Abstract
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 language | English (US) |
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Pages (from-to) | 31-44 |
Number of pages | 14 |
Journal | Chemical Physics |
Volume | 350 |
Issue number | 1-3 |
DOIs | |
State | Published - Jun 23 2008 |
Keywords
- Femtosecond spectroscopy
- Heat transport
- Interfaces
- Molecular monolayers
- Thermal conductance
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Spectroscopy
- Atomic and Molecular Physics, and Optics