Foto personale

Pagina personale di Maria Clelia RIGHI

Dipartimento di Scienze Fisiche, Informatiche e Matematiche
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

Loehle, S.; Righi, M. C. ( 2018 ) - Ab initio molecular dynamic simulation of tribochemical reactions involving phosphorus additives at sliding iron interfaces - LUBRICANTS - n. volume 6 - pp. da 31 a 40 ISSN: 2075-4442 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We performed, for the first time to our knowledge, fully ab initio molecular dynamics simulations of additive tribochemistry in boundary lubrication conditions. We consider an organophosphourus additive that has been experimentally shown to reduce friction in steel-on-steel sliding contacts thanks to the tribologically-induced formation of an iron phosphide tribofilm. The simulations allow us to observe in real time the molecular dissociation at the sliding iron interface under pressure and to understand the mechanism of iron phosphide formation. We discuss the role played by the mechanical stress by comparing the activation times for molecular dissociation observed in the tribological simulations at different applied loads with that expected on the basis of the dissociation barrier.

Restuccia, Paolo; Levita, Giacomo; Wolloch, Michael; Losi, Gabriele; Fatti, Giulio; Ferrario, Mauro; Righi, M. C. ( 2018 ) - Ideal adhesive and shear strengths of solid interfaces: A high throughput ab initio approach - COMPUTATIONAL MATERIALS SCIENCE - n. volume 154 - pp. da 517 a 529 ISSN: 0927-0256 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

We release a computational protocol to calculate two intrinsic tribological properties of solid interfaces from first principles, namely the adhesion energy, γ and the ideal interfacial shear strength, τ. These properties, which correspond to the energy required to separate two surfaces from contact and to the static friction force per unit area, respectively, are ruled by physical/chemical interactions between the surfaces in contact. First principles calculations based on Density Functional Theory (DFT) can accurately describe surface-surface interactions, offering the possibility to characterize the adhesive and shear strengths of materials in silico. We implemented the computational protocol as an AiiDA workflow (WF) that allows to obtain the γ and τ figures of merits in a high throughput manner. The software we produced uses a simple input file and most computational parameters determined automatically. To our best knowledge, this is the first time a high throughput approach has been used in tribology.

Wolloch, M.; Levita, G.; Restuccia, P.; Righi, M. C. ( 2018 ) - Interfacial Charge Density and Its Connection to Adhesion and Frictional Forces - PHYSICAL REVIEW LETTERS - n. volume 121 - pp. da 026804 a 026808 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We derive a connection between the intrinsic tribological properties and the electronic properties of a solid interface. In particular, we show that the adhesion and frictional forces are dictated by the electronic charge redistribution occurring due to the relative displacements of the two surfaces in contact. We define a figure of merit to quantify such a charge redistribution and show that simple functional relations hold for a wide series of interactions including metallic, covalent, and physical bonds. This suggests unconventional ways of measuring friction by recording the evolution of the interfacial electronic charge during sliding. Finally, we explain that the key mechanism to reduce adhesive friction is to inhibit the charge flow at the interface and provide examples of this mechanism in common lubricant additives.

Levita, Giacomo; Kajita, Seiji; Righi, M. C. ( 2018 ) - Water adsorption on diamond (111) surfaces: an ab initio study - CARBON - n. volume 127 - pp. da 533 a 540 ISSN: 0008-6223 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The physical and chemical adsorption of water at the (1 × 1) and (2 × 1)-reconstructed C(111) surfaces are investigated by means of first principles calculations and compared to hydrogen adsorption. The study aims at filling a gap of knowledge about the interaction of water with the most stable diamond surface. The calculated reaction energies and barriers indicate that the Pandey-reconstructed surface is almost inert towards water and hydrogen chemisorption in comparison with the unreconstructed surface and other low-index diamond surfaces. We also show that by increasing the amount of chemisorbed hydroxyl or hydrogen groups the stability of the Pandey reconstruction is progressively reduced with respect to the unreconstructed (111) surface, which becomes energetically more favourable above about 40% of adsorbate coverage. Our results provide a microscopic description of diamond surface passivation, which is very important for controlling macroscale phenomena, such as the friction reduction of diamond coatings in humid environments.

Levita, Giacomo; Righi, Maria C. ( 2017 ) - Effects of Water Intercalation and Tribochemistry on MoS2 Lubricity: An Ab Initio Molecular Dynamics Investigation - CHEMPHYSCHEM - n. volume 18 - pp. da 1475 a 1480 ISSN: 1439-4235 [Articolo in rivista (262) - Articolo su rivista]
Abstract

To date, no clear conclusion has been reached on the atomistic mechanisms that govern the observed decrease of lubricating capabilities of MoS2 in humid environments. Based on ab initio molecular dynamic calculations, we show that intercalated water molecules hinder the sliding motion of both regular and defective layers considerably, with the velocities decaying exponentially with time. However, in the presence of an applied load and exposed edge terminations, water is rapidly removed from the interface and is adsorbed on the edges either in undissociated form or as OH/H fragments. These outcomes suggest that the interlayer slipperiness can be reduced by the presence of water even in the absence of any chemical oxidation. Our work could help to set up more dedicated experiments to further tackle a technologically relevant issue for the applications of MoS2-based lubricants.

Loehlé, S.; Righi, M. C. ( 2017 ) - First principles study of organophosphorus additives in boundary lubrication conditions: Effects of hydrocarbon chain length - LUBRICATION SCIENCE - n. volume 29 - pp. da 485 a 491 ISSN: 0954-0075 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We apply first principle calculations to investigate the effects of the hydrocarbon chain length in additive molecules under boundary lubrication conditions. In these conditions, occurring in high-pressure applications, the thickness of the oil film becomes extremely thin, and the additive molecules remain confined between the two solid surfaces in contact. We consider two types of organophosphorous additives having the same phosphite group but hydrocarbon chains of different lengths. By comparing the molecular behavior under uniaxial stress applied, we elucidate the atomistic mechanisms that control the molecular capacity of maintaining an interfacial spacing under compression and the load-induced molecular dissociation. This insight is relevant not only for a rational design of lubricant additives but also to provide understanding on the activation mechanisms of tribochemical reactions.

Marchetto, Diego; Restuccia, Paolo; Ballestrazzi, Antonio; Righi, Maria Clelia; Rota, Alberto; Valeri, Sergio ( 2017 ) - Surface passivation by graphene in the lubrication of iron: A comparison with bronze - CARBON - n. volume 116 - pp. da 375 a 380 ISSN: 0008-6223 [Articolo in rivista (262) - Articolo su rivista]
Abstract

It has been recently reported that graphene is able to significantly reduce the friction coefficient of steel-on-steel sliding contacts. The microscopic origin of this behavior has been attributed to the mechanical action of load carrying capacity. However, a recent work highlighted the importance of the chemical action of graphene. According to this work graphene reduces the adhesion of iron interfaces by reducing the surface energy thanks to a passivation effect. The aim of the present work is to clarify the still debated lubricating behavior of graphene flakes. We perform pin-on-disc experiments using liquid dispersed graphene solution as a lubricant. Two different materials, pure iron and bronze are tested against 100Cr6 steel. Raman spectroscopy is used to analyze the surfaces after the friction tests. The results of these tests prove that graphene flakes have a beneficial effect on the friction coefficient. At the same time they show a tendency of graphene to passivate the native iron surfaces that are exposed during sliding as a consequence of wear.

Righi, Maria Clelia; Loehlé, S.; De Barros Bouchet, M. I.; Mambingo Doumbe, S.; Martin, J. M. ( 2016 ) - A comparative study on the functionality of S- and P-based lubricant additives by combined first principles and experimental analysis - RSC ADVANCES - n. volume 6 - pp. da 47753 a 47760 ISSN: 2046-2069 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Sulfur and phosphorus are key elements for the functionality of lubricant additives used in extreme pressure applications, such as synchronizer systems in cars. To understand their mechanism of action we combine first principles calculations and gas phase lubrication experiments. The surface spectroscopy analysis performed in situ after the tribological test indicates that iron sulfide (phosphide) is formed by rubbing steel-on-steel in the presence of organo-sulfur (-phosphorus) molecules. We, thus, study the effects of elemental sulfur and phosphorus on the interfacial properties of iron by spin-polarized density functional theory calculations. The results show that both the elements are very effective in reducing the adhesion and shear strength of iron. Sulfur is predicted to be more effective than phosphorus, especially at high pressure. Gas phase lubrication experiments confirm these results, indicating that the friction coefficient of iron-sulphide is lower than that of iron-phosphide and both S and P dramatically reduce the friction of steel-on-steel. These results indicate that the release of elemental sulfur and phosphorus may be the key mechanism to controlling the tribological properties of the metal interface and elucidate that the underling microscopic phenomenon is metal passivation. © 2016 The Royal Society of Chemistry.

Kajita, Seiji; Righi, M.C. ( 2016 ) - A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics - CARBON - n. volume 103 - pp. da 193 a 199 ISSN: 0008-6223 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Different hypotheses have been proposed to explain the mechanism for the extremely low friction coefficient of carbon coatings and its undesired dependence on air humidity. A decisive atomistic insight is still lacking because of the difficulties in monitoring what actually happens at the buried sliding interface. Here we perform large-scale ab initio molecular dynamics simulations of both undoped and silicon-doped carbon films sliding in the presence of water. We observe the tribologically-induced surface hydroxylation and subsequent formation of a thin film of water molecules bound to the OH-terminated surface by hydrogen bonds. The comparative analysis of silicon-incorporating and clean surfaces, suggests that this two-step process can be the key phenomenon to provide high slipperiness to the carbon coatings. The water layer is, in fact, expected to shelter the carbon surface from direct solid-on-solid contact and make any counter surface slide extremely easily on it. The present insight into the wettability of carbon-based films can be useful for designing new coatings for biomedical and energy-saving applications with environmental adaptability.

Kajita, Seiji; Righi, M. C. ( 2016 ) - Erratum to: Insights into the Tribochemistry of Silicon-doped Carbon-Based Films by Ab Initio Analysis of Water–Surface Interactions - TRIBOLOGY LETTERS - n. volume 62 - pp. da 657 a 657 ISSN: 1023-8883 [Articolo in rivista (262) - Articolo su rivista]
Abstract

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Righi, Maria Clelia; Zilibotti, Giovanna; Corni, Stefano; Ferrario, Mauro; Bertoni, Carlo Maria ( 2016 ) - First-Principle Molecular Dynamics of Sliding Diamond Surfaces: Tribochemical Reactions with Water and Load Effects - JOURNAL OF LOW TEMPERATURE PHYSICS - n. volume 185 - pp. da 174 a 182 ISSN: 0022-2291 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Ab initio molecular dynamics offers an unexpected tool to understand many aspects of complex and macroscopic phenomena, like friction, lubrication, and surface passivation through chemical reactions induced by load and confinement, as found in recent works (Zilibotti et al., in Phys. Rev. Lett. 111:146101, 2013; De Barros Bouchet et al., J Phys Chem C 116:6966, 2012). Here we review the results of first-principle molecular dynamics simulations of diamond interfaces interacting with water molecules, at different concentrations. We found that the molecular confinement induced by the applied load promotes water dissociation. The consequent surface passivation prevents the formation of carbon bonds across the interface, reducing adhesion and friction. The possibility to extend the use of an atomistic approach to understand the kinetics of tribochemical reactions and their effects on friction will also be discussed.

Levita, Giacomo; Restuccia, Paolo; Righi, Maria Clelia ( 2016 ) - Graphene and MoS2 interacting with water: A comparison by ab initio calculations - CARBON - n. volume 107 - pp. da 878 a 884 ISSN: 0008-6223 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Although very similar in many technological applications, graphene and MoS2 bear significant differences if exposed to humid environments. As an example, lubrication properties of graphene are reported to improve while those of MoS2 to deteriorate: it is unclear whether this is due to oxidation from disulfide to oxide or to water adsorption on the sliding surface. By means of ab initio calculations we show here that these two layered materials have similar adsorption energies for water on the basal planes. They both tend to avoid water intercalation between their layers and to display only mild reactivity of defects located on the basal plane. It is along the edges where marked differences arise: graphene edges are more reactive at the point that they immediately prompt water splitting. MoS2 edges are more stable and consequently water adsorption is much less favoured than in graphene. We also show that water-driven oxidation of MoS2 layers is unfavoured with respect to adsorption.

Kajita, Seiji; Righi, M.C ( 2016 ) - Insigths into the Tribochemistry of Silicon-doped Carbon-Based Films by Ab Initio Analysis of Water-Surface Interactions - TRIBOLOGY LETTERS - n. volume 61 - pp. da 1 a 17 ISSN: 1023-8883 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Diamond and diamond-like carbon are used as coating materials for numerous applications, ranging from biomedicine to tribology. Recently, it has been shown that the hydrophilicity of the carbon films can be enhanced by silicon doping, which highly improves their biocompatibility and frictional performances. Despite the relevance of these properties for applications, a microscopic understanding on the effects of silicon is still lacking. Here, we apply ab initio calculations to study the interaction of water molecules with Si-incorporating C(001) surfaces. We find that the presence of Si dopants considerably increases the energy gain for water chemisorption and decreases the energy barrier for water dissociation by more than 50 %. We provide a physical rational for the phenomenon by analyzing the electronic charge displacements occurring upon adsorption. We also show that once hydroxylated, the surface is able to bind further water molecules much strongly than the clean surface via hydrogen bond networks. This two-step process is consistent with and can explain the enhanced hydrophilic character observed in carbon-based films doped by silicon.

Restuccia, Paolo; Ferrario, Mauro; Sivestrelli, Pier Luigi; Mistura, Giampaolo; Righi, Maria Clelia ( 2016 ) - Size-dependent commensurability and its possible role in determining the frictional behavior of adsorbed systems - PHYSICAL CHEMISTRY CHEMICAL PHYSICS - n. volume 18 - pp. da 28997 a 29004 ISSN: 1463-9076 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Recent nanofriction experiments of xenon on graphene revealed that the slip onset can be induced by increasing the adsorbate coverage above a critical value, which depends on temperature. Moreover, the xenon slippage on gold is much higher than on graphene in spite of the same physical nature of the interactions. To shed light on these intriguing results we have performed molecular dynamics simulations relying on ab initio derived potentials. By monitoring the interfacial structure factor as a function of coverage and temperature, we show that the key mechanism to interpret the observed frictional phenomena is the size-dependence of the island commensurability. The latter quantity is deeply affected also by the lattice misfit, which explains the different frictional behavior of Xe on graphene and gold.

Restuccia, Paolo; Righi, Maria Clelia ( 2016 ) - Tribochemistry of graphene on iron and its possible role in lubrication of steel - CARBON - n. volume 106 - pp. da 118 a 124 ISSN: 0008-6223 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Recent tribological experiments revealed that graphene is able to lubricate macroscale steel-on-steel sliding contacts very effectively both in dry and humid conditions. This effect has been attributed to a mechanical action of graphene related to its load-carrying capacity. Here we provide further insight into the functionality of graphene as lubricant by analysing its tribochemical action. By means of first principles calculations we show that graphene binds strongly to native iron surfaces highly reducing their surface energy. Thanks to a passivating effect, the metal surfaces coated by graphene become almost inert and present very low adhesion and shear strength when mated in a sliding contact. We generalize the result by establishing a connection between the tribological and the electronic properties of interfaces, which is relevant to understand the fundamental nature of frictional forces.

Levita, Giacomo; Molinari, Elisa; Polcar, Tomas; Righi, Maria Clelia ( 2015 ) - First-principles comparative study on the interlayer adhesion and shear strength of transition-metal dichalcogenides and graphene - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 92 - pp. da 1 a 8 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Due to their layered structure, graphene and transition-metal dichalcogenides (TMDs) are easily sheared along the basal planes. Despite a growing attention towards their use as solid lubricants, so far no head-to-head comparison has been carried out. By means of ab initio modeling of a bilayer sliding motion, we show that graphene is characterized by a shallower potential energy landscape while more similarities are attained when considering the sliding forces; we propose that the calculated interfacial ideal shear strengths afford the most accurate information on the intrinsic sliding capability of layered materials. We also investigate the effect of an applied uniaxial load: in graphene, this introduces a limited increase in the sliding barrier while in TMDs it has a substantially different impact on the possible polytypes. The polytype presenting a parallel orientation of the layers (R0) bears more similarities to graphene while that with antiparallel orientation (R180) shows deep changes in the potential energy landscape and consequently a sharper increase of its sliding barrier.

De Barros-Bouchet, M.I; Righi, M.C.; Philippon, D.; Mambingo-Doumbe, S.; Le-Mogne, T.; Martin, J.M.; Bouffet, A. ( 2015 ) - Tribochemistry of phosphorus additives: Experiments and first-principles calculations - RSC ADVANCES - n. volume 5 - pp. da 49270 a 49279 ISSN: 2046-2069 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Organophosphorus compounds are common additives included in liquid lubricants for many applications, in particular automotive applications. Typically, organic phosphites function as friction-modifiers whereas phosphates as anti-wear additives. While the antiwear action of phosphates is now well understood, the mechanism by which phosphites reduce friction is still not clear. Here we study the tribochemistry of both phosphites and phosphates using gas phase lubrication (GPL) and elucidate the microscopic mechanisms that lead to the better frictional properties of phosphites. In particular, by in situ spectroscopic analysis we show that the friction reduction is connected to the presence of iron phosphide, which is formed by tribochemical reactions involving phosphites. The functionality of elemental phosphorus in reducing the friction of iron-based interfaces is elucidated by first principle calculations. In particular, we show that the work of separation and shear strength of iron dramatically decrease by increasing the phosphorus concentration at the interface. These results suggest that the functionality of phosphites as friction modifiers may be related to the amount of elemental phosphorus that they can release at the tribological interface.

Righi, M.C; Loehlé, S.; De Barros Bouchet, M.I.; Philippon, D.; Martin, J.M. ( 2015 ) - Trimethyl-phosphite dissociative adsorption on iron by combined first-principle calculations and XPS experiments - RSC ADVANCES - n. volume 5 - pp. da 101162 a 101168 ISSN: 2046-2069 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The reaction of trimethyl-phosphite, TMPi, with a clean Fe(110) surface has been investigated by ab initio calculations. The most stable configurations and energies are identified for both molecular and dissociative adsorption. The calculated reaction energies indicate that dissociation is energetically more favorable than molecular adsorption and we provide a description of the dissociation path and the associated energy barrier. In situ XPS analysis of adsorbed TMPi on metallic iron confirmed molecular chemisorption and dissociation at high temperature. These results shed light on the mechanism of phosphorus release from organophosphites at the iron surface, which is important for the functionality of these phosphorus-based additives, included in lubricants for automotive applications.

Righi, Mc ( 2014 ) - Ab initio investigation of tribochemical phenomena in solid and boundary lubrication ( National Meeting of the American Chemical Society - San Francisco - 10/08/2014) ( - ABSTRACTS OF PAPERS - AMERICAN CHEMICAL SOCIETY ) - n. volume 248 - pp. da 576 a 576 [Abstract in Atti di convegno (274) - Abstract in Atti di Convegno]
Abstract

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Levita, G.; Cavaleiro, A.; Molinari, E.; Polcar, T.; Righi, M. C. ( 2014 ) - Sliding Properties of MoS2 Layers: Load and Interlayer Orientation Effects - JOURNAL OF PHYSICAL CHEMISTRY. C - n. volume 118 - pp. da 13809 a 13816 ISSN: 1932-7447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Among the members of the transition metal dichalcogenides (TMD) family, molybdenum disulfide has the most consolidated application outcomes in tribological fields. However, despite the growing usage as nanostructured solid lubricant due to its lamellar structure, little is known about the atomistic interactions taking place at the interface between two MoS2 sliding layers, especially at high loads. By means of ab initio modeling of the static potential energy surface and charge distribution analysis, we demonstrate how electrostatic interactions, negligible in comparison with van der Waals and Pauli contributions at zero load, progressively affect the sliding motion at increasing loads. As such, they discriminate the relative stability and the frictional behavior of bilayers where the two monolayers defining the interface have a different relative orientation. In particular, for antiparallel sliding layers we observed a load-induced increase of both the depth of the minima and the height of the energy barriers compared to parallel ones, which may have important consequences for the fabrication of more efficient ultralow friction devices at the nanoscale.

Pierno, Matteo; Bignardi, Luca; Righi, Maria Clelia; Bruschi, Lorenzo; Gottardi, Stefano; Stöhr, Meike; Ivashenko, Oleksii; Silvestrelli, Pier Luigi; Rudolf, Petra; Mistura, Giampaolo ( 2014 ) - Thermolubricity of gas monolayers on graphene - NANOSCALE - n. volume 6 - pp. da 8062 a 8067 ISSN: 2040-3364 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Nanofriction of Xe, Kr and N2 monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. It was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N 2 confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates. © 2014 the Partner Organisations.

Zilibotti, Giovanna; Corni, S.; Righi, Maria Clelia ( 2013 ) - Load-induced confinement activates diamond lubrication by water - PHYSICAL REVIEW LETTERS - n. volume 111 - pp. da 146101 a 146105 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Tribochemical reactions are chemical processes, usually involving lubricant or environment molecules, activated at the interface between two solids in relative motion. They are difficult to be monitored in situ, which leaves a gap in the atomistic understanding required for their control. Here we report the real-time atomistic description of the tribochemical reactions occurring at the interface between two diamond films in relative motion, by means of large scale ab initio molecular dynamics. We show that the load-induced confinement is able to catalyze diamond passivation by water dissociative adsorption. Such passivation decreases the energy of the contacting surfaces and increases their electronic repulsion. At sufficiently high coverages, the latter prevents surface sealing, thus lowering friction. Our findings elucidate effects of the nanoscale confinement on reaction kinetics and surface thermodynamics, which are important for the design of new lubricants. © 2013 American Physical Society.

Zilibotti, G.; Corni, S.; Righi, M.C. ( 2012 ) - Formation energy of dangling bonds on hydrogenated diamond surfaces: A first-principles study - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 85 - pp. da 33406 a 33410 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We calculate the energy cost to create dangling bonds on hydrogenated diamond (001) surfaces by means of spin-polarized first-principle calculations. We demonstrate that the dangling bond formation energy depends on both the density and the arrangement of the dangling bonds already present on the surface. In particular, at low dangling bond density, hydrogen removal is less energetically costly than at high dangling bond density. We also find that adjacent dangling bonds are more stable in the antiferromagnetic configuration than in the ferromagnetic one. We provide quantitative information and a physical rationale of these phenomena. © 2012 American Physical Society.

Reguzzoni, M.; Fasolino, A.; Molinari, E.; Righi, M. C. ( 2012 ) - Friction by Shear Deformations in Multilayer Graphene - JOURNAL OF PHYSICAL CHEMISTRY. C - n. volume 116 - pp. da 21104 a 21108 ISSN: 1932-7447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present accurate calculations of friction in graphene films in configurations simulating the presence of an anchoring substrate. We find that a slider induces both out-of-plane and shear deformations, which increase with the thickness of the supported film. We elucidate the new frictional mechanism connected to shear layer motions, which is minimal for systems with the smallest number of layers

De Barros Bouchet, Maria-Isabel; Zilibotti, Giovanna; Matta, Christine; Righi, Maria Clelia; Vandenbulcke, Lionel; Vacher, Beatrice; Martin, Jean-Michel ( 2012 ) - Friction of diamond in the presence of water vapor and hydrogen gas. coupling gas-phase lubrication and first-principles studies - JOURNAL OF PHYSICAL CHEMISTRY. C - n. volume 116 - pp. da 6966 a 6972 ISSN: 1932-7447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Nanocrystalline diamond (NCD) has attracted much attention in recent years because of improvements in growth methodologies that have provided increases in both film thickness and growth rate, while preserving the outstanding mechanical properties of diamond material. We provide here some evidence, based on combined experimental and first-principles analyses, that ultralow friction of nanocrystalline diamond in the presence of water vapor is associated with OH and H passivation of sliding surfaces, resulting from the dissociative adsorption of H 2O molecules. The presence of these adsorbates (OH and H fragments) keeps the surfaces far apart preventing the formation of covalent bonds across the interface. H-passivated surfaces, resulting from the dissociative adsorption of H 2 molecules, appears to be more efficient in further reducing friction than OH-terminated surfaces. © 2012 American Chemical Society.

M. Reguzzoni;A. Fasolino;E. Molinari;M. C. Righi ( 2012 ) - Potential energy surface for graphene on graphene: Ab initio derivation, analytical description, and microscopic interpretation - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 86 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We derive an analytical expression that describes the interaction energy between two graphene layers identically oriented as a function of the relative lateral and vertical positions, in excellent agreement with first principles calculations. Thanks to its formal simplicity, the proposed model allows for an immediate interpretation of the interactions, in particular of the potential corrugation. This last quantity plays a crucial role in determining the intrinsic resistance to interlayer sliding and its increase upon compression influences the frictional behavior under load. We show that, for these weakly adherent layers, the corrugation possesses the same nature and z dependence of Pauli repulsion. We investigate the microscopic origin of these phenomena by analyzing the electronic charge distribution: We observe a pressure-induced charge transfer from the interlayer region toward the near-layer regions, with a much more consistent depletion of charge occurring for the AA stacking than for the AB stacking of the two layers. DOI:10.1103/PhysRevB.86.245434

Reguzzoni, Marco; Righi, Maria Clelia ( 2012 ) - Size dependence of static friction between solid clusters and substrates - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 85(20) - pp. da 1 a 4 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The presence of static friction between two solids implies that they managed to interlock. In the absence of contamination, the degree of interlocking results from a competition between interfacial and elastic interactions. Here we include this competition into a theory describing the depinning of adsorbed clusters. We find that the friction force can decrease with their size and derive an expression for the critical radius below which interlocking is large. The theory is validated through simulations of krypton and silicon clusters adsorbed on a variety of substrates. Our study provides a quantitative explanation of why shear forces can be nonextensive at small scales, as it has often been observed experimentally. © 2012 American Physical Society.

Zilibotti, G.; Ferrario, M.; Bertoni, C. M.; Righi, M. C. ( 2011 ) - Ab initio calculation of adhesion and potential corrugation of diamond (001) interfaces - COMPUTER PHYSICS COMMUNICATIONS - n. volume 182 - pp. da 1796 a 1799 ISSN: 0010-4655 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The system consisting of two diamond (001) surfaces in contact was studied by means of plane-wave/pseudopotential density functional calculations. Different hydrogen coverages, ranging from fully hydrogenation to bare surfaces, were considered. The adhesion energy was calculated as a function of both the separation and the lateral displacement of the two surfaces. The effects of dangling carbon bonds on the adhesion and potential corrugation are quantitatively discussed.

Zilibotti, G.; Righi, M.C. ( 2011 ) - Ab initio calculation of the adhesion and ideal shear strength of planar diamond interfaces with different atomic structure and hydrogen coverage - LANGMUIR - n. volume 27 - pp. da 6862 a 6867 ISSN: 0743-7463 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We propose a method to calculate the ideal shear strength τ of two surfaces in contact by ab initio calculations. This quantity and the work of adhesion γ are the interfacial parameters usually derived from tip-based friction force measurements. We consider diamond interfaces and quantitatively evaluate the effects of surface orientation and passivation. We find that in the case of fully passivated interfaces, γ is not affected by the orientation and the alignment of the surfaces in contact. On the contrary, τ does show a dependence on the atomic-scale roughness of the interface. The surface termination has a major impact on the tribological properties of diamond. The presence of dangling bonds, even at concentrations low enough to prevent the formation of interfacial C-C bonds, causes an increase in the resistance to sliding by 2 orders of magnitude with respect to the fully hydrogenated case. We discuss our findings in relation to experimental observations. © 2011 American Chemical Society.

Reguzzoni, M.; Ferrario, M.; Zapperi, S.; Righi, M. C. ( 2010 ) - Correction for Reguzzoni et al., Onset of frictional slip by domain nucleation in adsorbed monolayers - PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA - n. volume 107 - pp. da 5693 a 5693 ISSN: 0027-8424 [Abstract in rivista (266) - Abstract in Rivista]
Abstract

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Rosini, Marcello; Righi, Maria Clelia; Kratzer, Peter; Magri, Rita ( 2010 ) - In adsorption and diffusion on in-rich (2×4) reconstructed InGaAs surfaces on GaAs(001) ( 29th International Conference on Physics of Semiconductors, ICPS 29 - Rio de Janeiro (Brazil) - 27 July 2008 through 1 August 2008) ( - AIP Conference Proceedings ) - AIP CONFERENCE PROCEEDINGS - n. volume 1199 - pp. da 21 a 22 ISBN: 978-073540736-7 ISSN: 0094-243X [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

We investigate the potential energy surface (PES) and the adsorption properties of an In adatom on InAs (2×4) reconstructed wetting layers (WLs) deposited on a GaAs substrate. The results are then used to derive the diffusion properties of a single In adatom on the WLs. We find that: (i) the adsorbate diffusion is highly anisotropic; (ii) the adsorption sites within the As dimers have to be taken into account since they strongly affect diffusion; (iii) the most stable adsorption sites are the ones within the dimers and those located besides the in-dimers.

Reguzzoni, Marco; Ferrario, Mauro; Zapperi, Stefano; Righi, Maria Clelia ( 2010 ) - Onset of frictional slip by domain nucleation in adsorbed monolayers - PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA - n. volume 107 - pp. da 1311 a 1316 ISSN: 0027-8424 [Articolo in rivista (262) - Articolo su rivista]
Abstract

It has been known for centuries that a body in contact with a substrate will start to slide when the lateral force exceeds the static friction force. Yet the microscopic mechanisms ruling the crossover from static to dynamic friction are still the object of active research. Here, we analyze the onset of slip of a xenon (Xe) monolayer sliding on a copper (Cu) substrate. We consider thermal-activated creep under a small external lateral force, and observe that slip proceeds by the nucleation and growth of domains in the commensurate interface between the film and the substrate. We measure the activation energy for the nucleation process considering its dependence on the external force, the substrate corrugation, and particle interactions in the film. To understand the results, we use the classical theory of nucleation and compute analytically the activation energy which turns out to be in excellent agreement with numerical results. We discuss the relevance of our results to understand experiments on the sliding of adsorbed monolayers.

Manelli, O.; Corni, S.; Righi, M.C. ( 2010 ) - Water adsorption on native and hydrogenated diamond (001) surfaces - JOURNAL OF PHYSICAL CHEMISTRY. C - n. volume 114 - pp. da 7045 a 7053 ISSN: 1932-7447 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Understanding water interaction with diamond surfaces is fundamental for applications in tribology, device technology, and microelectronics operating in biological environments. In this paper, we provide a full microscopic description of the interaction of water with diamond (001) surfaces. We performed ab initio calculations within the framework of density functional theory including long-range van der Waals interactions. We considered both native and hydrogenated surfaces. We calculated the structure and the energetics for molecular and dissoiative adsorption, and in the case of exothermic proesses, we determined the energy barriers for dissoiation. Our results allow prediction of the formation of water islands on native surfaces that grow along and perpendicularly to dimer rows. Moreover, they highlight the role played by the water coverage in determining the adsorption mode (physisorption or dissoiation), suggesting an explanation for experimental results on similar Si(001) surfaces. Finally, we provide an understanding on the mechanism of carbon dangling bond passivation by water which is the key factor in determining the excellent tribological performances of diamond in humid environments as measured in experiments. © 2010 American Chemical Society.

ZILIBOTTI G; RIGHI M. C; M. FERRARIO ( 2009 ) - Ab initio study on the surface chemistry and nanotribological properties of passivated diamond surfaces (American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 79 - pp. da 1 a 10 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

Experimental findings indicate that the impressively low friction and wear of diamond in humid environments are determined by the surface passivation. In this paper, we investigate the relationship between the surface chemistry and the nanotribological properties of diamond surfaces. We consider the (2x1)-C(001) surface taking into account different terminations constituted of hydrogen, oxygen, and hydroxyl groups. We analyze the adsorbate geometry and the polarization of the surface bonds. We discuss the stability of the different surface terminations in different conditions, which account for the presence in the environment of H-2, O-2, and H2O molecules in different concentrations and we present the surface phase diagram. Finally, we report the calculated adhesion energy between the passivated surfaces and analyze its variation as a function both of the surface separation and of the surface relative lateral position. In this way, we provide information on the effect of the different adsorbates on the interaction between diamond surfaces and on the magnitude and anisotropy of friction forces.

Bertelli, Matteo; Löptien, P.; Wenderoth, M.; Rizzi, Angela; Ulbrich, R. G.; Righi, Maria Clelia; Ferretti, Andrea; Martin Samos, L.; Bertoni, Carlo Maria; Catellani, A. ( 2009 ) - Atomic and electronic structure of the nonpolar GaN (1 1̄ 00) surface - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 80 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a cross-section scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS) and ab initio density-functional theory simulations study of the cleaved nonpolar (1[overline 1]00) surface (m-plane) of n-type HVPE GaN free-standing quasisubstrates. Atomically resolved empty and filled states STM topographies show that no reconstruction occurs upon cleavage, as predicted by theory. STS measurements on clean and atomically flat cleaved surfaces (defect concentration sigmad<=2×10**12 cm−2) show that the Fermi energy is not pinned and the tunneling current flows through Ga-like electronic states lying outside the fundamental band gap. On surface areas with defect concentration sigmad>=3×10**13 cm−2, the Fermi energy is pinned inside the band gap in defect-derived surface states and tunneling through filled (empty) N-like (Ga-like) states takes place.

M. Rosini; M. C. Righi; P. Kratzer; R. Magri ( 2009 ) - Indium surface diffusion on InAs (2x4) reconstructed wetting layers on GaAs(001) - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 79 - pp. da 075302 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we present a study of In surface diffusion on InAs wetting layers deposited on the (001) surface of GaAs. The alpha2(2×4) and beta2(2×4) reconstructions stabilized by a high In concentration are considered. The low symmetry of the alpha2(2×4) reconstruction allowed us to understand the effect of the wetting-layer symmetry on the adsorbate diffusion. We find that (i) the diffusion coefficient value is larger for In motion on the alpha2 reconstruction than on the beta2 reconstruction. This is due to the presence on beta2 of an additional As dimer that rises locally the potential energy surface and offer an additional site to which the In adatom can bind strongly. (ii) The In adsorption sites located within the As dimers have to be taken into account properly for these specific reconstructions, since they greatly affect the value of the diffusion coefficient. This is in contrast to what happens for the other reconstructions reported in the literature. (iii) The adsorbate diffusion is highly anisotropic with the [[overline 1]10] direction favored over the [110] direction, due to the presence of low-potential channels along [[overline 1]10]. (iv) The anisotropy is slightly smaller on the alpha2 reconstruction than on the beta2 reconstruction because on the beta2 there is an additional diffusion channel along the [[overline 1]10] direction.

Rosini, Marcello; Righi, Maria Clelia; Kratzer, Peter; Magri, Rita ( 2009 ) - Indium surface diffusion on InAs (2×4) reconstructed wetting layers on GaAs(001) - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 79 - pp. da 075302 a 075309 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we present a study of In surface diffusion on InAs wetting layers deposited on the (001) surface of GaAs. The α2 (2×4) and β2 (2×4) reconstructions stabilized by a high In concentration are considered. The low symmetry of the α2 (2×4) reconstruction allowed us to understand the effect of the wetting-layer symmetry on the adsorbate diffusion. We find that (i) the diffusion coefficient value is larger for In motion on the α2 reconstruction than on the β2 reconstruction. This is due to the presence on β2 of an additional As dimer that rises locally the potential energy surface and offer an additional site to which the In adatom can bind strongly. (ii) The In adsorption sites located within the As dimers have to be taken into account properly for these specific reconstructions, since they greatly affect the value of the diffusion coefficient. This is in contrast to what happens for the other reconstructions reported in the literature. (iii) The adsorbate diffusion is highly anisotropic with the [1̄ 10] direction favored over the [110] direction, due to the presence of low-potential channels along [1̄ 10]. (iv) The anisotropy is slightly smaller on the α2 reconstruction than on the β2 reconstruction because on the β2 there is an additional diffusion channel along the [1̄ 10] direction. © 2009 The American Physical Society.

Bertelli, Matteo; Homoth, J.; Wenderoth, M.; Rizzi, Angela; Ulbrich, R. G.; Righi, Maria Clelia; Bertoni, Carlo Maria; Catellani, A. ( 2007 ) - Atomic and electronic structure of the cleaved 6H-SiC (11 2̄ 0) surface - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 75 - pp. da 165312 a 165319 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present a combined cross-section scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) and ab initio simulations study of the nonpolar (11 2̄ 0) cleaved surface of 6H-SiC. The experimental results show an unreconstructed surface in agreement with theory. Upon truncation, two surface bands appear inside the semiconductor band gap: one empty band localized on the Si atoms and one filled band on the C atoms. According to the STS experimental results on n -doped samples, the Fermi energy is pinned at the surface inside the band gap. By comparison of STM filled and empty states topographies we propose that on the fresh cleaved surface the Fermi level lies at the bottom of the Si-like band. The calculated STM images reproduce the experimental contrast of the 6H stacking sequence and its bias dependence very well. © 2007 The American Physical Society.

Righi, Maria Clelia; Ferrario, Mauro ( 2007 ) - Potential energy surface for rare gases adsorbed on Cu(111): Parameterization of the gas/metal interaction potential - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 19 - pp. da 305008 a 305015 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We propose a model potential function to describe the interaction between rare-gas atoms and a metal surface with parameters derived on the basis of ab initio calculations. We discuss the merits of the proposed functional form for applications in molecular dynamics studies of nanotribology. © IOP Publishing Ltd.

RIGHI M. C; M. FERRARIO ( 2007 ) - Potential energy surface for rare gases adsorbed on Cu(111):Parametrization of the gas/surface interaction potential (IOP Publishing Limited:Dirac House, Temple Back, Bristol BS1 6BE United Kingdom:011 44 117 9297481, EMAIL: custserv@iop.org, INTERNET: http://www.iop.org, Fax: 011 44 117 9294318 ) - JOURNAL OF PHYSICS. CONDENSED MATTER - n. volume 19 - pp. da 1 a 10 ISSN: 0953-8984 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We propose a model potential function to describe the interaction between rare-gas atoms and a metal surface with parameters derived on the basis of ab-initio calculations. We discuss the merits of the proposed functional form for applications in molecular dynamics studies of nanotribology.

RIGHI M. C; M. FERRARIO ( 2007 ) - Pressure Induced Friction Collapse of Rare Gas Boundary Layers Sliding over Metal Surfaces (American Institute of Physics:2 Huntington Quadrangle, Suite 1NO1:Melville, NY 11747:(800)344-6902, (631)576-2287, EMAIL: subs@aip.org, INTERNET: http://www.aip.org, Fax: (516)349-9704 ) - PHYSICAL REVIEW LETTERS - n. volume 99 - pp. da 1 a 4 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this Letter we show that friction of anticorrugating systems can be dramatically decreased by applying an external load. The counterintuitive behavior that deviates from the macroscopic Amonton law is dictated by quantum mechanical effects that induce a transformation from anticorrugation to corrugation in the near-surface region. We describe the load-driven modifications occurring in the potential energy surface of different rare gas-metal adsorbate systems, namely, Ar, Kr, Xe on Cu(111), and Xe on Ag(111), and we calculate the consequent friction drop for the commensurate Xe/Cu system by means of combined ab initio and classical molecular dynamics simulations.

Righi, Maria Clelia; Magri, Rita; Bertoni, Carlo Maria ( 2006 ) - Study of arsenic for antimony exchange at the Sb-stabilized GaSb(0 0 1) surface - APPLIED SURFACE SCIENCE - n. volume 252 - pp. da 5271 a 5274 ISSN: 0169-4332 [Articolo in rivista (262) - Articolo su rivista]
Abstract

In this paper we present a first-principle study on the energetics of a single As2 molecule on GaSb(0 0 1) reconstructed surface. In order to shed light into the mechanisms of anion exchange at the Sb-rich GaSb(0 0 1) surface, we studied firstly As2 adsorption and then As for Sb exchange. We identify a surface region where both the processes are energetically favored. The results of this twofold analysis can be combined to derive possible reaction paths for the anion exchange process. © 2005 Elsevier B.V. All rights reserved.

Righi MC; Pignedoli CA; Di Felice R; Bertoni CM; Catellani A ( 2005 ) - Combined ab initio and kinetic Monte Carlo simulations of C diffusion on the root(3)X root(3) beta-SiC(111) surface - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 71 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We investigate the kinetic behavior of a single C adatom on the root 3 x root 3 beta-SiC(111) surface by means of combined ab initio and kinetic Monte Carlo simulations. After identifying the metastable binding locations, we calculate the energy barriers the adatom must overcome when jumping among them. The presence of the root 3 x root 3 reconstruction creates considerable differences among the diffusion mechanisms that can be thermally activated. This has important implications for the C mobility on the surface, and therefore for SiC growth. The kinetic simulation at realistic temperatures and time scales revealed that C diffusion occurs mostly around the Si adatoms forming the root 3 x root 3 reconstruction. A reduced adatom mobility, as observed in many studies of surfactant-mediated growth, can favor the formation of a high density of nuclei, and thus promote a layer-by-layer growth. As a further result of the kinetic simulation we obtained the adatom diffusion coefficient, a macroscopic quantity accessible in experiments.

Righi, Maria Clelia; Pignedoli, C. A.; Di Felice, R.; Bertoni, Carlo Maria; Catellani, A. ( 2005 ) - Combined ab initio and kinetic Monte Carlo simulations of C diffusion on the √3 X √3 β-SiC(111) surface - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 71 - pp. da 075303 a 075310 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We investigate the kinetic behavior of a single C adatom on the √3 X √3 β-SiC(111) surface by means of combined ab initio and kinetic Monte Carlo simulations. After identifying the metastable binding locations, we calculate the energy barriers the adatom must overcome when jumping among them. The presence of the √3 X √3 reconstruction creates considerable differences among the diffusion mechanisms that can be thermally activated. This has important implications for the C mobility on the surface, and therefore for SiC growth. The kinetic simulation at realistic temperatures and time scales revealed that C diffusion occurs mostly around the Si adatoms forming the √3 X √3 reconstruction. A reduced adatom mobility, as observed in many studies of surfactant-mediated growth, can favor the formation of a high density of nuclei, and thus promote a layer-by-layer growth. As a further result of the kinetic simulation we obtained the adatom diffusion coefficient, a macroscopic quantity accessible in experiments. ©2005 The American Physical Society.

Catellani, A; Cicero, G.; Righi, Maria Clelia; Pignedoli, Carlo Antonio ( 2005 ) - First principles simulations of SiC-based interfaces ( 5th European Conference on Silicon Carbide and Related Materials, ECRSCRM2004 - Bologna; Italy - 31 August 2004 through 4 September 2004) ( - Silicon Carbide and Related Materials 2004 ) - n. volume 483-485 - pp. da 541 a 546 ISBN: 978-087849963-2 ISSN: 0255-5476 [Contributo in Atti di convegno (273) - Relazione in Atti di Convegno]
Abstract

We review some recent investigations on prototypical SiC-based interfaces, as obtained from first-principles molecular dynamics. We discuss the interface with vacuum, and the role played by surface reconstruction in SiC homoepitaxy, and adatom diffusion. Then we move to the description of a buried, highly mismatched semiconductor interface, the one which occurs between SiC and Si, its natural substrate for growth: in this case, the mechanism governing the creation of a network of dislocations at the SiC/Si interface is presented, along with a microscopic description of the dislocation core. Finally, we describe a template solid/liquid interface, water on SiC: based on the predicted structure of SiC surfaces covered with water molecules, we propose (i) a way of nanopatterning cubic SiC(001) for the attachment of biomolecules and (ii) experiments to reveal the local geometry of adsorbed water.

Righi MC; Magri R; Bertoni CM ( 2005 ) - First-principles study of Sb-stabilized GaSb(001) surface reconstructions - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 71 - pp. da 075323 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We report results of ab initio total-energy and electronic- structure calculations for the Sb-stabilized GaSb(001) surface. We consider different reconstruction models proposed in the literature on the basis of experimental observations in typical GaSb growth conditions and present the T=0 surface stability diagram. We found that dimer-based (4 x 3) reconstructions are favored over widely proposed (1 x 3)/c(2 x 6) models that have a similar structure, but do not satisfy the electron counting rule. We discuss also the stability of the beta 2(2 x 4) reconstruction for comparison with the As-based GaAs(001) and InAs(001) surfaces that present this phase in a wide range of surface preparation conditions. We predict the surface band structure of the GaSb(001) stable configurations and identify the nature of the surface bands. All the (4 x 3) reconstructions turned out to be semiconducting with an energy gap slightly smaller than the GaSb bulk value.

MC Righi; CA Pignedoli; R Di Felice; CM Bertoni; A Catellani ( 2005 ) - Kinetic Monte Carlo simulations of C diffusion on root 3x root 3 beta-SiC(111) based on ab initio calculations - COMPUTER PHYSICS COMMUNICATIONS - n. volume 169 - pp. da 50 a 53 ISSN: 0010-4655 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We performed ab initio calculations to identify and characterize the stationary points of the potential energy surface experienced by a C adatom deposited on the root 3- x root 3 beta-SiC(111) surface. A kinetic Monte Carlo simulation relying on the ab initio calculated parameters allowed us to follow C diffusion at realistic temperature and time scales. We found that the C diffusion occurs mostly around the Si adatoms characteristic of the root 3- x root 3- reconstruction.

Righi, Maria Clelia; Pignedoli, C. A.; Di Felice, R.; Bertoni, Carlo Maria; Catellani, A. ( 2003 ) - Ab initio Simulations of Homoepitaxial SiC Growth - PHYSICAL REVIEW LETTERS - n. volume 91 - pp. da 1361011 a 1361014 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

An ab initio simulation study of SiC homoepitaxial growth is presented. It is shown that the nonstoichiometric reconstruction plays a relevant role in favoring the attainment of high-quality films. It is observed that the energy gain upon surface stability can induce the reorganization of the deposited material into the crystalline structure, thus revealing that a surface-driven mechanism is able to stabilize defect-free layer deposition on Si-rich surfaces.

Serra, S.; Iarlori, S.; Tosatti, Erio; Scandolo, S.; Righi, Maria Clelia; Santoro, G. E. ( 2002 ) - Self-trapping vs. non-trapping of electrons and holes in organic insulators: Polyethylene - CHEMICAL PHYSICS LETTERS - n. volume 360 - pp. da 487 a 493 ISSN: 0009-2614 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We show, by electronic structure based molecular dynamics simulations, that an extra electron injected in crystalline polyethylene should fall spontaneously into a self-trapped state, a shallow donor with a large novel distortion pattern involving a pair of trans-gauche defects. Parallel calculations show instead that a hole will remain free and delocalized. We trace the difference of behavior to the intrachain nature of the hole, as opposed to the interchain one of the electron, and argue that applicability of this concept could be more general. Thus electrons (but not holes) should tend to self-trap in saturated organic insulators, but not for example in aromatic insulators, where both carriers are intrachain. © 2002 Elsevier Science B.V. All rights reserved.

Righi, Maria Clelia; Pignedoli, Carlo Antonio; Borghi, G.; DI FELICE, Rosa; Bertoni, Carlo Maria; Catellani, A. ( 2002 ) - Surface-induced stacking transition at SiC(0001) - PHYSICAL REVIEW. B, CONDENSED MATTER AND MATERIALS PHYSICS - n. volume 66 - pp. da 453201 a 453207 ISSN: 1098-0121 [Articolo in rivista (262) - Articolo su rivista]
Abstract

We present the ab initio results for the energetics of several SiC surfaces having different underlying bulk polytypes, to investigate the role of surface effects in the mechanisms of stacking inversion in SiC. We considered the Si adatom √3×√3 reconstruction for the cubic SiC(111) and the hexagonal SiC(0001) surfaces, taking into account the different subsurface bulk terminations compatible with the 4H and 6H polytypes, and allowing for two opposite stacking orientations of the topmost surface layer. Our investigation reveals that the energy differences among SiC polytypes are enhanced at the surface with respect to the bulk, and two-dimensional effects favor the formation of cubic SiC. We discuss the relevant role played by the surface energetics in the homoepitaxial growth of SiC.

Righi, Maria Clelia; Scandolo, S.; Serra, S.; Iarlori, S.; Tosatti, E.; Santoro, G. ( 2001 ) - Surface states and negative electron affinity in polyethylene - PHYSICAL REVIEW LETTERS - n. volume 87 - pp. da 076802 a 076806 ISSN: 0031-9007 [Articolo in rivista (262) - Articolo su rivista]
Abstract

The electronic properties of the surfaces of polyethylene (PE) were studied via first-principles calculations. It was confirmed that PE has a negative electron affinity and that PE surfaces possess a surface state whose energy lies in the forbidden bulk gap.