Less is More: Researchers Pinpoint Graphene’s Varying Conductivity Levels

Did you know that pencil lead may just end up changing the world? Graphene is the material from which graphite, the core of your No. 2 pencil, is made. It is also the latest “wonder material,” and may be the electronics industry’s next great hope for the creation of extremely fast electronic devices. Researchers at North Carolina State University have found one of the first roadblocks to utilizing graphene by proving that its conductivity decreases significantly when more than one layer is present.

Graphene’s structure is what makes it promising for electronics. Because of the way its carbon atoms are arranged, its electrons are very mobile. Mobile electrons mean that a material should have high conductivity. But NC State physicist Dr. Marco Buongiorno-Nardelli and NC State electrical and computer engineer Dr. Ki Wook Kim wanted to find a way to study the behavior of “real” graphene and see if this was actually the case.

“You can talk about the electronic structure of graphene, but you must consider that those electrons don’t exist alone in the material,” Buongiorno-Nardelli says. “There are impurities, and most importantly, there are vibrations present from the atoms in the material. The electrons encounter and interact with these vibrations, and that can affect the material’s conductivity.”

Buongiorno-Nardelli, Kim and graduate students Kostya Borysenko and Jeff Mullen developed a computer model that would predict the actual conductivity of graphene, both as a single layer and in a bilayer form, with two layers of graphene sitting on top of one another. It was important to study the bilayer model because actual electronic devices cannot work with only a single layer of the material present.

“You cannot make a semiconductor with just one graphite layer,” Buongiorno-Nardelli explains. “To make a device, the conductive material must have a means by which it can be turned off and on. And bilayer provides such ability.”

With the help of the high performance computers at Oak Ridge National Laboratories, the NC State team discovered both good and bad news about graphene. Their results appear as an Editor’s Suggestion in the April 15 edition of Physical Review B.

With a single layer of graphene, the mobility – and therefore conductivity – shown by the researchers’ simulations turned out to be much higher than they had originally thought. This good news was balanced, however, by the results from the bilayer state.

“We expected that the electrons’ conductivity in bilayer graphene could be somewhat worse, due to the ways in which the vibrations from the atoms in each individual layer interact with one another,” Mullen says. “Surprisingly, we found that the mobility of electrons in bilayer graphene is roughly an order of magnitude lower than in a single graphene sheet.”

“The reduction is substantial, but even this reduced number is higher than in many conventional semiconductors,” Borysenko adds.

Buongiorno-Nardelli says that the NC State researchers are turning their attention to remedying this problem.

“If we put the graphene on a substrate that can ‘siphon off’ some of the heat generated by the electric current, the crystal vibrations will decrease and the mobility will increase. Those are our next steps – running the simulations with graphene and substrates that have this property.”

The research was funded by the U.S. Department of Energy and the DARPA-CERA program. Buongiorno-Nardelli is a professor in the Department of Physics and joint faculty with Oak Ridge National Laboratory. Kim is a professor in the Department of Electrical and Computer Engineering. The physics department is part of NC State’s College of Physical and Mathematical Sciences. The Department of Electrical and Computer Engineering is part of NC State’s College of Engineering.

-peake-

Note to editors: The abstract of the paper follows.

“Electron-Phonon Interactions in Bilayer Graphene”

Authors: K. Borysenko, J.T. Mullen, K.W. Kim, et al, North Carolina State University

Published: April 15, 2011 in Physical Review B

Abstract:

Using calculations from first principles, we demonstrate that intrinsic carrier-phonon scattering in bilayer graphene is dominated by low energy acoustic (and acoustic-like) phonon modes in a framework that bears more resemblance with bulk graphite than monolayer graphene. The total scattering rate at low/moderate electron energies can be described by a simple two-phonon model in the deformation potential approximation with effective constants D_ac ≈ 15 eV and D_op ≈ 2:8 × 10⁸ eV/cm for acoustic and optical phonons, respectively. With much enhanced acoustic phonon scattering, the mobility of intrinsic bilayer graphene is estimated to be significantly smaller than that of monolayer.