Graphene: Putting Particle Physics at Your Fingertips?
BU researchers looking for new uses for old standby
It took a while for scientists to turn their attention to graphene, a strange and immensely promising carbon material that is only one atom thick and exists in the form of a honeycomb lattice. After all, says Antonio H. Castro Neto, a College of Arts and Sciences professor of physics, graphene can be made by running a graphite crystal over a piece of paper, so it has been around at least since 1564, when the pencil was invented. No doubt, explains the physicist, it was the material’s extreme slenderness that kept it literally out of sight — until a few years ago. Since then, however, it has been getting an awful lot of press. Last year alone, roughly 200 scientific papers were written about the bizarre properties and potential applications of graphene.
“It’s amazing,” says Castro Neto, who was part of the team that first imagined some of the strangest behavior of the material. “At this point, there are thousands of people around the world trying to figure out the full potential of something that almost no one was researching a couple of years ago.”
Castro Neto’s interest in graphene was piqued in 2005, when he read a paper written by Andre Geim of the University of Manchester, in England, where physicists had found a way to isolate the unique carbon lattice. At the time, Castro Neto was collaborating on other projects with physicists Francisco Guinea of Spain and Nuno M. R. Peres of Portugal, both of whom had come to BU as visiting researchers under the Quantum Condensed Matter Theory Visitors Program. Together, they took what Geim had shared about graphene and developed a theory that predicted something extraordinary: if graphene were subjected to a magnetic field, scientists would be able to observe effects that only occur with particles moving at a velocity close to the speed of light, although the electrons in graphene propagate at velocities 300 times slower than that. If the three researchers were correct, it would mean that graphene could be used as a kind of tabletop particle physics lab, a controlled environment in which scientists could test new physics ideas. Castro Neto contacted Geim and asked if their theory could be verified. Geim thought it could.
“At that point,” says Castro Neto, “if you look at the number of publications about graphene, you will see an explosion. Everyone was very excited.” Bennett Goldberg, a CAS professor of physics and department chair, was not surprised by that excitement. “One of the great attractions of graphene,” says Goldberg, “is that Castro Neto and colleagues have discovered entirely new physics and new materials science from something so commonplace — the scratch of a pencil on paper. This elegance is now coupled with tremendous technical and economic potential.”
An example of that potential, says Castro Neto, is in the manufacture of computer chips, now made of silicon. Graphene can be fashioned into chips that are much smaller than those made of silicon. More important, chips made of graphene would have the enormous advantage of generating almost no heat. He explains that computer circuitry heats up when electrons meet resistance while passing from one material, such as a transistor, to another material, such as the metal pathways that connect one transistor to another. With the graphene model, both the chips and the connecting circuitry could be literally “carved out” on graphene. With such very low resistance, he says, there would be very low heat.
And there are other applications. Graphene’s unusual electrical properties, Castro Neto says, can signal the presence of a single molecule of a foreign substance. “Imagine you have a terrorist who is carrying an explosive,” he says. “There are always some residual molecules in the vicinity, and if just one of those molecules hits the surface of graphene, it will be detected.”
He finds it ironic that so much excitement has been unleashed by something that other researchers had been throwing away until physicists in Geim’s lab decided to take a closer look. For years, Castro Neto says, researchers have used Scotch tape in their attempts to cleave graphite crystals, and having done that, they would throw the tape in the trash can.
“What Andre Geim did,” he says, “was look at what was left over on the Scotch tape. Until then, no one had done that, because when people are familiar with something, they don’t look at it closely.”
This year, Castro Neto’s research group is hot on the graphene trail, working with funding from the National Science Foundation. “We are trying to understand the basic electronic properties of this material,” he says. “We are thinking about how we can tailor the properties so it can be used in other applications. For example, can we make this material into a magnet? If so, we can use it for lots of applications involving magnetism. Can we make graphene a superconductor? We are looking at ways to modify it both structurally and chemically to change its electronic properties.”
There is, he says, one more thing about this unique lattice of carbon atoms: even though it is only one atom thick, it can actually be seen, appearing under a microscope as a strange violet-blue hue. That’s because when it is applied to a silicon oxide substrate that is exactly 300 nanometers thick, its wavelength is, coincidentally, a near-perfect match for the most sensitive cones in the human eye. “It is,” says Castro Neto, “the thinnest thing human eyes will ever see.”