Synthesizing carbon nanotubes in space

Abstract

Context. As the 4th most abundant element in the universe, carbon (C) is widespread in the interstellar medium (ISM) in various allotropic forms (e.g., fullerenes have been identified unambiguously in many astronomical environments, the presence of polycyclic aromatic hydrocarbon molecules in space has been commonly admitted, and presolar graphite as well as nanodiamonds have been identified in meteorites). As stable allotropes of these species, whether carbon nanotubes (CNTs) and their hydrogenated counterparts are also present in the ISM or not is unknown. Aims. We explore the possible routes for the formation of CNTs in the ISM and calculate their fingerprint vibrational spectral features in the infrared (IR). Methods. We study the hydrogen-abstraction/acetylene-addition (HACA) mechanism and investigate the synthesis of nanotubes using density functional theory (DFT). The IR vibrational spectra of CNTs and hydrogenated nanotubes (HNTs), as well as their cations, have also been obtained with DFT. Results. We find that CNTs could be synthesized in space through a feasible formation pathway. CNTs and cationic CNTs, as well as their hydrogenated counterparts, exhibit intense vibrational transitions in the IR. Their possible presence in the ISM could be investigated by comparing the calculated vibrational spectra with astronomical observations made by the Infrared Space Observatory, Spitzer Space Telescope, and particularly the upcoming James Webb Space Telescope.

Spitzer’s perspective of polycyclic aromatic hydrocarbons in galaxies

“…• Are PAHs related to other unexplained interstellar phenomena (e.g., the 2175Å extinction bump, the diffuse interstellar bands, the blue and extended red photoluminescence emission, and the "anomalous microwave emission"), other interstellar carbon species (e.g., C 60 , carbon chains, and possibly graphene and carbon nanotubes) [180][181][182][183] The distribution of PAHs in the SMC, derived from the Spitzer/IRAC photometric mapping data, does not follow that of C-rich AGB stars, instead, it correlates with molecular gas as traced by CO (ref. 155 ).…”

On carbon nanotubes in the interstellar medium

“…CNTs are actually more IR-active than graphene due to their cylindrical boundary condition. However, as mentioned earlier, the vibrational spectra of CNTs depend on their diameters and chiralities (e.g., see Chen & Li 2019). For SWNTs of diameters less than 2 nm, over 100 different structures are topologically possible.…”

Benzyne in V4334 Sqr: A Quest for the Ring with SOFIA/EXES

“…If benzyne is similarly reactive, this could deplete the abundance of benzyne below the level of detectability. However, spectral signatures at the putative wavelengths of carbon nanotubes (see Table 1 of Chen & Li 2019) are not evident in the Spitzer or SOFIA spectra of V4334Sgr. Unlike other BAGS, nondetection of PAHs in V4334Sgr may suggest that they are not extant in the dusty carbon-rich environment and possibly benzyne quickly transforms into other species.…”

“…Acetylene is present in V4334Sgr (Evans et al 2006). Chen & Li (2019) have shown production of carbon nanotubes, formed from benzene with the presence of C 2 H 2 (through a hydrogen abstraction and acetylene addition reaction) is possible. If benzyne is similarly reactive, this could deplete the abundance of benzyne below the level of detectability.…”