The phonon modes of 1.6-nm-thick Si square nanowires with  crystalline orientation are shown. The  and  facets are passivated by hydrogen and there are Si-Si dimers on the nanowire surface. The phonon dispersion and eigenvectors were calculated from a valence force field model of the Brenner type. There are 678 different phonon modes in this system of 226 atoms. The lowest ten phonon modes are considered to be purely acoustic ones. The higher-lying phonon modes represent mixed states of acoustic and optical phonons. The highest energy modes correspond to purely optical phonons.
 I. Bejenari, P. Kratzer, Phys. Rev. B 90, 045429 (2014).
 S. Mizuno and N. Nishiguchi, J. Phys.: Condens. Matter 21, 195303 (2009).
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