  Realizing tunable functional materials with built-in nanoscale heat flow
directionality represents a significant challenge with the potential to enable
novel thermal management strategies. Here we use spatiotemporally-resolved
thermoreflectance to visualize lateral thermal transport anisotropy in
self-assembled supercrystals of anisotropic Au nanocrystals. Correlative
electron and thermoreflectance microscopy reveal that heat predominantly flows
along the long-axis of the anisotropic nanocrystals, and does so across grain
boundaries and curved assemblies while voids disrupt heat flow. We finely
control the anisotropy via the aspect ratio of constituent nanorods, and it
exceeds the aspect ratio for nano-bipyramid supercrystals and certain nanorod
arrangements. Finite element simulations and effective medium modeling
rationalize the emergent anisotropic behavior in terms of a simple series
resistance model, further providing a framework for estimating thermal
anisotropy as a function of material and structural parameters. Self-assembly
of colloidal nanocrystals promises a novel route to direct heat flow in a wide
range of applications that utilize this important class of materials.
