  Tuning thermal transport in nanostructures is essential for many
applications, such as thermal management and thermoelectrics. Nanophononic
metamaterials (NPM) have shown great potential for reducing thermal
conductivity by introducing local resonant hybridization. In this work, the
thermal conductivity of NPM with crystalline Si (c-Si) pillar, crystalline Ge
(c-Ge) pillar and amorphous Si (a-Si) pillar are systematically investigated by
molecular dynamics method. The analyses of phonon dispersion and spectral
energy density show that phonon dispersions of Si membrane are flattened due to
local resonant hybridization induced by both crystalline and amorphous pillar.
In addition, a-Si pillar can cause larger reduction of thermal conductivity
compared with c-Si pillar. Specifically, when increasing the atomic mass of
atoms in pillars, the thermal conductivity of NPMs with crystalline pillar is
increased because of the weakened phonon hybridization, however, the thermal
conductivity of NPMs with amorphous pillar is almost unchanged, which indicates
that the phonon transports are mainly affected by the scatterings at the
interface between amorphous pillar and Si membrane. The results of this work
can provide meaningful insights on controlling thermal transport in NPMs by
choosing the materials and atomic mass of pillars for specific applications.
