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International audience; Surfaces of aluminum alloys are often coated with ultra-thin alumina films which form by self-limited selective oxidation. Although the presence of such films is of paramount importance in various applications, their structural and stability characteristics remain far from being known. In particular, on the NiAl(100) substrate, the observed structure has been tentatively assigned to a distorted θ-alumina polymorph, but the film stoichiometry, the nature of its surface and interface terminations, as well as the mechanisms that stabilize the θ phase remain unknown. Using a combined tight-binding/DFT genetic algorithm approach, we explicitly demonstrate that ultra-thin θ(100)-type films correspond to the structural ground state of alumina supported on the (2 × 1)-NiAl(100) substrate. Thus, experimentally observed θ-alumina films correspond to thermodynamic equilibrium, rather than being the result of kinetic effects involved in the alloy oxidation and film growth. They are favoured over other Al2O3 phases of dehydrated boehmite, pseudo-CaIrO3, γ, or bixbyite structures, which have recently been identified among the most stable free-standing ultra-thin alumina polymorphs. Moreover, our results prove that nonstoichiometry can be easily accommodated by the supported θ(100) film structure via an excess or deficiency of oxygen atoms at the very interface with the metal substrate. Dedicated DFT analysis reveals that the oxide-metal interaction at stoichiometric interfaces depends surprisingly little on the composition of the NiAl surface. Conversely, at oxygen-rich/poor interfaces, the number of additional/missing Al–O bonds is directly responsible for their relative stability. Finally the comparison between the experimental and theoretical electronic characteristics (STM and XPS) of supported θ(100)-type films provides clues on the detailed structure of the experimentally observed films.

International audience; With the help of a DFT+U approach, we have analyzed the characteristics of a series of pure and mixed 3d transition metal oxide honeycomb monolayers of M2O3 and MM′O3 stoichiometries (M, M′ = Ti, V, Cr, and Fe) deposited on a metal substrate (Me = Ag, Au, Pt). We show that the substrate-induced structural polarization, interfacial electron transfer, and oxide-metal interaction strength display general trends which are governed by the offsets between the oxide band structure and the metal Fermi level. They are the strongest for the least electronegative cations (Ti and V) deposited on supports with the largest work functions (Au, Pt), where the depletion of purely 3d Ti and V states provokes an increase of the cation oxidation state. Mixing generally induces electron transfers from the least (Ti and V) to the most (Cr and Fe) electronegative cations. However, the systematic delocalization of the Ti2O3 d valence electrons to the metal substrates limits any significant mixing-induced electronic rearrangements to the V-based compounds only. We show that these electronic effects directly impact the energetics of cationic mixing and are responsible for a dramatic destabilization of the mixed TiFeO3 monolayers compared to the bulk ilmenite phase, while they stabilize ordered mixed VFeO3 films which have no bulk equivalent. Our findings give general guidelines on how oxide electronic, magnetic, and reactivity characteristics can be efficiently engineered by tuning the oxide stoichiometry and the metal substrate, of direct interest for modern technologies

International audience; Observing non-classical nucleation pathways remains challenging in simulations of complex materials with technological interests. This is because it requires very accurate force fields that can capture the whole complexity of their underlying interatomic interactions and an advanced structural analysis able to discriminate between competing crystalline phases. Here, we first report the construction and a particularly thorough validation of a machine-learning force field for zinc oxide interactions using the Physical LassoLars Interaction Potentials approach which allows us to be predictive even for high temperature dynamical systems such as ZnO melt. Then, we carried out several types of crystallization simulations and followed the formation of ZnO crystal with atomistic precision. Our results, which were analyzed using a data-driven approach based on bond order parameters, demonstrate the presence of both prenucleation clusters and two-step nucleation scenarios, thus retrieving seminal predictions of non-classical nucleation pathways made on much simpler models. Dedicated calculations of high temperature ZnO free energy within a newly developed automated nonequi-librium thermodynamic integration method revealed the existence of a thermodynamic bias for the predicted non-classical nucleation scenarios.

Considering the importance of sub-monolayer transition metal oxides supported on another oxide in many industrial processes, with the help of a DFT + U approach, we provide information on the structural and electronic properties of pure M 2O3 and mixed MM′O3 oxide monolayers (M, M′ = Ti, V, Cr, Fe) supported on an α-Al2O3(0001) support. With their structure in the prolongation of the alumina corundum lattice, the monolayers have non-equivalent surface and interface cations, which leads to two different cation configurations in the mixed oxides. In all cases, the interfacial charge transfer is weak, but strong cation–cation electron redistributions may take place as in TiVO3, TiFeO3, VFeO3, and TiCrO3 in which actual redox processes lead to cation oxidation states different from the expected +3 value. We show that the tendency to mixing relies on the interplay between two very different driving forces. Cation–cation redox reactions, in most cases, strongly stabilise mixed configurations, but preference for a given cation position in the monolayer, because of surface energy reasons, may strengthen, weaken or even block the mixing tendency. By comparison with results obtained in bulk ilmenite, in free-standing monolayers and in MLs deposited on transition metal substrates, we evidence the flexibility of their electronic structure as a function of size, dimensionality and nature of support, as a lever to tune their properties for specific applications.

The Cr/α-Al

By combining differential conductance (dI/dV) spectroscopy with a scanning tunneling microscope and hybrid density functional theory simulations we explore the electronic characteristics of the (1×1) and (√3×√3)R30° terminations of the Cu2O(111) surface close to thermodynamic equilibrium. Although frequently observed experimentally, the composition and atomic structure of these two terminations remain controversial. Our results show that their measured electronic signatures, such as the conduction band onset deduced from dI/dV measurements, the bias-dependent appearance of surface topographic features, as well as the work function retrieved from field emission resonances unambiguously confirm their recent assignment to a (1×1) Cu-deficient and a (√3×√3)R30° nano-pyramidal reconstruction. Moreover, we demonstrate that due to a different localization of the screening charges at these Cu-deficient terminations, their electronic characteristics qualitatively differ from those of the stoichiometric (1×1) and O-deficient (√3×√3) terminations often assumed in the literature. As a consequence, aside from the topographic differences recently pointed out, also their electronic characteristics may contribute to a radical change in the common perception of the Cu2O(111) surface reactivity.

International audience; The Cr/α-Al2O3(0001) interface has been explored by X-ray photoemission spectroscopy, X-ray absorption spectroscopy (XAS) and ab initio first-principles calculations of core level shifts including final state effects. After an initial oxidation via a reaction with residual surface OH but no reduction of the alumina substrate, Cr grows in a metallic form without any chemical effect on the initially oxidized Cr. However, Cr metal lacks crystallinity. Long-range (reflection high energy electron diffraction) and short-range (XAS) order are hardly observed. Thus photoemission combined with atomistic simulations becomes a unique tool to explore the chemistry and environment at the Cr/alumina interface. Cr 2p, O 1s and Al 2s shifted components are all explained by the formation of moieties involving Cr3+ and/or Cr4+ and of metallic Cr0, which supports the previously found Cr buffer mechanism for poorly adhesive metals. Beyond the situation under study, the present data demonstrate the ability of a combined experimental and theoretical approach of core-level shifts to exhaustively describe the general case of disordered metal/oxide interfaces.

International audience; Thanks to a dedicated modelling of intensities, the depth sensitivity of X-ray photoemission is used to probe the segregation profile of aluminium at the (110), (100) and (111) low index surfaces of the body-centred Fe 0.85 Al 0.15 random alloy. Sputtered surface composition is close to the nominal bulk one, thus excluding preferential sputtering. Surface enrichment in aluminium upon annealing starts at around 700 K before reaching a stationary state above 1000 K. The average surface composition is close to Fe 0.5 Al 0.5 , corresponding to the B 2 CsCl structure on the phase diagram. The impacted depth, that is in the range of 2.5-3.5 nm, is quite significant. Although not evidenced previously in surface science conditions at FeAl single crystal surfaces, it is qualitatively in agreement with the segregation at grain boundaries and shear planes of Al-alloyed steels. This segregation tendency is rationalized through ab initio calculations.

Transition metal oxide layers on metal surfaces often show superior catalytic performance as compared to conventional, i.e., metal-on-oxide, systems. In this work, we studied the CO oxidation reaction over FeO supported by Pt(1 1 1) and observed strong size effects on the reactivity. The monolayer islands below 5 nm in size showed a light-off temperature for CO2 production 200 K lower than for the 30 nm islands. In an attempt to rationalize the size effect, we have performed oxygen desorption experiments and have combined them with an extended DFT analysis to provide insight into the bonding of various oxygen species in such systems. The theoretical results on isolated FeO2 islands indicate a substantially lower stability of the boundary oxygen atoms as compared with the same islands embedded into an FeO layer in qualitative agreement with experiment showing that the smaller the island, the lower the desorption temperature. To the best of our knowledge, the results demonstrate the first example of a size effect in oxidation catalysis on two-dimensional systems.

International audience; Despite the importance of cuprous oxide for industrial applications, the precise structure of its most stable reconstructed surface remains unknown. Here, by a combination of highresolution STM measurements and simulations based on a DFT-HSE approach, we challenge the previous interpretations of the Cu 2 O(111) (√ 3× √ 3)R30 • surface reconstruction, based on the removal of 1/3 of the under-coordinated surface oxygen atoms. We show that these models do not satisfy the shamrock-type features found by STM, i.e. their position in the Cu-O ring centers, the orientation towards the surface O cus atoms and the topographic height in the STM images. Moreover, they are thermodynamically unstable with respect to moving the vacancy subsurface, a property not recognized before which seems specific to the Cu 2 O (111) surface. We propose a nanopyramidal model of the (√ 3× √ 3)R30 • surface reconstruction which is free from all these shortcomings. Here, the shamrock protrusions seen in STM are formed by three copper adatoms located at the center of the Cu-O rings and capped by an oxygen atom. This structure profoundly differs from existing models of the Cu 2 O(111) surface, and would change common perceptions on its reactivity.

The formation of an ultra-thin aluminum oxide film at Fe0.85Al0.15(110) surface (A2 random alloy) has been studied by a variety of surface sensitive techniques (X-ray photoemission, low-energy electron diffraction, surface X-ray diffraction and scanning tunneling microscopy) supplemented by ab initio atomistic simulations. Since iron is not oxidized in the used conditions, the study focused on the coupling between aluminum oxidation and segregation processes. Compared to the bare surface, whose average composition (Fe0.6Al0.4) is closer to the B2-CsCl structure over a ∼ 3 nm depth, the oxidation hardly affects the subsurface segregation of aluminum. All the structural and chemical fingerprints point to an oxide film similar to that found on NiAl(110). It is a bilayer (∼ 7.5A thick) with a composition close to Al10O13 and a large (18.8 × 10.7)A 2 nearly rectangular unit cell; an almost perfect match between substrate periodicity and the (1 × 2) oxide supercell is found. Nevertheless, microscopy reveals the presence of anti-phase domain boundaries. Measured Al 2p and O 1s core level shifts match calculated ones; their origin and the relative contributions of initial/final state effects are discussed. The ubiquity of the present oxide on different supports asks for the origin of its stability.

International audience; Formation of mixed compounds at interfaces between materials often has significant impact on the properties of the junction. With the goal of assessing if interfacial oxide mixing improves the adhesion of anti-corrosive zinc coatings on silica, we report an ab initio study of the structural, electronic, and adhesion characteristics of interfaces between zinc and zinc silicate. We show that, regardless the precise thermodynamic conditions, ZnO-rich zinc sili-cate interfaces are more stable than SiO 2-rich ones because their formation does not require breaking strong Si-O bonds. Moreover, we find that zinc adhesion at such ZnO-rich interfaces is by at least a factor two larger than that of the zinc/silica contact. Due to a dense network of interfacial Zn-O bonds, this improvement does not produce any unfavorable decohesion effects in the zinc deposit. The formation of an interfacial silicate layer enables the suppression of the weak zinc/silica interface, which is responsible for the poor zinc adhesion. The cohesive cleavage within zinc predicted for the complex zinc/silicate/silica junction offers a promising improvement of performances of anti-corrosive zinc coatings of Si-rich steel grades, of crucial importance in steel-making and automotive industries.

International audience; (Sub)nanometre-thin alumina films are frequently encountered due to the self-limited oxidation of Al and its alloys, and seem to display an even larger structural variety than bulk alumina itself. While the nature of the underlying substrate and the oxidation kinetics are known to modulate the structure of supported films, understanding the intrinsic stability of freestanding films constitutes an important first step in itself, especially when the interaction with the substrate is rather weak. Using a combined tight-binding/DFT genetic algorithm approach, we identify particularly stable θ (100)-type films along with a host of novel stable thin film structures. Several of these correspond to cuts from relatively high energy bulk structures, e.g. dehydrated boehmite, pseudo-CaIrO 3 , defective rocksalt and LuMnO 3 , which are not commonly associated with alumina. DFT calculations allow to rationalize this stability reversal with respect to α-Al 2 O 3 in terms of low surface energies compared to α(0001) and to identify the underlying mechanisms: breaking a low density of relatively weak Al-O bonds, filling of Al surface vacancies, and polarity-induced relaxation of the whole film. These observations provide interesting insights into existing supported ultrathin films.

(Sub)nanometre-thin alumina films are frequently encountered due to the self-limited oxidation of Al and its alloys, and seem to display an even larger structural variety than bulk alumina itself. While the nature of the underlying substrate and the oxidation kinetics are known to modulate the structure of supported films, understanding the intrinsic stability of freestanding films constitutes an important first step in itself, especially when the interaction with the substrate is rather weak. Using a combined tight-binding/DFT genetic algorithm approach, we identify particularly stable θ(100)-type films along with a host of novel stable thin film structures. Several of these correspond to cuts from relatively high energy bulk structures, e.g. dehydrated boehmite, pseudo-CaIrO3, defective rocksalt and LuMnO3, which are not commonly associated with alumina. DFT calculations allow to rationalize this stability reversal with respect to α-Al2O3 in terms of low surface energies compared to α(0001) and to identify the underlying mechanisms: breaking a low density of relatively weak Al-O bonds, filling of Al surface vacancies, and polarity-induced relaxation of the whole film. These observations provide interesting insights into existing supported ultrathin films.

International audience; The understanding of interfacial effects and adhesion at oxide-metal contacts is of key importance in modern technology. Metal-silica interfaces specifically are relevant in electronics, catalysis and nanotechnology. However, adhesion at these interfaces is hindered by a formation of siloxane rings on the silica surface which saturate the dangling bonds at stoichiometric terminations. In this context, we report a thorough density functional theory study of the interaction between β-cristobalite and selected 3d transition metals under different oxygen conditions. For any given interface stoichiometry, we find a progressive decrease of the metal/silica interaction along the series, following the increase of metal electronegativity. Crucially, in presence of early transition metals (Ti or Cr) the surface siloxane rings are spontaneously broken, allowing for strong adhesion. Late transition metals interact only weakly with reconstructed surfaces, similarly to what was found for zinc. In absence of reconstruction, stoichiometric silica/metal contacts behave similarly to alumina/metal contacts, but display larger interactions across the interface. Based on these results, we show that early transition metal or stainless steel buffers can significantly improve the weak adhesion between silica and zinc, responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels.

Grain boundaries (GBs) are ubiquitous in solids. Their description is critical for understanding polycrystalline materials and explaining their mechanical and electrical properties. A GB in a 2D material can be described as a line defect and its atomic structures have been intensively studied in materials such as graphene. These GBs accommodate the relative rotation of two neighboring grains by incorporating periodic units consisting of nonhexagonal rings along the boundary. Zero-degree GBs, called domain boundaries (DBs), where there is only a lattice offset between two grains without any rotation, are rare in 2D van-der-Waals (vdW) bonded materials where the grains can easily move. However, this movement is not possible in 2D materials that have a strong epitaxial relationship with their substrate such as the M2O3 (2 × 2) honeycomb monolayers on noble metal (111) supports. Involving experimental and theoretical investigations, four main DBs are observed here in a monolayer of Ti2O3 supported on Au(111) and their atomic structures are solved. The DB formation energies explain why some DBs are more frequently observed than others. The strong epitaxial constraint from the Au(111) substrate stabilizes some unique Ti2O3 monolayer DB structures that are not observed in vdW-bonded 2D materials.

International audience; The formation of ultrathin copper oxide layers on an Au(111) surface is explored with scanning tunneling microscopy and density functional theory. Depending on the thermal treatment of as-grown Cu-O samples, a variety of thin-film morphologies is observed. Whereas 1D oxide stripes with Au[11 % 2] and Au[1 % 10] orientation emerge at 450 and 550 K annealing, respectively, a planar (2 2) Cu-O network with specific domain structure develops at higher temperature. The latter is ascribed to a Cu 3 O 2 honeycomb lattice with oxygen ions alternatingly located in surface and interface positions. Strain minimization and a thermodynamic preference for Cu-rich edges lead to the formation of structurally well-defined boundaries, delimiting either triangular, elongated or stripe-like Cu 3 O 2 domains. The low-temperature phases compirse complex arrangements of hexagonal and square Cu-O units, similar to those found in Cu 2 O(111) and (100) surfaces, respectively. The transitions between different thin-film phases are driven by Cu dissolution in the gold crystal and O 2 evaporation and therefore accompanied by a thinning of the oxide layer with increasing temperature.

International audience; The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels which are fundamental in the automotive industry, and important for rail transport, shipbuilding, or aerospace. With the goal of identifying possible levers for its improvement, we report an ab initio study of the effect of surface hydroxylation on the adhesion characteristics of model zinc/β-cristobalite interfaces, representative for various surface hydroxylation/hydrogenation conditions. We show that surface silanols resulting from dissociative water adsorption at the most stable stoichiometric (001) and (111) surfaces prevent strong zinc-silica interaction. However, dehydrogenation of such interfaces produces oxygen-rich zinc/silica contacts with excellent adhesion characteristics. They are due to a partial zinc oxidation and the formation of strong iono-covalent Zn-O bonds between zinc atoms and the under-coordinated excess anions, remnant of the hydroxylation layer. Interestingly, these interfaces appear as the most thermodynamically stable in a wide range of realistic oxygen-rich and hydrogen-lean environments. We also point out that the partial oxidation of zinc atoms in direct contact with the oxide substrate may somewhat weaken the cohesion in the zinc deposit itself. This fundamental analysis of the microscopic mechanisms responsible for the improved zinc wetting on pre-hydroxylated silica substrates furnishes useful guidelines towards practical attempts to improve adhesion.

International audience; The weak interaction between zinc and alumina is responsible for a poor performance of anticorrosive galvanic zinc coatings on modern advanced high strength steels. In this context, we report a theoretical study on the effect of realistic multicomponent metal buffers on the adhesion strength of a model α-alumina(0001)|zinc interface. Relying on results of ab initio calculations on relevant individual oxide|oxide, oxide|metal, and metal|metal interfaces (separation and interface energies), we determine by Monte Carlo simulations the thermodynamically preferred sequence of components in a multicomponent buffer, as a function of buffer composition and oxygen conditions. We find that stainless steel buffers considerably enhance the overall strength of the alumina|zinc interface. Most importantly, we show that a partial oxidation of multicomponent buffers, which is unavoidable under realistic conditions, does not degrade their performance. This advantageous property relies on the separation of metal and oxide components in the buffer and on the resulting suppression of weakly interacting oxide|zinc and moderately strong alumina|metal interfaces. More generally, owing to the possibility of selective oxidation and component segregation, multicomponent buffers appear as promising solutions for adhesion improvement at weakly interacting metal|oxide interfaces.

International audience; The control of adhesion at metal/oxide interfaces is of key importance in modern applications, whenever three-dimensional metal clusters or two-dimensional metal overlayers are to be synthesized on an oxide support. By focusing on the zinc/alumina system, we address here one of the long-standing issues in this context, which is the poor wetting of wide band-gap oxides by noble and post-transition metals. It has recently been recognized to have detrimental industrial consequences for the adhesion of anti-corrosive zinc coatings to new high strength steels grades. We have combined photoemission, thermal desorption and plasmonics with atomistic simulation to describe the energetics of zinc deposits on dry and hydroxylated α-Al2O3(0001) surfaces. Both experimental and computational results show that an activated reaction of the metal with the OH-covered surface, followed by hydrogen desorption, produces dispersed interfacial moieties involving both oxidized Zn species and under-coordinated oxygen ions, that lead to a significant improvement of adsorption/adhesion characteristics on the hydroxylated surface. In particular, the key role of interfacial under-coordinated anions, remnants of the hydroxylation layer, is highlighted for the first time, pointing to a general mechanism by which surface hydroxylation appears as a promising route towards a systematic improvement of wide band gap oxide wetting by metals.

We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1–4 ML thickness range.

International audience; We address herein the question of the termination of the Al-terminated α-Al 2 O 3 (0001) surface. Over decades, various analyses made by different groups repeatedly suggested the presence of a residual coverage of surface OH groups on Al 2 O 3 crystals and powders after annealing in vacuum. However, other authors came to contrary conclusions, thus maintaining a persistent blur on the issue. The present work examines the Cr/alumina interface via Cr K absorption edge analysis (Extended X-Ray Absorption Fine Structure and X-ray Absorption Near Edge Structure) and photoemission with the support of density functional (DFT) calculations. Experiments support the presence of surface OH groups to account for the observed environment as well as the oxidation state of Cr adatoms (Cr 3+). Following a comprehensive DFT-based analysis of Cr z+-O n H m configurations (z=0 to 6, n and m=0 to 3), the Cr 3+-O 2 H alumina-supported surface moieties are found to successfully fit the XAS edge calculations. Most importantly, the combination of experiment and theory that is developed unambiguously demonstrates the presence of surface OH groups on α-Al 2 O 3 (0001) after annealing in vacuum.

The experimental discovery and theoretical analysis of an epitaxial (2 \ifmmode\times\else\texttimes\fi{} 2) honeycomb $\mathrm{N}{\mathrm{b}}_{2}{\mathrm{O}}_{3}$ monolayer on a Au(111) surface is reported. The oxide monolayer is grown by Nb deposition and subsequent annealing in an oxidizing atmosphere. Scanning tunneling microscopy (STM) images show that the films form a well-ordered honeycomb lattice, and low energy electron diffraction patterns confirm that the films adopt a (2 \ifmmode\times\else\texttimes\fi{} 2) periodicity with respect to the Au(111) substrate. Density functional theory (DFT) modeling shows that the Nb atoms are located in Au(111) threefold hollow sites and the O atoms are located in on-top positions. DFT also demonstrates the existence of a strong interfacial interaction characterized by a large electron transfer towards the Au substrate, an increase of the Nb oxidation state, and substantial film rumpling. High-resolution STM images, supported by simulations, are able to discriminate between Nb atoms adsorbed in fcc or hcp hollow sites on the Au(111) substrate.

International audience; Silicate compounds are ubiquitous in nature and display a vast variety of structures and properties. Thin silicate films may also form under specific conditions at interfaces between metals and silica. In the present study, we focus on zinc silicate and present a thorough density functional theory-based study of polar and non-polar (001) surfaces of various stoichiometry of its tetrago-nal polymorph t-Zn 2 SiO 4. At the non-polar surfaces, the main features are the existence of the chain reconstruction at the ZnO termination, and the presence of unsaturated surface silanols at the SiO 2 termination. Stabilization of polar surfaces is provided by the formation of O 2− 2 peroxo groups, reduction of the surface or subsurface Si atoms or formation of Zn 2+ 2 groups, depending upon the surface stoichiometry. While the non-polar stoichiometric and ZnO rich terminations are the most stable in a large part of the accessible phase diagram, the SiO 2 termination is less stable due to the absence of siloxane group formation. We show that, while bulk Zn 2 SiO 4 is stable with respect to decomposition into the ZnO and SiO 2 oxides, the same is not true for ultra-thin films due to the fundamental difference of silicate and silica surface energies. Preliminary results show that a similar conclusion could be drawn Fe 2 SiO 4. This study opens towards a deeper understanding and possible improvement of zinc adhesion at silica surfaces, of crucial industrial importance.

Silicate compounds are ubiquitous in nature and display a vast variety of structures and properties. Thin silicate films may also form under specific conditions at interfaces between metals and silica. In the present study, we focus on zinc silicate and present a thorough density functional theory-based study of polar and non-polar (001) surfaces of various stoichiometry of its tetragonal polymorph t-Zn

International audience; PACS numbers : 61.46.-w, 68.55.-a, 71.15.Mb, 78.70.Dm The growth mode of strained epitaxial films relies on the interaction strength, the lattice matching and the mechanical response of the system. The present work focuses on the basic physics of supported nano-islands by examining the characteristics of MgO/Ag(100) taken as a case study. The combination of experiments and simulations highlights the existence of a small size regime in which, despite the largest adhesion and the smallest mismatch, the islands are the least distorted by the substrate. We assign this unexpected behavior to the enhanced island stiffness which makes the cost of elastic distortion prohibitive compared to the associated gain of MgO-Ag interaction energy. The analysis provides a general framework to predict and/or understand nanoscale effects on interfacial pseudomorphism. These are likely to hold whatever the nature of the deposit and substrate under consideration. It may have far reaching consequences on many properties of supported nano-objects.

International audience; Two-dimensional materials with a honeycomb lattice, such as graphene and hexagonal boron nitride, often contain local defects in which the hexagonal elements are replaced by four, five, seven, and eight-membered rings. An example is the Stone-Wales (S-W) defect, where a bond rotation causes four hexagons to be transformed into a cluster of two pentagons and two heptagons. A further series of similar defects incorporating divacancies results in larger structures of non-hexagonal elements. In this paper, we use scanning tunneling microscopy (STM) and density functional theory (DFT) modeling to investigate the structure and energetics of S-W and divacancy defects in a honeycomb (2 × 2) Ti 2 O 3 monolayer grown on an Au(111) substrate. The epitaxial rumpled Ti 2 O 3 monolayer is pseudomorphic and in a state of elastic compression. As a consequence, divacancy defects, which induce tension in freestanding films, relieve the compression in the epitaxial Ti 2 O 3 monolayer and therefore have significantly 2 lower energies when compared with their freestanding counterparts. We find that at the divacancy defect sites there is a local reduction of the charge transfer between the film and the substrate, the rumpling is reduced, and the film has an increased separation from the substrate. Our results demonstrate the capacity of the substrate to significantly influence the energetics, and hence favor vacancy-type defects, in compressively strained 2D materials. This approach could be applied more broadly, for example to tensile monolayers, where vacancy-type defects would be rare and interstitial-type defects might be favored.

International audience; The structural quality of oxide/oxide interfaces, their chemistry, and the nature of their electronic states are critical properties for new promising applications and require continuous advances in their fundamental understanding. To this goal, we have performed a theoretical study of a series of (0001) M2O3/M’2O3 interfaces (M, M’ = Al, Ti, V, Cr, Fe) between simple and transition metal oxides crystallizing in the corundum structure. Our DFT+U results reveal two qualitatively different mechanisms of interface charge redistribution: an interfacial oxidation–reduction reaction occurring at Ti2O3/V2O3 and Ti2O3/Fe2O3 contacts, and a much weaker electron transfer along anion-cation bonds which follows the difference of ionicity between the two constituting oxides at all interfaces. At variance with interfaces between sp semiconductors, the band bending does not propagate beyond one or two atomic layers. More generally, our results show that the nature of the interface electronic structure and the energetics of its formation can be tightly linked to the properties of the corresponding bulk oxides, thus providing a precious tool for designing oxide/oxide interfaces of required characteristics.

International audience; We have used a DFT + U approach to study the characteristics of a series of 3d transition-metal oxide monolayers (MLs) of M 2 O 3 stoichiometry (M = Sc, Ti, V, Cr, Mn, Fe, Co, and Ni) in freestanding and Au(111)-supported honeycomb structures. They include), and purely charge-transfer (Sc 2 O 3) oxides. We have found that besides a structural polarization, the interaction with the Au substrate induces an interfacial electron transfer and considerably modifies the electronic characteristics of the monolayers. In particular, in Ti 2 O 3 , V 2 O 3 , and Ni 2 O 3 , due to the purely d character of the relevant gap edges, these modifications may be rationalized in terms of a change of the cation oxidation state with respect to the corresponding bulk and freestanding ML reference. We propose a general conceptual framework that links the properties of the separate systems (band offsets and character of the states at the gap edges), the value of the interfacial electron transfer, and the electronic and energetic characteristics of the supported MLs. Our results and analysis furnish guidelines toward the tuning of electronic and magnetic characteristics beyond those imposed by the oxide stoichiometry, of direct interest for modern technologies.

International audience; With the advent of new steel grades, the galvanic protection by zinc coating faces a new paradigm. Indeed, enrichment in strengthening elements prone to oxidation such as Al, Mn, and Si, leads to the formation of oxide films that are poorly wet by zinc. We study herein routes for improvement of adhesion at the model Zn/α-Al 2 O 3 interface by metallic additions. As a first step, with help of ab initio results on adsorption characteristics of transition metal adatoms at α-alumina surfaces, we establish and rationalize clear trends in the behavior of both metal-alumina interaction strength and in the relative thermodynamic stability of configurations with weakly and strongly bound metal adatoms. The reasons of enhanced binding strength of transition metals, such as Cr, maintained regardless the precise alumina termination and surface charge state are pointed out. On these grounds, possible improvements of adhesion in realistic conditions are discussed. It is predicted that an enrichment in transition metals, such as Cr, may produce strongly adhesive interfaces that lead to cohesive cleavage.

International audience; The growing interest of modern technologies in oxide materials and the advances in the fabrication and control of ultra-thin oxide films raise new questions on how to use cation doping or mixing to engineer the properties of such two dimensional objects. With a special focus on their intrinsic properties in the absence of interaction with a substrate, we have modeled a series of free-standing pure M 2 O 3 and mixed MM'O 3 compounds (M, M' = Ti, V, Cr, Fe) in two-dimensional honeycomb structures within DFT+U and hybrid approaches. A systematic comparison with their bulk counterparts enabled the identification and discussion of their properties which are driven by their low dimensionality and peculiar atomic structure. On the one hand, it concerns the mixing-induced changes of cation oxidation states, which are controlled by the structure-dependent characteristics of their band edges. On the other hand, an overall increase of the energetic tendency for cationic mixing in low dimensions is highlighted. This comparison shows that low dimensionality enables stabilization of ternary oxides of compositions and properties with no bulk equivalents, thus of a direct interest for modern technologies.

The growing interest of modern technologies in oxide materials is in part due to the possibility of their doping and/or mixing, which often leads to artificial compounds with tunable properties, such as the gap width or the band edge positions. However, engineering of mixed oxide materials requires a better fundamental understanding of their structural, electronic, and ordering properties, well beyond low doping levels, and with reference to their parent materials. In the present work, a series of mixed $M{M}^{\ensuremath{'}}{\mathrm{O}}_{3}$ oxides $(M,{M}^{\ensuremath{'}}=\text{Al},\text{Ti},\text{V},\text{Cr},\text{Fe})$ in three corundum-type structures---ilmenite, ${\mathrm{LiNbO}}_{3}$, and C---is studied by means of first-principles DFT+$U$ simulations. We find that regardless of the precise atomic structure, the local structural and electronic characteristics of most of the compounds are very close to those of their parent corundum ${M}_{2}{\mathrm{O}}_{3}$ oxides. The two noticeable exceptions are ${\mathrm{TiVO}}_{3}$ and ${\mathrm{TiFeO}}_{3}$, for which the structural, electronic, and magnetic characteristics are consistent with a mixing-induced change of the cation oxidation states. We show that this actual oxidoreduction process can be rationalized by analyzing the relative band edge positions of the parent oxides. The formation energies of these mixed oxides correlate well with the experimental evidence and, within a mean-field approximation, allow predicting the thermodynamics of solid solutions ${M}_{2\ensuremath{-}2x}{M}_{2x}^{\ensuremath{'}}{\mathrm{O}}_{3}$ at finite temperature.

International audience; The weak interaction between zinc and silica is responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels. With the goal of identifying its microscopic origin, we report an extensive ab initio study of the structural, electronic, and adhesion characteristics of a variety of model zinc|β-cristobalite interfaces, representative for different oxidation conditions. We show that the weakness of the zinc-silica interaction at non polar interfaces is driven by the presence of surface siloxane rings. These latter are drastically detrimental to interface adhesion when intact and their breaking is impeded by a large energy barrier. Conversely, the characteristics of polar interfaces are principally driven by the capacity of zinc to screen the surface compensating charges and to form O-Zn bonds. This screening is especially efficient in oxygen-rich environment where the substrate-induced partial oxidation of the zinc deposit produces a considerable enhancement of interface adhesion. The identified microscopic mechanisms of interface interaction furnish precious guidelines towards practical attempts to improve adhesion. In particular, processes which enable breaking the surface siloxane rings are expected to noticeably reinforce the interaction at non-polar interfaces.

We report the existence of a two-dimensional electron system (2DES) at the $(001)$ surface of ${\mathrm{CaTiO}}_{3}$. Using angle-resolved photoemission spectroscopy, we find a hybridization between the ${d}_{xz}$ and ${d}_{yz}$ orbitals, not observed in the 2DESs at the surfaces of other $A{\mathrm{TiO}}_{3}$ perovskites, e.g., ${\mathrm{SrTiO}}_{3}$ or ${\mathrm{BaTiO}}_{3}$. Based on a comparison of the 2DES properties in these three materials, we show how the electronic structure of the 2DES (bandwidth, orbital energy order, and electron density) is coupled to different typical lattice distortions in perovskites. The orbital hybridization in orthorhombic ${\mathrm{CaTiO}}_{3}$ results from the rotation of the oxygen octahedra, which can also occur at the interface of oxide heterostructures to compensate strain. More generally, the control of the orbital energy order in 2DES by choosing different $A$-site cations in perovskites offers a gateway toward 2DESs in oxide heterostructures beyond ${\mathrm{SrTiO}}_{3}$.

International audience; Despite the application potential of nickel tungstate (NiWO4) in heterogeneous catalysis, humidity and gas sensing, etc, its surfaces have essentially remained unexplored. In this work, NiWO4 nanoparticles and films with the wolframite structure have been grown via a solid-state reaction of (WO3)3 clusters and a NiO(100) film on a Ni(110) crystal surface and characterized by a variety of experimental techniques, including x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and x-ray diffraction (XRD), combined with ab-initio density functional theory (DFT) calculations. NiWO4 grows initially as three-dimensional (3D) crystalline nanoparticles displaying mainly two crystalline facets vicinal to the (100) surface, which merge with increasing the (WO3)3 coverage into a quasi-continuous epitaxial film. The DFT results provide an account of the energetics of NiWO4 low index surfaces and highlight the role of faceting in the stabilization of extended polar (100) terraces. These combined experimental and theoretical results show that interaction with a metal substrate and vertical confinement may stabilize oxide nano-objects with high energy facets, able to enhance their reactivity.

Adhesion at zinc/alumina interface is a key issue in the field of steelmaking industry, where selective Al oxidation, followed by surface segregation of alumina islands, efficiently impedes wetting of anticorrosive Zn coating on the high strength steel grades. Relying on ab initio total energy calculations, we have examined adsorption of Zn adatoms on different terminations of α-Al2O3(0001) surface under both vacuum conditions and in the presence of surface hydroxyls. Surface configurations with strongly bound Zn and thermodynamic conditions necessary for their stability have been identified. We have shown the existence of a wide range of nonextreme oxygen-rich conditions under which Zn tends to spill over the alumina substrate as an array of strongly adsorbed adatoms, rather than to form metallic clusters weakly bound to the substrate. This effect has been assigned to surface non-neutrality, such as due to surface polarity, or to an excess of surface hydroxyls. Moreover, compared to its direct neighbors ...

International audience; We compare the performances of different DFT functionals applied to ultra-thin MgO(100) films supported on the Ag(100) surface, a prototypical system of a weakly interacting oxide/metal interface, extensively studied in the past. Beyond semi-local DFT-GGA approximation, we also use the hybrid DFT-HSE approach to improve the description of the oxide electronic structure. Moreover, to better account for the interfacial adhesion, we include the van de Waals interactions by means of either the semi-empirical force fields by Grimme (DFT-D2 and DFT-D2*) or the self-consistent density functional optB88-vdW. We compare and discuss the results on the structural, electronic, and adhesion characteristics of the interface as obtained for pristine and oxygen-deficient Ag-supported MgO films in the 1-4 ML thickness range.

Surface stress and energy are basic quantities in the Gibbsian formulation of the thermodynamic description of surfaces which is central in the formation and long-term behavior of materials at the nanoscale. However, their size dependence is a puzzling issue. It is even unclear whether they decrease or increase with decreasing particle size. In addition, for a given metal, estimates often span over an order of magnitude, far apart from bulk data, which, in the absence of any explicit size-dependence rule, escapes understanding. Here, we combine X-ray absorption and nanoplasmonics data with atomistic simulation to describe α-Al2O3(0001)-supported silver particles. By comparison to MgO(001)-supported and embedded silver, we distinguish epitaxial and surface stress. The latter is shown to dominate above 3 nm in size. Since the observation mostly relies on surface/bulk ratio, a metal-independent picture emerges that is expected to have far-reaching consequences for the understanding of the energetics of nanoparticles.

International audience; Scanning tunneling microscopy (STM) and hybrid density functional theory (DFT) have been used to study the stability and electronic characteristics of the Cu 2 O(111) surface. We challenge previous interpretations of its structure and composition and show that only appropriate (hybrid) calculations can correctly account for the relative thermodynamic stability of stoichiometric versus Cu-deficient terminations. Our theoretical finding of the stoichiometric surface to be most stable at oxygen-lean conditions is confirmed by an excellent matching between STM spectros-copy data and the calculated surface electronic structure. Beyond the specific case of the Cu 2 O(111) surface, and beyond the known deficiencies of GGA-based approaches in the description of oxide electronic structures, our work highlights the risk of an erroneous evaluation of the surface stability, in cases where the energetics and electronic characteristics are strongly coupled, as in a wide class of polar and/or non-stoichiometric oxide surfaces.

International audience; We review the manifestations of electrostatic interactions, and in particular polarity effects, in oxide ultra-thin films and two-dimensional nano-objects. We discuss the efficiency of various compensation mechanisms such as overall structural transformations, strong lattice relaxations, inhomogeneous charge redistributions, support effects, and/or the formation of 2D or 1D electron/hole gases.

International audience; In view of improving the performance of anti-corrosive zinc coatings, we report a first principles DFT study on the effect of a metal buffer on the adhesion characteristics at the weakly interacting alumina/zinc interface. With results obtained for metals across the first transition series (M = Cr, Fe, Ni) we show that such buffers may enhance considerably the interface strength, the effect being particularly well pronounced for Cr. Moreover, relying on a series of model MO x oxide buffers (x= 1, 3/2, 2), we demonstrate that buffer oxidation is in most cases detrimental to adhesion. Systematic analysis of the interfacial bonds enables to ascribe the predicted beneficial effect of metallic buffers to the formation of strong interfacial metal-oxygen and metal-zinc bonds. Reduction of the number of such bonds upon buffer oxidation drives the decrease of interface adhesion. If the oxidation of buffers composed of more reactive metals cannot be avoided, late transition elements may be more promising candidates for practical applications.

Thickness matters: Ultrathin oxide films on metals can greatly enhance catalytic activity, for example, in CO oxidation on an FeO(111) film grown on a Pt(111) substrate. Under the reaction conditions, the bilayer FeO film restructures to form a trilayer OFeO film (see picture). Experimental evidence for the structure/morphology of the film and theoretical modeling of the mechanism of its formation and CO oxidation on its surface are presented. (Figure Presented) © 2010 Wiley-VCH Verlag GmbH & Co. KGaA Weinhelm.

The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (ML) were studied using the nuclear inelastic scattering (NIS) of synchrotron radiation. Distinct evolution of vibrational characteristics with thickness that is revealed in the phonon density of states (PDOS) witnesses a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low energy part of the PDOS followed a linear dependence in energy that is characteristic for 2-dimensional systems. This dependence gradually transforms with thickness to the bulk ~E-square relation. Density functional theory phonon calculations perfectly reproduced the measured 1 ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long range magnetic order and stabilization of an unusual structural phase.

We studied CO oxidation on FeO(1 1 1) films on Pt(1 1 1) at submonolayer oxide coverages at ultrahigh vacuum and near-atmospheric pressure conditions. The FeO(1 1 1) bilayer islands are inert towards CO2 formation. In contrast, the FeO2-x trilayer structure shows substantial CO2 production that reaches a maximum at ≈40 % coverage at both pressure conditions. The results provide compelling evidence that the FeO2-x/Pt(1 1 1) interface is the most active in CO oxidation. Although FeO2-x boundaries possesses weakly bound oxygen species, strong binding of CO to Pt favors the reaction at the FeO2-x/Pt interface as compared to the FeO2-x/FeO one, thus giving a rationale to the reactivity enhancement observed in systems exposing metal/oxide boundaries. In addition, oxygen diffusion from the interior of an FeO2-x island to the active edge sites may be effective for the oxygen replenishment in the CO oxidation catalytic cycle.

While ${\mathrm{MoS}}_{2}$ and ${\mathrm{WS}}_{2}$ nanostructures gain an increasing importance in a number of recent technological applications, the control of their structure as a function of their size and their environment appears of prominent importance. In the present study which relies on first-principles simulations, we predict the dimerized ${1\mathrm{T}}^{\ensuremath{'}}$ structural phase to be the actual ground state of ${\mathrm{MoS}}_{2}$, ${\mathrm{WS}}_{2}$, and ${\mathrm{MoSe}}_{2}$ zigzag nanoribbons of small width and monolayer thickness. We assign this result to the competition between edge energy---which favors the nonpolar ${1\mathrm{T}}^{\ensuremath{'}}$ edges over the polar 1H edges---and the energy of atoms in the center of the ribbons---which favors the 1H ground state of the infinite monolayers. A metal-to-semiconductor transition accompanies the structural transition. At variance, ${\mathrm{ZrS}}_{2}$ zigzag ribbons are predicted to display the 1T structure whatever their width. In compounds of major technological importance, such structural and electronic flexibility associated with polarity effects opens the possibility for controlling the ribbon type during synthesis.

International audience; The structure of (Au) bipyramids, very promising for their specific plasmonic properties, is fully analyzed for gradual stages of growth. From combined use of HRTEM (High resolution transmission electronic microscopy) in specific conditions and SAED (selected area electronic diffraction) analysis, the atomic structure of these elongated objects of different sizes is extracted (from 2 to 60 nm). In the case of silver(I)-assisted growth from citrate capped seeds, a clear penta-twinned structure, made by the superimposition of specific pairs of crystallographic zones is found for the different size nanoparticles representing the different steps of the growing bipyramids. The experimental results were analyzed in the framework of a recent atomistic approach developed for metal-environment interactions to account for the stability of multi-twinned nanorods or bipyramids in a complex environment. Efficient in describing the multi-twinned nanoparticles and the drastic effects of environment on the stabilization of elongated multi-twinned shapes, this model supports the experimental features of bipyramids and nanorods formation from citrate seeds in function of the nature of the surface chemical ligands: citrate seeds in a growth solution prepared without silver(I) yields the formation of a few penta-twinned nanorods, while the silver(I)-assisted growth process mainly leads to penta-twinned bipyramids. The conservation of a bulk penta-twinned structure inherited from isotropic decahedral (i-Dh) seeds all along the growth is retrieved by this approach for two different environments.