One-dimensional (1D) materials such as various kinds of nanowires and nanotubes have attracted a lot of attention recently because of their potential applications in nanoelectronic and energy conversion devices [1–3]. As the size of low dimensional materials is confined to be comparable to the phonon mean free path, the thermal conductivity can be reduced due to boundary scattering. A theoretical study [4] further suggests that as the diameter of a Si nanowire becomes smaller than 20 nm, the phonon dispersion relation can be modified due to the boundary confinement and the phonon group velocities will be less than that of the bulk, which will also tend to reduce the thermal conductivity. A molecular dynamics (MD) simulation [5] has shown that for nanowires of cross sections ranging from 2.58 to 28.62 nm2, the thermal conductivity could be two orders of magnitude smaller than those of bulk Si crystals in a temperature range from 200 K to 500 K. In their paper, the authors also solved the Boltzmann transport equation (BTE) and showed that the MD results fit reasonably well with the BTE solutions. It is very important to experimentally verify the theoretical predictions and provide experimental data for the device design in nanoelectronic and nanoscale energy conversion devices. However, no systematic experimental studies of the size effect on the nanowire thermal conductivity have been published.

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