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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.