Ultrafast laser spectrometer measures heat flow through moleculesAugust 10, 2007CHAMPAIGN, Ill. - Global warming isn't the only heat scientists are feeling. Another area in which heat flow is becoming crucial is 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. "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." University of Illinois at Urbana-Champaign | |||||||||||||||||||||
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Related Heat Flow News Articles Quakes under Pacific floor reveal unexpected circulatory system Zigzagging some 60,000 kilometers across ocean floors, earth's system of mid-ocean ridges plays a pivotal role in many workings of the planet, from its plate-tectonic movements to heat flow from the interior, and the chemistry of rock, water and air. Plate tectonics may take a break Plate tectonics, the geologic process responsible for creating the Earth's continents, mountain ranges, and ocean basins, may be an on-again, off-again affair. Earth's heat adds to climate change to melt Greenland ice Scientists have discovered what they think may be another reason why Greenland's ice is melting: a thin spot in Earth's crust is enabling underground magma to heat the ice. They have found at least one "hotspot" in the northeast corner of Greenland -- just below a site where an ice stream was recently discovered. Nanotube forests grown on silicon chips for future computers, electronics Engineers have shown how to grow forests of tiny cylinders called carbon nanotubes onto the surfaces of computer chips to enhance the flow of heat at a critical point where the chips connect to cooling devices called heat sinks. Tectonic plates act like variable thermostat Like a quilt that loses heat between squares, the earth's system of tectonic plates lets warmth out at every stitch. Why a Rocky Mountain high? A University of Utah study shows how various regions of North America are kept afloat by heat within Earth's rocky crust, and how much of the continent would sink beneath sea level if not for heat that makes rock buoyant. High-resolution images herald new era in Earth sciences High-resolution images that reveal unexpected details of the Earth's internal structure are among the results reported by MIT and Purdue scientists in the March 30 issue of Science. New evidence shows New Madrid Seismic Zone may be cold and dying New results about the temperatures of rock deep below the New Madrid Seismic Zone in the central United States shed light on the puzzling questions of why large earthquakes happened there in 1811 and 1812 and when they may happen again. Seismologists measure heat flow from Earth's molten core into the lower mantle For the first time, scientists have directly measured the amount of heat flowing from the molten metal of Earth's core into a region at the base of the mantle, a process that helps drive both the movement of tectonic plates at the surface and the geodynamo in the core that generates Earth's magnetic field. Seismolgists get handle on heat flow deep in Earth Earth's interior is not a benign world that only stores the geologic history of our planet. Geologists now see the normally assumed placid inner Earth as a dynamic environment filled with exotic materials and substances roiling under intense heat and pressures. It is an environment that continues to evolve in interesting ways and one that has an impact on what happens at its surface. More Heat Flow News Articles |
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