Caltech Scientists Develop Cool Process to Make Better Graphene

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A new technique to produce graphene at room temperature could help pave the way for commercially feasible graphene-based solar cells and light-emitting diodes, large-panel displays, and flexible electronics.

Graphene could revolutionize a variety of engineering and scientific fields due to its unique properties, which include a tensile strength 200 times stronger than steel and an electrical mobility that is two to three orders of magnitude better than silicon. The electrical mobility of a material is a measure of how easily electrons can travel across its surface.

However, achieving these properties on an industrially relevant scale has proven to be complicated. Existing techniques require temperatures that are much too hot - 1,000 degrees Celsius - for incorporating graphene fabrication with current electronic manufacturing. Additionally, high-temperature growth of graphene tends to induce large, uncontrollably distributed strain - deformation - in the material, which severely compromises its intrinsic properties.

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"With this new technique, we can grow large sheets of electronic-grade graphene in much less time and at much lower temperatures," says Caltech staff scientist David Boyd. Boyd is the first author of a new study, detailing the new manufacturing process and the novel properties of the graphene it produces.

"Previously, people were only able to grow a few square millimeters of high-mobility graphene at a time, and it required very high temperatures, long periods of time, and many steps," says Caltech physics professor Nai-Chang Yeh, the Fletcher Jones Foundation Co-Director of the Kavli Nanoscience Institute and the corresponding author of the new study. "Our new method can consistently produce high-mobility and nearly strain-free graphene in a single step in just a few minutes without high temperature. We've created sample sizes of a few square centimeters, and since we think that our method is scalable, we believe that we can grow sheets that are up to several square inches or larger, paving the way to realistic large-scale applications."

The new manufacturing process might not have been discovered if not for a fortunate turn of events. In 2012, Boyd, then working in the lab of the late David Goodwin, at that time a Caltech professor of mechanical engineering and applied physics, was trying to reproduce a graphene-manufacturing process he had read about in a scientific journal.

There, heated copper is used to catalyze graphene growth. "I was playing around with it on my lunch hour," says Boyd, who now works with Yeh's research group. "But the recipe wasn't working. It seemed like a simple process. I even had better equipment than what was used in the original experiment, so it should have been easier for me."

During one of his attempts to reproduce the experiment, the phone rang. While Boyd took the call, he unintentionally let a copper foil heat for longer than usual before exposing it to methane vapor, which provides the carbon atoms needed for graphene growth. When later Boyd examined the copper plate using Raman spectroscopy, a technique used for detecting and identifying graphene, he saw evidence that a graphene layer had indeed formed. "It was an 'A-ha!' moment," Boyd says. "I realized then that the trick to growth is to have a very clean surface, one without the copper oxide."

As Boyd recalls, he then remembered that Robert Millikan, a Nobel Prize–winning physicist and the head of Caltech from 1921 to 1945, also had to contend with removing copper oxide when he performed his famous 1916 experiment to measure Planck's constant, which is important for calculating the amount of energy a single particle of light, or photons, Boyd wondered if he, like Millikan, could devise a method for cleaning his copper while it was under vacuum conditions.

The solution Boyd hit upon was to use a system first developed in the 1960s to generate a hydrogen plasma - that is, hydrogen gas that has been electrified to separate the electrons from the protons - to remove the copper oxide at much lower temperatures. His initial experiments revealed not only that the technique worked to remove the copper oxide, but that it simultaneously produced graphene as well.

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