One-Dimensional Pnictogen Allotropes inside Single-Wall Carbon Nanotubes

Abstract

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 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. 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 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.

Simulated Raman spectra of bulk and low-dimensional phosphorus allotropes

“…Various hypothetical bulk and layered P allotropes have been theoretically predicted [11][12][13][14] , notably using machine learning-force fields 15 . Nanoscale 1D nanoribbons and nanotubes also appear regularly in the literature 16 , and new nanoscale phases have been identified, for example when phosphorus is confined within the cavity of carbon nanotubes [17][18][19] .…”

“Missing” One-Dimensional Red-Phosphorus Chains Encapsulated within Single-Walled Carbon Nanotubes

“…The latter synthesis conditions were also suggested to result in the formation of black phosphorus nanoribbons inside the multiwalled CNTs (MWCNTs) with an inner diameter of about 4 nm. Zigzag phosphorus ladders were observed when the nanotube diameter exceeded ∼1.0 nm. , HRTEM images of phosphorus-filled nanotubes with diameters greater than 1.5 nm suggest double chains, but their structures were not defined . It was concluded that the host size has a more profound effect on the structure of the encapsulated phosphorus than the synthesis conditions; however, increasing synthesis temperature favors polymerization.…”

“…As well as confining the material to one dimension, nanotube shells can protect phosphorus from oxidation and additionally provide the chance to study an encased structure by high-resolution transmission electron microscopy (HRTEM). Filling of CNTs by phosphorus has been performed from the melt at 50 °C , or by chemical vapor transport at 500 °C. − The use of narrow (0.6–1.1 nm) single-walled CNTs (SWCNTs) allowed encapsulation of P 4 molecules from melted and gaseous phosphorus states, resulting in formation of single-stranded phosphorus chains , and generated previously unseen square columnar images by vapor condensation . The latter synthesis conditions were also suggested to result in the formation of black phosphorus nanoribbons inside the multiwalled CNTs (MWCNTs) with an inner diameter of about 4 nm.…”

The Role of Temperature on the Degree of End-Closing and Filling of Single-Walled Carbon Nanotubes

“…There is a special interest in using SWCNTs as enclosures for the encapsulation of payloads [6][7][8][9]. The nanotubes offer protection to the hosted material from the external environment, thus allowing, for instance, the encapsulation of air sensitive materials [10], gases [11], and even radionuclides [12,13]. Furthermore, a large variety of unprecedented structures have been observed by confinement of materials at the nanoscale [14].…”