Zero m phonons in MoS <inf>2</inf> nanotubes

Abstract

Phonon dispersions of single-wall MoS 2 nanotubes are calculated within a full symmetry implemented valence force-field model. For transversal, twisting, and breathing modes symmetry assignation, chirality, and diameter dependence of their frequencies and displacements are discussed. Tubular structure is found to be characterized by two Raman active modes: by the in-phase breathing mode sin full analogy to carbon nanotubesd with frequency approaching Brillouin scattering domain sas diameter approaches nmd, and by the high-energy breathing mode with sulfur shells breathing in phase, but out of phase relative to the molybdenum atoms. Likewise, the longitudinal rigid-shell mode, where sulfur shells vibrate out of phase whilst molybdenum atoms barely move, is predicted to be a fingerprint of the cylindrical configuration in infrared spectra. It is also found that twisting rigid-layer modes characterize chirality of the tubes. Finally, the large diameter limit is discussed and related to the measured Raman and infrared spectra of the layered structure. ©2005 The American Physical Society.