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Probing the Frontiers of Nanoscale Thermal Transport with Transient Thermal Gratings and Atomistic Green’s Function Simulations
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DOI：10.1021/accountsmr.5c00352.png)
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Abstract
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ConspectusThe relentless increase in power density and advancements in nanoengineering in modern electronic and energy conversion technologies have pushed thermal management to its physical limits, where ballistic transport phenomena and the wave nature of phonons become important. Our research tackles this frontier using a dual approach: advanced experimental tools to accurately probe thermal transport across ballistic and diffusive regimes, and predictive theoretical methods to resolve wave-based phonon transport with atomic precision.On the experimental front, transient thermal grating (TTG) spectroscopy has emerged as a crucial noncontact optical technique for probing complex material systems that are inaccessible to other methods, including new classes of materials such as two-dimensional (2D) covalent organic frameworks (COFs) and hybrid perovskites. Studying in-plane thermal transport in these materials is particularly challenging due to their small dimensions or fragility. On the theoretical front, the atomistic Green’s function (AGF) is a quantum-mechanical framework that describes phonon transport as wave propagation and accounts for interactions with the atomic structure, defects, and interfaces from first-principles, providing fundamental physical insights.This Account summarizes how we leveraged TTG and AGF for scientific discoveries. For instance, we used TTG to directly measure the in-plane thermal conductivity of novel 2D COFs, revealing a high value of ∼ 1.18 W/(m·K). We also applied TTG to unveil remarkably weak anisotropy of 1.5 in thermal conductivity of 2D hybrid perovskites. Furthermore, we developed TTG to uniquely characterize micrometer-thick metallic interfaces, smaller than the laser beam size. In the theoretical domain, we developed anharmonic AGF method for three-dimensional (3D) interfaces and discovered enhanced thermal interface conductance due to anharmonicity at the interface. We also found enhanced conductance due to a bridging effect caused by atomic mixing. We applied AGF to directly capture phonon Anderson localization in aperiodic superlattices and coherent phonon transport in periodic superlattices. The collective insights gained from these distinct experimental and theoretical advances are building a framework for the rational design of materials with tailored thermal transport properties, paving the way for next-generation solutions in electronics packaging, thermoelectrics, thermal insulation, and beyond.
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