Measuring and manipulating heat transport at the nanoscale

Understanding thermal transport at the nanoscale is fundamentally important for applications inenergy conversion and nanoelectronics. In this talk, I will describe some recent work that examines the intricate relationships between material microstructure and heat transport. In the first part, I will discuss measurements and modeling of thermal transport near individual grain boundaries in a polycrystal. Using a new ultrafast microscopy technique that can produce spatial maps of heat flow, we have observed interesting phonon scattering effects in the vicinity of disorder-rich grain boundaries. In the second part, I will discuss unusual aspects of heat transport in van der Waals (vdW) layered 2D materials. Through measurements of the thickness-dependent cross-plane thermal conductivity in MoS2, we have uncovered evidence of surprisingly long c-axis phonon mean-free-paths (~100s of nm) even at room temperature. Motivated by these results, we have explored how point-defects, namely intercalated lithium ions, can be used to drastically suppress cross-plane thermal transport in MoS2. An exciting consequence of this is the ability to engineer dynamically tunable thermal conductivity in a nanoscale 2D material, i.e. a thermal transistor. Finally, I will discuss prospects for creating artificial vdW heterostructures with extreme thermal properties, assembled from atomically-thin 2D layers