Thermometer Created for Nanotubes

Understanding how nanotubes heat up could make them useful for electronics.

For the first time, researchers have developed a way to accurately measure how heat flows within carbon nanotubes--tiny molecular wires that could someday be used to make circuits that are much faster and more energy efficient than today's. The results show that nanotube heating is more complicated than previously thought--a fact that could be crucial in enabling engineers to build carbon-nanotube electronics.

Phonons feeling: This illustration shows a nanotube being heated by a current of electrons (dark-gray arrow to the right). The electrons excite a vibrational mode of the carbon atoms, represented by the first red parabola. Energy flows to other vibrational modes (the other parabolas) at rates indicated by the width of each arrow. Each mode corresponds to a different temperature, ranging from 1,000 °C to 400 °C. Credit: IBM

Traditional semiconductors, such as silicon, undergo heating, says Phaedon Avouris, leader of IBM's nanoscale science and technology group in New York, where the work was carried out. "It's one of the limitations in improving speed," says Avouris. But the study published by his team today in Nature Nanotechnology "goes beyond the simple observation of heat" in carbon nanotubes, he says. "It goes to the atomic level of how heat is generated and dissipated."

In particular, Avouris's team found that when an electrical current is applied to a nanotube transistor, some atomic vibrations can produce heat of up to 1,000 °C, while other vibrations produce a relatively cool temperature of 400 °C. This is contrary to the behavior of most materials, which maintain a relatively uniform heat.

Moreover, the researchers found that the electrical properties of a nanotube, and the manner in which heat is transferred to a substrate made from the silicon dioxide, are both affected by the vibrations of atoms on the surface of the substrate. This means that the substrate used with nanotube transistors will play an important role in determining the electrical properties of the transistor, and the manner in which heat can be removed.

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Since about 1998, when the first carbon-nanotube transistor was demonstrated, researchers have dreamed of next-generation electronics made from such components. Nanotubes have novel properties that allow electrons to zip through them quickly, at low power, and researchers believe that they could act as the active component in transistors, outpacing those made of silicon in terms of speed, energy efficiency, and compactness. But understanding how nanotubes heat up when an electric current is passed through them has been a roadblock to building reliable nanotube circuits.

Mathais Steiner, a researcher at IBM's nanoscale science and technology group, who also worked on the project, says that in the past couple of years, scientists have turned their attention to the way that nanotubes heat, but this property has been difficult to measure. "The problem is that it's difficult to probe properties of the active channel [the region of nanotube used as the electrical switch in a transistor] because we're talking about one molecule," he says. "People were wondering how to get data and perform experiments. This is the first one to get results."