An investigation of the phonon properties of silicon nanowires

The phonon dispersion relations and density of states under the size confinement effect are crucial in order either to obtain accurate solutions of the phonon Boltzmann transport equation or to obtain properly quantum-corrected temperatures for low-dimensional materials. This work draws the confined phonon properties of silicon nanowires from the equilibrium molecular dynamics simulations. The simulation results show discrete acoustic phonon modes with smaller group velocities and many additional modes in the region of large wave numbers and small frequencies, compared to the continuous bulk counterparts. The latter shifts the distribution of phonon density of states toward the lower frequency. The lattice thermal conductivities of infinitely long silicon wires of diameter 4.1 nm, 7.6 nm, and 10.6 nm are next calculated using the non-equilibrium molecular dynamics simulations, with temperatures properly quantum corrected based on the confined phonon density of states. The lattice thermal conductivities are found to be significantly smaller than the bulk value and depend only weakly on the temperature, implying that the surface scattering strongly dominates over the phonon-phonon interaction.