Anisotropic Thermal\nTransport in Tunable Self-Assembled\nNanocrystal Supercrystals

Realizing tunable functional materials with built-in\nnanoscale\nheat flow directionality represents a significant challenge that could\nadvance thermal management strategies. Here we use spatiotemporally\nresolved thermoreflectance to visualize lateral thermal transport\nanisotropy in self-assembled supercrystals of anisotropic Au nanocrystals.\nCorrelative electron and thermoreflectance microscopy reveal that\nnano- to mesoscale heat predominantly flows along the long-axis of\nthe anisotropic nanocrystals, and does so across grain boundaries\nand curved assemblies while voids disrupt heat flow. We finely control\nthe anisotropy via the aspect ratio of constituent nanorods, and it\nexceeds the aspect ratio for nanobipyramid supercrystals and certain\nnanorod arrangements. Finite element simulations and effective medium\nmodeling rationalize the emergent anisotropic behavior in terms of\na simple series resistance model, further providing a framework for\nestimating thermal anisotropy as a function of material and structural\nparameters. Self-assembly of colloidal nanocrystals promises an interesting\nroute to direct heat flow in a wide range of applications that utilize\nthis important class of materials.