Vibrational and thermal properties of small diameter silicon nanowires

We present the results of vibrational and thermal properties for small diameter silicon nanowires (Si-NWs) from first principles calculations. Phonon spectrums of the Si-NWs are obtained based on the density functional perturbation theory. We found that heat-carrying acoustic branches exhibit "bending," which results from the strong interaction between acoustic and no-zero-frequency flexural modes. The bending of acoustic branches implies that the phonon group velocity (V =dω/dq) of Si-NWs is less than that of corresponding bulk silicon. Therefore, a lower lattice thermal conductivity of Si-NWs can be caused by the bending of acoustic phonon. In comparison with bulk silicon, optical branches of Si-NWs exhibit "blueshift," which is due to the high frequency vibration of silicon atoms at the edge of Si-NWs. From the obtained phonon spectrums, specific heat is calculated. The specific heat of Si-NWs is also lower than that of bulk silicon crystal. The reduction in the specific heat is due to the small magnitude of vibration density of states of low frequency phonons. In the temperature range from 100 to 1000 K, the Debye temperatures are obtained. We found that the Debye temperature of the Si-NWs is much higher than that in the corresponding bulk silicon. Especially, Debye temperature of tetrahedral Si-NW is nearly twice higher than that of bulk silicon. From the temperature dependence of Hamholtz free energy of Si-NWs, we find that the cagelike Si-NWs have higher thermal stability than the tetrahedral Si-NW.