Thermoreflectance Measurements of Non-Diffusive Transport In Bulk And Nanoscale Materials

Following the trend in miniaturization of devices to the sub-micron scale, thermal management has become increasingly important to device operation. Thermal management becomes even more concerning in nanoscale electronic devices, where there are several interfaces for heat to pass across before reaching a bulk-like heat sink. To mitigate these issues, existing devices are constantly being re-engineered and new materials are being sought to take advantage of their improved performance. Importantly, at small scales or during fast transients, heat transport deviates from the classic diffusive regime and its physics is still being understood. To examine the thermal performance of these structures and materials at different length scales and in different heating dynamics, suitable metrology techniques are needed. In this dissertation, heat transport within bulk and nanoscale materials as well as across interfaces is studied. To achieve this goal, a frequency domain thermoreflectance (FDTR) setup is established and three different extensions to this setup are presented that improve the metrology in nanoscale materials or non-diffusive transport. Having different variations of the FDTR setup enables us to selectively utilize these techniques depending on the sample structure and the questions one is investigating. By measuring different structures, we show that our setups are capable of examining thermophysical properties of different materials ranging from two dimensional to three dimensional materials, from dielectrics to metals, and from thermally isotropic to anisotropic. We, then, turn our attention to non-diffusive heat transport in two different structures: metallic Platinum-Cobalt multilayers, and Tungsten disulfide (WS2) crystal. In the case of metallic Platinum-Cobalt multilayers, we show that as the interface density increases and the layer thickness becomes comparable with the electron mean free path in these metals, a deviation from the diffusive theories governing heat transport in electron-mediated multilayers is observed for the first time. Finally, we show that strong non-diffusive heat transport in WS2 can be observed at room temperature as a function of heat spot size. This not only highlights unique transport features in this material, but also points to the susceptibility to misinterpret experimental data if non-diffusive transport is not considered.