Super Material Graphene More Easily Manufactured, Integrated into Mechanical Devices

It can't leap tall buildings in a single bound, but graphene - the super material that is stronger than steel, despite being only one atom thick - has enjoyed a research boom since scientists first isolated it less than a decade ago. Yet much of its promise has not been realized, in part due to earlier difficulties in fabricating the material, say researchers from Cornell University in a review article published in the Sept/Oct issue of the Journal of Vacuum Science and Technology B.

Recently, however, breakthroughs in creating graphene have led to a period of "remarkably rapid progress" in the field of graphene nanomechanics, write the authors. "It's been an evolution of new growth processes, new materials processing, lithography, and etching techniques that have advanced over the years," says Harold Craighead, Cornell University professor of engineering and applied and engineering physics and an author of the paper, along with Cornell University colleagues Robert A. Barton and Jeevak Parpia. The result is that graphene can now be grown and processed like many other materials, which has allowed scientists to integrate it into devices "more easily than was thought," Craighead says.

There are several ways that graphene, which consists of a single layer of carbon atoms in a honeycomb structure, can be manufactured. Scientists can slough off single layers of the material from a block of graphite by rubbing it against a hard surface, in a process called mechanical exfoliation. In the past it was painstakingly difficult to pick out the single layers of graphene from the much thicker chunks of graphite that are also produced in this process. But the discovery that the thickness of samples could be determined optically and in bulk, through Raman spectroscopy, has since made it easier for researchers to locate the fruits of their labor among the graphite rubble.

In addition to mechanical exfoliation, researchers can also grow graphene from scratch. A process called chemical vapor deposition (CVD) has proved an especially promising technique. At first researchers used CVD to grow the material on sheets of nickel, but this method produced too many layers. A copper substrate proved to be much more efficient at producing the single-layer graphene. An additional advantage is that the copper can subsequently be etched away, leaving a pure graphene sheet that can be used alone or placed onto other material substrates.

These and other advances have opened the door to a wider range of systems into which graphene can be incorporated. Among the devices for which this thin, durable material seems almost ideally suited are nanoelectromechanical systems (NEMS), which can be used as highly sensitive detectors of mass and position. Because graphene is both mechanically and electrically active, the authors write, it can be used to sense many things simultaneously, such as mass, stress, and charge. One proof-of-principle experiment suggests that circular graphene resonators could make mass sensors that perform better than some devices (such as quartz crystal microbalances) that are commonly used today to weigh the tiniest of particles.

In addition to sensors, Craighead's team is exploring the use of graphene as a membrane, with potential applications for electron microscopy. Graphene is impermeable to gases, so it can seal off a small pocket of air. It is also transparent to electrons, and its strength can withstand the vacuum pressure of an electron microscope. By encapsulating delicate objects such as cells, Craighead says, graphene could separate a cell-friendly aqueous environment from the microscope's vacuum, acting as "a thin window that would unite those two disparate worlds."

Despite the recent advances, Craighead says there is plenty of room for progress. "[Graphene is] still a new materials system," he says. "People are still exploring. It's still open-ended what new applications will emerge."