Thermal Transport in Organic-Inorganic Heterojunctions: Experimental Measurements and Computational Predictions
The objective of this work is to investigate thermal transport physics in organic-inorganic heterojunctions employing experimental and computational techniques. A self-assembled monolayer (SAM) junction, a two-dimensional array of ordered molecules sandwiched between two metal leads, is used as the primary investigative system. The SAM structure provides a convenient platform to isolate the junction transport properties that are otherwise difficult to characterize when using three-dimensional organic-inorganic hybrid materials with embedded interfaces. A combination of deposition and lift-off techniques are used to fabricate the SAM junctions and frequency domain thermoreflectance (FDTR) is employed to measure the junction thermal conductance. Molecular dynamics (MD) simulations are used to further probe the vibrational properties and intermolecular cooperative behavior of the junction components and identify their effects on the thermal transport trends. A statistical rough surface contact model is also developed to estimate the percentage contact area between two rough surfaces interacting through covalent bonds. The quality of contact at interfaces is crucial for the accurate interpretation of experimentally measured transport properties of such molecular junctions. I present the first-ever measurements of the thermal conductance of SAM junctions formed between metal leads with systematically mismatched phonon spectra. The experimental observation that junction thermal conductance (per molecule) decreases as the mismatch between the lead vibrational spectra increases, paired with results from MD simulations, suggest that phonons scatter elastically at the metal-SAM interfaces. Furthermore, a known discrepancy between measurements and MD predictions of SAM thermal conductance is resolved by using the contact mechanics model to predict the extent of areal contact in the metal-SAM-metal experimental junctions. Further investigations of the nature of scattering at the metal-SAM interfaces using MD simulations reveal limitations in this computational scheme to study mismatched SAM junctions. These are related to the classical nature of the MD simulations that allow vibrational coupling to occur at the metal-SAM-metal junction that is not present in the experimental system. I present methods to circumvent this limitation and validate the predicted thermal transport trends with analytical models. The statistical contact model is derived to predict areal contact between two rough surfaces interacting through covalent bonds and is validated using thermal conductance measurements of SAM junctions. The model is also capable of handling the contact between a two-dimensional surface and substrate, which is relevant for studying the contact of supported two-dimensional materials. I also present a methodology to extend this model to handle layered substrates (i.e. a substrate with layers of distinct atomic species), which are present in nanostructured devices. The effect of the cooperative behavior between molecules on thermal transport across molecular junctions is investigated using a binary SAM system. This system enables one to tailor the local molecular environment within the junction. A non-linear change in thermal conductance as a function of molecular composition of the SAM is observed indicating that the molecules are not independent heat channels. The thermal transport through the molecules that are weakly coupled to the leads is enhanced when strongly coupled molecules are placed in their vicinity. The per molecule thermal conductance increases with increasing average separation between the molecules, suggesting the challenge in predicting single molecule properties from parallel structures and vice-versa. The findings in this thesis can potentially be used to tailor transport properties of hybrid materials for devices such as thermoelectrics, light-emitting diodes, photovoltaics and electronic devices. Thermal management in such devices, which is crucial especially when they are extremely thin, can also be improved using design principles that enhance thermal transport across the organic-inorganic heterojunctions.