Ultrathin, Now Ultraflat: Ripple-Free Graphene May Hold Key to Material’s Mysteries

12/5/2009

 Graphene has been a hot topic in physics and materials science since its discovery five years ago. The sheets of carbon, just an atom thick, have a host of intriguing properties, including transparency, strength and a structure that lets electrons zip through almost unimpeded. Graphene’s characteristics and near two-dimensionality recommend it for use in next-generation displays, electronics or structural composites, but like many materials du jour, it has yet to find applications on a significant scale.

One problem slowing graphene’s rollout to implementation is an incomplete understanding of its physical, electronic and chemical properties. In 2007, for instance, researchers found that graphene was not truly planar but had a characteristic roughness in the form of nanometer-size surface ripples. (A nanometer is a billionth of a meter.) Some researchers have hypothesized that the ripples might hinder electron flow through graphene, but that assertion has proved difficult to test.

Now, a team from Columbia University, writing in the November 19 issue of Nature, reports creating ultraflat samples of graphene in which the material’s ripples are suppressed. (Scientific American is part of the Nature Publishing Group.) By comparing ordinary graphene with the new uncorrugated version, the study’s authors wrote, researchers should be able to unpack the effects of graphene’s roughness on its other properties.

Mikhail Katsnelson, a physicist at Radboud University Nijmegen in the Netherlands who did not contribute to the new research, has hypothesized that graphene’s ripples are one possible source for scattering of charge carriers in the material. That scattering, whatever the cause, limits graphene’s inherent ability to swiftly transport electrons.

If the flat graphene sample turns out to have the same electron mobility as regular rippled graphene, then the ripple-scattering theory would be sunk. “If, oppositely, the mobility will be much higher,” Katsnelson says, “it will be a direct confirmation of our hypothesis about ripples as the main limiting factor for electron mobility.” If that were the case, then ironing out graphene’s ripples would prove to be an important step toward using the novel material in electronics.

Tony Heinz, a Columbia physicist and study co-author, says that graphene’s ripples have also been implicated in other puzzling properties that the material exhibits. For instance, “they have been suggested as the reason for the difference in reactivity of mono- and multi- layer graphene,” Heinz says. Single graphene sheets are chemically reactive, but stacked layers of graphene—otherwise known as graphite, or pencil “lead”—are inert. “To date it has been difficult to provide definitive proof of the role of ripples, because ultraflat samples for comparison have not been available,” he adds.

Heinz says that the key to flattening graphene is simply to deposit it on an exceptionally smooth substrate. Whatever intrinsic tendency to crumple the substance may have, it is overcome by the interfacial interactions with an atomically flat surface. Heinz’s group used mica, a silicate mineral that can be cleaved to produce smooth surfaces many microns across. Once a flat mica terrace has been established, Heinz says, it is no more difficult to prepare graphene on that substrate than on traditional, somewhat rougher substrates such as silicon dioxide.