TDTR-Based Thermal Conductivity Mapping – Challenges and Applications

Over the past two decades, time-domain thermoreflectance (TDTR) has been developed into a versatile tool to measure the thermal conductivity of thin films and substrates. Variants of TDTR have been developed, to address the specific needs. For example, beam-offset TDTR has been developed to measure the in-plane thermal conductivity of the thin films. Dual-frequency TDTR has been developed to measure the thermal resistance of buried thin films and interfaces. These various TDTR variants are paramount to advance the knowledge of heat transport in different types of nanostructures and across interfaces. One variant of TDTR that is less explored is to use TDTR for thermal conductivity mapping. In this talk, we will present our recent work to develop and apply TDTR for thermal conductivity mapping. We will share the challenges we faced, including how to ensure smooth surfaces for mapping and how to interpret the results. We will also share our recent works to apply TDTR the map the thermal conductivity of twisted bi-layer graphene and electrodes in the Li-ion batteries. For the first example, we map the thermal conductivity CVD-grown twisted bilayer graphene on single-layer graphene substrates. With the mapping, we are able to accurately measure the thermal conductivity difference between the bilayer and single layer graphene, as a function of the twisted angle between the graphene flakes. In the second example, we map the thermal conductivity of graphite and LCO electrodes of Li-ion batteries. We discuss how we interpret the mapping results, including our measurements near the boundaries of the electrode powders. Our works suggest that TDTR-based thermal conductivity mapping could be a powerful tool to study heat transport in complex composites.