The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Green's function simulations.

We present an ab initio study which identifies dominant effects leading to thermal conductivity reductions in carbon and boron-nitride nanotubes with isotope disorder. Our analysis reveals that, contrary to previous speculations, localization effects cannot be observed in the thermal conductivity measurements. Observable reduction of the thermal conductivity is mostly due to diffusive scattering. Multiple scattering induced interference effects were found to be prominent for isotope concentrations *10%; otherwise, the thermal conduction is mainly determined by independent scattering contributions of single isotopes. We give explicit predictions of the effect of isotope disorder on nanotube thermal conductivity that can be directly compared with experiments.