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

Illustration of pump–probe spectroscopy of a colloidal semiconductor nanocrystal film, highlighting transient photoinduced heating effects. Heating following excitation above the band gap or at high fluences impacts transient spectra, relaxation kinetics, and carrier diffusivity measurements. The findings emphasize the importance of considering thermal contributions for accurate interpretation of spectroscopy measurements. View the article.

Thermal contributions are typically ignored in optical spectroscopy of semiconductor nanomaterials. However, such considerations are important for an accurate interpretation of spectroscopy measurements. Here, we identify signatures of transient photoinduced heating in optical pump–probe signals of colloidal semiconductor nanocrystal films. We find that lattice heating following excitation above the bandgap or at high fluences leads to a significant temperature-induced transient signal that impacts three aspects of pump–probe measurements: the transient spectra, relaxation kinetics, and spatiotemporally resolved carrier diffusivity. The effects are general across nanocrystal core material, appearing in both CdSe and PbS quantum dot films. This study proposes several methods for distinguishing simultaneous electronic and thermal contributions to transient measurements as well as guidelines for how to avoid misassignments. On the other hand, we discuss the ability to track both electronic and thermal transport as a largely missed opportunity that can be leveraged.

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.