Vibrational properties of boron-nitride nanotubes: effects of finite length and bundling

Abstract

This paper was presented in part at the Symposium of Microtechnologies for the New Millennium, Nanotechnology Conference, Gran-Canaria, Spain, May 2003. ; We present ab initio calculations of phonons in single-wall boron-nitride (BN) nanotubes. Raman and infrared (IR) active modes of isolated and infinitely long tubes are evaluated according to the nonsymmorphic rod groups of BN nanotubes. For tubes of finite length, the selection rules are less restrictive and give rise to additional modes, which may be observed in Raman and IR spectroscopy with an intensity depending on the tube length. Bundling of tubes is shown to have little effect on the phonon frequencies. However, arranging tubes in a large periodic array (larger than the wavelength of incoming light) gives rise to a strong frequency shift (longitudinal-optical-transverse-optical splitting) of certain modes due to the establishing of a macroscopic electric field. Modes of A1 symmetry experience a shift for laser light along the tube axis and E1 modes are split for light incidence in the perpendicular direction. ; This work was supported in part by the European Union under Grant HPRN-CT-2000-00 128, and in part by the Spanish Ministry of Science and Technology under Grant MAT2001-0946, and in part by the University of the Basque Country under Contract 00 206.215-13 639/2001. ; Peer reviewed