May 11, 2009

Making Square Centimeters of Graphene on Copper and Graphene Transistor Progress

1. Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils.

Graphene films of the order of centimeters were grown on copper substrates by chemical vapor deposition using methane. The films are predominantly single-layer graphene with a small percentage, <5%, of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. Graphene film transfer processes to arbitrary substrates were also developed, and top-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities as high as 4300 cm2V-1s-1 at room temperature.

The next step is to develop a technique for carefully transferring the carbon sheet from the copper to a semiconductor — like silicon. Initial attempts have resulted in limited success. “We are working on the fact that graphene is extremely hydrophobic and can float to the surface of a liquid… but it is also extremely delicate,” Piner said.

4 pages of supplementary material to the research paper.

2. MIT Technology Review reports on a new way to change the electronic properties of graphene could lead to ultrafast circuits.

Although the researchers haven't yet made a graphene circuit, they've demonstrated a way to control the amount of electron-rich molecules that are added to graphene to make so-called n-type transistors, a crucial electronic component.

In order to make a complex circuit, explains Stanford University chemistry professor Hongjie Dai, who led the work, engineers require transistors that are both electron rich (n-type) and electron poor (p-type). Previously, researchers had only demonstrated p-type graphene transistors, which are easier to make than n-type transistors because oxygen atoms readily bond to the edges of graphene ribbons producing "holes," electrons' positively charged counterparts. With both n- and p-types of graphene transistors, it will be possible to build complex circuitry, says Dai. His team collaborated with Jing Guo's group at the University of Florida and Peter Weber at Lawrence Livermore National Laboratory to develop the n-type graphene transistors.

To make the n-type graphene, the researchers exposed nanoribbons, which were deposited on a wafer of silicon and silicon dioxide, to ammonia and high heat, explains Dai. "We found that if you heat up these ribbons in ammonia, then you can actually get nitrogen into the ribbons, and nitrogen donates electrons to the graphene." While it seems like a simple trick, Dai says that it yielded somewhat unexpected results. "What's interesting is we didn't find a decrease in [electron] mobility," he says. This means that electrons were able to zip through the graphene at the same speeds as before, which is important since high electron mobility makes graphene an attractive material for future electronics.

3. MIT has an overview of current graphene research.

Anticipated Uses for Graphene Electronics

"Graphene could lead to faster computers that use less power, and to other sorts of devices for communications such as very high-frequency (radio-frequency-millimeter wave) devices," said Professor and physical chemist Rod Ruoff, one of the corresponding authors on the Science article.

"Graphene might also find use as optically transparent and electrically conductive films for image display technology and for use in solar photovoltaic electrical power generation."

Graphene, an atom-thick layer of carbon atoms bonded to one another in a "chickenwire" arrangement of hexagons, holds great potential for nanoelectronics, including memory, logic, analog, opto-electronic devices and potentially many others. It also shows promise for electrical energy storage for supercapacitors and batteries, for use in composites, for thermal management, in chemical-biological sensing and as a new sensing material for ultra-sensitive pressure sensors.

"There is a critical need to synthesize graphene on silicon wafers with methods that are compatible with the existing semiconductor industry processes," Ruoff said. "Doing so will enable nanoelectronic circuits to be made with the exceptional efficiencies that the semiconductor industry is well known for."

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