Date of Award
Physics and Astronomy
Mark E. Siemens, Ph.D.
Gold nanoparticles, Thermal boundary resistance, Transient thermoreflectance, Ultrafast pump/probe method
Experiments with nanoscale structures, designed to measure some of their thermal and optical properties, are the subjects of this dissertation. We studied the transport of thermal energy in systems of nanoparticles, and used the method of transient thermoreflectance to monitor those dynamics, and assess whether thermal transport features special to nanoscale systems emerged. This same method was also used to study the thermal transport of a single system of layered membranes. Optical properties were investigated using computational simulations of a nanoparticle system, using the method of finite-difference time-domain simulation.
In nanoparticle studies, there are two features of interest special to nanoscale systems: the transition from diffusive to ballistic thermal transport, and the presence of thermal resistance at interfaces. Our platform for studying these features was a system of gold nanoparticles, having citrate capping layers, and embedded in a polymer matrix. Our transient thermoreflectance method uses an ultrafast, infrared, pulsed laser to impulsively heat the particles, with an initial laser pulse, then measures the power of a second pulse, which reflects from the particles at a controlled delay time following the heating. The reflected power depends on the steadily decreasing temperature of the particles, and collecting this data over a range of delay times provides a picture of the nanoparticles' cooling dynamics. We have developed the first multilayer, spherical model of this diffusive cooling process, explicitly including interfacial boundary resistance. By adjusting it to our measurement results, we determine the amount of boundary resistance, the capping layer thickness, and the thermal conductivity of the matrix. Though we do not observe ballistic transport in this system, we have measured both the first value of the gold/polymer interfacial thermal resistance, and the capping layer thickness, and found both of them to significantly affect the transport of thermal energy.
Thermal boundary resistance was also a property of interest in our membrane systems, which consisted of a suspended bridge structure of a molybdenum (Mo) film deposited on a silicon nitride (SiN) substrate. This thermal isolation structure, designed and fabricated by our collaborators, enabled us to test whether the substrate material retains its bulk value of thermal conductivity and heat capacity at the nanoscale, which must otherwise be assumed in larger-scale experiments. Using transient thermoreflectance to monitor the thermal dynamics following impulsive heating of the upper surface of the metal film, we have measured the thermal boundary resistance present at the Mo/SiN interface, and found that, within experimental uncertainties, the bulk SiN conductivity and heat capacity values are retained.
In a separate study, we modeled the optical absorption properties of gold nanoparticles, in the visible range, using the method of finite-difference time-domain simulation. This method calculates the change induced in an incident pulse of visible light as it propagates past a particle, placed in a water matrix, and finds the fraction of the pulse's electromagnetic energy absorbed by the particle. The energy absorbed is determined by the dielectric properties of gold -- one picture of which is the Lorentz-Drude model, which derives dielectric properties from electron scattering behavior and resonances. Fitting this model to literature dielectric data, we predict an absorption spectrum which agrees with experimental values within several nanometers.
Green, Brian G., "Nanoscale Thermal Transport in Thermally Isolated Nanostructures" (2019). Electronic Theses and Dissertations. 1579.
Received from ProQuest
Brian G. Green
Nanoscience, Condensed matter physics