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

Abstract

Polycyclic aromatic hydrocarbons as well as other organic molecules appear among the most abundant observed species in interstellar space and are key molecules to understanding the prebiotic roots of life. However, their existence and abundance in space remain a puzzle. Here we present a new top-down route to form polycyclic aromatic hydrocarbons in large quantities in space. We show that aromatic species can be efficiently formed on the graphitized surface of the abundant silicon carbide stardust on exposure to atomic hydrogen under pressure and temperature conditions analogous to those of the interstellar medium. To this aim, we mimic the circumstellar environment using ultra-high vacuum chambers and investigate the SiC surface by in situ advanced characterization techniques combined with first-principles molecular dynamics calculations. These results suggest that top-down routes are crucial to astrochemistry to explain the abundance of organic species and to uncover the origin of unidentified infrared emission features from advanced observations.

ARTICLE

Received 24 Jul 2013 | Accepted 3 Dec 2013 | Published 21 Jan 2014

P. Merino1, M.vec2, J.I. Martinez3, P. Jelinek2, P. Lacovig4, M. Dalmiglio4, S. Lizzit4, P. Soukiassian5,6,J. Cernicharo1 & J.A. Martin-Gago1,3

Polycyclic aromatic hydrocarbons as well as other organic molecules appear among the most abundant observed species in interstellar space and are key molecules to understanding the prebiotic roots of life. However, their existence and abundance in space remain a puzzle. Here we present a new top-down route to form polycyclic aromatic hydrocarbons in large quantities in space. We show that aromatic species can be efciently formed on the graphitized surface of the abundant silicon carbide stardust on exposure to atomic hydrogen under pressure and temperature conditions analogous to those of the interstellar medium. To this aim, we mimic the circumstellar environment using ultra-high vacuum chambers and investigate the SiC surface by in situ advanced characterization techniques combined with rst-principles molecular dynamics calculations. These results suggest that top-down routes are crucial to astrochemistry to explain the abundance of organic species and to uncover the origin of unidentied infrared emission features from advanced observations.

DOI: 10.1038/ncomms4054

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

1 Centro de Astrobiologa INTA-CSIC, Carretera de Ajalvir, km.4, ES-28850 Madrid, Spain. 2 Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnicka 10, CZ-16200 Prague, Czech Republic. 3 Instituto Ciencia de Materiales de Madrid-CSIC, c/. Sor Juana Ins de la Cruz, 3, ES-28049 Madrid, Spain. 4 Elettra-Sincrotrone Trieste S.C.p.A., Area Science Park, S.S. 14, Km 163.5, I-34149 Trieste, Italy. 5 Commissariat lEnergie Atomique et aux Energies Alternatives, SIMA, DSM-IRAMIS-SPEC, Bt. 462, 91191 Gif sur Yvette, France. 6 Synchrotron SOLEIL, LOrme des Merisiers, Saint-Aubin, 91192 Gif sur Yvette, France. Correspondence and requests for materials should be addressed to J.A.M.-G. (email: mailto:[email protected]

Web End [email protected] ).

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The great advances experienced by radioastronomy during recent years have depicted the interstellar medium (ISM) and circumstellar envelopes (CSE) as supporting an active

and rich chemistry1. In these cosmic regions, both simple molecules, such as H2 or CO, and complex ones, such as C60 and polycyclic aromatic hydrocarbons (PAHs), are routinely detected by their characteristic infrared spectrum2,3. Advances in infrared spectroscopy, powered by space missions such as ISO and Spitzer, have revealed the existence of dust grains composed of silicates, SiC or oxides, among other substances, in such regions46. All this chemical complexity present in the ISM and in CSEs plays a major role in the evolution of galaxies, comets, the formation of planets and, ultimately, in the abiotic organic chemistry that preceded the origin of life on Earth7.

PAHs contain up to 20% of the carbon in the photodissociation regions8, and are present in any region subjected to ultraviolet radiation9. However, despite being so ubiquitous and playing such a crucial role in several astrophysical processes and environments, their formation in the interstellar and circumstellar media remains poorly understood. Proposed chemical routes leading to the formation of complex polyaromatic molecules, and particularly to large PAHs, in space cannot account for their abundance and thus efcient mechanisms for their formation are still an open issue. The main current theories invoke either combustion-like1012 or ultraviolet-drivenacetylenic polymerization and evaporation processes in most evolved objects1317. However, the efciency of such processes is still unknown8,9,18 and during the last 30 years an important amount of work has been performed to determine PAHs origin and physicochemical properties.

Unfortunately, a complete carbon inventory is difcult to determine, as many other important moleculessuch as carbon clusters (C2, C3, Cn), CH4 and C2H4, which could also be abundant in the inner regions of CSEshave not yet been quantitatively observed with sufcient detail. Within this context of chemical complexity in the ISM, new bottom-up assembling mechanisms based on concepts from nanotechnology have recently come into play. The catalytic role of interstellar dust grains in certain reactions has long been proposed; in their seminal work, Watson and Salpeter19 and later Hornekaer et al.20 studied the dust grains as the preferred sites for the formation of the most abundant molecule, H2.

In this article, we proposeand experimentally reproducean alternative and efcient mechanism based on a top-down approach for producing PAHs through the hydrogen processing of SiC dust grains, which is schematically shown in Fig. 1. Such SiC grains are formed in the inner regions (15 R*, R* stellar radius) of evolved stars by means of gas-phase condensation14,18. We show that in space, SiC crystallites possess a signicant amount of segregated carbon on their surfaces, primarily organized as graphitic overlayers. These grains travel through the CSE owing to the radiation pressure induced by the infrared photons of the central star, until they reach the cold external layers of the envelope. At this point (45 R*), they are exposed to ultraviolet radiation. PAH emission has been observed to be prominent in photodissociation regions where most molecules are photodissociated and dust grains are processed2,8. Here ultraviolet photons and H atoms produced by H2 photodissociation further process the grain surfaces. We show experimentally that atomic H interaction at elevated temperature (1,0001,300 K) on a graphene-terminated SiC surface, where graphene denotes a single layer of carbon atoms arranged in a honeycomb structure, triggers signicant surface erosion even at low atomic H exposures. Thus, the graphene layer at the SiC crystallite surface is etched, leading to the formation of broken graphene akes with sizes ranging from a few carbon rings to

large graphene areas. This provides a method to inject PAHs and other PAH-related species into the gas phase.

The potential of surface science methodology to mimic and characterize the essential characteristics of complex problems concerning molecules on surfaces has been previously demonstrated by explaining, among others, the fundamentals of catalysis21 or the formation of H2 on the stardust grains20. We simulate the conditions of interstellar space using ultra-high vacuum (UHV) chambers equipped with in situ atom-resolved scanning tunnelling microscopy (STM) and third-generation synchrotron radiation-based X-ray photoemission-spectroscopy (XPS), among others. The experimental results are conrmed by ab initio calculations. This theoryexperiment combination provides a state-of-the-art morphological, structural and spectroscopic description about molecular interactions on SiC crystallites under CSE conditions. In addition, SiC has also been widely studied in condensed matter research and nowadays it is well accepted that all SiC crystalline structures are covered with graphene after annealing in vacuum22,23.

ResultsGraphitized interstellar SiC. Dust grains produced in Carbon-rich asymptotic giant branch stars are abundant in SiC crystal-lites18. Their size can range from few nm in the proximities to the star to some mm. Extra-solar particles of SiC as big as 5 mm have been found in the interior of chondritic meteorites24,25. The most common circumstellar SiC polytypes, as found in the Murchison meteorite are 3C-SiC (80%) and 2H-SiC (16%). The size of the SiC grains in the ISM is sufciently large to present atomically reconstructed facets and their main chemical aspects can therefore be condently simulated by the use of SiC single crystals. We have used as an analogue to the dust grains a 6H-SiC(0001) single crystal, and the data were taken with a base pressure in the range of 10 11 mbar, similar to the conditions occurring in the densest regions of the ISM26 (see

Supplementary Note 1).

Because of temperature-induced Si sublimation and subsequent C segregation, the surface of SiC crystallites is always graphitized after annealing at typical temperatures of the ISM depletion zone (Fig. 1). In the case of SiC(0001), the surface after annealing at 1,500 K consists of a combination of atomically at terraces partially covered with single-layer graphene (Fig. 2a) and other areas with a carbonaceous termination consisting either of a mixture of Si and C atoms in sp2/sp3 conguration or bilayer graphene (BLG)23,27. The graphene termination of different SiC specimens after being submitted to high temperature has been observed in different previous studies, both for large grains or small nanoparticles22,23. Therefore, we conclude that the SiC grains present at CSE end with a graphene cover, as the one shown in Fig. 2a.

G/SiC exposed to atomic hydrogen. As the surface-graphitized SiC grains travel outwards into the ISM they cool (1,5001,000 K). During this trip, they are exposed to ultravioletphotodissociated atomic H with an approximate impinging rate of E1 1014 particles cm 2 s. These are the conditions we use to

simulate the process: a temperature of about 1,200 K while exposing the surface to an approximate dose of 4 Langmuir (L; 1 L 10 6 mbar sE1015 particles cm 2 s) of atomic H.

Figure 2ce shows STM images at different length scales with atomic resolution demonstrating that the graphene-covered SiC surface has been strongly modied. The upper part of the image in Fig. 2b still contains grapheme, whereas the lower region has been completely etched and roughens. In the etched part some remaining carbonaceous pieces can be observed (brighter features

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Si depletion, graphitization

Hydrogenation, G/SiC to G/H/SiC

PAH desorption

SiC nucleation

Processes occurring in the

circumstellar envelope

Carbon-rich red giant

2,000 K

1,500 K

1,200 K

1,000 K

3R* 5R* 1,500 K 1,200 K

1010 cm3

1011 cm3

3 year 6 year

8R* 750 K 109 cm3

10 year

R (R*) = 2R*

2,000 K 1012 cm31.5 year

Processes occurring

in the laboratory

T (K) =n (p cm3) = Time (year) =

2,100 K: CVD SiC formation

1,500 K:SiC graphitization

1,200 K: H-induced etching of graphene/SiC

1,000 K: PAH formation and desorption

Figure 1 | Qualitative sketch of the proposed process. The steps for the formation of interstellar PAHs and aromatic-aliphatic molecules in the envelope of an evolved star can be divided into four different stages. (1) Formation of SiC in the gas phase and condensation into micrometre- and nanometre-sized grains (T 2,000 K; 15 R*). (2) Annealing of the SiC dust grains due to the proximity of the star and the consequent promotion of surface C-rich

phases and graphene (T 2,0001,500 K, 15 R*). (3) Exposure of the surface to atomic hydrogen, promoting graphitization of the C-rich surface and H

passivation of the underlying buffer layer (T 1,5001,200 K; 520 R*). (4) Etching of the graphene by atomic hydrogen and thermally assisted

desorption of PAHs (T 1,2001,000 K, 520 R*). Temperature values are taken from ref. 36. Lower row: typical temperature in laboratory experiments.

in Fig. 2d). To emphasize the erosion process, we show in Fig. 2c a height prole of an etched surface recorded on Fig. 2b after dosing the crystallite with atomic H and for comparison a height prole of an as-grown surface. The peak-to-valley difference is about 3 , which is the size of an atomic step in the SiC surface, suggesting that a whole layer has been disrupted. It is clear that the corrugation of the proles strongly increases after H dosing at high temperature, indicating a strong erosion of the carbon termination.

In general, on H exposure at elevated temperature, the surface tends to reorganize from sp3 to sp2 conguration27, leading to a cracked and decoupled graphene layer from the SiC surface. Figure 2d shows some formed carbonaceous structures of different sizes remaining weakly bound to the surface. High-resolution images reveal some features that can be assigned to large PAHs (see Fig. 2e,f). These features show a molecular orbital-like electronic density and a periodicity of about 2.4 between bright lobes28. This distance corresponds to the lattice constant of graphene, indicating that these structures consist of a concatenation of benzene rings (see Supplementary Note 2 and Supplementary Figs 1 and 2). This situation leaves small patches of graphene weakly bound to a H-saturated substrate, ready to be ejected by thermal or photoexcitation processes. These molecules could be the origin of the recently proposed top-down interstellar carbon chemistry29 and the reason why SiC is abundant in evolved stars but normally gone in the ISM30.

Figure 3a,b shows some regions of the surface that have been heavily etched. Figure 3a displays an area where the graphene cover has been strongly shrunk to small patches. Only graphene islands remain in the centre of the image. The most probable

scenario is that the original ake covered the whole upper terrace (brighter area at the left side of Fig. 3a) and continued over another graphene layer into the lower terrace (darker area at the right side of Fig. 3a). The etching process has attacked the graphene in contact with the H-passivated substrate, and the process has stopped at the step edge where the overlayer becomes BLG. Thus, the presence of BLG can be considered as a protective lm with respect to H attack31,32. This could be the reason why this process does not disrupt the whole SiC grain. Figure 3b shows a graphene area where a small ake has been blown out from the surface and the contour of the etched region is visible. The akes exhibit rounded smooth contours, and high-resolution images show heavily distorted electronic states at the edges, which could originate from passivation through hydrogenation of the edge atoms33. In both images of Fig. 3 can be seen that the edge between graphene and eroded parts is strongly modied, indicating that H erosion takes place through edges and defects of the graphene layer. These structural defects are spots to intercalate H atoms into the graphene structure. In some parts of the surface, we have found evidences of H inserted between the SiC interface and the graphene layer. The Fig. 3c shows three bumps identied by their apparent height as H atoms adsorbed on the lower basal plane of graphene.

DFT and MD theoretical analysis. To shed some light on the etching of the graphene by atomic hydrogen, and the subsequent thermally assisted PAH formation and desorption at T 1,000 K

and beyond 5 R* (see Fig. 1), we have carried out density functional theory (DFT)-based atomistic molecular dynamics (MD)

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Graphene

Etched

4

3

1

0

Etched

Graphene

Height ()

2 Graphene

Quasi-6x6

0 5 10 15 20 25 30

Length (nm)

Figure 2 | Graphene etching on high-temperature and atomic H dose. (a) STM image (5 5 nm2) showing a graphene region V 100 mV; the small

mesh corresponds to the atomic honeycomb atomic lattice of graphene. (b) The image shows at the upper side a graphene plane, whereas at lower part the surface has strongly roughened. 40 40 nm2 V 100 mV. (c) Prole on STM images recorded before (blue) and after (black) being exposed to

atomic H with the surface kept at 1,200 K, taken along the dotted line indicated in. The total roughness (r.m.s.) of the surface increases from 0.22 to0.69 . (b,df) Series of STM images showing the graphene surface eroded after atomic H exposure at high temperature. (d) STM image (10 10 nm2)

recorded on the most eroded part that shows the formation on the surface of small molecules and nanostructures. V 200 mV (f) STM image

(2.2 4 nm2) zooming in on a detail of the surface showing protrusions corresponding to localized molecular orbitals of a PAH-like molecule. (e) Overlaid

optimized DFT model of a proposed PAH formed.

simulations. To fully understand the mechanism underlying the PAH formation and desorption, we have designed three heuristic theoretical procedures: rst, to mimic the behaviour of BLG, we have performed MD simulations (T 1,000 K) of two parallel

6 6 H-saturated graphene nanoribbons (with 132 atoms

each) distanced by 3.35 , see Supplementary Movies 13; second, to quantify the inuence of interlayered atomic hydrogen in the PAH formation and desorption process, we have repeated the MD calculations of the nanoribbon by including around0.5 ML atomic hydrogen content in between the interlayer region, see Supplementary Movies 46; and nally, to simulate the thermally assisted behaviour of a graphene single layer, we have performed MD calculations (T 1,000 K) of a 6 6 H-saturated

graphene nanoribbon on a (x) fully pre-optimized fragment of on-SiC graphene buffer layer (at a distance of 3.35 ), with the same interlayer atomic H content than in previous case (around0.5 ML), see Supplementary Movies 79. All these molecular

dynamics calculations have been carried out up to a total time of 250 fs (in steps of 1 fs) at 1,000 K. The fourth set of movies (Supplementary Movies 1012) is calculated with the same conditions than the last set; this is with a pre-optimized fragment of xed buffer layer but the timescale is larger, up to 1,000 fs.

From this exhaustive theoretical analysis, we can conclude that the highest PAH formation and desorption efciency is observed for the graphene single layer on the x on-SiC graphene buffer layer with a high atomic hydrogen content in between the interlayer region, as evidenced in the STM experiments. For this case, after 250 fs, MD process reveals a CC broken bond ratio of around 60%, to be compared with the 30 and 5% for the hydrogenated and clean graphene bilayer, respectively.

The lower PAH formation and desorption efciency for the hydrogenated BLG may be explained in terms of that the two free-to-move graphene layers forming the bilayered system can structurally follow, adapt and accommodate the vibrations

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Etched

Monolayer graphene

Bilayer graphene

Monolayer graphene

MD T = 103 K

(t = 250 fs)

Figure 3 | STM images after H dosing at 1,100 K of G/SiC and computer simulation. (a) 100 100 nm2, 1,100 mV. Wide view of a region where the

graphene has been reduced to a stripe with rounded edges on the centre of the image. On the right part of the image, a region of BLG seems unaltered. The upper inset shows a high-resolution image of a kink where single-layer graphene, BLG and the modied etched substrate coexist. (b) STM image of a hole in the middle of a graphene terrace. These holes are observed after H treatment at high temperatures and they evidence etching through edges and defects. 70 70 nm2, 1.1 V. (c) Atomically resolved STM image of a subsurface trimmer 2.5 2.5 nm2, 400 mV. (d) Initial and nal frames

of the graphene surface extracted from a movie of DFT molecular dynamics calculations of the G/H/SiC system. The distance between graphene layers expand from 7% at room temperature to about 62% at 1,000 K, and nally, after 250 fs (MD time), the topmost layer disrupt in the form of small carbonaceous species. In this case, benzene rings (blue oval), acetylene molecule (black oval), methylene radical (orange oval) or PAHs (red oval) can be identied.

thermally induced in each other, reducing in this way the efciency (with respect to the single-layer graphene). Graphene on SiC crystallites is supported on a strongly bonded (almost unaltered) buffer layer, which is unable to accommodate the thermally induced vibrations in the supported graphene single layer. Lower etching efciency in BLG has been experimentally conrmed in Fig. 3.

Figure 3d shows the structure of the starting graphene ake on the on-SiC graphene buffer layer before (left panel, t 0), and

after (right panel, t 250 fs) the MD calculation, showing the

initial stages of PAH formation and desorption. The MD simulations reveal that, at T 1,000 K, the atomic hydrogen

located in between the interlayer region starts to attack the already H-saturated edges of the nanoribbon, favouring the rupture of the external CC bonds, and the creation of some CH2 groups. As the time increases, the atomic hydrogen located in the interlayer region starts to attach to more internal CC bonds in the ribbon, inducing their rupture, as well as the formation of CH and C2H2 groups, and small aliphatic C3Hn chains. From around 200 fs, all the functional groups tend to separate from each other and desorbs by expansion, and some larger structures can be observed, such as some aromatic rings and larger PAHs.

In addition, to theoretically quantify the graphene-decoupling pattern by the intercalated hydrogen with temperature, we have performed an additional optimization calculation (at T 0 K), by

including the van der Waals interaction in a perturbative way (see ref. 34 and references therein), of an innite graphene bilayer with a 0.5-ML of intercalated atomic hydrogen content. The result of this optimization process shows the intercalated hydrogen atoms attaching to internal (basal) on-top carbon

positions leading to a net interlayer distance dilatation from3.35 for the pristine bilayered grapheneto an interlayer distance of 3.60 for the bilayered graphenewhich can be quantied in a net graphene decoupling of 7% with respect to the clean system. This value can be easily compared with the decoupling rates extracted from the molecular dynamics simulations (at T 1,000 K) of the previously mentioned calculations

(the pristine bilayered graphene and its H-intercalated form) after a molecular dynamics time of 250 fs. For these systems, the decoupling rates can be estimated to be around 19% for the pristine graphene bilayer at T 1,000 K (an average interlayer

distance dilatation from 3.35 to 3.98 ) and in around 62% for the graphene bilayer with intercalated atomic hydrogen at T 1,000 K (an average interlayer distance dilatation from 3.35

to 5.42 ). This behaviour clearly manifests that both the temperature and the intercalated atomic hydrogen content improve the graphene-decoupling efciency. Nevertheless, according to the previous numbers, the inuence of the intercalated atomic hydrogen is substantially higher (around three times higher) than the temperature, showing its important catalytic role in this graphene-decoupling reaction.

XPS C1s intensity on H exposition at high temperature. The above-proposed mechanism is based on topographical information obtained from high-resolution STM images and DFT calculations. To conrm them, and to acquire quantitative information, we explore the relevant chemistry by analysing high-resolution synchrotron radiation XPS spectra before and after H exposures. The C1s core level peak gives an indication of the

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atomic density of C at the surface region of a sample and the particular electronic environment (that is, bonding conguration)35. Figure 4 shows the C1s spectrum of the SiC before and after exposure to 550 l of atomic H at 1,250 K. This peak, as well as the Si2p peak (see Supplementary Fig. 3), can be decomposed into different components in good agreement with the scientic literature35,36 (see Supplementary Note 3 for Si2p curve components decomposition). The component appearing at 284.8 eV can be assigned to the graphene sp2 conguration and is the biggest contribution to the C spectra (58.2% of the total), consistent with the area estimation from the STM topography. The components located at lower binding energies (about 284.0 eV) are attributed to bulk carbides and the components at the left-hand side of the spectra to the buffer layer35. Figure 4 shows that the whole intensity of the C1s peak decreases on H exposure treatment, indicating that there is about a 30% of carbon desorption, which left the surface with lower density than the original one. Although this number cannot be directly extrapolated to the CSE, the high efciency of the presented erosion process makes these events highly probable to occur at the CSE regions.

If we perform the intensity analysis of the components of the C1s peak before and after H exposition, while keeping the sample at high temperatures, we can study the modication degree of every component. The C1s core level peak and its components indicate the quantity and density of carbon present on the sample. By tracking the changes in the intensity of every component, we can follow the modication of the carbon species upon H exposition. Therefore, in case the intensity of some particular component decreases, we can assume the number of carbon atoms in such bonding conguration is diminishing. The components of the C1s peak are divided in two ranges of modication after hydrogen treatment at high temperatures. The components at energies 284.8, 286.4 and 284.0 eV decrease,

respectively, by 19.4, 19.4 and 20.7% while the compo

nents at energies 285.25 and 285.7 eV change by 30.2 and 29.8%. This evidences a loss of carbon atoms on the surface in a disordered manner that leaves the surface with a smaller C density than before the hydrogen treatment. If the carbon atoms would leave the surface in a layer-by-layer mechanism, we would nd an XPS spectra modication that eventually would return the intensity and shape to its original spectra, indicating that a full layer has been eroded and a new one is exposed to the X-rays.

As the C1s peak intensity decreases only in two different values: approximately 20% for the graphene, bulk and S. C-Si 3

components, and approximately 30% for the surface S. C-Si 1

and S. C-Si 2 components, three values in the Si2p (B13.5, B 4

and B20%) indicates certain relation between the components sharing the same degradation. The less altered components pertain to the most buried electronic congurations as the hydrogen will mostly attack the atoms located at the surface vacuum interface. Therefore, the C bulk component is less altered than the surface S. C-Si 1 and S. C-Si 2 components. The exceptions are the graphene and the S. C-Si 3 components that are less altered than the other two surface components. This might arise because of a lower efciency of the etching process at the carbon atoms in such congurations.

Figure 5a shows the evolution of the lineshape of the C1s peak after subsequent different treatments involving exposition to H atmospheres at different temperatures. The original graphitization degree of the sample is around one monolayer. After a rst cycle, 50 L of atomic hydrogen while keeping the sample at 1,000 K, the C1s intensity decreased 5%. Subsequently, we exposed it to a second cycle, 90 L with a sample temperature of 1,200 K, that resulted in an extra 7% intensity loss. The third subsequent cycle was performed keeping the sample at room temperature (300 K) and submitting it to a hydrogen dose of

Graphitization 700C H:4(7) 2 min 900C H:5(7) 3 min RT H:6(7) 6 min 900C H:6(7) 9 min

sp2

Intensity (a.u.)

S. C-Si 1

S. C-Si 2

S. C-Si 3

Bulk SiC

288 287 286 285 284 283 Binding energy (eV)

287 285 284

Figure 5 | High-resolution evolution of the XPS core level peak of C1s. (a) Spectra corresponding to the exposure of a graphene covered SiC(1 ML) surface to atomic H at different temperatures during different times. (b) Spectra corresponding to the exposure of a partially graphene-covered SiC surface, surface to atomic H at different temperatures and doses: black curve (as graphitized), brown curve (rst two steps of a gure, that is, 2 min at 700 C and 4 10 7 mbar and later 3 min at 900 C and

5 10 7 mbar) and red curve (second two steps of a, that is, 6 min at 30 C

and 6 10 7 mbar followed by 9 min at 900 C and 6 10 7 mbar. The

photon energy for both spectra is 400 eV.

286

285

284

287

286

Binding energy (eV) Binding energy (eV)

Figure 4 | Synchrotron-based XPS spectra of C1s. The spectra are shown before (black curve) and after (red curve) H treatment (550 L) at 1,200 K. Beneath the experimental data, we show their decomposition into separate curve components. A decrease in the C peak of about 28% in the surface-related components indicates that those C species have been etched away through H processing. The component appearing at 284.8 eV can be assigned to the graphene sp2 conguration. The components located at lower binding energies (about 284.0 eV) are attributed to bulk carbides and the components at the left-hand side of the spectra to the buffer layer (between graphene and SiC). Photon energy: 400 eV.

Intensity (a.u.)

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220 L, and it resulted in an extra intensity loss of 3%. The last cycle performed on this sample consisted of 325 L of atomic hydrogen while the sample was kept at 1,200 K, and it resulted in an extra 3% decrease of the photoelectron intensity. In total, we dosed around 685 L of hydrogen on the sample, which caused a total intensity decrease of 19%. Importantly, in all the steps of the experiment using low hydrogen pressure, the XPS intensity was reduced after high-temperature dose. This experiment is an indication of the low-pressure etching efciency on SiC samples with graphene coverage around one monolayer.

The etching process also takes place on samples with a graphene layer covering less than one monolayer. In Fig. 5b, we see the C1s XPS core level spectra of a SiC sample whose degree of graphitization is lower than one monolayer. The intensity after submitting the sample to cycles of hydrogen treatment at high sample temperatures (1,0001,200 K) diminishes around 14%. The degree of modication of the hydrogen etching is dependent, in a non-trivial manner, on the hydrogen dose, the temperature of the sample during the exposition and the original degree of graphitization of the sample.

DiscussionSurface science aims at studying highly controlled systems and tracing the inuence of particular parameters in the nal product of a chemical transformation. This approach has produced outstanding results in different elds, such as solid-state chemistry or catalysis21. The environmental parameters in our simulation chambers, such as temperature, vacuum and gas composition, were as close as possible to the conditions occurring in the CSE. Of course, real CSEs have a much complex chemistry than the one gathered in our experimental ideal environmental simulation chambers. Molecules such as H2, CO or C2H2 are present in the proximities of evolved carbon-rich stars37, and electromagnetic radiations, covering large regions of the spectrum, have non-negligible intensities in the vicinities of the stars. These parameters might play a role in the mechanism proposed here for the formation of PAHs. However, in the case of the mechanism that we are proposing, the role of radiation and small molecules is not so important. First, because graphene absorbs electromagnetic radiation through excitation of vibrational phonon modes, which later decay through radiation emission, this process does not alter the intrinsic properties of graphene38. Second, because graphene is inert to most of the molecules present in the CSE, such as H2, CO, C2H2. We have performed exactly the same experiment using H2 instead of atomic H and no etching (not even molecular hydrogen adsorption) takes place. We have obtained exactly the same STM images as for the clean SiC surface (Fig. 2a).

This accurate methodology allows us to test the inuence of many parameters in the process. Our simulation experiments and calculations indicate that the size of the resulting PAHs strongly depends on the hydrogen concentration and the formation temperature. If atomic hydrogen is included in the system and temperature is much lower than 900 K, we just have adsorption of H on the surface. If the temperature exceeds 1,500 K, we have new graphene formation at the surface as it is etched away (the surface regenerates). To have efcient graphene erosion, temperature in the 9001,200 K range and high doses of atomic hydrogen are required. Thus, in our experiment performed in UHV chambers, we have been able to mimic the most important conditions for SiC dust grains in the CSE, and this kind of research opens the door to use UHV-model systems to understand the precise role of co-absorption of molecules, other than hydrogen, and radiation in dust-processing mechanisms (for further understanding of the UHV equivalence of the CSE, see Supplementary Note 1).

SiC is produced close to the evolved star as small-sized nanoparticles (typically few nm) and it has been proven that the crystallization rates of SiC grains can be very fast (Emm h 1) at high temperatures18. In our experiment, we have used single-crystal surfaces to simulate the process, and one might think that the surface chemical properties of large extended crystals and nanoparticles could be completely different. However, this is not the case. Just a few crystallographic unit cells are required for recovering the bulk electronic structure of SiC. There are several high-resolution transmission electron microscopic studies using SiC nanoparticles of diameter about 5 nm revealing that the graphitization process takes place efciently on high-temperature annealing39,40 all around the SiC nanoparticle22. Therefore, our single-crystal approach can be seen as a suitable atomic scale description of the astrochemical processes taking place on the surface of both, micrometre- and nanometre-sized, SiC grains populating the CSE.

The usual size of PAHs in the ISM is estimated to be in the range of hundreds of C atoms, in good agreement with the one that we have indentied in Fig. 2d, which has around 180 C atoms. Although Fig. 3d might give the erroneous impression that only small PAHs can be produced, we would like to remark that the PAH formed by our proposed mechanism can be quite large. This misleading idea is due to the reduced number of atoms that can be handled into the complex ab initio MD calculations we have performed.

Moreover, one might think that in our model, the etching process will last forever cracking the formed PAHs into smaller and smaller pieces as time goes on. There are several factors promoting a self-stopping of the etching of graphene into PAHs. The efciency of the process is strongly dependent on the underlying interface, as it is shown in the DFT calculated movies (see Supplementary Movies 3, 6 and 9). In other words, the graphene etching takes place efciently because it is supported on a SiC grain. Once the PAH is decoupled from the SiC, etching is not an efcient mechanism. The hydrogen concentration and the temperature decrease as the PAH travel outwards in the CSE. PAHs arrive to regions of ISM where the H concentration can be considered negligible. The same occurs with the temperature, which rapidly decrease below 900 K. We have recorded STM images of H deposited on graphene at 300 K where H clusters are spotted on top of C atoms. This H adsorption process is also known to occur on PAHs and it is called hyperhydrogenation. We have not seen any adsorption or erosion when H2 is used.

Also, the smaller PAHs have less probability of interacting with the atomic hydrogen and a higher hydrogen adsorption energy. DFT calculations show that as you decrease the size of the PAH, the atomic hydrogen capture probability decreases. In this sense, there are two factors inuencing the self-stopping hydrogenation of small PAHs. First, the size of the absorption area; the smaller the PAH, the lower the probability of scattering with an impinging adsorbate. Second, energetic considerations; the smaller a PAH, the higher the adsorption energy for an extra hydrogen (see Supplementary Note 4 and Supplementary Fig. 4). Therefore, graphene etching in the CSE is a self-limiting process yielding to signicantly large PAHs.

Finally, based on these experiments, we should not expect the formed ISM PAHs to have the identiable structures. Mixtures of PAHs, aromatic-aliphatic molecules and hyperhydrogenated PAHs molecules have been proposed to account for the astronomical infrared observations41. All these aromatic chemical species can be produced out of the above-presented processing mechanism of the SiC dust. We are aware that in our experiments we observe the surface of the SiC grains instead of the produced PAHs. However, the latter is difcult as a priori our mechanism does not favour a particular size of the produced

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species, and a global increase in mass detection experiments can just be expected.

In conclusion, we propose an alternative PAH top-down formation mechanism through graphene etching on the surface of SiC dust grains at ISM. This model is supported by a combination of different state-of-the-art experimental techniques with advanced computational methods. The good match between the available astrochemical information and the advanced surface science experimental methodology suggests that the problems of the former can be successfully understood through the techniques of the latter and opens the door to the investigation and modelling of other processes related to dust particles in ISM.

Methods

Scanning tunnelling microscope. STM experiments were performed in a UHV system with a base pressure 1 10 10 mbar using an Omicron variable

temperature scanning tunnelling microscope, which enables us to perform measurements while keeping the sample in the 40300 K range. Tungsten tips were used to acquire topography images in constant current mode. The chamber is equipped with an atomic H ux source that produces hydrogen by H2 thermal cracking (99% efciency, no protons). The gas passes through a tungsten capillary kept at 2,500 K by electron bombardment. The sample was placed at 150 mm from the capillary. The chamber contains an annealing stage that heats the sample through direct heating. After hydrogen deposition with the SiC sample at high temperature, it is transferred to the STM for local measurements.

Photoemission experiments. Synchrotron radiation XPS measurements were performed at the SuperESCA beamline of Elettra, the third-generation synchrotron light source in Italy. Here the top-up mode of operation of the storage ring generating synchrotron radiation provides a stable photon ux in a wide range of photon energies. The end station of SuperESCA consists of an UHV chamber with base pressure of 1 10 10 mbar. It is specically designed for high-ux and high-

resolution core level spectroscopy, particularly of adsorbates on single-crystal surfaces using soft X-ray synchrotron radiation. The atomic H doser of the STM apparatus was mounted in the chamber. The main chamber is equipped with a hemispherical electron analyser using an home-made delay line electron detection system. The overall energy resolution, comprising the analyser and the photon beam resolution, was better than 50 meV.

C1s core level peak decomposition into curve components. The XPS spectra analysis of the results obtained at the SuperESCA beamline was performed using the FITT program after a Shirley background subtraction. The high resolution of the experimental system permits us to decompose the peaks into its basic curve components; every component reects a particular electronic environment (that is, bonding conguration) and results from the convolution of a purely quantum Lorentzian energy distribution (full width at half maximum, 0.120.2 eV) with a Gaussian distribution (full width at half maximum, 0.40.7 eV)42. However, the carbon chemistry is very rich and the interpretation of the C1s spectra is not straightforward. The components corresponding to sp2- and sp3-bonding congurations measured for pure graphite and pure diamond, respectively, appear at very similar binding energies (284.68 and 285.0 eV, respectively)43,44.

We have decomposed the C1s XPS peak of the as-grown G/SiC sample (Fig. 4) in ve different basic components without using any asymmetry parameter. SiC is a wide-gap semiconductor and therefore we use symmetrical peaks for the tting. In Supplementary Fig. 1, we show the C1s and Si2p photoelectron peaks decomposed in its basic components before and after H treatment. The component appearing at 284.83 eV can be directly assigned to the graphene contribution (or C atoms in sp2 conguration), and is the biggest contribution to the C1s core level peak, with its58.2% of the total area. This component is also important for calibration of the energy width, since it is narrow enough to test the Lorentzian and Gaussian widths with a high precision. The components located at lower binding energies than the main peak (in our case 283.98 eV) are attributed normally to carbides, and we assign them to the C in the SiC bulk crystal35,36. There are three other components at higher binding energies, which can in principle be assigned to different congurations of C atoms in the supercial rearrangement that takes place on surface reconstruction35. We assign these XPS components to surface-related peaks and name them with an S. in the way that follows: S. C-Si 1 at 285.26 eV, S. C-Si 2 at 285.73 eV and S. C-Si 3 at 286.4 eV, respectively. We will later discuss the validity of this assumption.

Theoretical spatially distributed density of states. The molecular orbitals of the isolated PAHs have been calculated using the DFT calculation package FIREBALL. To resemble the experimental STM images of free PAHs, they have been taken as spatially distributed orbital densities, density of states, with suitable values of the orbital density three-dimensional isosurfaces to mimic the experimental morphology.

Molecular dynamics. The atomistic MD simulations have been performed by using the fast and efcient local-orbital DFT code FIREBALL. The temperature in these calculations is simulated by giving to each non-xed atom a random initial velocity. In particular, to mimic the physical conditions for the etching of the graphene by atomic hydrogen, the dynamical processes were simulated giving to all the systems an initial temperature T 1,000 K. Then, the atoms were free to move

using free MD up to a total time of 250 fs, corresponding to 250 MD steps. Three different graphenic congurations have been simulated by molecular dynamics:(a) two parallel 6 6 H-saturated at graphene nanoribbons (with 132 atoms each)

distanced by 3.35 ; (b) the same two parallel 6 6 H-saturated at graphene

nanoribbons (with 132 atoms each) distanced by 3.35 as in previous case (a), but this time by including around 0.5 ML atomic hydrogen content in between the interlayer region, which initially forms an homogenous atomic H-intercalated layer; and, nally, (c) a 6 6 H-saturated at graphene nanoribbon on a (x) fully

pre-optimized 6 6 fragment of a H-saturated on-SiC graphene buffer layer (at a

distance of 3.35 ), with the same interlayer atomic H content than in previous case (b).

Atomic hydrogen adsorption potential energy curves. The adsorption energy curves of atomic hydrogen in interaction with PAHs of different sizes have been obtained by using the accurate planewave DFT parametrization included in the QUANTUM ESPRESSO simulation package. In addition, to properly account the non-negligible dispersion forces appearing within this kind of interactions, we have used the DFT D approach also implemented in the mentioned planewave

simulation code. For this purpose, we have used a generalized gradient-corrected approximation for the electronic exchange-correlation description, and an empirical efcient van der Waals R 6 correction to add dispersive forces to conventional density functionals (DFT-D). Two different adsorption congurations have been analysed in the calculations: chemisorption and physisorption paths of atomic hydrogen approaching towards an on-top PAH carbon atom adsorption site.

References

1. Decin, L. et al. Warm water vapour in the sooty outow from a luminous carbon star. Nature 467, 6467 (2010).

2. Tielens, A. Interstellar polycyclic aromatic hydrocarbon molecules*. Annu. Rev. Astron. Astrophys. 46, 289337 (2008).

3. Cami, J., Bernard-Salas, J., Peeters, E. & Malek, S. E. Detection of C60 and C70 in a young planetary nebula. Science 329, 11801182 (2010).

4. Peeters, E., Allamandola, L., Hudgins, D., Hony, S. & Tielens, A. in Astrophysics of Dust, Vol. 309 (eds A. N. Witt, G. C. Clayton & B. T. Draine), 141162 (Astronomical Society of the Pacic, 2004).

5. Knacke, R. Carbonaceous compounds in interstellar dust. Nature 269, 132134 (1977).

6. Gail, H.-P., Zhukovska, S., Hoppe, P. & Trieloff, M. Stardust from asymptotic giant branch stars. Astrophys. J. 698, 11361154 (2009).

7. Caro, G. M. et al. Amino acids from ultraviolet irradiation of interstellar ice analogues. Nature 416, 403406 (2002).

8. Joblin, C. & Tielens, A. G. G. M. PAHs and the Universe: A Symposium to Celebrate the 25th Anniversary of the PAH Hypothesis in EAS Publications Series 46 (Cambridge University Press, 2011).

9. Tielens, A. The molecular universe. Rev. Mod. Phys. 85, 1021 (2013).10. Kwok, S., Volk, K. & Bernath, P. On the origin of infrared plateau features in proto-planetary nebulae. Astrophys. J. Lett. 554, L87L90 (2001).

11. Frenklach, M. & Feigelson, E. D. Formation of polycyclic aromatic hydrocarbons in circumstellar envelopes. Astrophys. J. 341, 372384 (1989).

12. Cherchneff, I., Barker, J. R. & Tielens, A. G. Polycyclic aromatic hydro-carbon formation in carbon-rich stellar envelopes. Astrophys. J. 401, 269287 (1992).

13. Jos, C. et al. Infrared space observatorys discovery of C4H2, C6H2, and benzene in CRL 618. Astrophys. J. Lett. 546, L123L126 (2001).

14. Fonfra, J. P., Cernicharo, J., Richter, M. J. & Lacy, J. H. A detailed analysis of the dust formation zone of IRC 10216 derived from mid-infrared bands of

C2H2 and HCN. Astrophys. J. 673, 445469 (2008).15. Pilleri, P., Montillaud, J., Bern, O. & Joblin, C. Evaporating very small grains as tracers of the UV radiation eld in photo-dissociation regions. Astron. Astrophys. 542, A69 (2012).

16. Contreras, C. S. & Salama, F. Laboratory investigations of polycyclic aromatic hydrocarbon formation and destruction in the circumstellar outows of carbon stars. Astrophys. J. Suppl. Ser. 208, 6 (2013).

17. Cernicharo, J. The polymerization of acetylene, hydrogen cyanide, and carbon chains in the neutral layers of carbon-rich proto-planetary nebulae. Astrophys.J. Lett. 608, L41L44 (2004).18. Frenklach, M., Carmer, C. & Feigelson, E. Silicon carbide and the origin of interstellar carbon grains. Nature 339, 196198 (1989).

19. Watson, W. & Salpeter, E. Molecule formation on interstellar grains. Astrophys.J. 174, 321340 (1972).

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NATURE COMMUNICATIONS | DOI: 10.1038/ncomms4054 ARTICLE

20. Hornekr, L., Baurichter, A., Petrunin, V., Field, D. & Luntz, A. Importance of surface morphology in interstellar H2 formation. Science 302, 19431946 (2003).

21. Sachs, C., Hildebrand, M., Vlkening, S., Wintterlin, J. & Ertl, G. Spatiotemporal self-organization in a surface reaction: from the atomic to the mesoscopic scale. Science 293, 16351638 (2001).

22. Ostler, M. et al. Direct growth of quasi-free-standing epitaxial graphene on nonpolar SiC surfaces. Phys. Rev. B 88, 085408 (2013).

23. Soukiassian, P. G. & Enriquez, H. B. Atomic scale control and understanding of cubic silicon carbide surface reconstructions, nanostructures and nanochemistry. J. Phys. Condens. Matter 16, S1611S1658 (2004).

24. Daulton, T. et al. Polytype distribution in circumstellar silicon carbide. Science 296, 18521855 (2002).

25. Daulton, T. et al. Polytype distribution of circumstellar silicon carbide: Microstructural characterization by transmission electron microscopy. Geochim. Cosmochim. Acta 67, 47434767 (2003).

26. Caro, G. M. et al. New results on thermal and photodesorption of CO ice using the novel InterStellar Astrochemistry Chamber (ISAC). A&A 522, A108 (2010).

27. Riedl, C., Coletti, C., Iwasaki, T., Zakharov, A. A. & Starke, U. Quasi-free-standing epitaxial graphene on SiC obtained by hydrogen intercalation. Phys. Rev. Lett. 103, 246804 (2009).

28. Repp, J., Meyer, G., Stojkovi, S. M., Gourdon, A. & Joachim, C. Molecules on insulating lms: scanning-tunneling microscopy imaging of individual molecular orbitals. Phys. Rev. Lett. 94, 026803 (2005).

29. Bern, O. & Tielens, A. G. Formation of buckminsterfullerene (C60) in interstellar space. Proc. Natl Acad. Sci. 109, 401406 (2012).

30. Whittet, D., Duley, W. & Martin, P. On the abundance of silicon carbide dust in the interstellar medium. Mon. Not. R. Astron. Soc. 244, 427431 (1990).

31. Zhang, Y., Li, Z., Kim, P., Zhang, L. & Zhou, C. Anisotropic hydrogen etching of chemical vapor deposited graphene. ACS Nano 6, 126132 (2011).

32. Diankov, G., Neumann, M. & Goldhaber-Gordon, D. Extreme monolayer-selectivity of hydrogen-plasma reactions with graphene. ACS Nano 7, 13241332 (2013).

33. Park, C. et al. Formation of unconventional standing waves at graphene edges by valley mixing and pseudospin rotation. Proc. Natl Acad. Sci. 108, 1862218625 (2011).

34. Dappe, Y. J. & Martnez, J. I. Effect of van der Waals forces on the stacking of coronenes encapsulated in a single-wall carbon nanotube and many-body excitation spectrum. Carbon N.Y. 54, 113123 (2013).

35. Emtsev, K. V. et al. Towards wafer-size graphene layers by atmospheric pressure graphitization of silicon carbide. Nat. Mater. 8, 203207 (2009).

36. Virojanadara, C., Yakimova, R., Zakharov, A. A. & Johansson, L. I. Large homogeneous mono-/bi-layer graphene on 6HSiC(0 0 0 1) and buffer layer elimination. J. Phys. D. Appl. Phys. 43, 374010 (2010).

37. Agndez, M. & Cernicharo, J. Oxygen chemistry in the circumstellar envelope of the carbon-rich star IRC 10216. Astrophys. J. 650, 374393 (2006).

38. Bonaccorso, F., Sun, Z., Hasan, T. & Ferrari, A. Graphene photonics and optoelectronics. Nat. Photon. 4, 611622 (2010).

39. Peng, T., Lv, H., He, D., Pan, M. & Mu, S. Direct transformation of amorphous silicon carbide into graphene under low temperature and ambient pressure. Sci. Rep. 3, 01148 (2013).

40. Oku, T., Niihara, K. & Suganuma, K. Formation of carbon nanocapsules with SiC nanoparticles prepared by polymer pyrolysis. J. Mater. Chem. 8, 13231325 (1998).

41. Bauschlicher, Jr C. et al. The NASA Ames polycyclic aromatic hydrocarbon infrared spectroscopic database: the computed spectra. Astrophys. J. Suppl. Ser. 189, 341351 (2010).

42. Zampieri, G. et al. Photoelectron diffraction study of the 6H-SiC(0001) sqrt[3] sqrt[3] R30 reconstruction. Phys. Rev. B 72, 165327 (2005).

43. Diaz, J., Paolicelli, G., Ferrer, S. & Comin, F. Separation of the sp^{3} and sp^{2} components in the C1s photoemission spectra of amorphous carbon lms. Phys. Rev. B 54, 80648069 (1996).

44. Wilson, J., Walton, J. & Beamson, G. Analysis of chemical vapour deposited diamond lms by X-ray photoelectron spectroscopy. J. Electron. Spectrosc. Relat. Phenom. 121, 183201 (2001).

Acknowledgements

We acknowledge funding from the following Spanish Grants: CSD2007-41 (NANOSELECT), CSD2009-38 (ASTROMOL), MAT2011-26534. M.S. and P.J. acknowledge GAAWM100101207. Access to the Elettra facility has been granted by the European Communitys Seventh Framework Programme (FP7/2007-2013) under Grant agreement number 226716. The research leading to these results has received funding from the European Union Seventh Framework Programme under Grant agreement number 604391 Graphene Flagship. P.M. acknowledges nancial support from the Rafael Calvo Rods programme. J.I.M. acknowledges CSIC-JAE fellowship programme, co-founded by the European Social Funding. M.S. and P.J. acknowledge GAAV project n. M100101207. We thank O. Bern for fruitful discussions. Background images for artistic assembly of Fig. 1 are from NASA.

Author contributions

All experiments were performed by P.M. who also co-wrote the manuscript, made the data analysis and discussed the main results. M.S. collaborated with STM measurements; DFT and MD calculations were performed by P.J. and J.I.M.; P.L., M.D. and S.L. contributed in the synchrotron radiation XPS set-up and measurements. P.S. participated in the project conception using SiC and helped with the XPS measurements. J.C. designed the main astrochemical idea, co-wrote the manuscript and discussed the results. J.A.M.-G. conducted the research, discussed all data and results and co-wrote the manuscript. All authors commented on the nal version of the manuscript.

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How to cite this article: Merino, P. et al. Graphene etching on SiC grains as a path to interstellar polycyclic aromatic hydrocarbons formation. Nat. Commun. 5:3054doi: 10.1038/ncomms4054 (2014).

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