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.
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!
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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.
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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."
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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.
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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!
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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.
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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.
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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).
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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.
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One thing we do know is that this
is a very exciting time for materials science!
