After the discovery of one form of carbon — fullerenes — was awarded the Nobel Prize in Chemistry in 1996, this year's Nobel Prize for Physics was awarded to Andre K. Geim and Konstantin S. Novoselov, both at the University of Manchester, U.K., for succeeding in producing, isolating, identifying and characterizing another form of carbon — graphene.
Anyone who has used an ordinary pencil has probably produced graphene-like structures, but without knowing it. A pencil contains graphite, and when it is moved on a piece of paper, the graphite is cleaved into thin layers that end up on the paper and make up the text or drawing that we are trying to produce. A small fraction of these thin layers will contain only a few layers or even a single layer of graphite, i.e. graphene.
Graphene, a single atomic layer of carbon, is the first two-dimensional (2D) crystalline material that has been identified and analyzed. This new material has a number of unique properties, which makes it interesting for both fundamental studies and future applications.
It is a transparent conductor which is one atom thin. It also gives rise to analogies with particle physics, including an exotic type of tunnelling. In addition, graphene has a number of remarkable mechanical and electrical properties. It is more than 100 times stronger than the strongest steel, and is very stretchable.
The thermal and electrical conductivity is very high and it can be used as a flexible conductor. For instance, the electrical conductivity is somewhat higher than the conductivity of copper. And on the thermal conductivity front, graphene conducts heat 10 times better than copper, and much higher than that of silver.
Different forms
Carbon can exist in several different forms. The most common form of carbon is graphite, which consists of stacked sheets of carbon with a hexagonal structure. Under high pressure, diamond is formed.
A new form of molecular carbon is the so called fullerenes. The most common, called C60, contains 60 carbon atoms and looks like a football (soccer ball) made up from 20 hexagons and 12 pentagons which allow the surface to form a sphere. The discovery of fullerenes was awarded the Nobel Prize in Chemistry in 1996.
A related quasi-one-dimensional form of carbon, carbon nanotubes, has been known for several decades and the single walled nanotubes since 1993. The electronic and mechanical properties of metallic single walled nanotubes have many similarities with graphene.
It was well known that graphite consists of hexagonal carbon sheets that are stacked on top of each other, but it was believed that a single such sheet could not be produced in isolated form. But in 2004, the two scientists showed that such a single layer called grapheme could be isolated and that it was stable.
Though the existence of grapheme-like structures was already known in the 1960s, isolating single layers proved difficult.
But the difficulty was not to fabricate the graphene structures, but to isolate sufficiently large individual sheets and to verify its unique 2D properties. This is what Geim, Novoselov, and their collaborators succeeded in doing.
What is graphene?
The electronic structure of grapheme, a single layer of carbon packed in a hexagonal (honeycomb) lattice, is rather different from usual three-dimensional materials. Its Fermi surface is characterized by six double cones. In intrinsic (undoped) graphene the Fermi level is situated at the connection points of these cones.
The electrical conductivity of intrinsic graphene is quite low, as density of states of the material is zero at the connection points of these cones.
But the Fermi level can however be changed by an electric field so that the material becomes either n-doped (with electrons) or p-doped (with holes) depending on the polarity of the applied field. Graphene can also be doped by adsorbing, for example, water or ammonia on its surface. The electrical conductivity for doped graphene is potentially quite high.
Graphene is practically transparent. In the optical region it absorbs only 2.3 per cent of the light. In contrast to low temperature 2D systems based on semiconductors, graphene maintains its 2D properties at room temperature.
The discovery
Before 2004, the isolation of stable sheets of graphene was not thought possible.
It was therefore a complete surprise when Andre Geim, Konstantin Novoselov and their collaborators from the University of Manchester (UK), and the Institute for Microelectronics Technology in Chernogolovka (Russia), succeeded in doing precisely this. They published their results in October of 2004 in Science .
They used a simple but effective mechanical exfoliation method for extracting thin layers of graphite from a graphite crystal with Scotch tape and then transferred these layers to a silicon substrate.
This method was first suggested and tried by another group (R. Ruoff's), but they were not able to identify any monolayers. The Manchester group was able to even identify flakes made up of a single layer, i.e. grapheme. Furthermore, they managed to pattern the graphene into a Hall bar and connect electrodes to it.
In this way they were able to measure both the (longitudinal) resistance and the Hall resistance.
An important piece of data was the ambipolar field effect where the resistance was measured as a function of an electric field applied perpendicular to the sample.
Once the technology to fabricate, identify, and attach electrodes to the graphene layers was established, both the Manchester group and other groups quickly made a large number of new experiments.
Apart from the exfoliation method, different ways of growing very thin carbon films were also studied. In particular, a group lead by W.A. de Heer at Georgia Tech was refining a method to burn off silicon from a Silicon Carbide (SiC) surface, leaving a thin layer of carbon behind.
A group at the University of Columbia lead by P. Kim investigated an alternative approach for making thin carbon layers. They attached a graphite crystal to the tip of an atomic force microscope and dragged it along a surface. In this way they were able to produce thin layers of graphite down to approximately 10 layers.
Research explosion
Since 2005, development in this research area has literally exploded, producing an increasingly growing number of papers concerning graphene and its properties. Double layers of graphene, which have different properties compared to (single layer) grapheme, have been studied thoroughly.
Apart from studying the mechanical strength, scientists discovered that light absorption in graphene is related to the fine structure constant as mentioned above.
Future applications
Graphene has a number of properties which makes it interesting for several different applications. It is an ultimately thin, mechanically very strong, transparent and flexible conductor.
Its conductivity can be modified over a large range either by chemical doping or by an electric field. The mobility of graphene is very high, which makes the material very interesting for electronic high frequency applications.
Recently it has become possible to fabricate large sheets of graphene. Using near-industrial methods, sheets with a width of 70 cm have been produced.
Since graphene is a transparent conductor it can be used in applications such as touch screens, light panels and solar cells, where it can replace the rather fragile and expensive Indium-Tin-Oxide (ITO). Flexible electronics and gas sensors are other potential applications.
The quantum Hall effect in graphene could also possibly contribute to an even more accurate resistance standard in metrology.
New types of composite materials based on graphene with great strength and low weight could also become interesting for use in satellites and aircraft. ( Excerpts from ‘2010 Nobel Prize Physics - Advanced Information' at www. nobelprize.org )
