One-Dimensional Pnictogen Allotropes Inside Single-Wall Carbon Nanotubes

The discovery of phosphorene, a single layer of black phosphorus, has kick-started tremendous research efforts investigating the pnictogen nanomaterials. Arsenene and antimonene have now also been identified and these two-dimensional nanomaterials display physical properties superior to graphene for some applications. Recently, single-wall carbon nanotubes (SWCNTs) have been filled with P₄ molecules from the melt and As₄ molecules from the vapor phase. Inside the confinement of the SWCNTs, polymerization reactions were observed yielding new one-dimensional pnictogen allotropes. Here we show using high-resolution electron microscopy that such nanostructures can also be observed upon filling SWCNTs from the vapor phase using red phosphorus as the source material. Using larger diameter SWCNTs and filling from the vapor phase favors the formation of double-stranded phosphorus zig-zag ladders observed here for the first time. SWCNTs were generally found to fill well with liquid phosphorus. However, substantial decreases in the filling yields were observed for both phosphorus and arsenic filling of narrow SWCNTs using the vapor route. The filling experiments with molten antimony gave very low filling yields. However, the antimony zig-zag ladder could be observed on two occasions suggesting that this structural motif dominates across the pnictogens. Computational predictions of the encapsulation energies of the various pnictogen nanostructures are consistent with the observed experimental trends and band gap calculations predict that the single-stranded zig-zag chains of all investigated pnictogens are fully metallic. Using SWCNTs with diameters greater than 1.5 nm displayed a plethora of complex new phosphorus nanostructures which highlights an exciting new avenue for future work in this area.