Exploring the Properties of World’s Strongest Known Material
Special from The Record
In a soundproof room in Pupin Hall, a futuristic-looking metallic apparatus is creating three-dimensional images of a material that may one day power a new generation of smaller and faster electronic devices. The miracle substance is graphene, a single-atom-thick sheet of carbon that has captivated scientists since it was first isolated in 2004.
A scanning tunneling microscope in Abhay Pasupathy’s lab used to measure the chemical properties of graphene. The tall cylindrical component on the right is a cryogenic chamber used to keep the ultra-strong carbon material at a temperature of minus-450 degrees Fahrenheit. Top left inset: A color-coded image of electrons around a nitrogen atom (red) embedded in the honeycomb-like surface of graphene (blue).
The imaging device is operated by physicist Abhay Pasupathy, who is studying the properties of graphene. He and his team want to understand how the ultra-thin, super-strong material behaves so scientists can continue to develop practical applications for it such as touchscreens, solar cells and sensors.
Scientists envision the honeycomb-like material as a replacement for silicon, the standard material in semiconductors. Because silicon is a three-dimensional structure, its size is a limiting factor. But graphene is two-dimensional, giving it the flexibility to be used in smaller devices and other innovative components.
“The beauty of graphene is that it’s chemically resistant to just about everything in normal everyday use,” says Pasupathy, an assistant professor of physics. But on its own, he acknowledges, “It’s almost too good.”
One area of Pasupathy’s research is studying what happens when foreign substances are added to graphene to affect how it conducts electricity—a process called “doping.”
Doping is a fundamental step in using silicon to make electronics. Elements such as phosphorus or boron are added to alter the number of electrons in regions of the material, allowing electrical current to be controlled. If scientists can figure out how to manipulate graphene similarly, many electronics applications would become possible.
In a paper published in the Aug. 19 issue of Science, Pasupathy, graduate student Liuyan Zhao and colleagues described doping graphene with nitrogen atoms and then imaged the doped substance for the first time, providing valuable insight into how the individual atoms and electrons behave. Pasupathy is an expert in scanning tunneling microscopy (STM), a technology that makes images of atomic-scale materials that are not visible to the human eye; STM is ideal for graphene, which is one-millionth the width of a strand of hair.
The apparatus, which keeps graphene at a cold temperature and free of vibration and microscopic water particles, sits in his lab. Instead of functioning like a standard microscope, the STM device measures the positions of electrons on the graphene, feeling the surface like a blind person reads Braille. That information is then used to make a 3-D snapshot of the surface of the material. Through this imagery, Pasupathy and his team showed that individual nitrogen atoms displace carbon atoms from the graphene lattice. They also showed how the nitrogen atoms actually “sit” in the lattice.
Columbia is at the forefront of graphene research. Pasupathy collaborates with physics professor Philip Kim, whose group was one of the first in the world to study graphene. And it was Columbia engineers and physicists who proved that graphene is the strongest material ever measured and discovered the point at which it breaks.
A native of Bangalore—known as the Silicon Valley of India—Pasupathy witnessed his hometown’s population explosion in the 1980s before attending the Indian Institute of Technology. He got his Ph.D. from Cornell University in 2004 and was a postdoctoral associate in physics at Princeton University before arriving at Columbia in 2009.
Physics was a natural path, “it runs in the family,” he says. His father is a physicist, and many of his relatives are scientists and engineers. Pasupathy won a Presidential Early Career Award for Scientists and Engineers in 2010 and was named a 2011 Sloan Research Fellow.
The next step for Pasupathy is to dope graphene with other atoms besides nitrogen and see what happens. Doped graphene might be used someday to make a sensor to detect poison gas. Graphene’s transparency makes it theoretically ideal for a touchscreen.
“We want to understand if it can be used as a chemical-sensing element, or whether you can use the nitrogen as a scaffold to put other fancy new elements on to make new materials based on this nitrogen-graphene,” he says. “What we really hope to do with this information is take graphene and add functionality to it.”
—by Beth Kwon