  The thermal conductivity of a freestanding single-crystal silicon membrane
may be reduced significantly by attaching nanoscale pillars on one or both
surfaces. Atomic resonances of the nanopillars locally and intrinsically couple
with the base membrane phonon modes causing these modes to hybridize and
flatten at each coupling location in the phonon band structure. The ensuing
group velocity reductions, which in principle may be tuned to take place across
silicon's full spectrum, lead to a lowering of the in-plane thermal
conductivity in the base membrane. Using equilibrium molecular dynamics
simulations, we report a staggering two orders of magnitude reduction in the
thermal conductivity at room temperature by this mechanism.
