Slow graphene down, speed computers up

02 February 2012 by David Shiga, writing for the New Scientist magazine

Astonishing conductivity helped the discoverers of graphene win the Nobel prize in physics in 2010. Now a way to switch off the easy flow of electrons in this wonder form of carbon is bringing super-fast graphene computers closer.

A sheet-like molecule just one carbon atom thick, graphene offers much less resistance to the flow of electrons than silicon. It has been hailed for its potential as the basis for computer circuits that operate at unprecedented speed. "It's an extremely promising material," says Konstantin Novoselov, who shared the Nobel prize with his co-discoverer, Andre Geim, both at the University of Manchester, UK.

But the ease of electron flow also creates a problem. To perform calculations, computers need to turn the flow of electricity on and off in their circuits. The gates that open and close to regulate the flow are called transistors. Making graphene-based transistors has proven difficult because it is such a good conductor.

Previous attempts have involved electrons confined to a single layer of graphene, but these still suffer from a leakage of electrons when the transistor is in its "off" state.

Quantum Tunnel

Now Novoselov and colleagues have found a way to overcome this leakage problem by sandwiching a layer of molybdenum disulfide between two layers of graphene. The molybdenum acts as an insulator, preventing electrons from flowing in the normal way from one graphene layer to the other. This constitutes an "off" state.

A quantum mechanical effect means a small number of electrons can "tunnel" through the molybdenum. This normally happens very rarely but applying a voltage across the barrier boosts the energy of the electrons, making tunneling much more probable – a sizable current starts to flow. This is the "on" state. By varying the voltage, the researchers could turn the flow on and off, making the device a transistor.

The graphene sandwich reduces leakage by a factor of 10 compared with previous graphene-based transistors. The team suggests reducing leakage further by increasing the thickness of the insulating layer. "It really opens a new dimension in our research," says Novoselov.

Journal reference: Science, www.sciencemag.org/content/335/6071/947