Graphene – a 2D sheet of carbon just one atom thick – is often touted as a candidate replacement for silicon as the electronic material of choice in the future thanks to its unique electronic and mechanical properties. All the more so because the material also has an unusually high intrinsic thermal conductivity. As electronic devices become ever smaller, local heating (which slows devices down) becomes more important, and silicon especially suffers in this respect. Materials with a higher thermal conductivity can spread this waste heat away more efficiently than materials with a lower thermal conductivity.

Graphene isotopes

Naturally occurring carbon materials, including graphene, are made up two stable isotopes, about 99% carbon-12 and 1% of carbon-13. The difference between the isotopes lies in the atomic mass of the carbon atoms, as carbon-13 has one more neutron than carbon-12.

Now, two teams, one led by Rodney Ruoff of the University of Texas and the other by Alexander Balandin at the University of California Riverside, have discovered that removing the carbon-13 content from normal graphene strongly modifies the crystal lattice of the material and significantly increases its thermal conductivity.

"Our result will help develop an accurate theory of heat conduction in graphene and other 2D crystals," explained Balandin. "This is because the change in thermal conductivity in graphene with different isotopic compositions will allow us to better understand how atoms of different masses scatter phonons (vibrations of the crystal lattice responsible for carrying away heat). This isotope scattering is easier to describe theoretically than the scattering caused by impurity atoms in a sample, which not only differ by mass but also by size and many other parameters."

Twice the conductivity

Using an optothermal laser Raman technique, originally developed in Balandin's lab and subsequently modified by Ruoff's group, the researchers found that the thermal conductivity of isotopically pure carbon-12 graphene (containing just 0.01% of carbon-13) was higher than 4000 Wm–1K–1 at a temperature of 320K and more than twice as high as the thermal conductivity in graphene sheets made up of half carbon-12 and half carbon-13. To compare, bulk copper, which is widely used to cool computer chips, has a thermal conductivity of around 400 Wm–1K–1.

The graphene samples studied were made by large-area chemical vapour deposition that allowed the researchers to create regions of film labelled with different ratios of carbon-12 to carbon-13. This means that areas with differing isotope ratios could be studied in the same experimental run.

When an object is illuminated with a laser beam, part of the incoming energy is reflected by the solid, part is transmitted through it and the rest is absorbed by the material. The researchers were interested in the fraction of energy absorbed because it heats up the material. Raman scattering signals can correspond to the emission or to the absorption of a phonon, and the ratio of these two signals can be used to determine the total number of phonons, which in turn, gives the lattice temperature.

New material of choice

"The interesting feature of this technique is that the temperature rise in graphene in response to laser heating is simply measured from the position of the Raman peaks we observed," Balandin told nanotechweb.org.

The result means that isotopically pure graphene may now be considered as the material of choice for some thermal management applications for its superior heat-transferring properties, says Ruoff.

The team, which also includes researchers from the Xiamen University in China, now plans to map out the thermal conductivity characteristics of graphene below room temperature.

Balandin, for his part, says that he is also going to be busy developing an accurate theoretical description of phonon-isotope scattering. "This would provide more physical insights into 2D phonon transport."

The work was reported in Nature Materials.