Ultrafast dynamics of heat flow across molecules

Zhaohui Wang, David G. Cahill, Jeffrey A. Carter, Yee Kan Koh, Alexei Lagutchev, Nak Hyun Seong, Dana D. Dlott

Research output: Contribution to journalArticlepeer-review


An ultrafast flash thermal conductance apparatus is used to study heat flow through aliphatic and aromatic molecules arranged in self-assembled monolayers (SAMs). The apparatus consists of a thin metal film which can be flash-heated by many hundreds of degrees in ∼1 ps using a femtosecond pulse. Heat flow from the metal surface into the SAM molecules is detected using vibrational sum-frequency generation (SFG) spectroscopy. The SAMs studied were alkanethiolates (AT) ranging from C6 to C24, benzenethiolate (BT) and benzylmercaptide (BMT). SFG in the CH-stretch region selectively probes transitions of the terminal methyl groups of AT and the CH moiety at the 4-position of the phenyl ring of BT and BMT (opposite the thiolate-surface bond). The SFG signal is sensitive to temperature-jump induced thermal disorder of the SAM and also to vibrational frequency shifts induced by the changing intramolecular vibrational populations. The SFG probe functions as a thermometer, and this thermometer is ∼1.5 Å thick with a response time of ∼1 ps. In the AT chains, a study of the length dependence is used to determine the rate heat flows across the metal-SAM interface and the rate of heat flow through the AT chains. The interface thermal conductance is 220 GW m-2 s-1. The AT molecular conductance is 50 pW K-1 or 0.3 eV s-1 K-1. Heat flow through the AT chains is ballistic with a velocity of 1 km/s. Heat flow into BMT is slower than in BT because BMT has one additional methylene linker group. The BT and BMT structures evidence a thermally-initiated surface rearrangement occurring in a few tens of picoseconds. These SAMs are strained and the phenyl rings cannot adopt the most stable staggered herringbone structure. After the T-jump, the SAM molecules have enough freedom to relax into more favorable configurations.

Original languageEnglish (US)
Pages (from-to)31-44
Number of pages14
JournalChemical Physics
Issue number1-3
StatePublished - Jun 23 2008


  • Femtosecond spectroscopy
  • Heat transport
  • Interfaces
  • Molecular monolayers
  • Thermal conductance

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Spectroscopy
  • Atomic and Molecular Physics, and Optics


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