Structural characteristics of Al2O3 ultra-thin films supported on the NiAl(100) substrate from DFTB-aided global optimization.

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.

Termination-dependent electronic structure and atomic-scale screening behavior of the Cu2O(111) surface.

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 × âˆš3)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/dVmeasurements, 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 (CuD) and a (√3 × âˆš3)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 × âˆš3) 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.

Theoretical study of metal/silica interfaces: Ti, Fe, Cr and Ni on ß-cristobalite.

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 ß-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.

Understanding the structural diversity of freestanding Al2O3 ultrathin films through a DFTB-aided genetic algorithm.

(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.

Surface thermodynamics of silicate compounds: the case of Zn2SiO4(001) surfaces and thin films.

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-Zn2SiO4. 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 SiO2 termination. Stabilization of polar surfaces is provided by the formation of O22- peroxo groups, reduction of the surface or subsurface Si atoms or formation of Zn22+ 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 SiO2 termination is less stable due to the absence of siloxane group formation. We show that, while bulk Zn2SiO4 is stable with respect to decomposition into the ZnO and SiO2 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 for Fe2SiO4. This study opens the way towards a deeper understanding and possible improvement of zinc adhesion at silica surfaces, of crucial industrial importance.

Influence of the support on stabilizing local defects in strained monolayer oxide films.

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) Ti2O3 monolayer grown on an Au(111) substrate. The epitaxial rumpled Ti2O3 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 Ti2O3 monolayer and therefore have significantly 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.

Effects of surface hydroxylation on adhesion at zinc/silica interfaces.

The weak interaction between zinc and silica is responsible for the 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, and aerospace. With the goal of identifying possible methods for its improvement, we report an ab initio study of the effect of surface hydroxylation on the adhesion characteristics of model zinc/ß-cristobalite interfaces, representative of 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 interactions. However, dehydrogenation of such interfaces produces oxygen-rich zinc/silica contacts with excellent adhesion characteristics. These are due to 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 provides useful guidelines towards practical attempts to improve adhesion.

Structural, electronic and adhesion characteristics of zinc/silica interfaces: ab initio study on zinc/ß-cristobalite.

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 on the structural, electronic, and adhesion characteristics of a variety of model zinc/ß-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 an 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 interactions 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.

Temperature-dependent phase evolution of copper-oxide thin-films on Au(111).

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[112[combining macron]] and Au[11[combining macron]0] 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 Cu3O2 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 Cu3O2 domains. The low-temperature phases compirse complex arrangements of hexagonal and square Cu-O units, similar to those found in Cu2O(111) and (100) surfaces, respectively. The transitions between different thin-film phases are driven by Cu dissolution in the gold crystal and O2 evaporation and therefore accompanied by a thinning of the oxide layer with increasing temperature.

Incorrect DFT-GGA predictions of the stability of non-stoichiometric/polar dielectric surfaces: the case of Cu2O(111).

Scanning tunneling microscopy (STM) and hybrid density functional theory (DFT) have been used to study the stability and electronic characteristics of the Cu2O(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 spectroscopy data and the calculated surface electronic structure. Beyond the specific case of the Cu2O(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.

Role of the environment in the stability of anisotropic gold particles.

Despite the long-lasting interest in the synthesis control of nanoparticles (NPs) in both fundamental and applied nanosciences, the driving mechanisms responsible for their size and shape selectivity in an environment (solution) are not completely understood, and a clear assessment of the respective roles of equilibrium thermodynamics and growth kinetics is still missing. In this study, relying on an efficient atomistic computational approach, we decipher the dependence of energetics, shapes and morphologies of gold NPs on the strength and nature of the metal-environment interaction. We highlight the conditions under which the energy difference between isotropic and elongated gold NPs is reduced, thus prompting their thermodynamic coexistence. The study encompasses both monocrystalline and multi-twinned particles and extends over size ranges particularly representative of the nucleation and early growth stages. Computational results are further rationalized with arguments involving the dependence of facet and edge energies on the metal-environment interactions. We argue that by determining the abundance and diversity of particles nucleated in solution, thermodynamics may constitute an important bias influencing their final shape. The present results provide firm grounds for kinetic simulations of particle growth.

Conditions for electronic reconstruction at stoichiometric polar/polar interfaces.

Relying on first principles simulations of ZnO(0 0 0 1)/MgO(1 1 1), MgO(1 1 1)/CaO(1 1 1) and AlN(0 0 0 1)/GaN(0 0 0 1) interfaces and examples taken from the literature, we discuss under which conditions stoichiometric polar/polar interfaces may display an electronic reconstruction. We point out the role of the three contributions to the interfacial polarization discontinuity--structure, valence and electronic terms--of interfacial strains, and of finite size effects. Depending upon their relative values, the interfaces may be polar (compensated by an electron reconstruction), non-polar, or polar uncompensated at low thickness. We stress that, in superlattices or heterostructures made of thin layers, the prediction of the interface polarity character from the bulk properties of the two materials may be questionable.

Properties of Pt-supported iron oxide ultra-thin films: similarity of Hubbard-corrected and hybrid density functional theory description.

We report a first principles study on the properties of Pt(111)-supported FeO(111) monolayer. We confront results issued from PBE+U and HSE06 approximations, and analyze the impact of the more accurate hybrid description of the electronic structure of the metal/oxide interface on a large variety of calculated characteristics of this system. In particular, we analyze the behavior of its work function and its consequences on the spontaneous charging of adsorbed Au adatoms. We also consider the FeO2 nano-oxide phase and its peculiar oxygen storage characteristics, responsible for the unusual catalytic properties of FeO(x)/Pt system. We show that while the hybrid approximation does indeed substantially improve the electronic characteristics of iron oxide, of individual Au adatoms, or oxygen molecules, its overall impact on the calculated properties of the composed FeO/Pt system is very small. We assign this to the relatively small effect of the hybrid approximation on the band structure alignment. This shows that the less computationally demanding DFT+U approximation remains a fully adequate tool in theoretical studies on this kind of systems. This is particularly important for calculations on realistic systems, with large-size reconstructions induced by the lattice mismatch at the interface between the two materials.

An efficient many-body potential for the interaction of transition and noble metal nano-objects with an environment.

We present a mean-field model for the description of transition or noble metal nano-objects interacting with an environment. It includes a potential given by the second-moment approximation to the tight-binding Hamiltonian for metal-metal interactions, and an additional many-body potential that depends on the local atomic coordination for the metal-environment interaction. The model does not refer to a specific type of chemical conditions, but rather provides trends as a function of a limited number of parameters. The capabilities of the model are highlighted by studying the relative stability of semi-infinite gold surfaces of various orientations and formation energies of a restricted set of single-faceted gold nanoparticles. It is shown that, with only two parameters and in a very efficient way, it is able to generate a great variety of stable structures and shapes, as the nature of the environment varies. It is thus expected to account for formation energies of nano-objects of various dimensionalities (surfaces, thin films, nano-rods, nano-wires, nanoparticles, nanoribbons, etc.) according to the environment.

Polarity in oxide ultrathin films.

Relying on first-principles calculations within density functional theory and on an analytical model for the electronic structure, we present an overview of specific electronic and structural features of polar ultrathin films. MgO(111) unsupported films of finite thickness are chosen as a generic system, in order to extract general concepts associated with polarity at the nanoscale, relate them to the well-known semi-infinite case, and unravel specific scenarios of polarity compensation which are not present for the latter. Size dependent behavior of the compensating charge and formation energy, changes in crystallographic structure, and the possibility of substantial lattice distortions throughout entire films are analyzed and discussed.

Prediction of uncompensated polarity in ultrathin films.

Relying on first principles simulations of stoichiometric MgO, ZnO, and NaCl (1x1) ultrathin (111) films, we demonstrate the existence of a critical thickness below which polarity is uncompensated: the surface charges are bulklike, and the total dipole moment and the formation energy grow linearly with thickness. This study reveals novel facets of the problematics of polarity akin to the nanoscopic size of the objects and opens stimulating perspectives on polar nanostructures with surface properties and reactivity unaffected by charge compensation as in macroscopic samples.

Using polarity for engineering oxide nanostructures: structural phase diagram in free and supported MgO(111) ultrathin films.

Using an ab initio total energy approach, we study the stability of free and Ag(111)-supported MgO(111) ultrathin films. We unravel a novel microscopic mechanism of stabilization of polar oxide orientations, based on a strong modification of the MgO structural phase diagram with respect to the bulk material. We predict that, at low thickness, films which are either unsupported or deposited on Ag(111) display a graphitelike Bk structure rather than the expected rocksalt one. Our results provide a consistent interpretation of recent experimental findings, exemplify the efficiency of this novel stabilization mechanism, and suggest new methods to engineer oxide nanostructures.

Stability of rocksalt (111) polar surfaces: beyond the octopole.

Stable polar oxide surfaces must be simultaneously electrostatically compensated and in thermodynamic equilibrium with the environment. As a paradigm, the MgO(111)-p(2x2) reconstructed surface is shown to involve combinations of Mg-covered terminations with peculiar insulating electronic structure, favored in O-poor conditions, and the O-terminated octopole, stabler in more O-rich environments. Such a picture, which could not have been foreseen by either experiments or simulations separately, goes beyond the Wolf model and reconciles the theory with the experimental data taken in variable thermodynamic conditions.