Towards ultimate impedance of phonon transport by nanostructure interface

Interface-induced reduction of thermal conductivity has attracted great interest from both engineering and science points of view. While nanostructures can enhance phonon scattering, the multiscale nature of phonon transport (length scales ranging from 1 nm to 10 µm) inhibits precise tuning of thermal conductivity. Here, we introduce recent advances toward ultimate impedance of phonon transport with nanostructures and their interfaces. We start by reviewing the progress in realizing extremely low thermal conductivity by ultimate use of boundary scattering. There, phonon relaxation times of polycrystalline structures with single-nanometer grains reach the minimum scenario. We then highlight the newly developed approaches to gain further designability of interface nanostructures by combining informatics and materials science. The optimization technique has revealed that aperiodic nanostructures can effectively reduce thermal conductivity and consequently improve thermoelectric performance. Finally, in the course of discussing future perspective toward ultimate low thermal conductivity, we introduce recent attempts to realize phonon strain-engineering using soft interfaces. Induced-strain in carbon nanomaterials can lead to zone-folding of coherent phonons that can significantly alter thermal transport.