Thermal Transport across Atomic and Molecular Junctions

This thesis reports on the development and demonstration of a novel experimental technique to investigate heat transport at the atomic and molecular scale. Before this work, there was no method able to reach these length scales in thermal transport experiments. The technique combines highly sensitive heat flux measurements with break junction methods, to simultaneously measure the thermal and electrical conductance of atomic scale contacts. This was achieved by using MEMS structures with an integrated microheater/temperature sensor as suspended substrates for performing break junction experiments in a high-vacuum scanning tunneling microscope. Thanks to the high thermal resistance of the MEMS sensors, the low noise laboratory environment and instrumentation, we obtained state-of-the-art sensitivities of 15 pW/K in a bandwidth of 50 Hz at a temperature difference of 50 K around room temperature. With this technique, we investigated the heat transport properties of atomic gold contacts. For the first time, we observed thermal conductance quantization at room temperature and demonstrated the validity of the Wiedemann-Franz law in single atom contacts. Within the uncertainties of the experiment, we could conclude that ballistic phonon transport in these systems contribute with less than 10% to the overall thermal conductance. We further confirmed these results by investigating platinum quantum point contacts and gold atomic junctions formed with Pt and Pt-Ir tips. Good agreement with the Wiedemann-Franz law was also found for gold-gold junctions in the presence of small organic molecules. These results suggest that heat transport in highly transmitting contacts is dominated by electrons and that phonons plat only a minor role. The experimental technique was further optimized and developed to characterize for the first time the thermal conductance of single organic molecules. We measured thermal transport across two model systems dithiol-oligo(phenylene ethynylene) (OPE3) and octane-dithiol (ODT), finding very good agreement with our theoretical simulations based on the Green’s Function scattering method and the predicted and measured literature values. In the case of ODT, our result fitted well with the thermal transport measurements performed on alkane chains of similar lengths, molecular dynamics and ab-initio simulations, suggesting that transport in this system is mostly dominated by the interface thermal resistance with the electrodes. These studies demonstrate that the experimental technique developed is suitable to investigate heat transport across molecules with different chemical backbones, therefore opening the door to systematic studies of the structure-property relationship. This represents a fundamental step to engineer phonon transport in nanoscale systems