Molecular Heat Flow Measured

A thermal measurement technique has been developed that can explore heat transport in extended molecules fastened at one end to a metal surface by using ultrafast lasers. Heat flow is crucial in the field of molecular electronics, where long-chain molecules attached to tiny electrodes are used to transport and switch electrons.

"How electrons flow through molecular wires has been studied, but less attention has been given to how the heat flows," said Dana Dlott, a physical chemist at the University of Illinois. He and engineering professor David Cahill and colleagues developed the new technique. "One of the problems has been the lack of a measurement technique that could operate over short distances, short time intervals and large temperature bursts.

A team of chemists and engineers at the University of Illinois has developed an ultrafast way of measuring temperature within a molecule by using femtosecond and picosecond lasers. Pictured are (from top, clockwise) Jeffrey Carter, Dana Dlott, Nak-Hyn Seong, Zhaohui Wang and Yee Kan Koh. (Photo: L. Brian Stauffer)

"The ability to selectively probe the atomic groups that terminate the chains allows us to investigate the transport of heat through the chain molecules themselves," Dlott said.

To study heat flow through long-chain hydrocarbon molecules anchored to a gold substrate, the researchers used an ultrafast laser spectrometer technique with picosecond time resolution (a picosecond is 1 million-millionth part of a second).

First, the flash from a femtosecond laser (a femtosecond is 1000th of a picosecond) heated the substrate to about 800 °C in one picosecond. This heat flowed quickly into the base of the hydrocarbon molecules and through the chains.

When heat reached the methyl groups at the ends of the chains, which were originally lined up in order, they began to shake and twist. An extremely sensitive form of coherent vibrational spectroscopy was used to probe this disordering. The researchers' work showed how the familiar concepts of heat transport do not apply at the level of individual molecules.

One interesting finding, for example, is that heating the molecule to 800 °C doesn't destroy it. "Because the molecule stays hot for only a billionth of a second, it doesn't have time to evaporate, burn up or chemically react," said Cahill, a Willett Professor of Materials Science and Engineering.

Another surprising finding is that heat moves ballistically -- that is, at a constant velocity -- through the molecule. Each time two more carbon atoms were added to the chains, the heat took a little longer, about one-quarter of a picosecond, to reach the end.

"Heat usually travels at different velocities as it diffuses through its surroundings," said Cahill, who also is a researcher at the Frederick Seitz Materials Science Laboratory and at the Coordinated Science Laboratory. "We found the leading edge of the heat burst traveled ballistically along the hydrocarbon chains at a velocity of 1 kilometer per second."

The researchers also determined the overall rate of heat flow in the molecule. They calculated a thermal conductance of 50 picowatts per degree Celsius. "This is a new way of measuring temperature within a molecule," Dlott said. "It’s the first step toward making a more precise thermometer with very high spatial resolution and with very high time resolution."

The work is reported in the Aug. 10 issue of the journal Science. With Dlott and Cahill, co-authors of the paper are postdoctoral research associates Zhaohui Wang, Alexei Lagutchev and Nak-Hyun Seong, and graduate students Jeffrey A. Carter and Yee Kan Koh. The work was funded by the US Department of Energy, the National Science Foundation and the Air Force Office of Scientific Research.

For more information, visit: www.uiuc.edu