Thermal Transport In Nanostructured Materials By Ultrafast Pump-Probe Techniques

Abstract

Thermal transport in nanostructured materials is crucial to nanotechnology-initiated applications such as electronics, solid-state energy conversion, and biomedical applications. At reduced size, the thermal properties of nanostructured materials can differ greatly from their bulk counterparts due to events such as the scattering of heat carriers. New experimental techniques, which can detect the nanoscale thermal properties, are needed to promote further study in this field. Pump-probe optical techniques, which utilize ultrafast laser pulses with a very short duration time and high-power objective lenses to achieve high temporal and spatial resolutions, make it feasible. Our studies were motivated to advance the understandings of thermal transport in nanostructured materials as functions of various structural parameters, utilizing pump-probe optical techniques and numerical/theoretical methods. In this dissertation, I have presented three research projects of thermal transport in different novel nanostructured materials including ultrathin films, nanoparticles, and nanocomposite. First, we extract the glass-like thermal conductivities of single-crystalline La0.5Sr0.5CoO2.9 (LSCO) epitaxial films with “built-in ordered oxygen vacancies”, through Time-domain thermoreflectance (TDTR) and linear extrapolation. Molecular dynamics simulation (MD) and Boltzmann Transport Equation (BTE) are applied to reveal the suppression mechanisms on thermal conductivity of LSCO due to structural parameters including the oxygen vacancies and orderings, film thickness, and substitution. Second, we study thermal transport across cetyltrimethylammonium bromide (CTAB) and polyethylene glycol (PEG) surfactants on gold nanorods (GNRs) in water solution, utilizing transient absorption (TA) technique. We notice a better thermal performance in PEG compared to that in CTAB on GNRs. Through a multiscale thermal modeling with the incorporation of MD simulation, we reveal such better thermal performance in PEG is due to water penetration and strong covalent bonding between GNR and PEG, which are not present in CTAB. Finally, we report thermal conductivities of direct-contact ZnO nanocrystal (NC) networks with infill materials of ZnO and/or Al2O3 by TDTR, as functions of contact radius between adjacent NCs, doping concentration, and the infill composition. A modified effective medium approximation model is applied to validate the experiment results and reveal the influences of various parameters on the thermal conductivity of this nanocomposite sample system.