Drawing a Model of the Future – in pencil
Pick up a graphite pencil and draw something. It might not sound high-tech, but you´ve probably created a nano-scale material that is fascinating and puzzling some of the best minds on the planet. In amongst your doodling will be fine flakes of graphene — a single layer of carbon atoms that could lead to the development of super-fast, super-small electronic devices.
Such technological applications are a long way off yet, as that will need significantly more understanding of what is happening at the nano-scale in order to control and utilise this material. And it´s that understanding which is taxing the mind of Professor Alan Kaiser of the MacDiarmid Institute and other researchers at scientific facilities around the world.
“Graphene is a remarkable material,” says Professor Kaiser. Although graphene has been around since we first started making marks with graphite in the 1500s, it was only five years ago that this special material was isolated and identified by Andre Geim and his group in Manchester. Before that, it was believed that a single layer of atoms could not exist in the free state. Another surprising feature of graphene is that, although it is just one atom thick, it can nonetheless be seen in an ordinary optical microscope.
In its natural form, graphene´s parent substance graphite is just high-quality coal, consisting of carbon atoms arranged in latticework layers. This structure allows large numbers of electrons to move within each layer, giving graphite electrically conducting properties useful, for example, in the operation of electrodes in steel furnaces.
Researchers have been trying to find ways of making graphite layers thinner. The approach can be surprisingly low-tech — pressing sticky tape to the surface of a graphite sample and peeling an individual layer of graphene away. Attempts to obtain larger pieces than small flakes have not been particularly successful.
Another approach takes a different direction, experimenting with building up a sheet of graphene by depositing it from a dissolved solution. This could provide a controlled way of producing sheets, rather than random flakes, making technological applications more achievable. However, the oxidation step required in the process results in regions of disorder forming amongst the regular honeycomb latticework of carbon atoms. Instead of having a smoothly conducting crystalline form, this tends to create small barriers where the electrical conductivity is reduced.
At this point, things get really strange, with the electrons undertaking quantum mechanical tunnelling or “hopping” to make their way through the barriers of the disordered regions. Drop the temperature below 50 K (minus 223 degrees Celsius), and there´s a lot of explaining to do, as the experimental results show a new mechanism of conduction. That´s where Professor Kaiser comes in.
“My role is to look at the electronic processes involved,” he says. Trying to build up an idea of what is happening at this very small scale requires a combination of an explanatory model and experimental evidence. And so the physicist pores over graphs of experimental results looking for the information that will help him test his ideas.
“I am interested in the meeting place between theory and experiment. What would be the next experiment to try? What would be the clincher to convince people that we were right?”
The models that Professor Kaiser proposes, serve to inform the experimental design that his collaborators work on, and vice versa, as each learns more about the material and its properties. This knowledge is vital if we are to take full technological advantage of the properties of graphene.
Carbon has the advantage of being a very stable material. It can be produced in a highly pure form, and the very thin nature of the graphene layers produces a very high surface-to-volume ratio. That makes it useful in applications such as biosensors. While graphite is typically a dark substance, graphene itself is transparent. That property and its electrical conductivity make it a very promising material for use in opto-electronics.
Graphene allows electronic devices to be developed that are just a single atom thick. The density of the electrons can be controlled by increasing and decreasing a gate voltage, and the remarkably high mobility of the electrons means that very fast transistors are possible.
Another possibility is building complete electronic circuits starting from a sheet of graphene, with the stability of the carbon rings enabling ultra-small components. Separate wires wouldn´t be needed — conducting channels could be formed on the graphene layer itself.
However, before we get to that, we need to understand what´s going on.
“There´s a lot more to do. We´re looking to the future.”