We report on the growth and formation of single-layer boron nitride dome-shaped nanostructures mediated by small iron clusters located on flakes of hexagonal boron nitride. The nanostructures were synthesized in situ at high temperature inside a transmission electron microscope while the e-beam was blanked (Figure 1). The formation process, typically originating at defective step-edges on the boron nitride support, was investigated using a combination of transmission electron microscopy, electron energy loss spectroscopy and computational modelling. The h-BN dome-shaped nanostructure of Figure 1 was used to simulate images of BN protrusions at various angles relative to the incident electron beam, by adjusting effectively the beam direction. Figures 2 presents simulated images for beam angles of 0° (2a), 30° (2b,c) and 50° (2d), respectively, relative to the h-BN plane normal, in comparison with experimentally observed features (Figures 2e-h). The image simulations are in striking agreement with the experimental images, consistent with the circular features being protrusions formed normal to the h-BN plane, whilst the hemispheres correspond to protrusions tilted with respect to the h-BN plane.  Computational modelling showed that the domes exhibit a nanotube-like structure with flat circular caps (Figure 3) and that their stability was comparable to that of a single boron nitride layer.

       Nanostructured carbon protrusions have been studied since 2001 [1-3], but the investigation of analogous BN structures has only just begun. In the present study, we have shown that even member rings are required for the formation of h-BN dome-shaped protrusions, but not in the form of active linear defects, containing B-B and N-N bonds, as observed recently in BN monolayers under electron beam irradiation [4]. Furthermore, according to our molecular simulations result the even members rings present in the half dome structure present B-B and N-N bonds (Figure 4). The BN dome-shaped nanostructures represent a new material that perhaps by hosting metal atoms may unveil new optical, magnetic, electronic or catalytic properties, emerging from confinement effects.

[1] Sharma, R.; et al. Journal of Electron Microscopy 2005, 54, 231-237.

[2] Chamberlain, T. W.; et al. Nature Chemistry 2011, 3, 732-737

[3] Nasibulin, A. G.; et al. Nature Nanotechnolgy2007, 2, 156-161.

[4] Cretu, O.; et al Nanoletters2014, 14, 1064-1068. 

Figures:

Figure 1: Bright field TEM images of single layer BN dome-like structures, formed after in situ heating. In some cases, b,c the nanostructures exhibited faceted topologies (feature size histogram inset: 2.6 - 4.4 nm).

Figure 2: Simulated TEM images (a-d) of the h-BN dome-like nanostructure presented in Figure 1 corresponding to incident beam directions of (a) 0°, (b-c) 30° and (d) 50° with respect to the h-BN plane; (e-h) Experimentally observed features for comparison with the simulations. The scale bars are 10 nm.

Figure 3: Stable h-BN dome protruding from an h-BN sheet, as predicted by molecular dynamics simulations, showing: (a) pictorial distribution of defects at the layer and cap interfaces; (b) histograms of the corresponding atomic rings; and (c) values of total energy (per B-N pair), the number of four-membered rings in the nanostructure, and three main types of connections between the squares in (BN)12, (BN)15 and (BN)18 cages, (BN) dome and a perfect single h-BN layer.

Figure 4: Schematic representation of the defects configuration of the dome-like structure.

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EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-tem-growth-of-single-layer-boron-nitride-dome-shaped-nanostructures-catalysed-by-iron-clusters/