Carbon and its allotropes have always fascinated me. Known as the “backbone” of organic chemistry, carbon comes in many shapes and sizes.
Most carbon allotropes preferentially form hexagonal structures, known as “aromatic” rings, which are highly stable. Sticking these hexagons together makes polycyclic aromatic hydrocarbons, planar carbon “frisbees” found in interstellar gas clouds (below, left).
Add more hexagons, and eventually the carbon will collapse into a truncated icosahedron. Known as “fullerenes,” this form of carbon resembles the nanoparticle version of a soccer ball.
Graphene, in particular, captures my interest. Consisting of a 2D sheet of hexagonally linked carbon atoms, graphene has a number of unique properties that have caused much excitement in the scientific community.
Its discovery in 2002 made it the first 2D material ever isolated [1]. In the wake of its discovery, graphene caused a surge of research into its use for flexible electronics, solar cells, light-emitting diodes (LEDs), supercapacitors, and high-strength lightweight composites.
Scientists researching the “wonder material” remark that it can do anything… except leave the laboratory.
Though its properties are exciting and the initial returns are promising, graphene still has a long way to go before it reaches widespread adoption in industry.
This post will serve as a more general overview. Before getting too specific, I wanted to introduce graphene conceptually, explain why it arouses such scientific curiosity, and highlight some of its more interesting properties. I’ll be sure to post more specific articles on interesting graphene-related topics in the future!
Jump Ahead!
Table of Contents
Material Properties
As a two-dimensional, atomically thin semi-metal, graphene’s physical properties are quite unorthodox. Here’s some of the characteristics that make graphene so interesting:
Mechanical Strength.
Graphene is the strongest material ever measured. In 2008, a group of researchers from Columbia University measured its tensile strength using an instrument called an atomic force microscope. With a strength of 130.5 Gigapascals, graphene is 150x more resistant to deformation than steel (0.840 GPa).
Graphene is often incorporated into materials to improve their mechanical strength. When a material is comprised of many components, it is referred to as a composite. Composites incorporating graphene are nanocomposites, meaning one of their components (in this case, graphene) has a dimension of less than 100 nanometers.
Graphene nanocomposites can be engineered into high strength, lightweight, biocompatible materials. That makes them ideal for tissue engineering applications. Graphene’s mechanical strength in particular allows it to form a sturdy regenerative scaffold that supports bone regeneration.
Thermal Conductivity.
Graphene has one of the highest measured thermal conductivities of any known material, at 5,000 Watts per meter-kelvin [3].
Graphene’s thermal conductivity makes it ideal for thermal interface materials. Graphene nanocomposites have been investigated for the efficient cooling of computers, power equipment, microelectronics, and optoelectronics.
Impermeability to Gas.
Graphene can be made selectively permeable to water vapor, blocking the passage of all other gases [4].
Andre Geim, the discoverer of graphene, fabricated thin films of graphene oxide, which he found were impermeable to gas molecules outside of water vapor. Such low rates of diffusion, especially for small atomic gases, are extremely rare.
Geim attributed this discovery to the low-friction passage of H2O through closely spaced graphene sheets. Diffusion of other molecules can be blocked by (i) reversible narrowing of capillaries at low humidity or (ii) clogging in the presence of water.
For fun, Geim and his co-authors sealed a bottle of vodka with their graphene membranes. The vodka became stronger and stronger over time; the water evaporated, but the alcohol was blocked by the membrane.
The lightweight nature of graphene, combined with its impermeability to gas, make it ideal for applications in distillation, separations chemistry, and environmental experiments involving gas separations.
Electrical Conductivity.
This is perhaps graphene’s greatest strength, and the property that arouses the most interest in the scientific community. Graphene conducts electricity better than any other material in the universe. To really understand graphene’s bizarre and amazing electrical properties, let’s take a look at the quantum chemistry concept of “electron bands.”
No, not that kind of band. Let’s start here: all of the electrons in an atom exist in electronic shells, particular energy levels in which an electron is allowed to exist. These can be visualized as the concentric “rings” in a Bohr model of the atom, shown below.
When individual atoms come together to form graphene, the behavior of these electrons changes. Instead of staying in distinct shells, electrons in graphene occupy a certain range of energies dependent on their momentum.
So, electrons with a particular momentum “x” are only permitted to have a certain energy “y.” Because momentum is variable, a range, or “band” of energies are available to all electrons.
We’re interested in two particular electron bands: the valence band, and the conduction band. Normally, electrons remain bound by their attraction to the nucleus, so they hang out in the low-energy valence band.
However, given enough energy, electrons can leap across the band gap, from the valence band to the conduction band (see below).
In the conduction band, electrons are released from their attraction to the nucleus and are free to roam around the material. That’s how we get the flow of electricity!
Let’s return to the electron’s band function. Remember that in graphene, an electron’s energy is a function of its momentum.
Graphene is a two-dimensional material; ergo, its electrons can have momentum in the x and y directions.
So, their energies are multivariable functions of both momenta.
This is where things get interesting. Graphene’s valence band and conduction band touch at several points in this “momentum space.”
These six intersection points are known as the Dirac points, and make the passage of electrons across the band gap much easier.
Graphene’s band structure approaching the Dirac point is linear, which forms a cone shape in three dimensions.
That linearity has important implications for graphene. Remember that electrons are incredibly small, with a mass registering 9.109×10-31 kilograms.
Turns out, that’s small enough to exhibit some wave-particle duality. The mass of an electron can vary, and its physical mass at any given point is given by the effective mass.
DANGER! MATH AHEAD
The effective mass of an electron is proportional to the second derivative of the band function it belongs to. That means the curvier that our electronic bands are, the higher the mass of the electron.
But straight lines don’t curve at all! You may or may not remember that the second derivative of a straight line is zero.
Since the band function approaching the Dirac points is linear, the effective mass of electrons entering the conduction band is zero. Zilch. Nada. As if they didn’t even exist.
How is that possible? After losing their mass, graphene’s electrons become massless quasi-particles, zipping across the material’s surface at mind-boggling speeds. This extraordinary electron mobility has led many to believe that graphene will eventually replace silicon as the gold standard in the semiconductor industry.
Now, what does all this mean for graphene? In summary:
- Points of intersection between the valence and conduction bands, known as Dirac points, allow electrons to easily pass from one band to the other.
- When electrons pass into the conduction band, they enter a state in which they are able to overcome electrostatic attraction to the nucleus, permitting their travel through the material. BOOM! Electricity!
- Electrons entering the conduction band lose their effective mass, giving graphene extraordinary electron mobility. In fact, electrons travel 45,000 times faster in graphene than in copper [5].
Biocompatibility.
The last property we’ll discuss is graphene’s biocompatibility. As an organic molecule, graphene is lightweight, flexible, and relatively nontoxic. Graphene has already been put to great use in bioengineering; in fact, we already discussed its use in regenerative scaffolds for bone tissue (see: mechanical strength).
Because of its biocompatibility, graphene has been explored for applications in biological sensing. Implantable biosensors, like common blood sugar monitors, can measure critical components of the chemical makeup in our bodies. Incorporation of graphene could allow the creation of a new class of flexible sensors that are safer for native tissue.
Graphene’s electrical conductivity make it ideal for biomimetic scaffolds in the brain. In addition to providing structural support, graphene can conduct electrical signals, facilitating communication between separated neurons.
Furthermore, graphene has shown the capacity to facilitate neural regeneration [6], effectively inducing differentiation in neural stem cells and accelerating the healing process.
One extreme example of the regenerative power of graphene was showcased in the treatment of spinal cord injury with graphene-based gel. Using a rat animal model, the Tour group at Rice University showcased a remarkable novel treatment, utilizing the conductive properties of graphene to restore motor function after impairment.
Watch the video to see for yourself!
The Other Side of the Narrative
Graphene as a technology has many detractors. It has thus far resisted commercialization, and still has many hurdles to overcome before the world can be populated with flexible electronics, graphene solar cells, high-strength graphene nanocomposites, and the like.
A popular narrative among the uninformed is that carbon nanotechnology is rather “stuck in the mud” and not going anywhere.
Bollocks. Many universities, companies, and laboratories around the globe remain dedicated to the exploration of this technology. New companies, such as Vorbeck and Nanowerk, have entered the manufacturing space and are ramping up production of graphene. Exciting synthetic pathways to graphene continue to crop up in literature. Interest and progress in graphene research remain strong.
Just because a material hasn’t reached widespread adoption in industry seventeen years after its discovery doesn’t mean it is dead or useless.
In fact, silicon itself was first purified in 1824 by Swedish chemist Jon Jakob Berzelius; the birth of the modern semiconductor industry took more than a century after that.
Materials innovations and industrial adaptation take time, especially when semiconductor companies have tied trillions of dollars into developing silicon technologies.
You can argue that graphene was oversold upon its discovery. And you’d be right.
One thing I have learned in my limited experience in science is that there is tremendous pressure on scientists to oversell. Their livelihood, their projects, and their funding depend on it.
Plus, graphene is damn cool and exciting. Scientists are humans too, and we can oversell the potential and delivery time for a new technology just as easily as any other human. You know how you told yourself you were going to wake up early and go to the gym last Saturday?
Unfortunately, innovation cannot be rushed. As materials scientists, we still have a few great leaps to make before our understanding of graphene can deliver on all of its promises.
No one knows when graphene will be adopted into mainstream industry. That will happen on its own timeline, when graphene becomes significantly cheaper and more effective than currently employed technologies. Anyone who pretends to know when this will happen is a liar.
But it will happen. And when it does, it is going to be awesome.
That’s all for now! I hope you enjoyed this brief introduction to graphene. As always, if you have any comments or questions for me, let me know! Till next time!
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- K. S. Nolosev, A. K. Geim, S. V. Morosov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov. “Electric Field Effect in Atomically Thin Carbon Films.” 2004. Science. 306 (5696)
- Lee, C., Wei, X., Kysar, J. W., Hone, J. “Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene.” 2008. Science. 321 (5887)
- A. A. Balandin, S. Ghosh, W. Bao, I. Calizo, D. Teweldebrhan, F. Miao, C. N. Lao. “Superior Thermal Conductivity of Single-Layer Graphene.” 2008. Nano Lett. 8 (3)
- R. R. Nair, H. A. Wu, P. N. Jayaram, I. V. Grigorieva, A. K. Geim. “Unimpeded Permeation of Water Through Helium Leak-Tight Graphene-Based Membranes.” 2012. Science. 335 (6067)
- J. H. Chen, C. Jiang, S. Xiao, M. Ishigami, M. S. Fuhrer. “Intrinsic and Extrinsic Performance Limits of Graphene on SiO2.” 2015. Nature Nanotechnology. 3 (4)
- J. S. Lee, A. Lipatov, L. Ha, M. Shekhirev, M. N. Andalib, A. Sinitskii, J. Y. Lim. “Graphene Substrate for Inducing Neurite Outgrowth.” 2015. Biochem Biophys Research Comm. 460 (2)
Very exciting and interesting article. Found you through the chemistry subreddit. Keep up the awesome work!