Evolutionary Selection Yields Roll-to-Roll Production Of Single Layer Graphene

Single-layer graphene sheets are getting supersized thanks to a new technique that allows one type of graphene seed to crush the competition and grow carbon crystals in its own orientation unimpeded by weaker rivals. Previously, sheets of graphene large enough to be rolled onto a metal or plastic dowel and distributed commercially were grown in multiple layers on a copper substrate . On the other hand, single-layer graphene sheets have been manufactured almost exclusively in small batches on finicky monocrystalline substrates or, in one case, by irradiating self-assembling monolayers of hydrocarbons. The new technique allows for one-atom-thick graphene sheets to be grown on polycrystalline substrates, no radiation required, with their size limited only by the width of the equipment.

Evolutionary Selection of Crystals

“It is truly the survival of the fittest crystal,” said Boris Yakobson, Rice University professor and co-author of the paper that details the new technique.

The primary obstacle to developing large graphene sheets the thickness of a single atom has been the tendency of carbon to form rings of five or seven atoms, called defects, rather than rings of six atoms, which disrupts the useful properties of the sheet. These defects form when carbon atoms of slightly different orientations meet while growing on a substrate; rather than one overpowering the other, they compromise and join together.

A democratic solution to overcrowding will never result in graphene with uniform conductive properties. A single-layer graphene sheet is like the Wild West—there’s no room for two crystal seeds in the same 2D space. Since the 1960s, scientists have been trying to figure out how to set conditions for crystal growth that allow only one seed to wear the sheriff’s hat.

Addition of Buffer Stream Allows One Graphene Seed to Dominate

Digital rendering of the deposition of hydrocarbons (from top nozzle) onto a nickel and copper alloy, with buffering stream (grey arrow) blowing carbon atoms to the front of the feed and creating an environment where only one crystal orientation of graphene seed can survive. (Image courtesy of Andy Sproles and the Oak Ridge National Laboratory, U.S. Department of Energy.)

By creating a special chemical vapor deposition (CVD) furnace that delivers hydrocarbon feedstock through a small nozzle to a slowly moving substrate while also pumping in a buffer stream of hydrogen and argon gases, researchers at Rice University and the Oak Ridge National Laboratory were able to blow carbon atoms to the substrate in front of a hydrocarbon deposit. The carbon atoms adhered to the substrate. Due to the continuing action of the buffer stream, one particular graphene seed was strong enough to steamroller competing crystal orientations and form a single, uniform layer. The resulting graphene sheet grows at approximately an inch per hour.

Applications Independent of Crystal Orientation

From left, researchers who developed the buffer stream technique for graphene production include Boris Yakobson, Ksenia Bets, and Nitant Gupta. (Image courtesy of Jeff Fitlow and Rice University.)

What does this new manufacturing technique mean for the world? According to the researchers who developed it, the graphene sheets resulting from their buffer stream process would have consistent but different orientations depending on the type of substrate and hydrocarbon precursor used. If manufacturers didn’t want to get picky about their substrate, the primary use for these large sheets of graphene would be cutting them into uniform pieces to put into different devices or device components. One potential application of highly conductive graphene is creating fast-switching analog circuits, which is increasingly necessary in the emerging field of biologically based robotics.

For more on potential applications of graphene, check out these waterproof graphene circuits .