In situ observation of small onion formation under electron irradiation of turbostratic BC2N and turbostratic BN

Turbostratic BC₂N and turbostratic BN samples were exposed to intense irradiation regimes in a high resolution electron microscope equipped with a field emission gun. EELS analysis was used to check the sp²-bonding type and the composition of the BC₂N starting materials. In both experiments, we observed a tendency for the basal planes to curl and form onion-like features. Based on the hypothesis that the onion formation occurs at a constant composition, a structural model is proposed for the innermost shell of a BC₂N onion. The overall polyhedral shape of the BN cages is explained within the frame of the octahedral model previously proposed for BN analogs to fullerenes.