Tuesday, November 18, 2008

Making Graphene More Practical

A novel process yields big pieces of single-ply graphene for smaller, faster electronics.

By Prachi Patel-Predd

Bigger, better graphene: A new way to coat large areas of silicon with single-layer sheets of graphene makes it easy to fabricate an array of field-effect transistors by depositing gold electrodes on top of the graphene. The SEM image (bottom) shows graphene spanning the seven-micrometer gap between the source and drain electrodes. Credit: Yang Yang Laboratory, UCLA

Researchers at the University of California, Los Angeles, have found a simple way to make large pieces of the carbon material graphene. Graphene, a flat, one-atom-thick sheet of carbon, can transport electrons at very high speeds, making it an attractive material for electronic devices. But producing sufficient quantities of large, uniform single-layer sheets of graphene has been a challenge. So far, processes to make graphene create small quantities of graphene flakes or films made of overlapping pieces.

Using the new method, presented online in Nature Nanotechnology, the researchers report making single-layer 0.6-nanometer-thick pieces that are tens of micrometers wide. Materials-science and engineering professor Yang Yang and his colleagues deposit the sheets on silicon wafers to make prototype field-effect transistors.

Testing at least 50 such transistors, the researchers found that the devices had an output current of a few milliamperes. That is 1,000 times higher than the output current of the devices that others have recently reported while using similar techniques to make graphene. "We believe this is a game-changing approach which will significantly improve graphene electronics in the future," Yang says.

Electrons flow through graphene sheets tens of times faster than they flow in silicon. The material could lead to electronic devices that are smaller, faster, and less power hungry than are those made of silicon. Thin and transparent, graphene is also a promising replacement for the indium tin oxide electrodes and the silicon thin-film transistors used in flat-panel displays.

The easiest way to make single sheets of graphene is by using adhesive tape to peel graphene flakes off of pieces of graphite, which is a stack of multiple graphene layers. This process results in a very small amount of tiny flakes of graphene. The pieces would have to be much larger for any practical use. "If you can coat an entire silicon wafer with a single sheet of graphene, then you can do lithography or patterning and have little devices," says James Tour, a chemistry professor at Rice University.

About two years ago, researchers came up with a chemical method that yields larger graphene pieces. They oxidize graphite to make graphite oxide and dissolve it in water. The oxygen atoms pry apart the individual graphene sheets, which get dispersed in the solution. After the researchers deposit the sheets on a substrate, the oxygen is removed using another chemical or by heating.