Ab initio vibrational and thermal properties of AlN nanowires under axial stress

The vibrational and thermal properties of small diameter AlN nanowires are investigated using first principles calculations. Upon applying external pressure, the nanowires suffer a stress induced phase transition from a würtzite (WZ) to a graphite-like (GL) phase. The thermal conductance displays a nearly identical behavior for all systems in the temperature regime governed by acoustical modes, while at higher temperatures the conductance is systematically enhanced for nanowires in the GL phase. The heat capacity points out the different phonon group velocities for the acoustical modes in the two structural configurations. Our DFT-based calculations are consistent with experimental data for bulk AlN, in terms of phonon spectrum and temperature dependent heat capacity in the large nanowire limit.