Once again, graphene has proven itself to be a rather special material. In a paper in Nature Communications, an international research team led by Fritz Aumayr from the Institute of Applied Physics at TU Wien in Austria report that the electrons in graphene are extremely mobile and can react very quickly.
The team discovered that impacting xenon ions with a particularly high electric charge on a graphene film causes a large number of electrons to be torn away from the graphene in a very precise spot. However, the one-atom-thick carbon material was able to replace the electrons within just a few femtoseconds. This resulted in extremely high currents, which would not be maintained under normal circumstances. Graphene’s extraordinary electronic properties make it a very promising candidate for future applications in the field of electronics.
The Helmholtz-Center Dresden-Rossendorf and the University of Duisburg-Essen, both in Germany, participated in the experiment alongside TU Wien. The international team received theoretical support from researchers in France and Spain, as well as from other staff at the Institute of Theoretical Physics.
“We work with extremely highly-charged xenon ions,” explains Elisabeth Gruber, a PhD student in Aumayr's research team. “Up to 35 electrons are removed from the xenon atoms, meaning the atoms have a high positive electric charge.”
These ions are fired at a free-standing single layer of graphene, which is clamped between microscopically small brackets. “The xenon ion penetrates the graphene film, thereby knocking a carbon atom out of the graphene – but that has very little effect, as the gap that has opened up in the graphene is then refilled with another carbon atom,” explains Gruber. “For us, what is much more interesting is how the electrical field of the highly-charged ion affects the electrons in the graphene film.”
This effect happens even before the highly-charged xenon ion collides with the graphene film. As the highly-charged ion approaches, it starts to tear electrons away from the graphene due to its extremely strong electric field. By the time the ion has fully passed through the graphene layer, it has a positive charge of less than 10, compared to over 30 when it started out. This shows that the ion is able to extract more than 20 electrons from a tiny area of the graphene film.
With these electrons missing from the graphene layer, the carbon atoms surrounding the point of impact of the xenon ions become positively charged. “What you would expect to happen now is for these positively-charged carbon ions to repel one another, flying off in what is called a Coulomb explosion and leaving a large gap in the material,” says Richard Wilhelm from the Helmholtz-Center Dresden-Rossendorf, who currently works at TU Wien as a postdoctoral assistant. “But astoundingly, that is not the case. The positive charge in the graphene is neutralized almost instantaneously.”
The only way this can happen is for a sufficient number of electrons to be replaced in the graphene within an extremely short time-frame of several femtoseconds (quadrillionths of a second). “The electronic response of the material to the disruption caused by the xenon ion is extremely rapid. Strong currents from neighboring regions of the graphene film promptly resupply electrons before an explosion is caused by the positive charges repelling one another,” explains Gruber. “The current density is around 1000 times higher than that which would lead to the destruction of the material under normal circumstances – but over these distances and time scales, graphene can withstand such extreme currents without suffering any damage.”
This extremely high electron mobility in graphene is of great significance for a number of potential applications. “The hope is that for this very reason, it will be possible to use graphene to build ultra-fast electronics. Graphene also appears to be excellently suited for use in optics, for example in connecting optical and electronic components,” says Aumayr.
This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.