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Observing Heat Move Through Molecules

Photonics Spectra
Sep 2007
In molecular electronics, long-chain molecules bound to electrodes are used as tiny switches. However, where electrons flow, heat goes, and the manner in which heat moves at the molecular level through materials used in electronic devices is of increasing concern.

Now researchers at the University of Illinois at Urbana-Champaign have reported using a technique dubbed ultrafast flash thermal conductance to study how heat travels from a substrate to the molecules attached to it — and onward to the tips of the molecules themselves.

Dana D. Dlott, David G. Cahill and their colleagues studied glass substrates that they coated with a 50-nm-thick layer of gold, using a 0.8-nm-thick layer of chromium to aid adhesion. Onto the gold layer, they applied densely packed self-assembled monolayers of long-chain hydrocarbon molecules.

The investigators heated the coated substrates at the glass-gold interface, using an 800-nm Ti:sapphire laser set to generate 500-fs pulses. The heat generated by one laser pulse flowed from the layer of gold into the base of the molecular chains, then traveled through the chains until it reached the methyl groups at the ends.

The scientists used broadband multiplex vibrational sum-frequency generation spectroscopy to measure the disordering effect on the methyl groups. In this technique, they aimed a femtosecond-scale pulse at a wavelength of 3.3 μm and a bandwidth of 150 cm–1 at the bound molecules. The beam coherently excited the alkane CH-stretch transitions in the 2850 to 3000 cm–1 range. A second, picosecond-long pulse with a bandwidth of 7 cm–1 interacted with the oscillating polarizations, producing a sum frequency signal that originated solely from the methyl groups at the ends of the molecules. When the heat reached these groups, it disturbed them out of alignment, which resulted in a sudden decrease of the sum frequency signal, enabling the scientists to measure the heat transfer rate.

They report in the Aug. 10 issue of Science that the gold layer reached thermal equilibrium at 800 °C within 1 ps, and that the heat traveled at a steady velocity of about 1 km/s through to the ends of the bound molecules. The gold layer stayed at a high temperature, but the heat dissipated entirely after several nanoseconds. The heat conductance through each of the ∼1011 molecular chains was 50 pW/K.

That branch of science involved in the study and utilization of the motion, emissions and behaviors of currents of electrical energy flowing through gases, vacuums, semiconductors and conductors, not to be confused with electrics, which deals primarily with the conduction of large currents of electricity through metals.
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