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Figure 3
HRTEM images and the helical-growth model of BNNTs. (a) An HRTEM image of a BNNT. (a′) The FFT pattern obtained from the HRTEM image in (a). (b) The helical-growth model for a BNNT (AAh BN helix with a zigzag structure) based on those of MWNTs (Lee et al., 2013BB24) and SWNTs (Lee et al., 2014BB23). An AAh BN nanoribbon seed (bottom) initiates and leads to helical growth comprising seven diagonal growths, which is assumed to adopt the heptagon cross-sectional morphology of an MWNT (Fig. S1). HG, DG and LG in (b) indicate helical growth, diagonal growth and lateral growth, respectively. (b′) A schematic explaining the facet growth of BN layers to maintain crystallinity. (c) An HRTEM image of a BNNT sample. The yellow arrows indicate left-handed scrolling. (c′), (c′′) Magnified images of the squares in (c). A′ in (c′′) indicates a disconnected layer. The white arrow in (c′) indicates helical edges where all of the wall layers are disconnected. The black arrows in (a) and (c) indicate the traces of mono-layer BN.

IUCrJ
Volume 8| Part 6| November 2021| Pages 1018-1023
ISSN: 2052-2525
https://doi.org/10.1107/S2052252521009118