Carbon Nanoribbons

Stanford chemists have developed a new way to make transistorsout of carbon nanoribbons. The devices could someday be integratedinto high-performance computer chips to increase their speed andgenerate less heat, which can damage today's silicon-based chipswhen transistors are packed together tightly.

 

The Dai group succeeded in making graphene nanoribbons lessthan 10 nanometers wide, which allows them to operate at highertemperatures. "People had not been able to make graphenenanoribbons narrow enough to allow the transistors to work athigher temperatures until now," Dai said. Using a chemical processdeveloped by his group and described in a paper in the Feburary 29,2009 issue of Science, the researchers have made nanoribbons,strips of carbon 50,000-times thinner than a human hair, that aresmoother and narrower than nanoribbons made through othertechniques.

 

Field-effect transistors are the key elements of computerchips, acting as data carriers from one place to another. They arecomposed of a semiconductor channel sandwiched between two metalelectrodes. In the presence of an electric field, a charged metalplate can draw positive and negative charges in and out of thesemiconductor. This allows the electric current to either passthrough or be blocked, which in turn controls how the devices canbe switched on and off, thereby regulating the flow of data.

 

Researchers predict that silicon chips will reach theirmaximum shrinking point within the next decade. This has prompted asearch for materials to replace silicon as transistors continue toshrink in accordance with Moore's Law, which predicts that thenumber of transistors on a chip will double every two years.Graphene is one of the materials being considered.

 

David Goldhaber-Gordon, an assistant professor of physics atStanford, proposed that graphene could supplement but not replacesilicon, helping meet the demand for ever-smaller transistors forfaster processing.

Dai said graphene could be a useful material for futureelectronics but does not think it will replace silicon anytimesoon.

Although researchers, including those in his own group, haveshown that carbon nanotubes outperform silicon in speed by a factorof two, the problem is that not all of the tubes, which can have1-nanometer diameters, are semiconducting, Dai said. "Depending ontheir structure, some carbon nanotubes are born metallic, and someare born semiconducting," he said. "Metallic nanotubes can neverswitch off and act like electrical shorts for the device, which isa problem."

 

On the other hand, Dai's team demonstrated that all of theirnarrow graphene nanoribbons made from their novel chemicaltechnique are semiconductors. "This is why structure at the atomicscale-in this case, width and edges-matters," he said.