Raman Spectroscopy of Single-Wall

Single-wall boron nitride nanotubes samples synthesized by laser vaporization of a hexagonal BN target under a nitrogen atmosphere are studied by UV and visible Raman spectroscopy. We show that resonant conditions are necessary for investigating phonon modes of BNNTs. Raman excitation in the UV (229 nm) provides preresonant conditions, allowing the identification of the A1 tangential mode at 1370 cm-1. This is 5 cm-1 higher than the E2g mode in bulk h-BN. Ab initio calculations show that the lower frequency of bulk h-BN with respect to large diameter nanotubes and the single sheet of h-BN is related to a softening of the sp2 bonds in the bulk due to interlayer interaction. Boron nitride nanotubes (BNNTs) have complementary properties to those of carbon nanotubes (CNTs). They can be considered as a single sheet of hexagonal BN (h-BN) rolled upon itself. BNNTs possess a band-gap larger than 5.5 eVsindependent of their diameter and chirality.1 This makes BNNTs promising materials for applications in nanoelectronics2 and optoelectronics (blue-light emitters, excitonic lasers, etc.). The experimental study of their properties has been limited for a long time by the lack o