There’s no doubt that the discovery of graphene is one sweet breakthrough. The remarkable material offers everything from faster, cooler electronics and cheaper lithium-ion batteries to faster DNA sequencing and single-atom transistors. Researchers at Rice University have made graphene even sweeter by developing a way to make pristine sheets of the one-atom-thick form of carbon from plain table sugar and other carbon-based substances. In another plus, the one-step process takes place at temperatures low enough to make the wonder material easy to manufacture.
Zhengzong Sun, a fourth-year graduate student in the lab of Rice chemist James Tour, found that depositing carbon-rich sources on copper and nickel substrates produced graphene in any form he desired, including single-, bi- or multilayer sheets. Sun and his colleagues also found that the process adapts easily to producing doped graphene, which allows the manipulation of the material’s electronic and optical properties – an important factor in making switching and logic devices using the material.
For pristine graphene, Sun started with a thin film of poly (methyl methacrylate) (PMMA) – better known as Plexiglass – spun onto a copper substrate that acted as a catalyst. Under heat and low pressure, hydrogen and argon gas was flowed over the PMMA for 10 minutes, reducing it to pure carbon and turning the film into a single layer of graphene. Sun was able to control the thickness of the PMMA-derived graphene by changing the gas-flow rate.
Sugar, sugarWhen he turned to other carbon sources, including a fine powder of sucrose – aka table sugar – is when things got interesting, says Sun. "We thought it would be interesting to try this stuff," he said. "While other labs were changing the metal catalysts, we tried changing the carbon sources."
Sun put 10 milligrams of sugar (and later fluorene) on a square-centimeter sheet of copper foil and subjected it to the same reactor conditions as the PMMA. It was quickly transformed into single-layer graphene. Sun said he had expected defects in the final product, given the chemical properties of both substances (a high concentration of oxygen in sucrose, five-atom rings in fluorene); but he found potential topological defects would self-heal as the graphene formed.
"As we looked deeper and deeper into the process, we found it was not only interesting, but useful," Sun said.
Low temp processImportantly, the process allows large-area, high-quality graphene to be grown from a number of carbon sources at temperatures as low as 800 degrees Celsius (1,472 F). As hot as that may seem, the difference between running a furnace at 800 and 1,000 degrees Celsius is significant, Tour said.
"At 800 degrees, the underlying silicon remains active for electronics, whereas at 1,000 degrees, it loses its critical dopants," said Tour.
Sun also tried to grow graphene on silicon and silicon oxide, which raised the possibility of growing patterned graphene from a thin film of shaped copper or nickel deposited onto silicon wafers, but that approach proved unsuccessful.
Doped graphene opens more possibilities for electronics use, Tour said, and Sun found it fairly simple to make. Starting with PMMA mixed with a doping reagent, melamine, he discovered that flowing the gas under atmospheric pressure produced nitrogen-doped graphene. Pristine graphene has no bandgap, but doped graphene allows control of the electrical structure, which the team proved by building field-effect transistors.
"Each day, the growth of graphene on silicon is approaching industrial-level readiness, and this work takes it an important step further," Tour said.