Many of today's consumer electronics rely on microscopic machines. These tiny devices are found in smart-phone motion sensors, inkjet printheads, and the switches that activate some display pixels, to name just a few components.
Machine making: A transparent sheet of graphene is stretched over the surface of this silicon wafer. The graphene can oscillate over holes in the silicon beneath, acting as a nanomechanical device called a resonator.
Shrinking these electromechanical machines down to the nanoscale would enable new devices, such as extremely sensitive chemical sensors, incredibly precise accelerometers, and super-fast integrated circuit switches. In an important step toward this goal, researchers at Cornell University have made large arrays of nanoscale resonators using graphene.
An atom-thin form of carbon called graphene is among the most promising materials for making nanoelectromechanical systems (NEMS). Graphene is the strongest known material, and the most electrically conductive. Graphene's atom-thin size means it is also incredibly lightweight and can move very fast. Cornell physics professor Paul McEuen says graphene can be used to build large numbers of nanodevices with equipment developed for etching silicon chips on flat wafers. But building mechanical nanomachines from graphene is challenging, and most of the devices created so far have been one-offs.
McEuen and fellow Cornell professor Harold Craighead have now shown that they can make graphene nanodevices called resonators on the surface of a silicon wafer. Each resonator is made of a film of graphene that oscillates back and forth, like a trampoline moving up and down, in response to a mechanical force applied to its surface or to an electrical field.
The Cornell group first etched trenches into the surface of a silicon wafer. They then topped the wafer with a film of graphene grown on top of copper. The graphene sticks to the surface of the silicon wafer like plastic cling wrap would. The researchers finally add electrical contacts to the graphene to complete the resonators. The work is described online in the journal Nano Letters.
"We're making large numbers of identical resonators, which demonstrates a transition from a lab experiment to a technology," says McEuen. Previous nanoresonators made at this scale were either much thicker and less sensitive, or they had to be made one at a time. "The two major obstacles in implementing nanodevices are scale-up and reproducibility in performance," says Alex Zettl, professor of physics at the University of California, Berkeley. Zettl has made similar devices from carbon nanotubes, including a radio made from a single carbon nanotube. "Using single-layer graphene allows many devices to be made in one shot, with similar performance," Zettl says.
Graphene nanoresonators could make very sensitive chemical detectors or accelerometers. The suspended graphene films respond dramatically when any weight is added—even just a molecule or an atom. "It couples very strongly to the outside world," which makes for a good sensor, says McEuen.
Rod Ruoff, professor of mechanical engineering at the University of Texas at Austin, who pioneered the graphene growth-and-transfer technique used by the Cornell group, says this work demonstrates that this type of graphene performs well in nanomechanical systems. But Ruoff says he sees room for improvement in the performance of the resonators.
The Cornell researchers are now working to push the graphene resonators to their ultimate performance limits. The crystalline structure of graphene, which determines its strength and electrical conductivity, is not perfect in the Cornell devices made so far.
The researchers also hope to take advantage of quantum effects that occur at the nanoscale. This could improve their sensitivity, McEuen says.
By Katherine Bourzac
From Technology Review