Vibrational Mismatch of Metal Leads Controls Thermal Conductance of Self-Assembled Monolayer Junctions

Abstract

We present measurements of the thermal conductance of self-assembled monolayer (SAM) junctions formed between metal leads (Au, Ag, Pt, and Pd) with mismatched phonon spectra. The thermal conductance obtained from frequency domain thermoreflectance experiments is 65 ± 7 MW/m(2) K for matched Au-alkanedithiol-Au junctions, while the mismatched Au-alkanedithiol-Pd junctions yield a thermal conductance of 36 ± 3 MW/m(2) K. The experimental observation that junction thermal conductance (per molecule) decreases as the mismatch between the lead vibrational spectra increases, paired with results from molecular dynamics (MD) simulations, suggest that phonons scatter elastically at the metal-SAM interfaces. Furthermore, we resolve a known discrepancy between measurements and MD predictions of SAM thermal conductance by using a contact mechanics model to predict 54 ± 15% areal contact in the Au-alkanedithiol-Au experimental junction. This incomplete contact obscures the actual junction thermal conductance of 115 ± 22 MW/m(2) K, which is comparable to that of metal-dielectric interfaces.

Thermal Conductance across Phosphonic Acid Molecules and Interfaces: Ballistic versus Diffusive Vibrational Transport in Molecular Monolayers

“…This is consistent with Majumdar's findings, providing further evidence that conductance through the non-F21PA phosphonic acid molecules is ballistic. 76 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 13 important to note that PFBPA terminates in fluorine, similar to F21PA, and as such we believe that the difference in trends between these molecules is not due to interfacial bonding effects.…”

“…al. 76 For the heaviest molecule, F21PA, we observe a large reduction in thermal 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 conductance across the metal/F21PA/sapphire interface, which we attribute to diffusive scattering in the F21PA molecule and not to interfacial bonding effects. The effective thermal conductivity of this single F21PA molecule is remarkably similar to the thermal conductivity of various polymers, suggesting that the diffusive scattering mechanisms that govern heat transport in disordered polymers may remain the same down to the single molecule level.…”

A Theoretical Review on Interfacial Thermal Transport at the Nanoscale

“…One of the main characteristics of SAMs is to bridge the two different PDOSs of the materials on each side. The ITC was observed to increase as the mismatch between the vibrational spectra of the materials decreases, and the ITC obtained from FDTR (frequency domain thermoreflectance) experiments can be up to 65 ± 7 MW m −2 K −1 for matched Au–alkane dithiol–Au junctions, while the ITC of mismatched Au–alkane dithiol–Pd junctions is only 36 ± 3 MW m −2 K −1 . SAMs are more widely used to tune the ITC of interfaces with highly mismatched vibrational properties .…”

“…a) Schematic diagram of the transfer printing process to create a SAM junction at the Au–Qz interface. Reproduced with permission . Copyright 2015, American Chemical Society.…”

“…To match different materials to form an ideal interface, different SAMs with various end‐group functionalities have been applied to tune the bonding strength and vibrational coupling. The SAM end‐groups play a dominant role in enhancing the ITC; furthermore, the additional factor of the SAM chain length has also been investigated in many papers. Sun et al observed that the overall ITC increased with the SAM chain length when the number of carbon atoms was no more than 12.…”