Graphene

by Chris Woodford. Last updated: January 5, 2023.

If the 20th century was the age of plastics, the 21st century seems set to become the age of graphene—a recently discovered material made from honeycomb sheets of carbon just one atom thick. Science journals have been running out of superlatives for this wondrous stuff: it's just about the lightest, strongest, thinnest, best heat- and electricity- conducting material ever discovered. And if we're to believe the hype, it promises to revolutionize everything from computing to car tires and solar cells to smoke detectors. What is this strange and remarkable new stuff? Let's take a closer look!

Photo: Growing graphene on a platform. Photo courtesy of Argonne National Laboratory and US Department of Energy published on Flickr.

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What is graphene?

In school you probably learned that carbon comes in two basic but startlingly different forms (or allotropes), namely graphite (the soft, black stuff in pencil "leads") and diamond (the super-hard, sparkly crystals in jewelry). The amazing thing is that both these radically different materials are made of identical carbon atoms. So why is graphite different to diamond? The atoms inside the two materials are arranged in different ways, and this is what gives the two allotropes their completely different properties: graphite is black, dull, and relatively soft (soft and hard pencils mix graphite with other materials to make darker or fainter lines); diamond is transparent and the hardest natural material so far discovered.

If that's what you learned in school, you probably finished your studies quite a while ago, because in the last few years scientists have discovered various other carbon allotropes with even more interesting properties. There are fullerenes (discovered in 1985; hollow cages of carbon atoms, including the so-called Buckyball, Buckminsterfullerene, made from a kind of football-shaped cage of 60 carbon atoms), nanotubes (discovered in 1991; flat sheets of carbon atoms curled into amazingly thin, hollow tubes one nanometer in diameter)—and (drum roll) graphene (discovered in 2004).

Artwork: Carbon nanotubes are one of the more recently discovered forms of carbon. Each tube is about 50,000 times thinner than an average human hair. Various other names were considered for nanotubes, including microtubules, tubulins, and Iijima tubes (named for one of their discoverers).

So what exactly is graphene? Peer inside lots of familiar solid materials (including most metals) and you'll find what's known as a crystal lattice (another name for a solid's internal, crystalline structure): lots of atoms arranged in a regular, endlessly repeating, three-dimensional structure a bit like an atomic climbing frame, only instead of bars there are invisible bonds between the atoms that hold them together. Diamond and graphite both have a three-dimensional structure, though it's completely different: in diamond, the atoms are tightly bonded in three-dimensional tetrahedrons, whereas in graphite, atoms are bonded tightly in two-dimensional layers, which are held to the layers above and below by relatively weak forces.

Artworks: 1) Diamond has a strong 3D (three-dimensional) crystal lattice based on a repeating tetrahedron (left). The red blobs are the carbon atoms and the gray lines are the bonds that join them together. (Bonds are invisible, but we draw them like this so we can visualize them more easily.) 2) Graphite has a much weaker structure based on layers of tightly bonded hexagons. The layers are weakly joined to one another by van der Waals forces (blue dotted lines—only a few of which are shown for clarity).

Graphene is a single layer of graphite. The remarkable thing about it is that its crystalline structure is two-dimensional. In other words, the atoms in graphene are laid out flat, like billiard balls on a table. Just like in graphite, each layer of graphene is made of hexagonal "rings" of carbon (like lots of benzene rings connected together, only with more carbon atoms replacing the hydrogen atoms around the edge), giving a honeycomb-like appearance. Since the layers themselves are just one atom high, you'd need a stack of about three million of these layers to make graphene 1mm thick! [1]

Artwork: Graphene has a flat crystal lattice made from interlinked hexagons of carbon atoms (red blobs) tightly bonded together (black lines).

Graphene or graphenes?

People talk about "graphene" the way they talk about "plastic," but it's important to remember that scientists are working on many different kinds of graphene-based materials (just like there are many different kinds of plastics), all of which are a little bit different and designed to do different things. There are many different ways of preparing graphene, for example, and they produce different thicknesses and purities of graphene-based materials. Apart from simple, single-layer sheets—classic graphene—there are lots more complex forms, including multilayer graphene (which, confusingly, is different from graphite), quantum dots, aerogels, and superlattices. And, if that's not enough, there's potentially an infinite number of graphene-based composites and polymers. Can we really capture the richness of all this in a single word?

In this article, I've followed the convention of calling the material "graphene," but it's as well to remember that this very new, fast-evolving substance has many different angles and aspects—and the word graphene will ultimately come to refer to a very wide range of different materials. One day, it may be common to talk about "graphenes" the way we now speak of "plastics."

Photo: A pencil like this is a wooden shaft filled with a stick of soft graphite, a type of carbon made from strongly bonded layers of atoms that are very weakly held together by van der Waals forces. As you drag your pencil along the page, the thin layers of graphite shear off and stay behind, making the black line you can see. Now if you could shave off a super-thin layer of graphite, just one atom high, what you'd have would be graphene. There are tiny specks of graphene in any pencil mark like this, but since they're only one atom high, you'll be doing well to spot them!

What is graphene like?

People are discovering and inventing new materials all the time, but we seldom hear about them because they're often not that interesting. Graphene was first discovered in 2004, but what's caused such excitement is that its properties (the way it behaves as a material) are remarkable and exciting. Briefly, it's super-strong and stiff, amazingly thin, almost completely transparent, extremely light, and an amazing conductor of electricity and heat. It also has some extremely unusual electronic properties.

General properties

Graphene is an amazingly pure substance, thanks largely to its simple, orderly structure based on tight, regular, atomic bonding. Graphene is pure carbon. And, because carbon is a nonmetal, you might expect graphene to be one too. In fact, it behaves much more like a metal (though the way it conducts electricity is very different), and that's led some scientists to describe it as a semimetal or a semiconductor (a material mid-way between a conductor and an insulator, such as silicon and germanium). Even so, it's as well to remember that graphene is extraordinary—and quite possibly unique.

Strength and stiffness

If you've ever scribbled with a soft pencil (something like a 4B), you'll know that graphite is horribly soft. [2]

That's because the carbon layers inside a stick of graphite shave off very easily. But the atoms within those layers are very tightly bonded so, like carbon nanotubes (and unlike graphite), graphene is super-strong—even stronger than diamond! Graphene is believed to be the strongest material yet discovered, and is often described as "200 times stronger than steel." [3] Remarkably, it's both stiff and elastic (like rubber), so you can stretch it by an amazing amount (25 percent of its original length) without it breaking. That's because the flat planes of carbon atoms in graphene can flex relatively easily without the atoms breaking apart. [4]

No-one knows quite what to do with graphene's super-strong properties, but one likely possibility is mixing it with other materials (such as plastics) to make composites that are stronger and tougher, but also thinner and lighter, than any materials we have now. Imagine an energy-saving car with super-strong, super-thin, super-light plastic body panels reinforced with graphene; that's the kind of object we might envisage appearing in a future turned upside down by this amazing material!

Thinness and lightness

Something that's only one atom thick is bound to be pretty light. Apparently, you could cover a football field with a sheet of graphene weighing just four grams—although it's pretty unlikely anyone has actually tried! [5] According to my quick calculations, that means if you could cover the entire United States with graphene, you'd only need a mass of around 1500–2000 tons. That might sound a lot, but it's only about as much as about 1500 cars—and it's completely covering one of the world's biggest countries!

Heat conductivity

As if super strength and featherweight lightness aren't enough, graphene is better at carrying heat (it has very high thermal conductivity) than any other material—better by far than brilliant heat conductors such as silver and copper, and much better than either graphite or diamond. [6] Again, we're most likely to discover the benefit of that by using graphenes in composite materials, where we could use them to add extra heat-resistance or thermal conductivity to plastics or other materials. [7]

Electrical conductivity

This is where graphene starts to get really interesting! Materials that conduct heat very well also conduct electricity well, because both processes transport energy using electrons. The flat, hexagonal lattice of graphene offers relatively little resistance to electrons, which zip through it quickly and easily, carrying electricity better than even superb conductors such as copper and almost as well as superconductors (unlike superconductors, which need to be cooled to low temperatures, graphene's remarkable conductivity works even at room temperature). Scientifically speaking, we could say that the electrons in graphene have a longer mean free path than they have in any other material (in other words, they can go further without crashing into things or otherwise being interrupted, which is what causes electrical resistance). [8] What use is this? Imagine a strong, light, relatively inexpensive material that can conduct electricity with greatly reduced energy losses: on a large scale, it could revolutionize electricity production and distribution from power plants; on a much smaller scale, it might spawn portable gadgets (such as cellphones) with much longer battery life. [9]

Electronic properties

Photos: Advances in nanotechnology, including the development of graphene, will drive faster, smaller, cheaper computers. Picture by courtesy of Argonne National Laboratory published on Wikimedia Commons under a Creative Commons Licence.

Electrical conductivity is just about "ferrying" electricity from one place to another in a relatively crude fashion; much more interesting is manipulating the flow of electrons that carry electricity, which is what electronics is all about. As you might expect from its other amazing abilities, the electronic properties of graphene are also highly unusual. First off, the electrons are faster and much more mobile, which opens up the possibility of computer chips that work more quickly (and with less power) than the ones we use today. (In 2016, MIT researchers floated the possibility of optical graphene chips that might be a million times faster than the ones we use today.) Second, the electrons move through graphene a bit like photons (wave-like particles of light), at speeds close enough to the speed of light (about 1 million meters per second, in fact) that they behave according to both the theories of relativity and quantum mechanics, where simple certainties are replaced by puzzling probabilities. That means simple bits of carbon (graphene, in other words) can be used to test aspects of those theories on the table top, instead of by using blisteringly expensive particle accelerators or vast, powerful space telescopes. [10]

Optical properties

As a general rule, the thinner something is, the more likely it is to be transparent (or translucent), and it's easy to see why: with fewer atoms to battle, photons are more likely to penetrate through thin objects than thick ones. (That's nothing like the whole story, however. The reason why you can see through thick glass but not very thin metal is quite a bit more complex than this. [11]) As you might expect, super-thin graphene, being only one atom thick, is close to transparent; in fact, graphene transmits about 97–98 percent of visible light (compared to about 80–90 percent for a basic, single pane of window glass). [12] Bearing in mind that graphene is also an amazing conductor of electricity, you can start to understand why people who make solar panels, LCDs, and touchscreens are getting very excited: a material than combines amazing transparency, superb electrical conductivity, and high strength is a perfect starting point for applications like these. [13]

Impermeability

Sheets of graphene have such closely knit carbon atoms that they can work like super-fine atomic nets, stopping other materials from getting through. That means graphene is useful for trapping and detecting gases—but it might also have promising applications holding gases (such as hydrogen) that leak relatively easily from conventional containers. One of the drawbacks of using hydrogen as a fuel (in electric cars) is the difficulty of storing it safely. Graphenes, potentially, could help to make fuel-cell cars running on hydrogen a more viable prospect. [14]

On the other hand, if you pepper tiny holes into graphene to make it porous, you get make a meshlike material called holey graphene that can work like an electrical semiconductor or a very fine, physical sieve. Still very new, it's already starting to find exciting applications in new forms of energy storage (such as supercapacitors) and water filters that could reduce pressure on the planet by helping us turn ocean water into safe, clean drinking water.

Artwork: Ordinary graphene (bottom) and porous, holey graphene (top), which has a variety of improved properties. Artwork courtesy of NASA.

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How can we use graphene?

We can answer that question in at least three different ways. First, because graphene has so many excellent properties, and because all those properties probably aren't needed in the same material (for the same applications), it makes sense to start talking about different types of graphene (or even different graphenes) that are being used in different ways or being optimized for particular purposes. So we're likely to see some graphenes being developed for structural uses (in composites materials), some being optimized to make the most of their extraordinary electron-carrying properties (for use in electronic components), others where we make the most of low-resistivity (in energy-saving power systems), and still others where excellent transparency and electrical conductivity are the important things (in solar cells and computer displays).

Photo: Computer memory chips like this might become smaller and faster if graphene replaces the silicon we currently use.

Second, we can see graphene as an exciting replacement for existing materials that have been pushed to their physical limits. Silicon transistors (the switching devices used as memories and "decision-making" logic gates in computers), for example, have consistently become smaller and more powerful over the last few decades, following a trend known as Moore's law, but computer scientists have long expressed concerns that the same rate of progress can't continue as we approach basic limitations imposed by the laws of physics. Some scientists are already imagining smaller and faster transistors in which silicon is replaced by graphene, taking computer devices even closer to the absolute limits of physics. Others, including engineers at IBM, think that's an unlikely development.

In theory, we could use graphene to make ballistic transistors that store information or switch on and off at super-high speeds by manipulating single electrons. In much the same way, graphene could revolutionize other areas of technology constrained by conventional materials. For example, it could spawn lighter and stronger airplanes (by replacing composite materials or metal alloys), cost-competitive and more efficient solar panels (replacing silicon again), more energy-efficient power transmission equipment (in place of superconductors), and supercapacitors with thinner plates that can be charged in seconds and store more energy in a smaller space than has ever previously been possible (replacing ordinary, chemical batteries entirely). Companies such as Samsung, Nokia, and IBM are already developing graphene-based replacements for such things as touchscreens, transistors, and flash memories, though the work is at a very early stage.

Third, and most exciting of all, is the likelihood that we'll develop all kinds of brand-new, currently unimaginable technologies that take advantage of graphene's amazing properties. In the 20th century, plastics didn't simply replace older materials such as metal and wood: for better or worse, they completely changed our culture into one where disposability and convenience overtook durability. If graphenes lead us to ultra-light, ultra-thin, strong, transparent, optically and electrically conducting materials, who knows what possibilities might lie ahead. How about super-lightweight clothes made of graphenes, wired to batteries, that change color at the flick of a switch? Or an emergency house built for disaster areas, with graphene walls so strong and light that you can fold it up and carry it in a backpack?

Our graphene future?

Is it full-steam ahead to a future where graphene rules the world? Maybe—or maybe not. It's important not to get carried away with the hype: most of the exciting work on graphene has so far been done on a very small scale in chemical and physics laboratories. Most of the research is still what we'd describe as "blue sky": it could be many years or even decades before it can be developed practically, let along cost-effectively. By the same token, it's still very early days for basic scientific research into graphene. Forgetting all the amazing applications for a moment, there's doubtless much more exciting science to emerge. For example, it's now believed that graphene is only the first of numerous materials with a two-dimensional crystal lattice—and there could well be lots more similarly structured materials just waiting to be discovered. [15] One thing we do know is that this is a very exciting time for materials science!

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On this website

You might like these other articles on our site covering similar topics:

• Alloys
• Atoms
• Composites and laminates
• Materials science
• Nanotechnology
• Plastics
• Materials: A full list of all our materials science articles.

Other websites

General

• Wired: Graphene gallery: Some great photos of graphene from research labs around the world.
• Nobel Prize in Physics 2010: A great set of background material about the discovery of graphene and the prize awarded to Andre Geim and Konstantin Novoselov.

Research groups

This is a small but representative selection of institutions at the cutting-edge of graphene research around the world:

• University of Manchester: National Graphene Institute: The UK's new specialist research institute, home to Andre Geim, Kostya Novoselov, and 230 other researchers working on graphene-related materials.
• Kim Group, Department of Physics, Harvard University: Another active research group, led by Philip Kim.
• National University of Singapore Centre for Advanced 2D Materials and Graphene Research Centre: Directed by Antonio H. Castro Neto.
• Hone Group: Columbia University: Headed by James Hone.
• Cambridge Graphene Centre: Directed by Andrea C. Ferrari.

Books

Introductions and popular science

• The Superstrong, Superthin, and Superversatile Material That Will Revolutionize the World by Les Johnson, Joseph E. Meany. Prometheus, 2018.
• The Graphene Revolution: The weird science of the ultra-thin by Brian Clegg. Icon. 2018.
• An Introduction to Graphene and Carbon Nanotubes by John E. Proctor et al. CRC, 2017.

Academic

• Graphene: Fabrication, Characterizations, Properties and Applications by Hongwei Zhu et al (eds). Academic Press/Elsevier, 2018. What is graphene, how do you make it, and what can you use it for—cutting-edge answers to these questions in this up-to-date collection of papers.
• Graphene: Synthesis and Applications by Wonbong Choi and Jo-won Lee (eds). CRC Press, 2016. Covers how graphene can be made and what it can be used for.
• Graphene, Carbon Nanotubes, and Nanostructures: Techniques and Applications by James E. Morris and Krzysztof Iniewski (eds). CRC Press, 2013. A collection of recent research papers.
• Graphene: Fundamentals and emergent applications by Jamie H. Warner et al. Newnes/Elsevier, 2012. A very well illustrated guide that takes us from the latest in graphene science to the latest in graphene technology—what will we do with it in the future?
• Graphene and Its Fascinating Attributes by Swapan K. Pati, Toshiaki Enoki, C. N. R. Rao (eds). World Scientific, 2011. An introduction to the synthesis and scientific properties of graphene.
• Carbon nanomaterials by Yury Gogotsi. CRC/Taylor & Francis, 2006. An introduction to nanotubes, fullerenes, and graphene.

Articles

Introductions

• Carbon Wonderland by Andre Geim and Philip Kim. Scientific American, April 2008, pp90–97. A great overview by two of the leading graphene scientists. An easy-to-understand, non-technical overview.
• Graphene: the perfect atomic lattice Nobelprize.org. 13 Sep 2011. A summary of graphene pitched at a similar "popular-science" level as this article.
• Why graphene is the stuff of the future by Andre Geim. New Scientist, 5 October 2010. A quick summary of graphene's potential.

More technical introductions

• [PDF] Graphene: exploring carbon flatland by Andre Geim and Allan MacDonald. Physics Today, August 2007. A much more detailed explanation (you may find it too complex if you don't have a physics degree).
• Graphene-Based Materials by Dan Li and Richard B. Kaner, Science, Vol. 320, No. 5880, May 30, 2008, pp. 1170–1171.
• Graphene: Status and Prospects by A. K. Geim, Science, Vol. 324, No. 5934, June 19, 2009, pp. 1530–1534.
• 2D materials and van der Waals heterostructures by K. Novoselov et al, Science, Vol 353, Issue 6298, July 2016. What other graphene-like materials are on the horizon?

News

• Graphene Made in a Flash From Trash by Charles Q. Choi. IEEE Spectrum, 5 February 2020. A new technique promises to turn any carbon-based material into graphene.
• 3D Printed Graphene Aerogel Offers Highest-Ever Capacitance for a Supercapacitor by Dexter Johnson. IEEE Spectrum. 23 October 2018. Graphene is back on track as the material of choice for making energy-dense supercapacitors.
• Carbon Nanomaterials Could Push Copper Aside in Chip Interconnects by Dexter Johnson. IEEE Spectrum. 11 December 2017. Researchers explore if graphene could replace "chunky" copper wires in tomorrow's computer chips.
• Graphene Composites Go Big by Charles Q. Choi. IEEE Spectrum. 18 May 2015. Graphene composites are super-strong and have excellent conductivity.
• Bend It, Charge It, Dunk It: Graphene, the Material of Tomorrow by Nick Bilton. The New York Times. 13 April 2014. A good, accessible review of graphene's material properties and commercial promise.
• Is graphene really a wonder-material? by David Shukman, BBC News, 15 January 2013. Can graphene really deliver on the hype? Or would the huge sums of money being pumped into researching it be better spent elsewhere?
• Graphene: Patent surge reveals global race by David Shukman, BBC News, 15 January 2013. The race is on to commercialize graphene.
• Graphene supercapacitor breaks storage record: Physics World, November 26, 2010. How graphenes are improving the performance of energy-storing supercapacitors to the point where they can compete with batteries.

Videos

• Graphene and the Carbon Revolution: by Dr Jonathan Hare, Vega Science Trust. Another of Jonathan's excellent, crystal-clear video introductions. (7 minutes.)
• On the Verge of a Graphene Gold Rush: The New York Times/Reuters, January 15, 2014. A quick (3-minute) introduction to the commercial potential of graphene.

Notes and references

1. ↑   A typical layer of graphene is about 0.3 nanometers, so you'd need a stack of 3 million layers to reach 1mm.
2. ↑   Graphite comes in at around 1–2 on the Mohs hardness scale, compared to diamond, which scores a perfect 10. That means it's softer than "classic" metals like silver and gold (themselves fairly soft), but harder than super-soft metals like sodium and lithium.
3. ↑   This often-quoted statement clearly needs some explaining. How much graphene? What kind? How do we define "stronger"? What do we mean by "steel"? The original claim was apparently based on theoretically perfect crystalline graphene, not what you might produce or encounter in the real world. In 2008 and 2013, Columbia University engineers demonstrated that "pristine graphene is the strongest material ever measured." But, as they noted in Science in 2008, these are still essentially lab measurements measured with an atomic-force electron microscope, not real-world comparisons of equivalent graphene and steel structures or objects.
4. ↑   "Very high deformation" (a figure of 25 percent) is quoted in Graphene Science Handbook: Mechanical and Chemical Properties by Mahmood Aliofkhazrae et al (eds), p. 62 (itself quoting Failure Processes in Embedded Monolayer Graphene under Axial Compression by Androulidakis et al, 2014.
5. ↑   This is another often-repeated graphene factoid. If a square meter of graphene weighs 0.77 milligrams and the area of an American football field is 110m × 49m or 5390 square meters, we'd need just over 4 grams to do the job. You'll find this calculation in various places, including Vertically-Oriented Graphene: PECVD Synthesis and Applications by J. Chen et al, p.13.
6. ↑   "thermal conductivity of approximately 2000~4000 W/m/K is obtained for freely suspended graphenes; this value is the highest of any known material" according to Electronic and Thermal Properties of Graphene and Recent Advances in Graphene Based Electronics Applications.
7. ↑   See for example Researchers improve thermal conductivity of common plastic by adding graphene coating by Bob Yirka, Phys.org, 29 October 2014 and Graphene Spreads the Heat by Ravi Prasher, Science, 9 April 2010.
8. ↑   I've simplified this, somewhat! For very much more detail, see, for example Universal mobility characteristics of graphene originating from charge scattering by ionised impurities by Jonathan Gosling et al, Communication Physics, 2021.
9. ↑   For some examples of using graphene in energy generation and storage, see Graphene for batteries, supercapacitors and beyond by Maher F. El-Kady et al, Nature Reviews Materials, 2016 and An overview of graphene in energy production and storage applications by Dale A.C. Brownson et al, Journal of Power Sources, Volume 196, Issue 11, 2011.
10. ↑   See the video lecture Relativity, Quantum Physics, and Graphene by Philip Kim, Harvard University, August 31, 2016.
11. ↑   For a good discussion of "solid state" topics like this, see "Glass planes and aluminium foils" in Why Things Are the Way They Are by B.S. Chandrasekhar. Cambridge University Press, 1998, p.163.
12. ↑   Graphene is nothing like transparent, absorbing about 2.3 percent of visible light, according to Geim and colleagues.
13. ↑   One example of where things might be heading: Transparent graphene electrodes might lead to new generation of solar cells, MIT, June 5, 2020.
14. ↑   Layered Graphene Sheets Could Solve Hydrogen Storage Issues, NIST, March 16, 2010.
15. ↑   Other 2D-sheet materials held together by van der Waals forces in a similar way to graphene are reviewed by Novoselov et al in 2D materials and van der Waals heterostructures, Science, 29 July 2016. They include hexagonal boron nitride (hexa-BN), the superconducting metal niobium diselenide (NbSe2), and quite a few others.