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At molecular scale, vibrational couplings define heat conduction
James
E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
9/23/2004
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CHAMPAIGN,
Ill. — Too much heat can destroy a sturdy automobile engine or
a miniature microchip. As scientists and engineers strive to make ever-smaller
nanoscale devices, from molecular motors and switches to single-molecule
transistors, the control of heat is becoming a burning issue.
The shapes of molecules really matter, say scientists from the University
of Illinois at Urbana-Champaign and the University of Scranton who timed
the flow of vibrational heat energy through a water-surfactant-organic
solvent system. The rate at which heat energy moves through a molecule
depends specifically on the molecule’s structure, they found.
“The flow of vibrational energy across a molecule is dependent
upon where and how the energy is deposited,” said Dana Dlott,
a professor of chemistry at Illinois and a co-author of a paper to appear in the journal Science,
as part of the Science Express Web site, on Sept. 23. “Unlike
normal heat conduction, different excitations may travel across the
molecule along different paths and at different rates.”
To monitor energy flow, Dlott and his colleagues – Scranton chemistry
professor John Deak, Illinois postdoctoral research associate Zhaohui
Wang and graduate student Yoonsoo Pang, and Scranton undergraduate student
Timothy Sechler – used an ultrafast laser spectrometer technique
with picosecond time resolution.
The system the scientists studied is called a reverse micelle, and consisted
of a nanodroplet containing 35 water molecules enclosed in a sphere
of surfactant (sodium dioctyl sulfosuccinate) one molecule thick that
was suspended in carbon tetrachloride. The ultrafast laser technique,
developed at Illinois, monitored vibrational energy flow as it moved
from water, through the surfactant shell out to the organic solvent,
atom by atom.
When the researchers deposited energy in the nanodroplet, the vibrations
moved through the surfactant and into the carbon tetrachloride within
10 picoseconds. However, when the energy was deposited directly into
the surfactant, the vibrations required 20 to 40 picoseconds to move
into the carbon tetrachloride. Even though the distance was shorter,
the energy transfer took significantly longer.
“This is opposite of what you would think in terms of simple and
ordinary heat conduction,” Dlott said. “To explain this
strange result, we have to analyze the energy transfer in terms of specific
vibrational couplings that occur through a vibrational cascade.”
There are hundreds of different vibrations in the water-surfactant-organic
solvent system, Dlott said. “When energy moves through molecules,
the detailed structure of the molecules and the way the vibrations interact
are extremely important.”
When the water was excited by a laser pulse, the scientists report,
much of the energy was immediately moved to the surfactant, which then
efficiently transferred the energy to the carbon tetrachloride. But
when the surfactant was excited by the laser, the energy took a different
path among the atoms, delaying the transfer to the carbon tetrachloride.
“The movement of vibrational energy within and between molecules
is a fundamental process that plays a significant role in condensed
matter physics and chemistry,” Dlott said. “In designing
nanoscale devices, the shapes of the molecules must be designed not
only to be small and fast, but also to efficiently move heat.”
The National Science Foundation, the Air Force Office of Scientific
Research and the U.S. Department of Energy supported this work.
