Cleaning graphene : a first quantum/classical molecular dynamics approach

Graphene outstanding properties created a huge interest in the condensed matter community and unprecedented fundings at the international scale in the hope of application developments. Recently, there have been several reports of incomplete removal of the polymer resists used to transfer as-grown graphene from one substrate to another, resulting in altered graphene transport properties. Finding a large-scale solution to clean graphene from adsorbed residues is highly desirable and one promising possibility would be to use hydrogen plasmas. In this spirit, we couple here quantum and classical molecular dynamics simulations to explore the kinetic energy ranges required by atomic hydrogen to selectively etch a simple residue, a CH3 group, without irreversibly damaging the graphene. For incident energies in the 2-15 eV range, the CH3 radical can be etched by forming a volatile CH4 compound which leaves the surface, either in the CH4 form or breaking into CH3+H fragments, without further defect formation. At this energy, adsorption of H atoms on graphene is possible and further annealing will be required to recover pristine graphene.Comment: 9 figures, 27 page

Concepts, Capabilities, and Limitations of Global Models : A Review

International audienceFor researchers wishing to generate an understanding of complex plasma systems, global models often present an attractive first step, mainly due to their ease of development and use. These volume averaged models are able to give descriptions of plasmas with complex chemical kinetics, and without the computationally intensive numerical methods required for spatially resolved models. This paper gives a tutorial on global modeling, including development and techniques, and provides a discussion on the issues and pitfalls that researchers should be aware of. Further discussion is provided in the form of two reviews on methods of extending global modeling techniques to encompass variations in either time or space

Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation

Catalysis makes possible a chemical reaction by increasing the transformation rate. Hydrogenation of carbon-carbon multiple bonds is one of the most important examples of catalytic reactions. Currently, this type of reaction is carried out in petrochemistry at very large scale, using noble metals such as platinum and palladium or first row transition metals such as nickel. Catalysis is dominated by metals and in many cases by precious ones. Here we report that graphene (a single layer of one-atom-thick carbon atoms) can replace metals for hydrogenation of carbon-carbon multiple bonds. Besides alkene hydrogenation, we have shown that graphenes also exhibit high selectivity for the hydrogenation of acetylene in the presence of a large excess of ethylene.This study was financially supported by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and CTQ2012-32315); and Generalidad Valenciana (Prometeo 21/013) is gratefully acknowledged.Primo Arnau, AM.; Neatu, F.; Florea, M.; Parvulescu, V.; García Gómez, H. (2014). Graphenes in the absence of metals as carbocatalysts for selective acetylene hydrogenation and alkene hydrogenation. Nature Communications. 5:1-9. https://doi.org/10.1038/ncomms6291S195Dreyer, D. R. & Bielawski, C. W. Carbocatalysis: heterogeneous carbons finding utility in synthetic chemistry. Chem. Sci. 2, 1233–1240 (2011).Machado, B. F. & Serp, P. Graphene-based materials for catalysis. Catal. Sci. Technol. 2, 54–75 (2012).Schaetz, A., Zeltner, M. & Stark, W. J. Carbon modifications and surfaces for catalytic organic transformations. ACS Catal. 2, 1267–1284 (2012).Su, D. S. et al. Metal-free heterogeneous catalysis for sustainable chemistry. ChemSusChem 3, 169–180 (2010).Chauhan, S. M. S. & Mishra, S. Use of graphite oxide and graphene oxide as catalysts in the synthesis of dipyrromethane and calix[4]pyrrole. Molecules 16, 7256–7266 (2011).Dreyer, D. R., Jarvis, K. A., Ferreira, P. J. & Bielawski, C. W. Graphite oxide as a carbocatalyst for the preparation of fullerene-reinforced polyester and polyamide nanocomposites. Polym. Chem. 3, 757–766 (2012).Dreyer, D. R., Park, S., Bielawski, C. W. & Ruoff, R. S. The chemistry of graphene oxide. Chem. Soc. Rev. 39, 228–240 (2010).Pyun, J. Graphene oxide as catalyst: application of carbon materials beyond nanotechnology. Angew. Chem. Int. Ed. 50, 46–48 (2011).Rourke, J. P. et al. The real graphene oxide revealed: stripping the oxidative debris from the graphene-like sheets. Angew. Chem. Int. Ed. 50, 3173–3177 (2011).Sun, H. et al. Reduced graphene oxide for catalytic oxidation of aqueous organic pollutants. ACS Appl. Mater. Interf. 4, 5466–5471 (2012).Dreyer, D. R., Jia, H. P. & Bielawski, C. W. Graphene oxide: a convenient carbocatalyst for facilitating oxidation and hydration reactions. Angew. Chem. Int. Ed. 49, 6813–6816 (2010).Dreyer, D. R., Jia, H. P., Todd, A. D., Geng, J. X. & Bielawski, C. W. Graphite oxide: a selective and highly efficient oxidant of thiols and sulfides. Org. Biomol. Chem. 9, 7292–7295 (2011).Hayashi, M. Oxidation using activated carbon and molecular oxygen system. Chem. Rec. 8, 252–267 (2008).Jia, H. P., Dreyer, D. R. & Bielawski, C. W. C-H oxidation using graphite oxide. Tetrahedron 67, 4431–4434 (2011).Kumar, A. V. & Rao, K. R. Recyclable graphite oxide catalyzed Friedel-Crafts addition of indoles to alpha, beta-unsaturated ketones. Tetrahedron Lett. 52, 5188–5191 (2011).Soria-Sanchez, M. et al. Carbon nanostructure materials as direct catalysts for phenol oxidation in aqueous phase. Appl. Catal. B Environ. 104, 101–109 (2011).Verma, S. et al. Graphene oxide: an efficient and reusable carbocatalyst for aza-Michael addition of amines to activated alkenes. Chem. Commun. 47, 12673–12675 (2011).Yu, H. et al. Solvent-free catalytic dehydrative etherification of benzyl alcohol over graphene oxide. Chem. Phys. Lett. 583, 146–150 (2013).Holschumacher, D., Bannenberg, T., Hrib, C. G., Jones, P. G. & Tamm, M. Heterolytic dihydrogen activation by a frustrated carbene-borane Lewis pair. Angew. Chem. Int. Ed. 47, 7428–7432 (2008).Staubitz, A., Robertson, A. P. M., Sloan, M. E. & Manners, I. Amine- and phosphine-borane adducts: new interest in old molecules. Chem. Rev. 110, 4023–4078 (2010).Stephan, D. W. & Erker, G. Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More. Angew. Chem. Int. Ed. 49, 46–76 (2010).Poh, H. L., Sanek, F., Sofer, Z. & Pumera, M. High-pressure hydrogenation of graphene: towards graphane. Nanoscale 4, 7006–7011 (2012).Sofo, J. O., Chaudhari, A. S. & Barber, G. D. Graphane: A two-dimensional hydrocarbon. J. Phys. Chem. B 75, 153401 (2007).Elias, D. C. et al. Control of graphene’s properties by reversible hydrogenation: evidence for graphane. Science 323, 610–613 (2009).Despiau-Pujo, E. et al. Elementary processes of H2 plasma-graphene interaction: a combined molecular dynamics and density functional theory study. J. Appl. Phys. 113, 114302 (2013).Xu, L. & Ge, Q. Effects of defects and dopants in graphene on hydrogen interaction in graphene-supported NaAlH4. Int. J. Hydrogen Energy 38, 3670–3680 (2013).Perhun, T. I., Bychko, I. B., Trypolsky, A. I. & Strizhak, P. E. Catalytic properties of graphene material in the hydrogenation of ethylene. Theor. Exp. Chem. 48, 367–370 (2013).Hummers, W. S. & Offeman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 80, 1339 (1958).Dhakshinamoorthy, A., Primo, A., Concepcion, P., Alvaro, M. & Garcia, H. Doped graphene as a metal-free carbocatalyst for the selective aerobic oxidation of benzylic hydrocarbons, cyclooctane and styrene. Chem. Eur. J. 19, 7547–7554 (2013).Latorre-Sanchez, M., Primo, A. & Garcia, H. P-doped graphene obtained by pyrolysis of modified alginate as a photocatalyst for hydrogen generation from water-methanol mixtures. Angew. Chem. Int. Ed. 52, 11813–11816 (2013).Primo, A., Sanchez, E., Delgado, J. M. & Garcia, H. High-yield production of N-doped graphitic platelets by aqueous exfoliation of pyrolyzed chitosan. Carbon N. Y. 68, 777–783 (2014).Stankovich, S. et al. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon N. Y. 45, 1558–1565 (2007).Pumera, M. & Wong, C. H. A. Graphane and hydrogenated graphene. Chem. Soc. Rev. 42, 5987–5995 (2013).Teschner, D. et al. The roles of subsurface carbon and hydrogen in palladium-catalyzed alkyne hydrogenation. Science 320, 86–89 (2008).Bridier, B., Lopez, N. & Perez-Ramirez, J. Molecular understanding of alkyne hydrogenation for the design of selective catalysts. Dalton Trans. 39, 8412–8419 (2010).Flick, K., Herion, C. & Allmann, H. Palladium-haltiger Trägerkatalysator zur selektiven katalytischen Hydrierung von Acetylen in Kohlenwasserstoffströmen. EP764463-A; EP764463-A2; DE19535402-A1; JP9141097-A; CA2185721-A; KR97014834-A; MX9604031-A1; US5847250-A; US5856262-A; TW388722-A; MX195137-B; CN1151908-A; EP764463-B1; DE59610365-G; ES2197222-T3; KR418161-B; CN1081487-C; JP3939787-B2; CA2185721-C (1997).Gartside, R. J. et al. Improved olefin plant recovery system employing a combination of catalytic distillation and fixed bed catalytic steps. WO2005080530-A1; EP1711581-A1; BR200418414-A; MX2006008045-A1; JP2007518864-W; KR2007005565-A; CN1961059-A; IN200604063-P1; KR825662-B1; JP4376908-B2; CA2553962-C; IN251202-B; SG124072-A1; SG124072-B; CN1961059-B (2005).Wegerer, D. A., Bussche, K. V. & Vandenbussche, K. M. Selective Co oxidation for acetylene converter feed Co CONTROL. US2012294774-A1; US8431094-B2 (2102).Chernichenko, K. et al. A frustrated-Lewis-pair approach to catalytic reduction of alkynes to cis-alkenes. Nat. Chem. 5, 718–723 (2013).Vile, G., Bridier, B., Wichert, J. & Perez-Ramirez, J. Ceria in hydrogenation catalysis: high selectivity in the conversion of alkynes to olefins. Angew. Chem. Int. Ed. 51, 8620–8623 (2012).Ambrosi, A. et al. Metallic impurities in graphenes prepared from graphite can dramatically influence their properties. Angew. Chem. Int. Ed. 51, 500–503 (2012).Ambrosi, A. et al. Chemical reduced graphene contains inherent metallic impurities present in parent natural and synthetic graphite. Proc. Natl Acad. Sci. USA 109, 12899–12904 (2012).Vile, G., Almora-Barrios, N., Mitchell, S., Lopez, N. & Perez-Ramirez, J. From the lindlar catalyst to supported ligand-modified palladium nanoparticles: selectivity patterns and accessibility constraints in the continuous-flow three-phase hydrogenation of acetylenic compounds. Chemistry 20, 5849–5849 (2014).Gurrath, M. et al. Palladium catalysts on activated carbon supports—Influence of reduction temperature, origin of the support and pretreatments of the carbon surface. Carbon N. Y. 38, 1241–1255 (2000).Stephan, D. W. ‘Frustrated Lewis pairs’: a concept for new reactivity and catalysis. Org. Biomol. Chem. 6, 1535–1539 (2008).Stephan, D. W. Frustrated Lewis pairs: a new strategy to small molecule activation and hydrogenation catalysis. Dalton Trans. 17, 3129–3136 (2009).Chase, P. A., Jurca, T. & Stephan, D. W. Lewis acid-catalyzed hydrogenation: B(C6F5)3-mediated reduction of imines and nitriles with H2. Chem. 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A brief overview on Molecular Dynamics simulations of plasma-surface interaction in reactive ion etching

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A brief overview on Molecular Dynamics simulations of plasma-surface interaction in reactive ion etching

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Molecular dynamics simulations of hydrogen plasma interaction with graphene

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Gravure des semi-conducteurs III-V par plasmas inductifs chlorés

This thesis work is part of an emerging area of research: plasma etching for photonics and optoelectronics. The control of etching processes requires to predict the energies and fluxes of plasma species as function of the discharge parameters, and to understand the mechanisms of plasma-surface interaction. This work addresses both aspects through fluid modeling and atomistic simulations. We have developed molecular dynamics simulations to understand the fundamental mechanisms that govern the sputtering of two III-V semiconductors (GaAs and GaN) by low energy Ar ions. This numerical study, confronted to a series of experiments, shows that the composition of bombarded materials is modified over a few tens of angstroms and that sputtered Ga atoms leave the surface with energies sufficient to damage the etching sidewalls and passivation layers, in processes dominated by ion bombardment. We have also worked on fluid simulations (two-dimensional and global) to understand the dynamics of inductive chlorine discharges and to study the transport of species within the plasma bulk. Comparisons between model and experiments show that the 2D fluid model overestimates the charged particles densities but predicts fairly well the neutral and ionic composition of the plasma. The global model is the first step toward the modeling of the low power regime of inductive chlorine plasmas ; it has allowed us to study the instabilities that develop at the E to H transition.Ce travail de thèse s'inscrit dans un domaine de recherche émergeant : la gravure par plasma pour la photonique et l'opto-électronique. La maîtrise des procédés de gravure passe par la prédiction des énergies et des flux d'espèces produites dans le plasma en fonction des paramètres de la décharge, ainsi que par la compréhension des mécanismes d'interaction plasma-surface. Ce travail aborde ces deux aspects au travers de modélisations fluides et de simulations atomistiques. Nous avons développé des simulations de dynamique moléculaire pour comprendre les mécanismes fondamentaux qui régissent la pulvérisation de deux semiconducteurs III-V (GaAs et GaN) par des ions Ar faiblement énergétiques. Cette étude numérique, confrontée à une série d'expériences, montre que la composition des matériaux bombardés est modifiée sur quelques dizaines d'angströms et que les atomes de Ga pulvérisés quittent la surface avec des énergies suffisantes pour endommager les flancs de gravure et briser les couches de passivation, notamment dans les procédés dominés par bombardement ionique. Nous avons également travaillé sur des simulations fluides (bi-dimensionnelles et globales) pour comprendre la dynamique des décharges inductives chlorées et étudier le transport des espèces au sein du plasma. Des confrontations modèle/expérience montrent que le modèle fluide 2D surestime les densités des particules chargées mais prédit de façon satisfaisante la composition neutre et ionique du plasma. Le modèle global constitue le premier pas vers une modélisation du régime basse puissance des plasmas inductifs chlorés ; il nous a permis d'étudier les instabilités qui se développent à la transition E-H

Global model of instabilities in low pressure inductive chlorine discharges

International audienceExperimental studies have shown that low-pressure inductive discharges with attaching gases are subject to instabilities near the transition between capacitive (E) and inductive (H) modes. A global model, consisting of two particle and one energy balance equations, has been previously proposed to describe the instability mechanism. This model, which agrees qualitatively well with experimental observations, leaves significant quantitative differences. In this paper, the model is revisited with Cl2 as the feedstock gas. A novel treatment of inductive power deposition is evaluated and chlorine chemistry is included. Old and new models are systematically compared. It is found that the alternative inductive coupling description slightly modifies the results. The effect of gas chemistry is even more pronounced. The instability window is smaller in pressure and larger in absorbed power, the frequency is higher and the amplitudes of oscillations are reduced. The feedstock gas is weakly dissociated (≈16%) and Cl2+ is the dominant positive ion, which is consistent with the moderate electron density during the instability cycle

Low energy Ar<SUP>+</SUP> bombardment of GaN surfaces : A statistical study of ion reflection and sputtering

International audienceStatistical molecular dynamics simulations are performed to analyze the sputtering of w-GaN (wurtzite) and z-GaN (zinc blende) surfaces under 100 eV Ar ion bombardment. Ion reflection and physical sputtering mechanisms are investigated as a function of the ion impact angle and the crystalline nature of samples. The probability of ion reflection is lower for the w-GaN phase and increases with the angle of incidence &#952;i. As &#952;i becomes more glancing, the reflected ions become more energetic and their angular distribution tends to narrow. The sputtering yields of w-GaN and z-GaN surfaces are maximum for &#952;i = 45°. For near-normal incidence, the probability of sputtering is smaller for the w-GaN phase, suggesting that the atomic arrangement in the pristine state modifies the characteristics of the momentum transfer occurring between the ion and the surface atoms during the collision cascade. Atomic nitrogen sputters preferentially and represents 87% to 100% of sputtered species due to its lower mass. These statistical results differ from the predictions of continuous ion bombardment simulations since the surfaces are not allowed to evolve self-consistently during the gathering of impact statistics

Alternative solutions for nm-precision etching: H2 Plasmas Modification of Si/ SiN thin-films

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