Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

Merino, Pablo; Švec, Martin; Martínez, José I.; Jelı́nek, Pavel; Lacovig, Paolo; Dalmiglio, Matteo; Lizzit, Silvano; Soukiassian, P.; Cernicharo, J.; Martin-Gago, J.A.

doi:10.1038/ncomms4054

Cited by 72 publications

(67 citation statements)

References 42 publications

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“…In particular, recent ultra-high vacuum experiments with carbonaceous grains show that hydrogen atoms attached to the grain surface can efficiently react and produce a large variety of organic molecules, from PAHs to acetylene (Merino et al 2014). We note that C 2 H 2 photodissociation produces C 2 H and the reaction of C 2 H 2 with C + forms the observed hydrocarbon ion l-C 3 H + , an important gas-phase precursor of C 3 H 2 and C 3 H. In addition, laboratory experiments performed by Alata et al (2014) show that the photodestruction of hydrogenated amorphous carbon (HAC) grains, observed in the diffuse medium (Duley & Williams 1983), also leads to the production of small hydrocarbons (such as CH 4 ) that can trigger the gasphase formation of other hydrocarbons.…”

Section: Pah/hac Photodestruction and Grain Surface Chemistrymentioning

confidence: 99%

The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR

Cuadrado

Goicoechea

Pilleri

et al. 2015

A&A

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124

177

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Context. Carbon chemistry plays a pivotal role in the interstellar medium (ISM) but even the synthesis of the simplest hydrocarbons and how they relate to polycyclic aromatic hydrocarbons (PAHs) and grains is not well understood. Aims. We study the spatial distribution and chemistry of small hydrocarbons in the Orion Bar photodissociation region (PDR), a prototypical environment in which to investigate molecular gas irradiated by strong UV fields. Methods. We used the IRAM 30 m telescope to carry out a millimetre line survey towards the Orion Bar edge, complemented with ∼2 × 2 maps of the C 2 H and c-C 3 H 2 emission. We analyse the excitation of the detected hydrocarbons and constrain the physical conditions of the emitting regions with non-LTE radiative transfer models. We compare the inferred column densities with updated gas-phase photochemical models including 13 CCH and C 13 CH isotopomer fractionation.Results. Approximately 40% of the lines in the survey arise from hydrocarbons (C 2 H, C 4 H, c-C 3 H 2 , c-C 3 H, C 13 CH, 13 CCH, l-C 3 H, and l-H 2 C 3 in decreasing order of abundance). We detect new lines from l-C 3 H + and improve its rotational spectroscopic constants. Anions or deuterated hydrocarbons are not detected, but we provide accurate upper limit abundances:Conclusions. Our models can reasonably match the observed column densities of most hydrocarbons (within factors of <3). Since the observed spatial distribution of the C 2 H and c-C 3 H 2 emission is similar but does not follow the PAH emission, we conclude that, in high UV-flux PDRs, photodestruction of PAHs is not a necessary requirement to explain the observed abundances of the smallest hydrocarbons. Instead, gas-phase endothermic reactions (or with barriers) between C + , radicals, and H 2 enhance the formation of simple hydrocarbons. Observations and models suggest that the [C 2 H]/[c-C 3 H 2 ] ratio (∼32 at the PDR edge) decreases with the UV field attenuation. The observed low cyclic-to-linear C 3 H column density ratio (≤3) is consistent with a high electron abundance (x e ) PDR environment. In fact, the poorly constrained x e gradient influences much of the hydrocarbon chemistry in the more UV-shielded gas. The inferred hot rotational temperatures for C 4 H and l-C 3 H + also suggest that radiative IR pumping affects their excitation. We propose that reactions of C 2 H isotopologues with 13 C + and H atoms can explain the observed [C 13 CH]/[ 13 CCH] = 1.4 ± 0.1 fractionation level.

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Section: Pah/hac Photodestruction and Grain Surface Chemistrymentioning

confidence: 99%

The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR

Cuadrado

Goicoechea

Pilleri

et al. 2015

A&A

Self Cite

124

177

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“…They also can accelerate ions that impact and implant into the grain surfaces (e.g., Heck et al 2009). Such shock heating and bombardment can disrupt the SiC grains (Van de Steene & van Hoof 2003), in analogy to our experimental conditions, producing graphite and reduced carbon nanostructures which can lead to C 60 , and even possibly PAHs (Merino et al 2014). Therefore, interstellar fullerenes can be created from shocked, circumstellar SiC grains, without the need for dehydrogenation, carbon atom elimination, and any dimensional constraint on the precursor material, such as PAH size (Berné et al 2015).…”

Section: Implications For Interstellar/circumstellar C 60mentioning

confidence: 79%

Formation of Interstellar C60 From Silicon Carbide Circumstellar Grains

Bernal¹,

Ziurys²,

Haenecour³

et al. 2019

Proceedings of the 74th International Symposium on Molecular Spectroscopy

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We have conducted laboratory experiments with analog crystalline silicon carbide (SiC) grains using transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS). The 3C polytype of SiC was used-the type commonly produced in the envelopes of asymptotic giant branch (AGB) stars. We rapidly heated small (∼50 nm) synthetic SiC crystals under vacuum to ∼1300 K and bombarded them with 150 keV Xe ions. TEM imaging and EELS spectroscopic mapping show that such heating and bombardment leaches silicon from the SiC surface, creating layered graphitic sheets. Surface defects in the crystals were found to distort the six-membered rings characteristic of graphite, creating hemispherical structures with diameters matching that of C 60 . Such nonplanar features require the formation of five-membered rings. We also identified a circumstellar grain, preserved inside the Murchison meteorite, that contains the remnant of an SiC core almost fully encased by graphite, contradicting long-standing thermodynamic predictions of material condensation. Our combined laboratory data suggest that C 60 can undergo facile formation from shock heating and ion bombardment of circumstellar SiC grains. Such heating/bombardment could occur in the protoplanetary nebula phase, accounting for the observation of C 60 in these objects, in planetary nebulae (PNs) and other interstellar sources receiving PN ejecta. The synthesis of C 60 in astronomical sources poses challenges, as the assembly of 60 pure carbon atoms in an H-rich environment is difficult. The formation of C 60 from the surface decomposition of SiC grains is a viable mechanism that could readily occur in the heterogeneous, hydrogen-dominated gas of evolved circumstellar shells.

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“…XPS spectra were calibrated to 284.8 eV according to the C 1s core level binding energy (BE). 27 Si 2p core-level spectra were fitted using Figure 1d). The penetration depth of the X-ray source (~10 nm) is less than the accumulated thickness of these oxidized agglomerates and therefore does not detect fresh Si-Si bonds.…”

Section: Resultsmentioning

confidence: 99%

Impact of Silicon Nanocrystal Oxidation on the Nonmetallic Growth of Carbon Nanotubes

Rocks

Mitra

Macías-Montero

et al. 2016

ACS Appl. Mater. Interfaces

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Carbon nanotube (CNT) growth has been demonstrated recently using a number of nonmetallic semiconducting and metal oxide nanoparticles, opening up pathways for direct CNT synthesis from a number of more desirable templates without the need for metallic catalysts. However, CNT growth mechanisms using these nonconventional catalysts has been shown to largely differ and reamins a challenging synthesis route. In this contribution we show CNT growth from partially oxidized silicon nanocrystals (Si NCs) that exhibit quantum confinement effects using a microwave plasma enhanced chemical vapor deposition (PECVD) method. On the basis of solvent and a postsynthesis frgamentation process, we show that oxidation of our Si NCs can be easily controlled. We determine experimentally and explain with theoretical simulations that the Si NCs morphology together with a necessary shell oxide of ∼1 nm is vital to allow for the nonmetallic growth of CNTs. On the basis of chemical analysis post-CNT-growth, we give insight into possible mechanisms for CNT nucleation and growth from our partially oxidized Si NCs. This contribution is of significant importance to the improvement of nonmetallic catalysts for CNT growth and the development of Si NC/CNT interfaces.

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Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation

Cited by 72 publications

References 42 publications

The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR

The chemistry and spatial distribution of small hydrocarbons in UV-irradiated molecular clouds: the Orion Bar PDR

Formation of Interstellar C60 From Silicon Carbide Circumstellar Grains

Impact of Silicon Nanocrystal Oxidation on the Nonmetallic Growth of Carbon Nanotubes

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