One-Dimensional Pnictogen Allotropes inside Single-Wall Carbon Nanotubes

The discovery of phosphorene, a single layer of black phosphorus, has accelerated the investigation of pnictogen nanomaterials, leading to the recent identification of arsenene and antimonene. These two-dimensional nanomaterials display physical properties superior to those of graphene for some applications. Recently, single-wall carbon nanotubes (SWCNTs) have been filled with P-4 molecules from the melt and As-4 molecules from the vapor phase. Confined within SWCNTs, polymerization reactions yielded 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, the vapor phase favors the formation of double-stranded phosphorus zigzag ladders observed here for the first time. Overall, however, SWCNTs were generally found to fill more efficiently with liquid phosphorus; substantial decreases in the filling yields were observed for both phosphorus and arsenic filling of narrow SWCNTs using the vapor route. Attempts to extend the pnitogen series using molten antimony gave very low filling yields. However, the antimony zigzag ladder was 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 zigzag chains of all investigated pnictogens are fully metallic. Using SWCNTs with diameters of >1.5 nm revealed a plethora of complex new phosphorus nanostructures, which highlights an exciting new avenue for future work in this area.