Vibrational mismatch of metal leads controls thermal conductance of self-assembled monolayer junctions

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/m² K for matched Au-alkanedithiol-Au junctions, while the mismatched Au-alkanedithiol-Pd junctions yield a thermal conductance of 36 ± 3 MW/m² 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/m² K, which is comparable to that of metal-dielectric interfaces.