The mines of lead pencils actually consist of graphite – and therefore also contain graphene, i.e. layers of individual graphite atoms.
Graphene is a material whose existence was long thought to be impossible. It consists of pure carbon and is only one layer of atoms in thickness. Researchers from Jülich are investigating the properties of graphene and looking for applications.
Bare mountain sides, rough stone walls, grazing sheep: Borrowdale is a barren region of the Lake District in the north west of England. There’s no hint of the technological revolution that took place here when sixteenth-century farmers discovered a soft black mineral in the remote region. The material’s use lay in cutting it into thin bars and wrapping it with string: the lead pencil was thus invented, a handy writing tool set to replace goose quills and inkwells.
Image above: The mines of lead pencils actually consist of graphite – and therefore also contain graphene, i.e. layers of individual graphite atoms.
Its name is unjustified, however: the shiny black material at its core is not lead but pure graphite. And this form of carbon could now be the catalyst for another innovative boost. In 2004, researchers at the University of Manchester succeeded in producing an exotic material from graphite that is only one atomic layer in thickness: graphene. The material is flexible yet extremely tough and robust. At room temperature, it also conducts electrons faster than any other material. For these reasons, it is expected to be suitable for a whole range of applications: extremely robust composite materials, lightening-fast transistors, and highly sensitive sensors, for example.
“Graphene has a purely two-dimensional structure. It consists of carbon atoms that all lie in one plane and are linked together by solid molecular bonds to form a lattice. They form a six-sided honeycomb pattern,” explains Prof. Stefan Blügel from Jülich’s Peter Grünberg Institute (PGI-1). “Physicists had described the material theoretically as early as the 1970s. The fact that it actually exists was long thought impossible, however. It was therefore a sensation when it was first produced successfully – especially since its production was so easy.”
Blügel is alluding to the sticky-tape method that the team of Manchester physicists – headed by Andre Geim and Konstantin Novoselov – used to produce the atomic carbon layers. The base material was a tiny graphite flake. This material consists of innumerable graphene layers clinging to each other in piles. Using simple sticky tape of the kind that can be found in virtually any office, the researchers peeled individual layers from the flake and transferred them to a silicon disc where they could be investigated intensively. In 2010, they both received the Nobel Prize in Physics for their work on graphene.
Stefan Blügel remembers: “Theorists went and opened their drawers, pulling out their old calculations. Finally, they had the opportunity to review the validity of their models using a real system.”
One outstanding property was of particular interest to the experts: the electrical conductivity of graphene. Every carbon atom in the molecular honeycomb contributes one electron, and these electrons pool like a lake above and below the carbon layer, according to Blügel: “What’s particularly interesting, however, is that the electrons in these lakes can move as if they had no mass. They can be accelerated effortlessly. And this results in the extraordinary conductivity of the material.”
The charged particles shoot through graphene at around 0.3 % of the speed of light – that’s almost 1,000 kilometres per second. Although these speeds are not quite as high as those that can be achieved in large particle accelerators, they move so quickly that the rules of Albert Einstein’s theory of special relativity apply to them. This means that graphene permits exotic phenomena to be studied in the laboratory which would otherwise only occur in expensive accelerator facilities. Graphene is not only extremely conductive but also extraordinarily tough. Mixing it into plastics would undoubtedly increase the durability of such composite materials considerably. And when foreign molecules come into contact with the carbon layer, they produce tiny waves that roll over the electron lake, making graphene an ideal material for sensitive sensors. In the field of microelectronics, in particular, there is great hope that graphene might replace the standard semiconductor silicon. But the dawn of the carbon age is a while off yet.
“I know of no established application of graphene in the electronics sector,” says Dr. François Bocquet (PGI-3). And this is due to the prominent property of the carbon lattice mentioned earlier: high electron conductivity. Conventional transistors on computer chips consist of the semiconductor material silicon. These components can be switched between two states: on and off. The flow of electricity in a transistor based on graphene, however, can never be switched off completely. It is like a leaky tap that constantly drips water. Such components are therefore not suitable for digital circuits: “If all transistors in a logical circuit are constantly switched on, they cannot be used to make computations,” says Bocquet.
The Jülich researcher is therefore working on influencing the conductivity of graphene, for example by means of targeted impurities: these are atoms of different materials introduced to the graphene, a method called doping that is well-established in semiconductor technology. Another possibility of controlling conductivity is using chemical procedures to produce graphene – making use of the material on which the graphene is deposited. After all, the sticky-tape method is no longer the only route to producing the coveted material. Chemists have developed much more elegant processes involving the surface deposition of graphene from gases that contain carbon.
“This usually leads to interactions between the substrate material and the graphene. We were able to show that the atomic distance between the two is decisive in terms of how strong the influence on conductivity is,” explains Bocquet. He says that while his work is essentially basic research these insights may still make a major contribution to potential applications.
The electronic components that Dr. Ilia Valov (PGI-7) is working on could also profit from the carbon lattice: “We develop data storage systems that require extremely little energy, switch very quickly, and take up remarkably little space. We’re talking about the nanometre range.” In contrast to normal computer memory systems, these ReRAMs maintain their information even if the power is switched off.
Valov and his colleagues were able to show that ReRAMs behave like tiny little batteries. But this also comes with a number of disadvantages regarding long-term stability, says the researcher: “We’re familiar with these phenomena from the conventional lithium batteries: they age because the substances inside them damage the electrodes. This harmful influence in the cell chemistry also occurs in our memory systems.”
And this is where graphene comes into play: Ilia Valov places it on the electrodes of his memory elements as a thin protective layer that allows him to keep the cell chemistry in check. Graphene is ideally suited to this: the layer is so thin that it hardly interferes with the functionality of the component. And simultaneously, it is impenetrable for the components of the cell that would damage the electrode. As Valov says: “The storage cells are more stable as a result. Although we have to live with slightly poorer switching times, I still believe that this structure represents the way forward.
The coming years will show what applications the atomic carbon layers are most suited to. As for the graphite from the northern English mines, it had a totally different purpose for a period of time: the black mineral was perfectly suited to lining moulds for cannon balls. This application, at least, has now been consigned to the history books.
Cardiac cells and nerve cells have one thing in common: they pass on information in the form of weak electric impulses. Nanoresearcher Dmitry Kireev from Jülich’s Institute of Complex Systems (ICS-8) wants to eavesdrop on this whispering in the cells and thus gain access to the information. He is therefore developing sensors based on graphene, which in future will be implanted in the body. “This way, signals can be picked up directly in the central nervous system and forwarded externally,” explains the microtechnology expert. Patients would be able to use these signals, for example, to control leg prostheses. The Jülich sensors have a long way to go yet, however. Laboratory experiments are currently under way in which Kireev grows cardiac cells and nerve cells on pieces of transparent plastic foil about the size of a postage stamp. Gold-coloured lines lead from the edge of the foil towards the centre. “These are feed lines; the actual graphene sensors are located in the centre,” explains Kireev. After just a few days, the cells start to fire off their characteristic impulses. The graphene serves as a tiny electrode which collects the electric signals. The advantages of the material: it is extremely sensitive to the weak cell impulses, biocompatible, and can be applied to a flexible substrate – all of which are important prerequisites for use inside the body. And therein lies the next challenge: measuring the cell signals in a natural organ rather than in the laboratory.
Images: Forschungszentrum Jülich, Forschungszentrum Jülich/Ralf-Uwe Limbach, Graphics: SeitenPlan