All That Matters

Today it was announced that the 2010 Nobel prize in physics goes to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the two-dimensional material graphene.”

Geim’s and Novoselov’s work on graphene has been frequently predicted for the Nobel prize, although interestingly graphene has been studied long before they entered the field. Studies on graphene go back at least to the 1970s, and the name for this atomically thin layer of carbon came into more wider use in the 1980s.

A model of graphene. Image by AlexanderAlUS via Wikimedia Commons.

So what is the big deal with Geim’s and Novoselov’s research? Well, they developed a really simple method to fabricate graphene. Graphene is a close relative of graphite. Graphite consists of layers of carbon where in each layer the carbon atoms arrange as hexagons. These layers can be visualized as sheets of chicken wire.

Graphene is nothing but a single one of those sheets that make up graphite. The method Geim and Novoselov developed in 2004 to extract graphene is stunningly simple. Take a graphite pencil and write with it on a piece of paper. Then take a post-it note and use it to lift off tiny pieces of graphite. Look under the microscope and identify the single layer ones, and that’s it! But of course, in the meantime more efficient fabrication technologies for graphene have been developed.

As Geim, Novoselov, and many others consequently demonstrated, graphene is a unique material, fundamentally different to graphite. It is highly conducting, and electrons can travel through it for long distances without being deflected. This makes it interesting for fast transistors, and this is the point also of Geim and Novoselov’s ground-breaking first paper on graphene published in Science in 2004. Graphene shows also some interesting electronic properties owing to its electronic band structure, even the fractional quantum Hall effect.

And then of course the electronic bonds in graphene are very strong, which not unlike carbon nanotubes makes it an excellent structural material. Then there are possible applications in molecular sensing and many others. All this makes graphene highly interesting for researchers from many scientific areas. However, some of the rationale expressed by the Nobel Committee strikes me a bit odd, evidenced by this tweet: “According to Nobel Committee, practical applications for graphene include touch screens, fast transistors & DNA sequencing. #nobelprize.”

Flakes of graphene. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials 6, 183-191 (2007).

Indeed, I agree that graphene has potential in all these areas. But we still have to see those promised applications. The last application in this list, DNA sequencing, is from a Nature paper less than a month old!

As for transistors, well, the edges of graphene cause a lot of problem, and so does fabrication. I recently blogged about attempts to use nanowires to make graphene transistors, which are still very far off commercial uses as well. And when it comes to the band structure properties of graphene such as the so-called Dirac point, well, topological insulators show similar physics but could be far more promising.

Graphene is a highly interesting and versatile material with cool properties. But when it comes to applications, well, we will see whether an all-rounder such as graphene will be able to beat incumbents. This is certainly far from clear yet. So please let’s stay realistic on the practical implications of graphene.

Overall of course, I am very happy for Geim and Novoselov, they certainly deserve the prize. At the same time I find it interesting that Sumio Iijima‘s discovery of carbon nanotubes hasn’t been rewarded yet.

In any case, it is a great week for UK science, with Nobel prizes in medicine and physics going to UK institutions. This recognition shows the high standard of UK science, which is presently in severe danger from government budget cuts.

Reference:
Novoselov, K., & Geim, A. (2004). Electric Field Effect in Atomically Thin Carbon Films Science, 306 (5696), 666-669 DOI: 10.1126/science.1102896

Further reading:
Geim, A., & Novoselov, K. (2007). The rise of graphene Nature Materials, 6 (3), 183-191 DOI: 10.1038/nmat1849

This post was chosen as an Editor’s Selection for ResearchBlogging.org

Solar energy is obviously one of the key renewable energy resources available to us. At the same time researchers are hitting against a glass ceiling. A famous 1961 paper by William Shockley (who co-invented the transistor) and Hans Queisser comes to the conclusion that for a semiconductor such as silicon the maximum conversion efficiency of solar energy into electricity will never be more than about 30%.

Dye-sensitized solar cells. A design similar to these solar cells is now used to demonstrate the creation and extraction of multiple charge carriers per photons.

One reason for this limit is that each light particle only excites one electron. Even if the electron has enough energy to excite two electrons, all this energy is lost and only one electron is excited. And this is the case for pretty much all present commercial solar cell technology. Fortunately, however, there are possible exceptions. Bruce Parkinson and colleagues from the University of Wyoming in the USA have now built a photovoltaic cell that at certain wavelengths of light can generate more than one electron per photon of light. Their approach promises to beat the Shockley-Queisser limit and could lead to solar cells with considerably enhanced efficiency.

In silicon and other semiconductors, if a photon excites an electron all excess energy is predominantly lost as heat. Of course, there are attempts to harvest the heat generated in solar cells, and such approaches could beat the Shockley-Queisser limit. And so could nanostructured materials that use for example plasmonic effects. But a more direct solution would be if the excess energy could be used to excite more than one electron in the first place.

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Quantum computers are so highly sought after because they can solve complex mathematical problems and parallel computer operations such as code breaking really fast. Attempts to build quantum computers come in many flavours and use different kinds of quantum states, ranging from trapped atoms, superconductors to semiconductors such as gallium arsenide or diamond. The approach with diamond is particularly promising as it uses the spin of an electron to perform quantum computing operations, which means that such devices could be nicely tied in with conventional electronics and its control of electron transport itself.

Reading single spins. The information of a single spin in a reservoir can be read by a silicon single electron transistor island. Reprinted by permission from Macmillan Publishers Ltd. Nature (2010). doi:10.1038/nature09392

Now,  Andrea Morello, Andrew Dzurak and colleagues from the University of New South Wales in Sydney in collaboration with researchers from Melbourne and Aalto University in Finland have achieved a major step towards such quantum computers using the most straightforward electronic material available: silicon. In a paper in Nature they demonstrate the reading of a single electron’s spin with a silicon electronic circuit. “Until this experiment, no-one had actually measured the spin of a single electron in silicon in a single-shot experiment,” says Morello. Ronald Hanson from the TU Delft, who works on the competing approach using impurities within diamond, agrees: “this is a result that the silicon quantum computing community has been waiting for for a long time.”

The principle of how the electron spin is read with a silicon circuit is relatively straightforward. It uses a charge reservoir, where the electrons are stored in phosphorus impurities that are implanted into the silicon. Next to the reservoir is a single electron transistor, which is a tiny silicon transistor that can detect the presence of a single electron.

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