When someone scribes a line with a humble pencil the resulting mark includes bits of the hottest new material in physics and nanotechnology - graphene.
Graphene is a single layer of carbon atoms arranged in a honeycomb lattice that could allow electronics to process information and produce radio transmissions 10 times better than silicon-based devices. It is among the strongest materials known and has an attractive array of benefits. These sheets have potential as electrodes for solar cells, for use in sensors, as the anode electrode material in lithium batteries and as efficient zero-band-gap semiconductors.
Mass production of graphene
Research on graphene sheets has been restricted, though, due to the difficulty of creating single-layer samples for use in experiments. But in a recent study published, researchers from University of California's (UCLA) California NanoSystems Institute (CNSI) propose a method which can produce graphene sheets in large quantities.
Led by Yang Yang, a professor of materials science and engineering at the UCLA Henry Samueli School of Engineering, and Richard Kaner, a UCLA professor of chemistry and biochemistry, the researchers developed a method of placing graphite oxide paper in a solution of pure hydrazine (a chemical compound of nitrogen and hydrogen), which reduces the graphite oxide paper into single-layer graphene.
Such methods have been studied by others, but this is the first reported instance of using hydrazine as the solvent. The graphene produced from the hydrazine solution is also a more efficient electrical conductor. Field-effect devices display output currents three orders of magnitude higher than previously reported using chemically produced graphene.
The coverage of the graphene sheets can be controlled by altering the concentration and composition of the hydrazine solution. This hydrazine method also preserves the integrity of the sheets, producing the largest area graphene sheet yet reported, 20 micrometers by 40 micrometers. A micrometer is one-millionth of a meter, while a nanometer is one billionth of a meter.
The scientists believe these graphene sheets are by far the largest produced. Chemically converted graphene can now be studied in depth through a variety of electronic tests and microscopic techniques not previously possible.
There are two methods currently used for graphene production - the drawing method and the reduction method, each with its own drawbacks. In the drawing method, layers are peeled off of graphite crystals until one is produced that is only one-atom thick. When likely graphene suspects are identified from the peeled layers, they must be extensively studied to conclusively prove their identity. In the reduction method, silicon carbide is heated to high temperatures (1100° C) to reduce it to graphene. This process produces a small sample size and is unlikely to be compatible with fabrication techniques for most electronic applications.
"This technology (hydrazine reduction) utilizes a true solution process for graphene, which can dramatically simplify preparing electronic devices," said Yang, who is also faculty director of the Nano Renewable Energy Center at the CNSI. "It thus holds great promise for future large-area, flexible electronics."
Graphene for radio electronics
Princeton University has built transistors - tiny on-off switches - on their printed graphene crystals. Their transistors displayed high performance and were more than 10 times faster than silicon transistors in moving "electronic holes" - a key measure of speed. They suggest that the new technology could find almost immediate use in radio electronics, such as cell phones and other wireless devices that require high power output. Depending on the level of interest from industry, the technique could be applied to wireless communication devices within a few years they predict.
A research team from Manchester University in the UK has now demonstrated highly transparent and highly conductive films that can be produced cheaply by dissolving chunks of graphite into graphene and then spraying the suspension onto a glass surface. The research team has demonstrated what it believes to be the first liquid crystal devices with graphene electrodes. It is believed that only a few small, incremental steps remain for this technology to reach a mass production stage.
Graphene could replace indium tin oxide
Graphene could also replace indium tin oxide as an electrode material in displays. Transparent conducting films are an essential part of many gadgets including common liquid crystal displays for computers, TVs and mobile phones. The underlying technology uses thin metal-oxide films based on indium. But indium is becoming an increasingly expensive commodity and its supply is expected to be exhausted within just 10 years.
Also read an earlier article Graphene - highest mobility and processable.
For more attend Printed Electronics USA 2008 and Printed Electronics Europe 2009.
Top image: Two overlapping images of the same graphene sheet produced by hydrazine reduction; the top image was produced using atomic force microscopy, while the bottom was produced with scanning electron microscopy. This is the first reported instance of a graphene sheet being large enough for both tests to be run on the same specimen. (Image credit: Vincent Tung, Matthew Allen, Adam Stieg of UCLA).
References: Princeton University, UCLA's California NanoSystems Institute
