Amun passed his doctoral dissertation defense titled: “Thermal Transport across Nanoscale Vacuum Gaps and the Transition to Phonon Heat Conduction”
The investigation of thermal transport across extremely small vacuum gap distances is of both practical and fundamental significance. Previous theoretical studies have predicted that in the near-field, or when the emitter-receiver separation is less than the thermal wavelength, thermal radiation can exceed Planck’s blackbody limit by up to several orders of magnitude due to radiation tunneling of evanescent electromagnetic waves. This enhancement has been verified by several recent experimental efforts providing exciting opportunities for the development of thermophotovoltaic energy conversion, passive radiative cooling, and nanoscale thermal management. While experimental findings for gap distances above 10 nm have observed good agreement with near-field thermal radiation theory within the fluctuational electrodynamics (FE) framework, the underlying physics of thermal transport in the sub-10-nm (i.e., extreme near-field) gap regime is still in significant debate. The aforementioned knowledge gap poses a fundamental question: How does thermal radiation transition to phonon heat conduction at contact? To investigate this question, the finite dipole model is combined with FE to elucidate the magnitude of extreme near-field thermal radiation. Then, an experimental setup based on a custom-built high-vacuum scanning probe microscope and platinum nanoheaters is developed to enable heat transfer measurements across sub-10-nm vacuum gaps. By implementing this experimental setup, thermal transport between a silicon tip and platinum nanoheater separated by single-digit nanometer vacuum gaps is found to be more than three orders of magnitude larger than FE predictions. The atomistic Green’s function for phonon transport illustrates that acoustic phonon tunneling links the measured gap and contact thermal transport mechanisms. The results of this dissertation suggest a direct correlation between the tip-substrate force and acoustic phonon transmission, such that interfacial heat transfer may be engineered using external force stimuli.