Journal Article

new regime of nanoscale thermal transport: Collective diffusion increases dissipation efficiency

Gu, Xiaokun, Murnane, Margaret M., Nardi, Damiano, Frazer, Travis D., Chao, Weilun, Kapteyn, Henry C., Falcone, Roger W., Yang, Ronggui, Hoogeboom-Pot, Kathleen M., Hernandez-Charpak, Jorge N., Anderson, Erik H.

2015

Overview

Significance A complete description of nanoscale thermal transport is a fundamental problem that has defied understanding for many decades. Here, we uncover a surprising new regime of nanoscale thermal transport where, counterintuitively, nanoscale heat sources cool more quickly when placed close together than when they are widely separated. This increased cooling efficiency is possible when the separation between nanoscale heat sources is comparable to the average mean free paths of the dominant heat-carrying phonons. This finding suggests new approaches for addressing the significant challenge of thermal management in nanosystems, with design implications for integrated circuits, thermoelectric devices, nanoparticle-mediated thermal therapies, and nanoenhanced photovoltaics for improving clean-energy technologies. Understanding thermal transport from nanoscale heat sources is important for a fundamental description of energy flow in materials, as well as for many technological applications including thermal management in nanoelectronics and optoelectronics, thermoelectric devices, nanoenhanced photovoltaics, and nanoparticle-mediated thermal therapies. Thermal transport at the nanoscale is fundamentally different from that at the macroscale and is determined by the distribution of carrier mean free paths and energy dispersion in a material, the length scales of the heat sources, and the distance over which heat is transported. Past work has shown that Fourier’s law for heat conduction dramatically overpredicts the rate of heat dissipation from heat sources with dimensions smaller than the mean free path of the dominant heat-carrying phonons. In this work, we uncover a new regime of nanoscale thermal transport that dominates when the separation between nanoscale heat sources is small compared with the dominant phonon mean free paths. Surprisingly, the interaction of phonons originating from neighboring heat sources enables more efficient diffusive-like heat dissipation, even from nanoscale heat sources much smaller than the dominant phonon mean free paths. This finding suggests that thermal management in nanoscale systems including integrated circuits might not be as challenging as previously projected. Finally, we demonstrate a unique capability to extract differential conductivity as a function of phonon mean free path in materials, allowing the first (to our knowledge) experimental validation of predictions from the recently developed first-principles calculations.

• Publisher: National Academy of Sciences
• Subject: Conductivity / cooling / Dispersion / electronic circuits / Heat / high harmonic generation / Integrated circuits / mean free path spectroscopy / nanoscale thermal transport / NANOSCIENCE AND NANOTECHNOLOGY / nondiffusive transport / Photovoltaics / Physical Sciences / Physics / solar energy / ultrafast X-rays