All That Matters

Photo of the electronic device used to measure the high electron mobility in ZnO. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials (2010). doi:10.1038/nmat2874

Even if you don’t know much about this compound, everybody is familiar with zinc oxide (ZnO). It is a white powder used as the UV-light absorbing component in many sun lotions (and the part of the sun lotion that leaves those white marks on clothes), as an antibacterial agent in some baby powders, in rubbers where it promotes vulcanisation, or as white pigment in colours, and for many other products.

And even now ZnO remains one of the most widely studied oxygen-containing oxide compounds — and this is not going to change anytime soon. In a paper that is now published in my journal Nature Materials, Atsushi Tsukazaki, Masashi Kawasaki and colleagues from Tohoku University in Japan accomplished growing ZnO to such high purity that its electrons can move at extremely high speeds. The material is so clean that it shows quantum effects that are only known from a few very pure compounds. Darrell Schlom from Cornell University in the USA, who works on the growth of oxides, is quite enthusiastic: “Oxides have the unfortunate stigma of being associated with dirt, bricks, and toilet bowls. I love this rags to riches story because it shows that oxides can be clean, so clean that with ZnO they have broken into the most exclusive and elite club that was reserved for just half a dozen of the world’s cleanest materials. This is the greatest achievement of the year for oxides!”

Indeed, the ZnO thin films that Tsukazaki and colleagues grew are so clean that electrons in them move so fast that the researchers were able to observe the so-called Fractional Quantum Hall Effect (FQHE), a first for any oxide compound. The FQHE is a sign that electrons are in quantum states that can be used in quantum computing, and by showing the FQHE, ZnO has established itself as a candidate material for such schemes.

[…]

This morning I read an article by the Scientific American editor David Biello on an important topic: the importance of rare earth elements for our economy, and the power of those few countries that export them on a larger scale. (disclaimer: Scientific American is part of Nature Publishing Group, my employer)

David hits an important point there. But to my mind, the problem is far more critical and fundamental than this single, focussed example suggests, and we need to act on it soon.

Salt production at Salar de Uyuni. This salt flat harbours 50% of the world's lithium reserves. Image by Ricampelo via Wikimedia Commons.

The issue is that rare earth elements such as neodymium are essential to green energy and our economy. Neodymium is part of Nd₂Fe₁₄B, a powerful permanent magnet that is used for electromotors, read heads of hard disk drives, etc. Each wind turbine apparently uses 300 kg of neodymium, each Toyota Prius about 1 kg. At present, China produces 97% of all neodymium.

And this is the problem. China has implemented export controls for its rare earth elements resources. In a recent diplomatic spat with Japan, they temporarily restricted the export of rare earth elements to Japan. But the Chinese should not take all the blame for a little realpolitik. Heard of the 1973 oil crisis?

[…]

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