Less is more: Researchers pinpoint graphene's varying conductivity levels
(糖心视频Org.com) -- 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鈥檚 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鈥檚 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 鈥渞eal鈥� graphene and see if this was actually the case.
鈥淵ou can talk about the electronic structure of graphene, but you must consider that those electrons don鈥檛 exist alone in the material,鈥� Buongiorno-Nardelli says. 鈥淭here 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鈥檚 conductivity.鈥�
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.
鈥淵ou cannot make a semiconductor with just one graphite layer,鈥� Buongiorno-Nardelli explains. 鈥淭o make a device, the conductive material must have a means by which it can be turned off and on. And bilayer provides such ability.鈥�
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鈥檚 Suggestion in the April 15 edition of 糖心视频ical Review B.
With a single layer of graphene, the mobility 鈥� and therefore conductivity 鈥� shown by the researchers鈥� 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.
鈥淲e expected that the electrons鈥� 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,鈥� Mullen says. 鈥淪urprisingly, we found that the mobility of electrons in bilayer graphene is roughly an order of magnitude lower than in a single graphene sheet.鈥�
鈥淭he reduction is substantial, but even this reduced number is higher than in many conventional semiconductors,鈥� Borysenko adds.
Buongiorno-Nardelli says that the NC State researchers are turning their attention to remedying this problem.
鈥淚f we put the graphene on a substrate that can 鈥榮iphon off鈥� some of the heat generated by the electric current, the crystal vibrations will decrease and the mobility will increase. Those are our next steps 鈥� running the simulations with graphene and substrates that have this property.鈥�
More information: 鈥淓lectron-Phonon Interactions in Bilayer Graphene鈥� K. Borysenko, J.T. Mullen, K.W. Kim, et al, North Carolina State University, Published: April 15, 2011 in 糖心视频ical 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 Dac 鈮� 15 eV and Dop 鈮� 2:8 脳 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.
Provided by North Carolina State University
