“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. “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.”
As reported in the Aug. 10 issue of the journal Science, Dlott, engineering professor David Cahill and colleagues at Illinois have now developed an ultrafast thermal measurement technique capable of exploring heat transport in extended molecules fastened at one end to a metal surface.
“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 1,000th of a picosecond) heated the substrate to about 800 degrees Celsius 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’ study showed how the familiar concepts of heat transport do not apply at the level of individual molecules.
One cool finding, for example, is that heating the molecule to 800 degrees Celsius 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, both on the Illinois campus.
“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.”
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 U.S. Department of Energy, the National Science Foundation and the Air Force Office of Scientific Research.
James E. Kloeppel | University of Illinois
Shape matters when light meets atom
05.12.2016 | Centre for Quantum Technologies at the National University of Singapore
Climate cycles may explain how running water carved Mars' surface features
02.12.2016 | Penn State
Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...
A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.
Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...
In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.
“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...
The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.
The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...
Broadband rotational spectroscopy unravels structural reshaping of isolated molecules in the gas phase to accommodate water
In two recent publications in the Journal of Chemical Physics and in the Journal of Physical Chemistry Letters, researchers around Melanie Schnell from the Max...
16.11.2016 | Event News
01.11.2016 | Event News
14.10.2016 | Event News
05.12.2016 | Power and Electrical Engineering
05.12.2016 | Materials Sciences
05.12.2016 | Power and Electrical Engineering