Focus: Spotlight on Nanotubes
Think of it as track lighting on the smallest possible scale. Physicists recently discovered that a tiny tube-like molecule of carbon can produce light when electricity passes through it. Now, the same team has captured images of the precise spot from which the light shines, and by varying the applied voltages, the researchers have even moved the spot back and forth along the 3-nanometer-wide molecule. Described in the 13 August PRL, the effect provides a new tool for studying the inner workings of nanotubes, which might someday serve as the building blocks for molecular electronic circuits.
A carbon nanotube is a cylinder of carbon atoms that resembles a miniscule roll of chicken wire. As in other semiconductors such as silicon, electric current arises in a nanotube when negatively charged electrons flow in one direction or the positively charged “holes” left by absent electrons flow in the other. When a nanotube bridges the gap between two electrodes, known as the source and the drain, the type of flow–electrons or holes–depends on the voltage applied to a third, “gate” electrode lying beneath the tube and separated from it by a thin insulating layer. And if the gate voltage is set just right, equal numbers of electrons and holes can flow through the tube in opposite directions at the same time. When they collide in the narrow channel, the electrons fall into the holes to make infrared light, as Phaedon Avouris of IBM’s T.J. Watson Research Center in Yorktown Heights, New York, and his team demonstrated last year.
Now Avouris, IBM’s Marcus Freitag, and their colleagues have found that the gate not only controls the numbers of electrons and holes, but it also controls the position of the spot where the electrons and holes collide. The researchers varied the gate voltage on a 50-micron-long nanotube from -40 volts to 0 volts and back again. Using a microscope and an infrared camera, they observed that as the voltage increased, a bright spot appeared near the drain, moved along the tube to the source, and disappeared. When the gate voltage decreased from 0 volts, the spot reappeared near the source and traveled back the other way.
The spot is controllable because, instead of zipping through like bullets, the electrons and holes diffuse through the long, narrow nanotube like a creeping fog. Just how far the clouds of electrons and holes extend from either end of the tube depends on the precise gate voltage. At low gate voltages, the cloud of holes reaches much farther across the nanotube than the cloud of electrons, and the two meet close to the drain. As the voltage increases, this meeting point–where electrons and holes annihilate to produce light–moves toward the source.
“It’s very elegant,” says Bruce Weisman, a physical chemist at Rice University in Houston, Texas. “This is a powerful new tool for learning about conduction processes in these nanotube devices.” Tobias Hertel of Vanderbilt University in Nashville, Tennessee, agrees and notes that the bright spot where holes and electrons collide is permeated with very strong electric fields. So researchers might be able to study the distribution of electrons around defects and impurities in nanotubes, Hertel says, by sweeping the bright spot past them.
Adrian Cho is a freelance science writer in Grosse Pointe Woods, Michigan.