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Atom sized membrane

1st April 2013


In October 2004, researchers from the University of Manchester in England and Chernogolovka in Russia announced that they had discovered the world’s first single-atom-thick fabric.

The team, led by professor Andre Geim at the University of Manchester, succeeded in extracting individual planes of carbon atoms from graphite crystals. This led to the production of the thinnest possible fabric, which they named graphene. The resulting atomic sheet is stable, highly flexible, strong and remarkably conductive. The nanofabric belongs to the family of fullerene molecules, but is its first 2D member.

At the time, the one atom thick gauze of graphene, which resembles chicken wire, became a topic of great interest to physicists around the world. However, it could only exist on top of other materials and doubts were expressed as to whether it could ever exist in the free state without the need of such a support. Now, however, physicists at the University of Manchester and the Max-Planck Institute in Germany have created a new kind of free-hanging membrane out of graphene. Still only one atom thick, it is thought that it could find numerous uses, for example in new drug development.

To create the membrane, the team used a combination of microfabrication techniques that are more typically used in the manufacturing of microprocessors.

To begin with, a metallic scaffold was placed on top of a sheet of graphene, which in turn was placed on a silicon chip. The chip was then dissolved in acids, leaving the graphene hanging freely in air, or a vacuum from the scaffold.

The resulting membranes are the thinnest material possible and maintain a remarkably high quality, according to the researchers.

Geim – who works in the School of Physics and Astronomy at the University of Manchester – and his fellow researchers have also found the reason for the stability of such thin materials, a stability which was previously presumed to be impossible.

They report in Nature that graphene is not perfectly flat, but instead gently crumpled out of plane, which helps stabilise otherwise intrinsically unstable ultra-thin matter.

Geim and his colleagues believe that the membranes they have created can be used like sieves, to filter light gases through the atomic mesh of the chicken wire structure, or to make miniature electro-mechanical switches.

However, it is also thought it may be possible to use them as a non-obscuring supports for electron microscopy to study individual molecules. This has significant implications for the development of medical drugs, as it will potentially allow the rapid analysis of the atomic structures of bio-active complex molecules.

“This is a completely new type of technology – even nanotechnology is not the right word to describe these new membranes,” he said. “We have made proof-of-concept devices and believe the technology transfer to other areas should be straightforward. However, the real challenge is to make such membranes cheap and readily available for large-scale applications.”

Graphene in the real world

As Elab went to press, Geim and his team revealed details of the world’s smallest transistor. Just one atom thick and less than 50 atoms wide, the graphene transistor is described in Nature Materials.

The Manchester scientists developed an early graphene transistor just after their discovery of graphene, a result that other groups have subsequently reproduced. However, these graphene transistors were very ‘leaky’, which has limited possible applications and ruled out important ones, such as their use in computer chips and other electronic circuits with a high density of transistors.

Now the Manchester team has found an elegant way around the problem and made graphene-based transistors suitable for use in future computer chips. Geim and his colleagues have shown for the first time that graphene remains highly stable and conductive even when it is cut into strips of only a few nanometres wide. This is significant because all other known materials – including silicon – oxidise, decompose and become unstable at sizes tens times larger. This poor stability of these materials has been the fundamental barrier to their use in future electronic devices – and this has threatened to limit the future development of microelectronics.

“We have made ribbons only a few nanometres wide and cannot rule out the possibility of confining graphene even further – down to maybe a single ring of carbon atoms,” says Geim.

The research team suggests that future electronic circuits can be carved out of a single graphene sheet. Such circuits would include the central element or ‘quantum dot’, semitransparent barriers to control movements of individual electrons, interconnects and logic gates – all made entirely of graphene.

Geim’s team have proved this idea by making a number of single-electron-transistor devices that work under ambient conditions and show a high-quality transistor action.

“At the present time no technology can cut individual elements with nanometre precision. We have to rely on chance by narrowing our ribbons to a few nanometres in width,” says Leonid Ponomarenko, who is leading this research at the University of Manchester. “Some of them were too wide and did not work properly whereas others were over-cut and broken.”

But Ponomarenko is optimistic that this proof-of-concept technique can be scaled up: “To make transistors at the true-nanometre scale is exactly the same challenge that modern silicon-based technology is facing now. The technology has managed to progress steadily from millimetre-sized transistors to current microprocessors with individual elements down to tens nanometres in size. The next logical step is true nanometre-sized circuits and this is where graphene can come into play because it remains stable – unlike silicon or other materials – even at these dimensions,” he said.





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