We have shown conclusive evidence of diffusive thermal transport in a single molecule of a phosponic acid (henicosafluorododecyl-phosphonic acid (F21PA)) between various metals and a sapphire substrate. thermal transport across single molecules is typically assumed ballistic, where the thermal resistance is dominated by the molecular contacts. We show that the large molecular weight phosphonic acid F21PA, vibron scattering in the molecule can lead to additional thermal resistance at metal/PA/sapphire interfaces, indicative of diffusive thermal transport. This work, in which Dr. John Gaskins was first author, was recently published in The Journal of Physical Chemistry C (The Journal of Physical Chemistry C 119, 20931-20939 (2015)), and was in collaboration with Dr. Sam Graham from Ga Tech.
Abstract
The influence of planar organic linkers on thermal boundary conductance across hybrid interfaces has focused on the organic/inorganic interaction energy rather than on vibrational mechanisms in the molecule. As a result, research into interfacial transport at planar organic monolayer junctions has treated molecular systems as thermally ballistic. We show that thermal conductance in phosphonic acid (PA) molecules is ballistic, and the thermal boundary conductance across metal/PA/sapphire interfaces is driven by the same phononic processes as those across metal/sapphire interfaces without PAs, with one exception. We find a more than 40% reduction in conductance across henicosafluorododecyl-phosphonic acid (F21PA) interfaces, independent of metal contact, despite similarities in structure, composition, and terminal group to the variety of other PAs studied. Our results suggest diffusive scattering of thermal vibrations in F21PA, demonstrating a clear path toward modification of interfacial thermal transport based on knowledge of ballistic and diffusive scattering in single monolayer molecular interfacial films.
Acknowledgements
P.E.H. is grateful for support from the Office of Naval Research Young Investigator Program (N00014-13-4-0528).
patrickehopkins.com 
Leave a Reply