Session: 17-15-01: Society-Wide Micro/Nano Poster Forum

Paper Number: 100227

100227 - Comprehensive Investigation of Phonon Scattering and Phonon Coherence in Nanomesh Structures 

To date, superlattices (a superstructure formed by stacking up two or more different materials layer by layer) and nanomeshes (thin films containing densely arranged nanoholes) are the two most studied nanostructures that have demonstrated significant phonon coherence. Notably, several types of coherent phonon transport behaviors have been successfully demonstrated in superlattices experimentally and theoretically. For instance, the lattice thermal conductivity of various types of superlattices was shown to increase when the interface density increases, i.e., the period thickness decreases. This is in contrast to the incoherent (or, particle-like) phonon transport picture, which dictates that the lattice thermal conductivity should decrease with increasing interface density due to increased phonon-interface scattering.  In addition, GaAs/AlAs superlattices displayed an almost linear increase with the number of periods in Luckyanova et al.'s experimental work. This is another significant manifestation of coherent phonon transport, which was also observed in atomistic modeling. Moreover, significant coherent phonon localization was also observed in experiments and atomistic simulations, further proving the significance of coherent phonons in superlattice systems.   

Unlike the success in superlattices, the experimental observation of phonon coherence in nanomesh structures was much rarer. Only a few experiments displayed coherent phonon behavior at very low temperatures. Therefore, more investigations are needed to understand why it is much harder to observe coherent phonon behaviors in nanomesh structures. In this work, we report our comprehensive investigations of phonon thermal transport in graphene nanomeshes (as a representative two-dimensional material) and silicon nanomeshes (as a representative three-dimensional material) with different hole sizes, hole shapes, surface or edge roughness of holes, and periodicity/aperiodicity of holes. A few immediate conclusions can be drawn from our nonequilibrium molecular dynamics simulations of those structures. First, a two-dimensional material with one-dimensional hole edges can preserve phonon coherence better than three-dimensional material with two-dimensional hole surfaces. Second, edge/surface roughness renders it harder to preserve phonon coherence in nanomeshes. Third, aperiodicity not only affects the transport of coherent phonons but also blocks incoherent phonons.  To gain a better understanding of the mechanism of reduced thermal conductivity of nanomeshes with aperiodically arrange holes, we have performed particle-based phonon Monte Carlo simulations of nanomeshes of the same dimensions, in addition to continuum-level, finite element simulations of macroscopic meshed structures containing macro-sized holes. It was found that incoherent phonon backscattering and coherent phonon localization are the two primary reasons for the reduced thermal conductivity of aperiodic nanomeshes. More investigations of the detailed spectral phonon properties are ongoing, aiming at revealing the nature of coherent phonon and incoherent phonon transport in nanomesh structures. 

Presenting Author: Haoran Cui University of Nevada, Reno

Presenting Author Biography: Haoran Cui is a Ph.D. student at the University of Nevada, Reno.

Authors:

Haoran Cui University of Nevada, Reno
Tengfei Ma University of Nevada, Reno
Yan Wang University of Nevada, Reno