Graphene shows exceptional ability as heat conductor; NANOTECHNOLOGY

Researchers, led by Alexander Balandin at the University of California-Riverside, USA, have demonstrated that graphene--sheets of carbon only one atom thick--can also be an excellent heat conductor.

Since the discovery of the material in 2004, researchers have beenable to demonstrate that it is a good electrical conductor, as well as having good strength. It has also been surmised that graphene could show interesting heat conduction properties too, based on the behaviour of carbon nanotubes.

However, graphene can be difficult to work with and researchers had previously had problems determining the material's thermal properties using traditional techniques, such as attaching heaters and other devices to it.

To this end, the team had to invent a new way to measure thermal conductivity in order to study the material. They are now using this novel approach to investigate how graphene's thermal properties could be used to cool ultrafast silicon chips.

The new measurement technique uses a laser to both heat the graphene and measure its temperature. To achieve this, sheets of the material are suspended across trenches, measuring micrometres, which have been cut into a silicon oxide surface. The sheets are pinned-down at both ends by layers of graphite, which also act as heat sinks.

Next, the centre of the sheet is exposed to a beam of laser light,which heats the graphene and changes the frequencies at which its carbon atoms vibrate. Some of the laser light changes frequency as it undergoes Raman scattering off the vibrating atoms and the size of thefrequency shift is proportional to the temperature of the illuminated region, the researchers say.

By measuring the frequency shift as a function of laser power, theteam was able to calculate that the thermal conductivity of grapheneis 5300 W * (m * [K.sup.-1]) at room-temperature. This is the highest known value of any solid: 50% higher than carbon nanotubes and morethan ten times greater than metals like copper.

The researchers think that the material's high thermal conductivity is caused by the relative ease with which atomic vibrations can move through graphene compared to other materials. The scientists are now devising a theory that explains this phenomenon.

Due to the material's high thermal conductivity, flat shape and ability to be integrated with silicon, it could find potential applications in removing heat from electronics devices.

For further information, contact: Alexander Balandin, Engineering 2435, University of California, Riverside, CA 92521, USA; tel: 951-827-2351; fax: 951-827-2425; E-mail: alexander.balandin@ucr.edu; Internet: www.ucr.edu