Formation of small single-layer and nested BN cages under electron irradiation of nanotubes and bulk material

Abstract

An experimental evidence for the formation of small BN cage-like molecules, under electron-irradiation experiments of BN samples, is presented. Depending on the starting material, either close-packed agglomerates of small "fullerenes", or small nested "fullerenes" with up to six layers are found as irradiation derivatives. The overall polyhedral shape of the BN cages is explained within the frame of the octahedral model previously proposed for BN analogs to fullerenes. The diameters of the smallest and most observed cages range from 0.4 to 0.7 nm, and are close to those of the B 12 N 12 , B 16 N 16 and B 28 N 28 octahedra which were predicted to be magic clusters for the BN system from electronic structure calculations.The discovery of the C 60 soccer ball [1] followed by that of larger fullerenes and carbon nanotubes [2] raised the curtain on a new class of nano-objects based on layered materials with predicted unique physical properties. Recently, the synthesis of BN [3][4][5][6], BCN [7,8], and MoS 2 [9] nanotubes and closed shell nanoparticles has generalized the idea already admitted for carbon that for a small assembly of atoms the network bends and curls into a close structure in order to eliminate the highly unfavorable dangling bonds. Nested concentric BN polyhedra have been successfully synthesized by reaction of BCl 3 with NH 3 in a laser beam [10,11]. However, in spite of theoretical predictions for the stability of the hybrid B 24 C 12 N 24 molecule [12] and the B 12 N 12 , B 16 N 16 , B 28 N 28 molecules [13,14], there has been, so far, no experimental evidence for the stability of structures analogous to C 60 and other small carbon fullerenes in layered materials other than graphite.Strong irradiation in an electron microscope allows structural fluidity and a rearrangement of the structure caused by the momentum transfer of high-energy electrons to the nuclei (knock-on collisions). It is now well known that carbon material, whatever its initial structural state -namely amorphous, turbostratic or well-graphitized, bulk crystallites, nanocages, or nanotubes -can be transformed into onion-like structures when submitted to such intense electron irradiation [15,16]. Carbon onions can have very different sizes and consist of concentric polyhedral fullerene cages; single-or double-layer small fullerene cages can also be formed [17]. On the other hand, only disordered spherical layered structures with incompletely closed layers were shown to form under electron irradiation of crystalline BN [18], MoS 2 crystals [19], and turbostratic BC 2 N [20]. We present transformations obtained when submitting to intense irradiation regimes two kinds of BN samples, a turbostratic BN sample and a BN nanotube powder. Depending on the starting material, two different irradiation products are observed: small nested polyhedra or close-packed arrangements of small single-layer cage-like clusters. The size and shape of the smallest observed cage is compared to that of the smallest high-symmetry polyhedral clusters predicted t...

Atom by atom: HRTEM insights into inorganic nanotubes and fullerene-like structures

“…Nonetheless, unlike the almost perfectly spherical carbon onions, mediumsize (Ϸ20 nm in diameter) BN IF structures were found to be strongly facetted (11). When produced in smaller sizes by electron irradiation, the basic structure of the BN particles exhibits rectangular shapes, attributed to a 3D octahedral structure (10,12), which is obtained by placing (B-N) 2 squares in the six corners.…”

Photoabsorption spectra of boron nitride fullerenelike structures

“…The fabrication of BN nanotubes 1 and nested cages 2 was reported in the mid-1990s, soon after the discovery of carbon-based fullerenes and related carbon nanostructures. BN fullerenes were synthesized for the first time in 1998 by Stéphan et al 3 These isoelectronic analogs of carbon structures are of interest to both theorists and experimentalists in materials science because of their potential applications in future nano-and optoelectronic devices and also as lubricants. The superiority of BN based structures over the carbon ones would result from their better thermal and chemical stabilities.…”

Theoretical infrared, Raman, and optical spectra of theB36N36cage

“…The vertical and adiabatic ionization potentials as well as static dipole polarizability are also reported. [3,4,5,6,7,8,9,10].On the experimental side, there are also a few reports of synthesis of hollow BN structures [10,11,12,13,14].In a series of experiments by Oku and coorkwers, boron nitride clusters of different sizes were reported [10,11,12].Perhaps the most interesting result of these works was the production of B 24 N 24 in abundance. We performed theoretical calculations [8,9] …”

“…On the experimental side, there are also a few reports of synthesis of hollow BN structures [10,11,12,13,14].…”