The dimensionality effect on phonon localization in graphene/hexagonal boron nitride superlattices

Phonon localization, a largely elusive phenomenon, has a great promise for improving current applications like thermoelectric materials and thermal barrier coatings. By freezing the out-of-plane atomic motion in graphene/hexagonal boron nitride (hBN) superlattices and random multilayers, i.e aperiodic superlattices, we are able to completely isolate the effect of the third dimension on phonon scattering and localization in 2D materials. In particular, we find much more prominent phonon heat conduction and localization when atomic motions in the third dimension are frozen. Rigorous spectral phonon transmission and scattering phase space analyses reveal that the phase-breaking anharmonic scatterings can significantly hinder the occurrence of phonon localization. Phonon participation ratio calculations further reveal that the flexural modes-arising from the degree of freedom of the third dimension-are rather extended, in contrast to the vastly localized in-plane modes in graphene/hBN random multilayers. These two factors altogether greatly obstruct the observation of coherent phonon localization in realistic materials. This work will be useful for guiding the search for nanostructures possessing significant phonon localization behaviors.