At first, Boyd could not figure out why the technique was so successful. He later discovered that two leaky valves were letting in trace amounts of methane into the experiment chamber. "The valves were letting in just the right amount of methane for graphene to grow," he says.

The ability to produce graphene without the need for active heating not only reduces manufacturing costs, but also results in a better product because fewer defects - introduced as a result of thermal expansion and contraction processes - are generated.

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This in turn eliminates the need for multiple postproduction steps. "Typically, it takes about ten hours and nine to ten different steps to make a batch of high-mobility graphene using high-temperature growth methods," Yeh says. "Our process involves one step, and it takes five minutes."

Work by Yeh's group and international collaborators later revealed that graphene made using the new technique is of higher quality than graphene made using conventional methods: It is stronger, because it contains fewer defects that could weaken its mechanical strength, and it has the highest electrical mobility yet measured for synthetic graphene.

The team thinks one reason their technique is so efficient is that a chemical reaction between the hydrogen plasma and air molecules in the chamber's atmosphere generates cyano radicals - carbon-nitrogen molecules that have been stripped of their electrons. Like tiny superscrubbers, these charged molecules effectively scour the copper of surface imperfections providing a pristine surface on which to grow graphene.

The scientists also discovered that their graphene grows in a special way. Graphene produced using conventional thermal processes grows from a random patchwork of depositions. But graphene growth with the plasma technique is more orderly. The graphene deposits form lines that then grow into a seamless sheet, which contributes to its mechanical and electrical integrity.

A scaled-up version of their plasma technique could open the door for new kinds of electronics manufacturing, Yeh says. For example, graphene sheets with low concentrations of defects could be used to protect materials against degradation from exposure to the environment. Another possibility would be to grow large sheets of graphene that can be used as a transparent conducting electrode for solar cells and display panels.

Another possibility, she says, is to introduce intentional imperfections into graphene's lattice structure to create specific mechanical and electronic attributes. "If you can strain graphene by design at the nanoscale, you can artificially engineer its properties. But for this to work, you need to start with a perfectly smooth, strain-free sheet of graphene," Yeh says. "You can't do this if you have a sheet of graphene that has uncontrollable defects in different places."

Along with Yeh and Boyd, additional authors on the paper, "Single-Step Deposition of High-Mobility Graphene at Reduced Temperatures," include Caltech graduate students Wei Hsiang Lin, Chen Chih Hsu and Chien-Chang Chen; Caltech staff scientist Marcus Teague; Yuan-Yen Lo, Tsung-Chih Cheng, and Chih-I Wu of National Taiwan University; and Wen-Yuan Chan, Wei-Bing Su, and Chia-Seng Chang of the Institute of Physics, Academia Sinica. Funding support for the study at Caltech was provided by the National Science Foundation, under the Institute of Quantum Information and Matter, and by the Gordon and Betty Moore Foundation and the Kavli Foundation through the Kavli Nanoscience Institute. The work in Taiwan was supported by the Taiwanese National Science Council.

* Ker Than, Contributing writer, California Institute of Technology

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