Graphene for real-world devices

New research in phonon scattering sheds more light on graphene as a replacement for silicon

08-May-2014 - USA

graphene, a one-atom-thick form of the carbon material Graphite, has been hailed as a wonder material — strong, light, nearly transparent, and an excellent conductor of electricity and heat. But a number of practical challenges must be overcome before it can emerge as a replacement for silicon and other materials in microprocessors and next-generation energy devices.

Li Shi, The University of Texas at Austin

Schematic to model phonon scattering by boundary in a multilayer graphene ribbon where the group velocity and wave vector are not collinear because of the highly anisotropic structure.

One particular challenge concerns the question of how graphene sheets can be used in real devices.

"When you fabricate devices using graphene, you have to support the graphene on a substrate and doing so actually suppresses the high thermal conductivity of graphene," said Li Shi, a professor of mechanical engineering at The University of Texas at Austin, whose work is partially funded by the National Science Foundation (NSF).

Thermal conductivity is critical in electronics, especially as components shrink to the nanoscale. High thermal conductivity is a good thing for electronic devices fabricated from graphene. It means the device can spread the heat it generates to prevent the formation of local hot spots. However, in the case of graphene, when the needed supporting materials are also used, graphene loses some of the superhigh thermal conductivity that is predicted for its idealized state when it is freely suspended in a vacuum.

In a paper published in September 2013 in the Proceedings of the National Academy of Sciences, Shi, along with graduate research assistant Mir Mohammad Sadeghi and post-doctoral fellow Insun Jo, designed an experiment to observe the effects of thermal conductivity when the thickness of graphene supported on an amorphous glass layer was increased.

They observed that thermal conductivity increased as the number of layers grew from a single one-atom layer to as thick as 34 layers. However, even at 34 layers, the thermal conductivity had not recovered to the point where it was as high as bulk graphite, which is an excellent heat conductor.

These findings are leading Shi and others to explore novel ways of supporting or connecting graphene with the macroscopic world, including three-dimensional interconnected foam structures of graphene and ultrathin graphite, or the use of hexagonal boron nitride, which has nearly the same crystal structure as graphene.

"One of our objectives is to use graphene and other layered materials to make flexible electronic devices," Shi explained. "And those devices will be made on plastic substrates, which are flexible, but also have very low thermal conductivity. When you run current through the devices, a lot of them fail. The heat cannot be dissipated effectively, so it becomes very hot and it just melts the substrate."

Melting isn't the only problem. As temperatures get higher, the flexible polymer substrate can become a molten and rubber-like material that breaks the electronic materials built on top and causes tiny conducting wires in electronic devices to easily fail.

"In general, a hot chip is not good for the devices," Shi said. "The transistors will switch slower and will require more power."

Shi has been exploring the physical properties of graphene-based materials for more than a decade. He co-authored a 2001 paper in Physical Review Letters that reported the first measurement of high thermal conductivity in individual carbon nanotubes, a cousin of graphene. He also co-authored a 2010 paper in Science that provided critical insight into the thermal conductivity and thermal transport in single layer graphene supported on a substrate.

Shi is trying to answer fundamental questions about how phonons — the vibrations of atoms in solids — transport heat. Phonons are like electrons or photons (light particles), in that they carry heat energy. However, much less is known about phonons because their effects are less apparent at the macro-scale at which we live.

"This fundamental study allowed us to understand the intrinsic physics of the scattering of lattice waves," Shi said.

Shi's experiments let his team infer how phonons scatter as a function of thickness of the graphene layers, based on observations of how the thermal conductivity varied with different numbers of layers.

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