IBM's discovery is based on a new experimental set up that simultaneously monitors both the electrical and optical properties of nanotubes. They also made use of optical spectroscopy to discover the new mechanism. "We used optical laser microscopy to monitor how the heat was generated and dissipated in the nanotube as we heated it up electrically," said Steiner.
When the current was first turned on, it excited very high-frequency phonons, which dissipated by exciting lower-frequency vibrations. When this happens, an equilibrium is eventually reached where all the different vibrational modes achieve the same temperature. Instead, the researchers found a non-equilibrium distribution of phonons. Each displayed distinctly different temperatures.
"Here was a new steady state--a nonstatistical distribution of the energy inside the nanotube," said Avouris . "Some of the vibrations were hot, and some of the vibrations were colder, which is very unusual in nature."
After analyzing their observations, the researchers discovered the new mode whereby vibrations were directly coupling to the silicon dioxide substrate. This was accomplished through a new mechanism that allows heat to jump the gap by virtue of coupled electrical fields.
"The vibrations of the silicon dioxide [substrate] create an electrical field that couples with the electrical field generated by the current in the carbon nanotube, allowing its energy to be directly coupled to the vibrations of the silicon dioxide lattice--which amounts to a new cooling mechanism that we believe can be exploited for future carbon-based transistors," said Avouris.
IBM's heat-dissipation mechanism challenges long-standing scientific paradigms asserting that very small transistor channels made of one-dimensional materials are exceedingly difficult to cool. The researchers are now characterizing the new mechanism for transferring heat to substrates from both carbon nanotube transistor channels, and from nanoribbon channels formed by graphene monolayers of carbon.