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The Origins of Buckyballs in Space

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The Origins of Buckyballs in Space News & views sufficient ultraviolet irradiation. The first astronomical source in which Astrochemistry fullerene was detected was the star Tc 1 (ref. 7). Puzzlingly, however, the emission associated The origins of with fullerene came from a location far away from the star and its ultraviolet photons, whereas the PAH emissions were closer to the buckyballs in space star. On the basis of the previously reported laboratory experiments, this is the opposite of what should happen if fullerene forms from Alessandra Candian PAHs in this source8. So how can the locations The spectroscopic fingerprints of buckyballs have been of the emissions be explained? observed in space, but questions remain about how these Bernal and co-workers now report that large molecules form. Laboratory experiments have revealed fullerene also forms readily from silicon car- bide (SiC), which has been proposed to be the a possible mechanism. first carbonaceous material to condense out of old, carbon-rich stars9. The authors rapidly heated grains of the crystalline form of SiC that A long-standing mystery in astronomical different formation route for fullerene. is found in highest abundance in meteorites10, spectroscopy concerns diffuse interstellar The carbon atoms in fullerene are arranged and irradiated them with xenon ions, mimick- bands, a family of absorption features seen in in the shape of a football, a molecular struc- ing the heating caused by shock waves around the spectra of the interstellar medium of the ture that is remarkably stable but also difficult old stars. Milky Way and of other galaxies. First observed to construct. Fullerene has been made in the Using a transmission electron microscope to almost 100 years ago, the origin of any of the laboratory in experiments designed to probe image the surfaces of the samples down to the bands was unknown until 2015, when four of the chemistry that occurs in carbon-rich stars: subnanometre scale, the scientists observed them were assigned1 to the cation of buck- carbon in the form of graphite was vaporized that the grain material had altered notably as + ministerfullerene (C60 ; the uncharged mol- into a high-density helium flow, producing car- a result of its treatment (Fig. 1). Silicon atoms ecule is often referred to simply as fullerene, bon clusters4. The discovery that fullerene was had percolated to the outer layers of the or colloquially as a buckyball). Fullerene and among the reaction products led to the award grains, leaving behind what looked like sheets its analogue, C70, are by far the biggest mol- of the Nobel Prize in Chemistry to Harry Kroto, of carbon atoms in a hexagonal ‘chicken-wire’ ecules detected in space, raising the question Richard Smalley and Robert Curl in 1996. arrangement — that is, graphene sheets. of how such large species can form in those However, the range of temperatures required The transformation of the outer layers of rarified conditions. Researchers have sug- to create fullerene in this way is quite specific2; SiC into graphene sheets at high temperatures gested that fullerene forms in the outflows of outside that range, molecules known as poly- had been reported11 previously for a different old, carbon-rich stars known as asymptotic cyclic aromatic hydrocarbons (PAHs) are form of SiC from that studied by Bernal and col- giant branch stars2 — the temperatures and produced instead. These molecules are 2D sec- leagues. However, Bernal et al. also observed densities of these outflows promote chemi- tions of a single layer of graphite (a graphene the formation of hemispherical structures stry similar to that of combustion. This could sheet), decorated with hydrogen atoms. Sub- with diameters similar to that of fullerene. lead to the formation of soot, which can contain sequent experiments5,6 have shown that PAHs Their work thus provides a convincing new fullerene-like structures. Writing in Astrophysi- that contain more than 60 carbon atoms are mechanism for the formation of fullerene in cal Journal Letters, Bernal et al.3 propose a very converted into fullerenes when exposed to evolved stars. Bernal et al. report another piece of evidence supporting the idea that SiC grains are rapidly heated and bombarded with ions in evolved stars. They have identified a fragment of the Murchison meteorite — a highly studied Silicon meteor ite that is rich in organic compounds carbide — in which the ratio of carbon-12 to carbon-13 Graphene isotopes is typical of material from an old, car- bon-rich star. This indicates that the fragment was not produced during or after the forma- tion of the meteorite, but instead is stardust that originated in an old star. The fragment has a core of SiC surrounded by graphene C60 sheets. However, previous analyses12 of graphite-containing stardust found evidence only of titanium carbide cores, rather than SiC cores. This raises the question of how common SiC cores are in graphite-containing Figure 1 | Evidence of a mechanism for the formation of buckminsterfullerene in space. Bernal et al.3 heated grains of silicon carbide (SiC) and bombarded them with ions, mimicking the conditions experienced stardust. by the dust around old stars. Using a transmission electron microscope, the scientists observed that the outer The rapid heating of SiC grains in the layers of SiC had transformed into graphene sheets, as shown in this idealized grain. They also observed presence of hydrogen can lead to the forma- 13 the formation of hemispherical structures with diameters similar to that of buckminsterfullerene (C60) on tion of PAHs . Bernal and colleagues’ findings the surface of the grains. Their work thus reveals a convincing process through which C60 could form in the therefore suggest that the thermal conversion outflows of old stars. of SiC to graphene sheets in evolved stars 490 | Nature | Vol 574 | 24 October 2019 ©2019 Spri nger Nature Li mited. All ri ghts reserved. ©2019 Spri nger Nature Li mited. All ri ghts reserved. could be the first step in the formation of large carbon-containing molecules in general: sub- Optical physics sequent (or simultaneous) exposure of the graphene to atomic hydrogen produces PAHs, whereas ion bombardment produces fuller- Light trapping gets a boost ene. Alternatively, PAH molecules might be molecular intermediates in the formation of carbon soot, which can then be broken down Kirill Koshelev & Yuri Kivshar by ultraviolet irradiation to make PAHs again14. The efficiency of Bernal and colleagues’ The ability of structures called optical resonators to trap light fullerene-forming mechanism is unknown, is often limited by scattering of light off fabrication defects. A raising the question of how many SiC grains physical mechanism that suppresses this scattering has been are needed to account for the observed abun- reported that could lead to improved optical devices. See p.501 dance of fullerene molecules in space. If there aren’t enough grains, then a further mechanism will be required to explain the abundance of Devices called optical resonators confine light, fullerene. By contrast, if there are too many but for only a limited time because of unavoida- Merging BIC SiC grains, what happens to the ‘excess’ fuller- ble light emission. On page 501, Jin et al.1 report ene molecules produced, given that they are that such emission can be greatly reduced by notoriously difficult to degrade? More experi- using the interference of light waves known as BIC ments and detailed modelling of the formation bound states in the continuum. Such waves are of fullerene and of other carbon-containing akin to exotic electron waves that were intro- large molecules from SiC grains are needed to duced in the theory of quantum mechanics 2 understand this process, and to quantify its almost a century ago . The authors’ finding Quality factor importance in old stars. could have many technological implications The launch of the James Webb Space for nanophotonics, quantum optics and non- Telescope in 2021 will provide powerful new linear optics — the study of how intense light tools for studying old stars, among other interacts with matter. astronomical objects. Observations of fuller- Interference is a common wave phenomenon Wavevector ene-containing sources7,8 such as Tc 1 will be in physics, whereby two or more waves pass able to constrain the regions in which SiC through one another to produce a combined Figure 1 | Increasing the quality factor of an grains, fullerene and PAHs are present, provid- waveform. Consider the case in which these optical resonator. Jin et al.1 report simulations of ing more clues about how large mol ecules are waves are correlated with one another, either and experiments on a light-trapping device known actually formed. Further analysis and model- because they come from the same source or as an optical resonator. The key characteristic of a ling of the routes involved will eventually allow because they have almost the same frequency. resonator is the quality factor — a measure of the astronomers to suggest the identities of the If the crest of one wave coincides with the crest efficiency of light trapping. This quantity varies with the wavevector, which describes the velocity other mysterious molecules responsible for of another wave, the combined amplitude will and propagation direction of a wave. The authors the diffuse interstellar bands. be the sum of the individual amplitudes. And used their resonator to trap light in the form of if the crest of one wave meets the trough of waves called bound states in the continuum (BICs). Alessandra Candian is at the van’t another wave, the combined amplitude will They then combined these BICs into a single state: Hoff Institute for Molecular Sciences, be the difference in the individual amplitudes. a merging BIC. As this graph shows, a merging BIC University of Amsterdam, 1090 GD These two scenarios are called constructive and increases the quality factors of all waves that have Amsterdam, the Netherlands, and at the destructive interference, respectively.
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