To measure the heat of a nanotube, the researchers relied on standard optical spectroscopy techniques. In other words, they shined a tightly focused laser beam onto a single nanotube transistor while passing an electrical current through the target. When the light hits the nanotube, it is absorbed and scattered in ways that reveal the manner in which the nanotube’s atoms are vibrating. These atomic vibrations, known as phonons, occur at different frequencies, or modes. And these modes, explains Avouris, indicate temperatures inside the nanotube.
The reason why the IBM team was able to achieve results that have eluded others, explains Avouris, is its degree of sensitivity. The researchers built their experiment to specifically probe a single nanotube, a process that requires a tight focus of light, and to physically isolate one nanotube from all the others. But simply looking at the results from a single spectroscopy study wouldn’t be enough, says Avouris. So his team looked at a combination of different measurements, or “thermometers,” including the characteristics of electron transitions within the nanotube. Combining these measurements produced enough data to construct a complete heat-flow model.
It has been a challenge to get an accurate view of the thermal properties of nanotubes, especially in relation to a substrate and the surrounding environment, says Eric Pop, a professor of electrical engineering at the University of Illinois, Urbana-Champaign. “The work confirms ideas that had been floating around for a while, but that people weren’t too sure about.” This includes the idea that a nanotube’s environment plays a strong role in determining how it dissipates heat. “Maybe a year ago, it would have been fair to say that nanotubes are excellent heat conductors, but now we know they’re [also] actually quite sensitive to their environment,” Pop says.
For engineers, the IBM findings are significant because they point the way to important heat-management techniques of circuits. Since the substrate has shown to be an important factor in the heat flow and electrical properties of nanotubes, engineers could start to think about modifying them, wrapping materials around nanotubes, or exploring different materials to bond nanotube transistors to a substrate.