Double-helix structure in multiwall boron nitride nanotubes

Abstract

A new nanotube structural form is reported that resembles a double helix in multiwall boron nitride nanotubes (MW-BNNT) grown by a carbon-free chemical-vapor-deposition process as documented by evidence obtained by transmission electron diffraction and microscopy. The double-helix structure is found in MW-BNNTs exhibiting the same chirality in its different walls. The MW-BNNTs deviate from the structure of ideal nested coaxial cylindrical tubes. Most significantly, bright- and dark-field electron imaging reveals regular zigzag dark and bright spots on the side walls of the nanotubes. The repeating distance between the bright, or dark, spots is related to the chiral angle of the nanotube. Electron diffraction patterns recorded from individual nanotubes show additional diffraction spots belonging to the 201 zone axes, which are not allowed in a perfectly cylindrical nanotube. These additional diffraction spots become asymmetrical as smaller sections of the nanotube are probed. A series of diffraction patterns recorded along the tube axis showed that the imperfections giving rise to these spots move in a regular fashion around the circumference of the tube. It is shown that all experimental evidence supports the structure model of two helices; one is polygonal in cross section and highly crystalline and the other is circular and less ordered. It is further suggested that the double-helix structure is a result of stronger wall-wall interactions associated with the ionic bonding in boron nitride.

Tensile Tests on Individual Multi‐Walled Boron Nitride Nanotubes

“…The similar defect‐induced strength reduction should be expected to take place in BNNTs. As mentioned above, the present BNNT samples have frequent cross‐section variations along both length and radius directions 21, 22. At the boundaries between faceted polygons and cylinders along the tube axes some pentagonal and heptagonal defects are supposed to exist to counterbalance the perimeter difference.…”

“…The continuous shell breakage of the inner tubes during pullout is resulted from the structural peculiarities of the present multi‐walled BNNT samples. These have been demonstrated to frequently have cross‐section variations between a faceted polygon and a cylinder along both tube axis and tube radius directions (also see Figure S2 in SI) 8, 21, 22. Each cross‐section variation along the tube axis will result in obstruction during a pullout.…”

Thermal conduction mechanisms in boron nitride nanotubes: Few-shell versus all-shell conduction

“…25 The cross section of the calculated singlewalled NTs ͑SWNTs͒ was taken as S =2R␦, 26 where R is the NT radius, and ␦ = 3.35 Å is the layer separation in bulk hexagonal BN. 27 The thermal conductivity of the measured multiwalled NTs ͑MWNTs͒ was estimated by incorporating the geometrical area between the inner and the outer radii as the actual cross section. 13 The computational burden of the calculation prevents us from analyzing the MWNT with a diameter of Ϸ30 nm considered in the experiment.…”

Boron Nitride Nanotubes

“…Obviously, the dislocation-like line defects should exist between the fragmented facets. The coexistence of cylindrical and polygonized cross sections in multiwalled BNNTs has recently become a subject of a detailed crystallographic study by Celik-Atkas et al [111,112] and by the present authors.…”