2:30 PM - DS03.04.04
Novel Thermal Behaviors from Nanostructured Heat Sources—Experiments and Theory on Directional Channeling
Joshua Knobloch1,Brendan McBennett1,Hossein Honarvar1,Albert Beardo Ricol2,Jorge Hernández-Charpak1,Travis Frazer1,BegoÃ±a Abad Mayor1,Lluc Sendra Molins2,Javier Bafaluy2,Weilun Chao3,Mahmoud Hussein4,Juan Camacho2,F. Xavier Alvarez2,Henry Kapteyn1,Margaret Murnane1
STROBE, JILA, University of Colorado Boulder1,Universitat Autònoma de Barcelona2,Lawrence Berkeley National Laboratory3,University of Colorado Boulder4
Material systems with nanometer feature sizes exhibit exotic properties and behaviors beyond those observed for bulk materials. Moreover, at nanometer length scales, conventional macroscopic models of materials often fail to accurately describe their physical behavior while traditional measurement techniques struggle to precisely access their functional properties. In particular, thermal transport in nanoscale geometries can differ significantly from bulk models. As the length scales approach the dominant mean free paths of the heat carriers, the phenomenological model of heat diffusion no longer accurately captures the observed behavior, even for nanoscale heat sources on a bulk substrate [1,2]. In such situations, thermal transport cannot be accurately predicted for nanodevices: fundamental models are often too computationally intensive for multi-scale complex geometries while phenomenological models require too many fitting parameters or calibrations to be predictive. Observing and modeling nanoscale heat flow in complex systems is critical to not only improve existing technologies—such as thermal management in nanoelectronics and improved thermoelectrics—but also for new thermal diodes and communication devices.
In this work, we advance both experimental characterization and theoretical predictions for heat flow away from nanoscale heat sources. By utilizing tabletop sources of ultrafast, coherent extreme ultraviolet (EUV) pulses produced via high harmonic generation , we uncover non-diffusive thermal transport behavior away from nanostructured heat sources of varying geometries on a variety of materials. In previous work, we revealed a novel and counter-intuitive thermal transport behavior—that closely-packed nanoscale hot spots cool faster than when widely spaced [2,4]. Here, we map the transient, non-diffusive heat flow away from both 1D- and 2D-confined nanoscale heat sources on bulk silicon experimentally demonstrating, for the first time, that closely-spaced 2D-confined heat sources cool faster than widely spaced ones, and that this effect is larger in the 2D- than the 1D-confined geometry . Additionally, we measure the heat dissipation efficiency from similar heat source geometries on a diamond substrate, where longer mean free paths and higher amounts of normal scattering are present.
To understand and predict these novel behaviors, we apply advanced mesoscopic and microscopic theoretical models to capture the interplay between heat source geometry and heat dissipation efficiency. From a mesoscopic viewpoint, we demonstrate that the kinetic-collective model, based on a hydrodynamic-like transport equation with ab initio calculated parameters, predicts the full thermomechanical response of both 1D- and 2D-confined nanostructured transducers on silicon without the need for geometrical fitting parameters . This hydrodynamic identifies two relaxation time-scales and their associated fundamental mechanisms: specifically, a fast time-scale due to the thermal boundary resistance and a longer time scale arising from hydrodynamic-like effects. From a microscopic viewpoint, we utilize advanced atomic-level simulations to capture the fundamental mechanism responsible for the counter-intuitive behavior of enhanced cooling efficiency in closely-spaced heat sources observed by previous experiments [3-5]. Specifically, we show that for heat source periodicities below the phonon mean free path of the substrate, an enhancement of in-plane phonon-phonon scattering leads to increased cross-plane conduction, or directional channeling of thermal transport, a novel phenomenon.
 Nat. Mater. 9, 26 (2010).  Proc. Natl. Acad. Sci. 112, 4846 (2015).  Science 280, 1412 (1998).  Phys. Rev. Appl. 11, 024042 (2019).  ACS Nano 15, 13019 (2021).  Proc. Natl. Acad. Sci. 118, e2109056118 (2021).