Realizing tunable functional materials with built-in nanoscale heat flow directionality represents a significant challenge that could advance 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 nano- to mesoscale 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 nanobipyramid 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 an interesting route to direct heat flow in a wide range of applications that utilize this important class of materials.

Ordered arrays of nanocrystals, called supercrystals, have attracted significant attention owing to the collective quantum effects arising from the coupling between neighboring nanocrystals. In particular, lead halide perovskite nanocrystals are widely used because of the combination of the optical properties and faceted cubic shape, which enables the formation of highly ordered supercrystals. The most frequently used method for the fabrication of perovskite supercrystals is based on the self-assembly of nanocrystals from solution via slow evaporation of the solvent. However, the supercrystals produced with this technique grow in random positions on the substrate. Moreover, they are mechanically soft due to the presence of organic ligands around the individual nanocrystals. Therefore, such supercrystals cannot be easily manipulated with microgrippers, which hinders their use in applications. In this work, we synthesize mechanically robust supercrystals built from cubic lead halide perovskite nanocrystals by a two-layer phase diffusion self-assembly with acetonitrile as the antisolvent. This method yields highly faceted thick supercrystals, which are robust enough to be picked up and relocated by microgrippers. We employed X-ray nanodiffraction together with high-resolution scanning electron microscopy and atomic force microscopy to reveal the structure of CsPbBr3, CsPbBr2Cl, and CsPbCl3 supercrystals assembled using the two-layer phase diffusion technique and explain their unusual mechanical robustness. Our findings are crucial for further experiments and applications in which supercrystals need to be placed in a precise location, for example, between the electrodes in an electro-optical modulator.

Spatiotemporal thermoreflectance microscopy locally probes lateral thermal transport in ultrathin colloidal nanocrystal supercrystals. In stark contrast to typical materials, the thermal diffusivity ...

Halide perovskite supercrystals, also known as superlattices, are electronically coupled low-dimensional materials, such as nanocrystals, quantum dots, or nanoplatelets that offer collective optical and electronic properties distinct from those of their constituents. The intrinsic dielectric properties and defect t

UCL Discovery is UCL's open access repository, showcasing and providing access to UCL research outputs from all UCL disciplines.

Halide perovskite supercrystals, also known as superlattices, are electronically coupled low-dimensional materials, such as nanocrystals, quantum dots, or nanoplatelets that offer collective optical and electronic properties distinct from those of their constituents. The intrinsic dielectric properties and defect t

Halide perovskite supercrystals, also known as superlattices, are electronically coupled low-dimensional materials, such as nanocrystals, quantum dots, or nanoplatelets that offer collective optical a...

Realizing tunable functional materials with built-in nanoscale heat flow directionality represents a significant challenge that could advance thermal management strategies. Here we use spatiotemporall...