All That Matters

Domains in a ferroelectric material, where electric charges have a different orientation. Here, there are two separate sets of domains. The cross-hatched patterns indicate domains in the plane, the rounder shapes are domains where the polarization points out of the plane. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials 7, 209-215 (2008).

Insulators might seem pretty boring materials for an electronic device such as a computer memory, because by the very nature of their definition, they don’t conduct any electrical current. But some insulators show some pretty intriguing properties. Amongst them are the so-called ferroelectrics.

Dipoles in a ferroelectric. During switching, positive and negative charges interchange.

A ferroelectric is a material where positive and negative electrical charges, are permanently separated along a common direction. These are the positive and negative ions that make up the crystal. Their order leads to an overall electrical polarization of the material. This can only happen in an insulator, because if the crystal would enable electrical charges to move around the separated plus and minus charges could be compensated easily by such movements of electrons.

In some special materials, ferroelectricity and magnetism occur simultaneously. These are known as multiferroics, and I blogged about their potential applications before. In particular, the dipoles in a ferroelectric can be switched by an electric field, which makes them attractive for electronic applications as ferroelectrics can be used to permanently store information as a new form of computer memory.

But how can the electric polarization in a ferroelectric be switched? There are two options. One mechanism is similar to what happens in a magnet. If an electric field is applied, new domains with a polarization aligned in direction of the external field form (see figure below). These domains gradually replace the old ones. This process is abrupt, because as the new domains expand, the ions in the crystal swap places in a single process.

The second possibility of switching electric polarization is a continuous mechanism. There, the positive and negative ions move slowly in opposite direction. First, the electric polarization weakens, vanishes, and then builds up again in opposite direction. This process occurs without the involvement of any domains. Of these two processes, the domain-based switching is far more favourable, which is why the switching process without domains hasn’t been observed before. Two independent papers now both claim to have seen switching without domains. […]

Stretchable electronic arrays. The LED sheets can be twisted by 720 degrees and considerably stretched. The wavy metal wires are visible in the image on the on the right. Reprinted by permission from Macmillan Publishers Ltd. Nature Materials (2010). doi:10.1038/nmat2879

Take a piece of silicon, try to bend it and it will break. Stretch a thin film of gold and it will rupture. Conventional metals and semiconductors are brittle and not elastic at all. But these are properties that you need when you want to use electronic devices in unusual places and for unusual applications. In biomedicine for example, if you want to put a diagnostic sensor on top of a muscle. In electronics, when you want to put a large-scale solar cell on the curved top surface of a car.

Sure, you can make a thin film and warp it around a cylinder, and if you do this with electronic circuits it is called flexible electronics. Organic electronics and very thin metal films on plastic can do this. But you cannot fit a two-dimensional sheet on a sphere without stretching it. For such applications you need what is called stretchable electronics, which is different to the flexible electronics that has been around for a while.

The latest milestone has been achieved by John Rogers and colleagues from the University of Illinois in Urbana-Champaign. They demonstrate (disclaimer: in my journal, Nature Materials) a fully biocompatible and implantable stretchable structure containing large arrays of light-emitting diodes and photodetectors. The sheets are stretchable and can be twisted by more than 720 degrees without damage, and can be brought into almost any desirable shape or configuration, says Rogers. “This advance suggests a technology that can complement features available with organic light emitting diodes, where peak brightness and lifetime are limited, and conventional inorganic LEDs, where relatively thick, brittle supports restrict the way that they can be integrated together and the substrates that can be used.”

[…]

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