The researchers led by Konstantin Novoselov in Manchester and Alexander Balandin in California obtained their results by looking at how the thermal properties of copper films change as graphene synthesized by chemical vapour deposition (CVD) was placed on top of the films. To better understand the results, the team correlated the thermal conduction data with optical and scanning electron microscopy spectra of the samples.

Graphene is a flat sheet of carbon atoms arranged in a honeycombed lattice. It has been attracting the attention of scientists and engineers alike since it was first created in 2004 thanks to its unique electronic and mechanical properties that could come in useful for making a host of novel electronics devices. These include transistors that are faster than any that exist today thanks to the fact that electrons move extremely fast in the carbon material.

High intrinsic thermal conductivity

Graphene by itself has a very high intrinsic thermal conductivity of above 2000 Wm–1K–1 at room temperature, a value that is even greater than that of diamond (the best heat conductor known). Although the thermal conductivity of graphene does drop a little when it is placed on a substrate, it still remains high. The thermal conductivity decreases because phonons – quantized vibrations of the crystal lattice that transport heat – couple across the different atomic layers in the carbon material and this so-called phonon scattering disrupts heat conduction.

However, since graphene is so thin, the researchers did not expect that it would dramatically change the overall average thermal conductivity of copper when placed on it. “The fact that we observed a substantial increase in the thermal conductivity of the graphene-copper heterofilms took us by surprise at first,” says Balandin. “But, after carefully examining the grain sizes in copper before and after graphene had been deposited, we then realized that the act of coating graphene (via CVD) at high temperatures in the copper actually causes the grains in the copper films to grow.”

This grain size increase is larger than in reference copper films that had been simply heat treated at the same temperatures as those employed in the CVD process, he explained. The larger grain sizes in copper coated with graphene result in higher thermal conductivity.

Better at dissipating unwanted heat

Since copper is traditionally used as the interconnect material in modern computer chips, and has a thermal conductivity of around just 400 Wm–1K–1, capping these interconnects with graphene could help them better dissipate unwanted heat, says the team. Excess heat is a big problem in modern devices that are based on conventional silicon circuits – and the problem is set to get worse as devices become ever smaller.

The researchers say that they have only looked at relatively thick copper foils so far and would now like to investigate how heat conduction properties change in nanometre-thick copper films coated with graphene. Such structures would be prototypes of real graphene-copper hybrid interconnects, said Balandin.

“On the theoretical side, we have already developed a relatively simple model to explain how thermal conductivity scales with copper grain size and now plan to put forward a more accurate theory to predict the effect in downscaled copper-graphene hybrids,” he added.

The research is detailed in Nano Letters DOI: 10.1021/nl404719n.

Further reading

Isotopic purity boosts graphene's heat conduction (Jan 2012)
Graphene-based composites could cool down electronics (Feb 2012)
'Few-layer' graphene keeps its cool (May 2010)