The Power in Your Pencil
Graphite, the black sheep of the carbon family, outshines diamonds
The first time I went to art class, the teacher, a man called Barrington, told us that everything we could see was made of atoms. Everything. And that if we could understand that, we could begin to be artists. The room went quiet. He asked for questions, but all of us were struck dumb, wondering if we were in the right class. He continued his introduction to art by holding up his pencil and proceeding to draw a perfect circle on the piece of paper he had taped to the wall. There was general excitement and sighs of relief from the assembled pupils. Perhaps we were in an art class after all.
“I’ve transferred atoms from the pencil to the paper,” he observed. He then gave us a speech about the wonders of graphite as a material for artistic expression.
“It is important to note,” he said, “that although diamond is culturally revered as the superior form of carbon, it is in fact incapable of deep expression, and unlike graphite no good art can come from diamond.”
What he subsequently thought of Damien Hirst’s diamond-encrusted skull, For the Love of God, valued at £50 million, I can only guess.
But in describing the relationship between the two forms of carbon, diamond and graphite, as a rivalry, he was certainly correct. The battle between dark, expressive, utilitarian graphite and sublime, cool, hard, glinting diamond has been raging since antiquity. In terms of cultural value, diamond has long been the winner, but that may be set to change. A new understanding of graphite’s inner structure has made it a source of wonder.
Thirty years after my introduction to graphite by my art teacher, I met Professor Andre Geim, one of the world’s foremost carbon experts, in his fluorescent-lit office on the third floor of the Physics Department of Manchester University. I wish I could say that, like Barrington, he too used only graphite to express himself, but when he opened his desk drawer I saw that it was full of ballpoint pens and whiteboard markers. In his thick Russian accent, Andre said, “There is no such thing as a perfect circle, Mark,” leaving me a little unsure as to whether he had understood the point of my story. Then he fished out of the drawer a small red leather presentation case and said, “Have a look at that while I make the coffee.”
Inside the case was a disk of pure gold the size of a biscuit, decorated with the relief portrait of a man. As I weighed the heavy disk in my hand I found it almost obscenely metallic: gold is the full-fat cream of the metal world. I was taken aback by the decadence of the material. The man depicted was Alfred Nobel, and the inscription on the medal announced to the world that Andre Geim’s team had received the 2010 Nobel Prize for physics, for his groundbreaking work on graphene, a two-dimensional version of graphite and a marvel of the materials world. As I waited for Andre to return with the coffee, I pondered his cryptic answer. Perhaps he was suggesting that although his last ten years of research on carbon had been circular, he had not ended up where he started.
Carbon is a light atom with six protons and usually six neutrons in its nucleus. Sometimes it contains eight neutrons, but in this form, known as carbon-14, the atomic nucleus is unstable, and so the element falls apart through radioactive decay. Because the rate of this decay is consistent over long periods of time, and because this form of carbon finds its way into many materials, measuring its presence in a material allows us to work out that material’s age. This scientific method, known as carbon dating, has thrown more light than any other on our ancient past. The true ages of Stonehenge, the Turin Shroud, and the Dead Sea Scrolls have all been revealed by this form of carbon.
Radioactivity aside, the nucleus plays a back-seat role in carbon. In terms of all of its other properties and behavior, it is the six electrons that surround and shield the nucleus that are important. Two of these electrons are deeply embedded in an inner core near the nucleus and play no role in the atom’s chemical life—its interaction with other elements. This leaves four electrons, which form its outermost layer, that are active. It is these four electrons that make the difference between the graphite of a pencil and the diamond of an engagement ring.
The simplest thing a carbon atom can do is share each of these four electrons with another carbon atom, forming four chemical bonds. This solves the problem of its active four electrons: each electron is partnered off with a corresponding electron, belonging to another carbon atom. The crystal structure produced is extremely rigid. It is a diamond.
The biggest diamond yet discovered is located in the Milky Way in the constellation of Serpens Cauda, where it is orbiting a pulsar star called PSR J1719–1438. It is an entire planet five times the size of Earth.
Diamonds on Earth are minuscule by comparison. The biggest yet found is the size of a football. Extracted from the Cullinan mine in South Africa, it was eventually presented to King Edward VII in 1907 on his birthday and is now part of the crown jewels of the British monarchy. This diamond was formed far below the surface of the Earth at a depth of approximately three hundred kilometers, where, over the course of billions of years, the high temperatures and pressures converted a largish-sized carbon rock into the huge diamond. The diamond was then most likely carried to the surface of our planet during a volcanic eruption, where it lay inert and undisturbed for millions of years until it was discovered a mile underground.
Each diamond is, in fact, a single crystal. In a typical diamond there are about a million billion billion atoms (1,000,000,000,000, 000,000,000,000), perfectly arranged and assembled into this pyramidal structure. And it is this structure that accounts for its remarkable properties. In this formation, the electrons are locked into an extremely stable state, and this is what gives it its legendary strength. It is also transparent, but with an unusually high optical dispersion, which means that it splits light that enters it into its constituent colors, giving it its bright rainbow sparkle.
The combination of extreme hardness and optical luster makes diamonds almost flawless as gemstones. Because of their hardness, virtually nothing can scratch them, and so they keep their perfectly faceted shape and pristine sparkle not just throughout the lifetime of the wearer but throughout the lifetime of a civilization—through rain or shine, whether worn in a sandstorm, hacking through a jungle, or just doing the washing up. Even in antiquity diamond was known to be the hardest material in the world. The word diamond is derived from the Greek adamas, meaning “unalterable” or “unbreakable.”
But diamonds are not forever, at least on the surface of this planet. It is, in fact, diamond’s sibling structure, graphite, that is the more stable form, and so all diamonds, including the Great Star of Africa in the Tower of London, are actually turning slowly into graphite. This is distressing news for anyone who owns a diamond, although they can be reassured that it will take billions of years before they see an appreciable degradation of their gems.
The structure of graphite is radically different from diamond. It consists of planes of carbon atoms connected in a hexagonal pattern. Each plane is an extremely strong and stable structure, and the bonds between the carbon atoms are stronger than those in diamond—which is surprising, given that graphite is so weak that it is used as a lubricant and as lead in pencils.
The conundrum can be explained by noting that within the graphite layers each carbon atom has three neighbors with which it shares its four electrons. In the diamond structure, each carbon atom shares its four electrons with four atoms. This gives the individual graphite layers a different electronic structure and stronger chemical bonding than diamond. The flip side, though, is that each atom in graphite has no electrons left over to form strong bonds between its layers. Instead, these layers are held together by the universal glue of the material world, a weak set of forces generated by fluctuations in the electric field of molecules, called van der Waals forces. This is the same force that makes Blu-Tack sticky. The upshot is that when graphite is put under stress, it is the weak van der Waals forces that break first, making graphite very soft. This is how a pencil works: as you press it on the paper you break the van der Waals bonds and layers of graphite slide across one another, depositing themselves on the page. If it weren’t for the weak van der Waals bonds, graphite would be stronger than diamond. This was one of the starting points for Andre Geim’s team.
When Andre Geim came back with the coffee, I was still holding his gold Nobel Prize medal in my hand. I felt vaguely guilty despite the fact that it was he who had given it to me to look at. He put the coffee down, took the medal out of my hand, and replaced it with a lump of pure graphite from the Plumbago Mines of Cumbria. He had, he said, obtained the graphite direct from the mine, which was just up the road, geographically speaking, from his office at Manchester University. Then he showed me how his research group had made a single sheet of hexagonal carbon.
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 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 four or five times created yet thinner wafers of graphite. Finally he announced that he had made some graphite that was one atom thick. I looked at the piece of sticky tape he was holding. It had a few black smudges on it, and, not wanting to downplay the significance of it, I peered intently. “Of course,” he said with a smile, “you cannot see it. At that scale it is transparent.” I nodded exaggeratedly as he took me next door to the microscope, which would allow us to see these atomic layers of graphite.
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 tunneling, 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 its most intriguing claim to fame is that it is a two-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.
Although my art teacher, Barrington, didn’t know it when he claimed that graphite was a superior form of carbon to diamond, he was right in almost every technical sense. He was right also about the importance of the atomic nature 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 airplanes, rockets, and even—who knows?—to space elevators.
Reposted from Stuff Matters by Mark Miodownik. Copyright (c) 2014 by Mark Miodownik. Used by permission of Houghton Mifflin Harcourt Publishing Company. All rights reserved.
