Graphene, a two-dimensional sheet made of pure carbon, is 200 times stronger than steel. A new experiment at Columbia University in New York City has for the first time directly measured the strength of two-dimensional carbon and found it to be unprecedentedly strong. Carbon sheets only a single atom thick might be used in making super-lightweight composite materials.
The study of carbon exemplifies this symbiotic relation between science and engineering. Scientists and engineers have worked together for centuries to fill our material world with amazing devices, from airplanes to microscopes, from atom bombs to blenders. Generally scientists probe the inner workings of nature, increasingly at a microscopic level, while engineers snatch up the new basic knowledge and convert it into the sophisticated innovative products that characterize our nanotech society.
Carbon is, of course, one of the most important elements for both living and non-living things. In its myriad chemical combinations it provides the inner scaffolding for our body and for all the proteins and chemical processes that make life possible. Carbon in pure form is rarer but still noticeable. Bulk three-dimensional carbon can appear in the form of graphite, which consists of loosely bound sheets of carbon atoms (making graphite a good pencil-writing substance and a good lubricant), and diamond, the more elaborately bonded web of carbon with unequaled hardness.
In the last few decades scientists have discovered carbon with other dimensionalities. For example, buckyballs are molecules in the shape of a soccer ball and contain 60 carbon atoms. This nearly-perfectly-round molecule, whose official name, buckminsterfullerene, is practically a zero-dimensional form of carbon; that is, it resembles a point . The discovery of carbon-60 molecules won three chemists a Nobel prize but it hasn’t yet led to any practical applications.
Then one-dimensional carbon tubes were discovered. These tubes, only nanometers wide (billionths of a meter) but microns (millionths of a meter) long, have very interesting electrical, optical, heat, and mechanical properties. Engineers and scientists working together are trying to turn carbon nanotubes into useful elements in micro-circuitry, either because of the tubes’ tunable electrical properties (they can be conductor, like a metal, or a semiconductor depending on the way they’re grown) or because they might be able to carry away waste heat from hotspots in microchips.
Still more recently, only a few years ago, single-atom-thick sheets of carbon were discovered. Again, scientists and engineers are working together to explore new materials and exploit new properties of this marvelous material, referred to as graphene. Carbon nanotubes are really just rolled up version of graphene.
In the new experiment at Columbia, mechanical engineer James Hone and his colleagues Changgu Lee, Xiaoding Wei, and Jeffrey W. Kysar, stretched an ultrapure, ultrathin sliver of graphene across a hole drilled in a plane of silicon. Then they lowered a diamond-tipped needle. The needle is part of a sensor called an atomic force microscope, or AFM, which, while scanned above a microscopic sample, will adjust its position to maintain a constant tension. The probe’s tiny motions can be converted into a map of the sample itself. Or the motion of the probe can be used to measure the force operating between the probe and sample.
In this case the probe tip pushes down into the graphene sheet and measures the reaction force. (See the accompanying drawing to see what this looks like at the atomic level: http://www.aip.org/png/2008/304.htm) The probe measures the strength of the material, the force needed to break the material. (The accompanying figure shows a graph of the strengths of many materials along with their densities, or their mass per volume.) Organic materials (those containing carbon) like wood and polymers often have a small density and a small strength. Metals have a higher strength, but composite materials, like epoxy, will have just as much strength but weigh a lot less. That’s why they’re used in auto bodies and bullet-proof vests. On this same chart, graphene is way off by itself, with a middle-level density but a record high strength. The new results were reported last week in Science magazine.
Dr. Hone says that his new measurements will serve to reinforce the theories formulated by physicists in their own work. The result of this ongoing synergy between scientists and engineers might be even stronger materials yet to come.