Novel Metrologies of Thermophysical Properties on the Nanoscale

The measurement of thermal properties on the micro- and nano-scale is essential for understanding fundamental thermophysical phenomena such as heat flow and phase change. Existing measurement techniques (such as Time Domain (TDTR), Frequnency Domain (FDTR), or Steady State (SSTR) Thermoreflectance) or simulation techniques (such as lattice dynamics (LD) or molecular dynamics (MD)) have offered key insights into some of the fundamental principles of heat transfer over the last few decades. Several properties and length-scales have persisted which cannot be simulated or measured however. Similarly, there is little consensus within the nanoscale thermal measurement community as to the correct means by which measurement error and experimental uncertainty should be characterized.

Over the course of my studies at The University of Virginia, I have done work on several topics in this area. These topics can be broken down into three primary thrusts. To begin, I present several advancements in the analysis of existing measurement techniques (``Development of new analysis methodologies for existing measurement techniques''). As a prerequisite to this, I identify and outline the sources of measurement error and uncertainty associated with the existing measurement techniques, as these serve to limit whether a given thermal property can reasonably be measured. I next expand TDTR for use as a depth-dependent thermal conductivity measurement, and present a hybrid fitting analysis method for simultaneous analysis of TDTR and SSTR data. In both cases, additional thermal properties can be extracted which could not have otherwise been measured. Next, I will discuss computational advancements (``Advancements to computational techniques''), starting with Spectral Heat Flux (SHF) and disorder analysis. These tools helped to elucidate the underlying mechanisms of thermal boundary resistance (TBR), highlighting the effects of scattering within the two materials on either side of an interface. I have also developed a method of exciting specific targeted phonon modes in an effort to explore the effects on TBR, based on the hypothesis that overexciting modes could increase scattering and lead to a reduction in TBR. Finally, I present the development of two new measurement techniques (``Development of new experimental metrologies''). The first is an optical pump-probe experiment for defect detection, taking advantage of the exceptional signal-to-noise available using lock-in detection and the ability of defects to strongly affect the optical and electronic properties of materials. The second is a pump-probe micro-scale calorimetry technique, wherein the pump beam melts a portion of the sample, and the temporal signal is analyzed to determine the latent heat.