Nanoscale Thermal Transport at Solid-liquid Interfaces By

This thesis focuses on the experimental study of nanoscale thermal transport across solid-liquid interfaces in both nanoparticle system and planar thin film system. Thermal conductance of solid-liquid interfaces, G, will be measured using time-domain thermo-reflectance and pump-probe transient absorption. Interface thermal conductance, G, relates the temperature drop T ∆ at an interface to the flux of heat F that crosses the interface, T G F ∆ =. In nanoparticle systems, using pump-probe transient absorption measurement, we find that nanoparticles, ranging in size from 3-24 nm with widely varying hydrophilic surface chemistry, give thermal conductances G ~ 100-300 MW m-2 K-1 for the particle-water interfaces, approximately an order of magnitude larger than the conductance of the interfaces between alkanethiol-terminated AuPd nanoparticles and toluene. The relatively large thermal conductances between particle-water interfaces indicate that the thermal coupling between hydrophilic nanoparticles and water is strong regardless of the self-assembled stabilizing group. In planar systems, using time-domain thermoreflectance, we find that the thermal conductance between water and planar hydrophilic surfaces ranges between 100 and 180 MW m-2 K-1 , which is in good agreement with the nanoparticles systems. While in hydrophobic-water interfaces, interface thermal conductance is smaller, ranging between 45 and 65 MW m-2 K-1 , indicating that the thermal coupling between hydrophobic surfaces and water is weaker than with hydrophilic surfaces. The Kapitza length-the thermal conductivity of water divided by the thermal conductance per unit area of the interface – at hydrophobic interfaces (10-12 nm) is a factor of 2-3 larger than the Kapitza length at hydrophilic interfaces (3-6 nm). We also utilized the pump-probe transient absorption measurement to probe thermal transport in Au-core polymer-shell nanoparticles. The addition of an organic co-solvent to the iv suspension causes the polystyrene component of the polymer shell to swell and this change in the microstructure of the shell increases the effective thermal conductivity of the shell by a factor of approximately 2. The corresponding timescale for the cooling of the nanoparticle decreases from 200 ps to approximately 100 ps. The theoretical calculations of heat transport in nanoscale systems may not be reliable if those calculations are based on the thermal properties of bulk phases. Furthermore, interface thermal conductance, which usually can be negligible in large system, can have a dramatic effect in heat dissipation at the nanoscale. v For my family, my wife vi ACKNOWLEDGEMENTS