AAh BN crystal, basic structure of boron nitride nanotubes

Abstract

AAh boron nitride (BN) crystal, assigned to an orthorhombic space group (No. 31, Pm21), is reported here. This new AAh BN crystal exhibits a `linear' morphology for high-resolution transmission electron microscopy (HRTEM) and a (non-hexagonal) `diagonal' electron-diffraction pattern, which have been experimentally demonstrated in this article. It is also demonstrated that this new crystal is the basic structure of multi-walled BN nanotubes (BNNTs) existing in the form of a helix. The helical AAh BNNTs exist in a metastable phase owing to 〈200〉 texture growth of the orthorhombic crystal, where the energy is ∼15 meV higher than that of stable AB or AA′ BN. It is shown that the typical scanning electron microscope `fluffy cotton-like' morphology of BNNTs is due to secondary growth of diverse BN sheets (including mono-layers) on incoherently scrolled wall strands of BNNTs, providing further evidence for the helical structure with HRTEM evidence for a left-handed helix.

Keywords

• aah bn crystals
• boron nitride nanotubes
• morphology
• transmission electron microscopy
• electron diffraction
• helical structures