26 June 2012
Researchers have confirmed that they can change the electronic properties of graphene by simply introducing strain into the material. The phenomenon, which had been predicted in theory but only seen in only a handful of experiments to date, could be used to make electronic devices such as transistors as well as optoelectronics components.
Graphene – a honeycombed lattice of carbon just one atom thick – is an excellent conductor of electricity thanks to the fact that electrons whizz through the material at extremely high speeds with little resistance. Such a property means that the material might be used to make transistors that are faster than any that exist today. However, graphene’s extreme conductivity is also a problem because devices made from the material remain conducting even when switched off. This not only wastes power, but also means that such devices cannot be packed onto computer chips in the same way that silicon components are today.
Lack of bandgap
Although graphene is a semiconductor (albeit a special type of one), it is unlike familiar materials such as silicon because it does not have an energy gap between its valence and conduction bands. Such a band gap allows a semiconductor to switch the flow of electrons on and off. Researchers have proposed various schemes to overcome this problem – for example, by cutting graphene into nanoscale ribbons or dots, or chemically modifying the material to make it properly semiconducting. While these schemes work in principle, opening a band gap in graphene in this way also damages the material so much that finished devices no longer have high electron mobility.
Now, Joseph Stroscio of NIST in the US and colleagues are saying that they can modify the electrical properties in graphene by simply stretching the material. The researchers say that distorting graphene appears to have the same effect as applying a strong magnetic field and produces quantum dots in the material. Quantum dots are tiny semiconducting structures in which electrons are confined in all three dimensions and have unique electronic and optoelectronic characteristics. Creating semiconducting regions in graphene by modifying the material’s shape might now give scientists the best of both worlds, says the team: high speed and a bandgap.
Stroscio and co-workers performed their experiments on graphene “drumheads”, obtained by placing graphene flakes over a series of holes (measuring around a micron across) deeply etched in a silicon dioxide wafer. The technique is rather like placing a sheet of plastic cling film over a tiny muffin tin. The silicon dioxide is grown atop conducting silicon, which also acts as a gate electrode when voltage is applied between the silicon and the graphene.
The researchers then strained the graphene by pulling it up with the attractive force from a scanning tunnelling microscope (STM) probe tip and pulling the sample down at the same using the electrostatic force from the silicon gate. “The graphene was pulled in opposite directions, but we could control which force was larger, thereby changing the drumhead shape,” explained Stroscio.
Fictitious magnetic field
When the graphene is stretched or strained in this way, charge carriers (electrons and holes) in the material begin to move in circles rather than simply travelling in straight lines, as is usually the case. “Mathematically, the applied strain can be likened to a fictitious magnetic field,” said Stroscio, “and this field makes the charges travel in circles, just as they would in a real magnetic field.”
According to calculations performed by the researchers, the graphene drumhead is strained in the region around the STM probe tip, which produces a shape that looks like a circus tent as it pulls up the graphene sheet. The charge carriers are thus confined around the apex of the tent, in the same way as they would be confined in a quantum dot.
“Our experiments are the first to show that strained graphene confines charge carriers like in a quantum dot,” Stroscio toldnanotechweb.org. “Normally you would have to cut out a nanosized piece of graphene to make a quantum dot out of the material, but our work shows that you can achieve the same thing with strain-induced pseudo-magnetic fields.”
The team, which includes scientists from the University of Maryland in the US and the Korea Research Institute of Standards and Science in Seoul, is now performing more comprehensive simulations to find out whether the size of the quantum dots in graphene can be tuned. “We are also busy looking at how intense the fictitious magnetic fields can become, and engineering different strain fields in graphene,” revealed Stroscio.
The current results are reported in Science.