Using morphology and structure to tune solid-state thermal properties
Diffusive phonon transport in nanostructured materials has been a subject of intense interest and micro-fabricated platforms have been used to measure the thermal conductivity of nanowires. In this work, we demonstrate how the limits of heat transport can be tested in three novel material systems by extending this platform to probe material structure and provide a direct correlation to their thermal properties.
Phonons are lattice vibrations and their scattering in solids has largely been explained like collision of particles. Since the development of nanostructures, diffusive boundary scattering from large surface-to-volume ratio materials has been studied in nanowires and superlattices. To beat this diffusive scattering limit, we designed integrated silicon nanowires with rough surfaces with 30% reduction in thermal conductivity. Subsequently, we took a significant step further by making nanostructures with broadband roughness close to the dominant phonon wavelength (1-10 nm) at room temperature. The decrease in thermal conductivity of intrinsic silicon by a factor of ~30 from 140 W/m-K to 5 W/m-K in this sub-diffusive regime might be due to multiple scattering stemming from coherent phonon wave effects. Transmission Electron Microscopy (TEM) based techniques including three- dimensional tomography were then used to map out the morphology and find that we can reduce the thermal conductivity to as low as 1 W/m-K, while preserving the single-crystalline core, which is as low as that amorphous silicon or silica. Correlating the surface roughness and porosity to the measured thermal conductivity opens up a new paradigm to observing wave physics in thermal phonons at room temperature in nanomaterials.
Secondly, the platform developed previously was extended to be compatible with TEM, allowing us to characterize the crystal structure of measured nanowires. While phonon optics experiments in the 1970s showed a crystallographic direction dependent thermal conductivity, we performed the first 1-1 mapping of nanowire growth direction and thermal conductivity in Bismuth Nanowires. In the boundary scattering regime with diameter 100 nm, a nanowire in the [-102] direction had k = 8.5 W/m-K, ~6 times higher than a nanowire in the  direction with k = 1.5 W/m-K.
Finally, this thesis also studies tapered Vanadium Oxide beams to study asymmetric phonon physics that manifest in temperature dependent thermal rectification. The interplay between electrons and phonons and the possibility of asymmetric scattering rates prompted us to look closely at the existence of Metal-Insulator interfaces that could result in thermal rectification. Between 150K and 340K, the Vn O2n-1 phases could be either metallic or insulating with nanoscale domains. We performed high resolution Auger spectroscopy on single-crystal Vanadium Oxide beams that show a stoichiometry variation and measured thermal rectification as high as 22%. The rectification behavior turns off (<4%) once the whole beam reaches the insulating phase, higher than 340K.
Our platform thus couples materials characterization, especially TEM, with thermal property measurement to enhance understanding of thermal phonons.