The thermal conductivity of graphene strongly depends on the material’s isotopic composition. So say researchers in the US and China, who have shown that graphene made from pure carbon-12 has a much higher thermal conductivity than normal graphene – which contains about 1% carbon-13. As well as helping to develop an accurate theory of heat conduction in 2D materials, the result means that isotopically pure graphene could be ideal for cooling tiny components in electronic circuits.

Graphene is a 2D sheet of carbon just one atom thick that has a range of unique electronic and mechanical properties. It is a semiconductor and is often touted as a replacement for silicon as the material of choice for electronics in the future, thanks in part to its exceptional ability to conduct heat. As electronic devices become ever smaller, local heating becomes more of a problem, and silicon in particular suffers in this respect. Materials such as graphene have a higher thermal conductivity and so can remove this waste heat more efficiently than materials such as silicon with a lower thermal conductivity.

Graphene isotopes

Two stable isotopes of carbon occur in nature – carbon-12 comprising about 99% of naturally occurring carbon and carbon-13 about 1%. These concentrations are also found in the graphene made and studied in labs. 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 from normal graphene strongly modifies the crystal lattice of the material and significantly increases its thermal conductivity.

Heat travels through crystalline materials such as graphene by way of lattice vibrations called phonons. Atoms with different masses scatter phonons differently, and therefore thermal-conductivity studies of graphene with varying isotopic compositions should help physicists gain a better understanding of how atomic mass affects the transport of heat. “Our result will help develop an accurate theory of heat conduction in graphene and other 2D crystals,” explains Balandin. “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,” he adds.

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% carbon-13) is higher than 4000 Wm^–1 K^–1 at a temperature of 320 K, while graphene with 1% carbon-13 has a conductivity of 2500 Wm^–1 K^–1. The thermal conductivity drops to about 2000 Wm^–1 K^–1 in graphene sheets made up of half carbon-12 and half carbon-13. In comparison, bulk copper, which is widely used to cool computer chips, has a thermal conductivity of about 400 Wm^–1 K^–1.

The graphene samples studied were made using large-area chemical vapour deposition, which allowed the researchers to create regions of film with different ratios of carbon-12 to carbon-13. This meant 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.

“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,” says Balandin.

New material of choice

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

The work is described in Nature Materials.