What is graphene?


About the author: Rui Filipe Serra Maia is a PhD student in Geosciences at Virginia Tech. Check out his profile on the VTSuN student page.

Have you ever imagined winning a Nobel Prize by using the most powerful particle accelerator? Probably yes! Have you ever imagined winning a Nobel Prize by using the fastest super computer? Probably yes! But, have you ever imagined winning a Nobel Prize sitting at home by only using adhesive tape and a lump of graphite, such as the tip of your pencil? Probably not! But that was all that the Nobel laureate of Physics, 2010 used. Surprised? Well, they had just produced for the first time a super strong and super light material that has been revolutionizing fields from electronics to energy and drug delivery.

Do you want to know a little bit more about one of the biggest scientific revolutions of recent decades? Here we go!

Graphene is composed of an amazing element called carbon. Well, humans are mostly made of carbon… Our houses are mostly heated by energy stored in carbon fuels… Diamonds are made of carbon… Our pencils are made of carbon… So is graphene. So why is graphene so different and important? Graphene is made of one single layer of carbon atoms densely packed in a regular sp2-bonded atomic scale, which forms a structure in the shape of a honeycomb, and the distance between each carbon atom is 0.142 nm. Because of its single layer shape, graphene is the only carbon structure in which each atom is exposed for a chemical reaction on both sides.

 

3D model of graphene's honeycomb structure (The Guardian November 13, 2011).
3D model of graphene’s honeycomb structure (The Guardian November 13, 2011).

The honeycomb structure of graphene causes electrons to behave as if they had no mass (known as Dirac fermions). These electrons move about 300 times slower than the speed of light in vacuum, allowing us to observe relativistic effects normally observed in particle accelerators! These Dirac fermions behave as though the speed of light was only 106 m·s-1 (as opposed to 3×108 m·s-1).

In electrical conductors such as copper, electrical resistance increases with the length of the wire because electrons run into more and more impurities in the copper metal while traveling through the wire. However, the presence of impurities in graphene does not cause any increase in electrical resistance. In fact, in graphene nanoribbons electrons travel ballistically and smoothly along the material’s edges behaving more like photons in an optical fiber.

These properties make graphene very useful to study some fundamental physical properties because it exhibits so many interesting–and weird–physical phenomena. Graphene can, for example, change its perpendicular conductivity when exposed to a magnetic field (known as an anomalous “quantum Hall effect”) and it has been used to show a very counterintuitive and rare quantum effect called “Klein tunneling”, in which electrons can pass through a graphene barrier as if it weren’t there. Both phenomena exist because the carbon atoms in graphene are divided in two different families, and electrons travelling through a graphene sheet cannot jump from one family to the other, leading to a new quantum variable. This new quantum variable is called pseudospin, which is inactive in most materials. In graphene, this quantum variable is active and leads to these and other unusual properties.

The mechanical properties of graphene are also very surprising. Besides being impermeable and highly elastic, graphene appears to be one of the strongest materials in nature. Its breaking strength is more than 300 times higher than a (hypothetical) steel film of the same thickness (we can’t make steel films this thin), which is even higher than diamond. At the same time, it is also very stretchable, which combined with its optical properties allows for the production of the flexible screen displays that have been in our dreams.

You should have a good idea of what graphene is by now. With very simple tools commonly found in offices, Andre Geim and Konstantin Novoselov created one of the most promising nanomaterials, opening a tremendous number of possibilities that we had never believed to be possible.

Stay in touch for future VTSuN blog posts on Graphene! Coming up: Applications of Graphene!

 

The author acknowledges Dr. Jean Heremans, from the Department of Physics of Virginia Tech, for his valuable contribution to understand and explain the quantum mechanical phenomena approached in this post.

 

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