s foremost pencil museum at Keswick in Cumbria.
§§ One good source of high-quality graphite for producing graphene is so-called Kish graphite, which is deposited on the surface of molten iron during the steel-making process.
¶¶ The name ‘graphene’ was not dreamed up by Geim, but had been in use since the 1980s to describe the layers in a piece of graphite and that also formed tiny carbon ‘nanotubes’ when in a more stable rolled structure. But no one believed that a flat sheet of graphene could be separated or that it would remain as a stable substance.
2
THE ESSENCE OF MATTER
Atoms everywhere
To discover where the concept of atoms came from, we need to go back around 2,500 years in time. The Ancient Greeks had two main competing theories on the nature of matter. In the 5th century BC , Empedocles, a philosopher based in the Sicilian city of Agrigentum, introduced the concept of the four elements: earth, water, air and fire. Nearly 100 years later, Aristotle, probably the most famous of all the Greek philosophers, added a fifth element called quintessence (also sometimes called the ether). This, he felt, was necessary as the dominant cosmology of the time considered everything above the orbit of the Moon to be unchanging and
eternal – requiring something more stable than the Earthly elements.
The competitor theory dated back to around the same time that Empedocles was coming up with the elements. Another philosopher, Democritus, supported by his teacher Leucippus (if he existed * ), suggested that everything was made up of atoms – literally ‘uncuttable’ † fragments of matter, the smallest possible components of stuff.
In principle, the two theories were compatible – just as now we are quite happy to talk about both the elements, which reflect the different structures of atom, and of atoms themselves. Philosophically, though – which is the only thing that mattered in Ancient Greece – the two theories had significant differences of approach. Atomists thought that each different type of substance had its own specific variant of atom that made it the material that it was. So, wood atoms would be different from cheese atoms – each was assumed to have a distinctive shape. Those who supported the theory of the four/five elements not only rejected this, but from Aristotle onwards were unhappy with the whole concept of atoms because of something they implied.
The problem with atoms, as this theory imagined them, was that if they did exist there must also be empty space in between them. Relatively few three-dimensional shapes can fill all of space without leaving gaps of nothingness in between. There certainly aren’t enough such shapes for there to be one for each type of matter. But Aristotle was convinced that nature did not support the existence of a void or vacuum.
One of Aristotle’s arguments against the existence of the void was remarkably like Newton’s first law of motion. Aristotle commented that if a void existed, ‘No one could say why something moved will come to rest somewhere; why should it do so here rather than there? Hence it will either remain at rest or must move on to infinity unless something stronger hinders it.’ And he was quite right – it does. He just thought that this was nonsense, because in the normal world, such behaviour is usually restricted by friction and air resistance.
After a degree of toing and froing, it was the atomic theory that lost out among the Ancient Greek philosophers. This is not as shocking as it sounds. Perhaps surprisingly to us now, the five elements were a more useful scientific theory than was the version of atomic theory proposed by Democritus and Leucippus. We now know that Aristotle was wrong – but at least his theory made predictions about the way different materials behaved.
For example, if you burn a piece of wood it gives off the air-like smoke plus fiery flames and, if it’s green wood, it will exude watery liquids as it is heated. You are left with earth-like ash. The element theory seemed to explain this, assuming wood was made up of a combination of the elements. By comparison, the Ancient Greek atomic theory didn’t stand up to experiment. By allowing every type of material to have its own kind of atom, it didn’t really reflect how one substance could be transformed into another. Consequently, Aristotle’s five elements were to hold sway until Newton’s time and beyond.
Dalton’s discovery
By 1800, though, new discoveries were starting add to in far more elements than the original four or five, and the way that those elements combined in specific ways seemed to imply that the elements were themselves constructed from fundamental building blocks. John Dalton, who we’ve already met as a leading light of the early Manchester science scene, suggested that some matter was made up of multiple atoms of one kind of element, while other, more complex, substances combined different types of atoms in what we’d now call compounds.
Dalton was a remarkable character. He was a Quaker, which made it impossible for him to attend an English university, with places then only available to members of the Church of England. What education he had came primarily from reading and from the instruction of those around him. He appears to have been quite confident of his knowledge, as he was already acting as a schoolteacher by the time he was fifteen. It’s not at all clear how he came by his atomic theory, though there is some evidence that it was influenced by his studies of different gases and liquids and the way that they interact.
Where the Ancient Greeks had been prepared to dream up different shapes for the atoms, Dalton had no concept of what an atom was like. Even though his theory was widely adopted, because it proved immensely useful, many of his contemporaries did not even believe that atoms per se really existed, but rather thought that they provided a useful model to describe how the different elements interacted. It wasn’t until the early 20th century that there would be widespread acceptance that atoms truly existed.
At the heart of Dalton’s big idea was the concept of atomic weight. He gave relative weights to different elements, starting with the lightest – hydrogen – which was allocated a weight of 1. Each element’s atom was a distinct building block of matter that had its own weight (a multiple of that of hydrogen) and an individual tendency to react with other atoms to form compounds in simple ratios, reflected in whole numbers of different atoms.
Elements and their atomic weights, from Dalton’s A New System of Chemical Philosophy.
To the modern mind, familiar with subatomic particles, it might seem obvious that there was something significant about the way Dalton’s atomic weights were exactly multiples of the weight of hydrogen. Mathematically, when a series of items are all multiplies of the same value, there is an implication of an underlying numerical structure – and in the case of atoms, it’s easy for us to think that Dalton must surely have seen that this implied either that all the atoms were made of hydrogen, or that all atoms had some other set of internal components – but this doesn’t seem to have occurred to Dalton or his contemporaries.
Manchester historian of science James Sumner points out that Dalton did not actually believe that his atomic weights were exact multiples. When describing water as combining hydrogen and oxygen, Dalton notes that these atoms’ weights ‘are as 1:7, nearly …’. Furthermore, says Sumner, ‘This use of “nearly” recurs in many of Dalton’s proposed proportions, and would perhaps have diverted attention away from any thoughts of a compositional principle.’
What’s more, ‘Dalton was strongly concerned with the physical sizes of the atoms, assuming them to be something like spheres: conceptualising an oxygen atom as somehow like a grouping of seven(ish) hydrogen atoms would be of no explanatory help here.’ In practice, it seems that Dalton’s concern was not so much to explore the fundamental structure of matter as to produce a pragmatic mechanism, based on the idea of component particles, to explain the behaviour of gases when mixed together. From this he was able to speculate on how elements combine. The atomic theory emerged from what Dalton had only originally intended to have a significantly smaller application.
Dalton’s precise theory suffered from plenty of errors, based as it was on relatively crude measurements. His atomic weights were often inaccurate and he didn’t realise that, for example, oxygen in the air was in the form of a molecule with a pair of atoms held together by chemical bonds. His ideas for the number of atoms in molecules were often well away from the values we now expect. There was no good way to determine what the ratio of the elements was and Dalton worked on a self-imposed ‘rule of greatest simplicity’ – which had no evidence to back it up. He merely assumed that the simplest possible combination of elements was correct unless there was something to suggest otherwise.
Dalton thought, for example, that water, which the familiar modern chemical formula of H2 O identifies as two hydrogen atoms to one atom of oxygen, was a ‘binary’ atom with one atom each of hydrogen and oxygen. Similarly, he thought that ammonia (NH3 ) had one atom of azote (nitrogen) to one of hydrogen, and, most dramatically, he was far from the mark in suggesting that alcohol, presumably the ethanol (C2 H6 O) of alcoholic drinks, consisted of three carbon atoms and one atom of nitrogen.
He wouldn’t, incidentally, have been happy with representations such as H2 O. He disliked this approach, introduced by the Swedish chemist Jöns Jacob Berzelius, and always used his own symbols, which in a style reminiscent of the Ancient Greek atomic c
oncept, portrayed each element as a circle with a different shape inside it.
We shouldn’t be too hard on Dalton, though, for his inability to spot the true proportions of elements in compounds. Not only did he have very limited equipment, even by the standards of 1800, he was working at the leading edge of scientific discovery for the day. Many of the elements he was working with had only been identified as distinct substances during the preceding three decades. Oxygen and nitrogen, for example, were first identified in the 1770s. And it was not until Einstein wrote a paper in 1905 on Brownian motion – the way that small particles such as those ejected from pollen grains bounce around in water as they are buffeted by the water molecules – that there was good evidence to suggest that atoms and molecules were real things.
No one had been sure what caused Brownian motion, named after the Scottish botanist Robert Brown. ‡ It had even been thought for a while that it might be due to the life force in the pollen grains, until it was pointed out that it occurs with totally inanimate matter. But Einstein suggested that the motion was due to the constantly moving water molecules repeatedly bashing into the much larger particles, collectively causing the tiny specks to undergo a random dance § through the liquid. What convinced many that this was more than mere speculation was that Einstein backed up the idea with mathematics, showing that the observed behaviour was just what would be expected if molecules (and hence atoms) really existed, as described by Dalton’s theory.