Atomistic investigation of phonon transport through nanoparticles

Phonon conduction across solid-solid interfaces has been commonly estimated via the acoustic mismatch model (AMM) and the diffuse mismatch model (DMM) in several forms. These theories are mainly applied to planar interfaces and are well understood, however, an accurate description of phonon scattering by non-planar interfaces has yet to be found. This subject is of particular interest for improvement of embedded nanoparticle composites that have demonstrated a great potential as low-cost but high figure-of-merit thermoelectric materials. Specifically, a nanoparticle can be viewed as a combination of two non-planar interfaces that, in addition to reflection and transmission, can induce phonon wave trapping, localization, and lagged dissipation effects. In this work, we apply atomistic phonon wave-packet simulations to uncover these effects with wavevector and mode dependence. We found two unreported phenomena: (1) phonon lensing and (2) phonon localization. Regarding (1), we found that spherical interfaces focus a uniform phonon wavefront similar to optical lenses focusing light. However, the spherical aberration of the phonon wave is much more diffuse, suggesting significant interference effects. Furthermore, localization of phonon energy (2) inside the NP is observed and is strongest when (1) is most prominent, suggesting a unique geometric interference phenomenon that affects both phonon wave dynamics and energy propagation.