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Here we present a combined theoretical and experimental study of hydrogen adsorption on Zn-functionalized MgO surface. We have systematically compared infrared spectra recorded at increasing hydrog...

The morphology, epitaxy, band-alignment and optical response of silver nanoparticles on ZnO(1010) has been analysed by microscopy, photoemission, electron diraction and dier-ential reectivity. At 700 K, clusters tend to nu-cleate and grow exclusively along [010]-oriented polar steps and corners, and not on at terraces or non-polar [001] steps. This preference leads to a specic epitaxy Ag(111)[110] ZnO[010](1010) with a large lattice mismatch along the dense row of the metal. The experiments have been rationalized through ab ini-tio simulations. A much lower adsorption energy is obtained for the O-terminated steps with a charge transfer from silver to oxygen, that corroborates the variation of ionization energy observed in photoemission and the presence of cationic silver. This anisotropy of growth reverberates in the plasmonic response of the metallic particles. The growth and epitaxy of metals on the polar (0001) and non-polar (1010) ori-entations of ZnO is discussed in terms of sur-face/step polarity compensation in the light of the present ndings.

Crystalline KOH undergoes an antiferroelectric (AFE) proton ordering phase transition at low temperatures, which results in a monoclinic bilayer structure held together by a network of weak hydrogen bonds (HBs). The Curie temperature shifts up when the compound is deuterated, an effect that classical MD is not able to catch. For deeper insights into the transition mechanism, we carry out ab initio MD simulations of KOH and KOD crystals by including quantum effects on the nuclei through Feynman path integrals. The geometric isotope effect and the evolution of the lattice parameters with temperature agree with the experimental data, while the purely classical description is not appropriate. Our results show that deuteration strengthens the HBs in the low-T AFE ordered phase. The transition is characterized by the flipping of OH/OD groups along a bending mode. Above the transition, the system is driven into a dynamical disordered paraelectric phase.

The performance of different approximate algorithms for computing anharmonic features in vibrational spectra is analyzed and compared on model and more realistic systems that present relevant nuclear quantum effects. The methods considered combine approximate sampling of the quantum thermal distribution with classical time propagation and include Matsubara dynamics, path integral dynamics approaches, linearized initial value representation, and the recently introduced adaptive quantum thermal bath. A perturbative analysis of these different methods enables us to account for the observed numerical performance on prototypes for overtones and combination bands and to draw qualitatively correct trends for the numerical results obtained for Fermi resonances. Our results prove that the unequal performances of these approaches often derive from the method employed to sample initial conditions and not, as usually assumed, from the lack of coherence in the time propagation. Furthermore, as confirmed by the analysis reported in Benson and Althorpe, J. Chem. Phys. 130, 194510 (2021), we demonstrate, both via the perturbative approach and numerically, that path integral dynamics methods fail to reproduce the intensities of these anharmonic features and follow purely classical trends with respect to their temperature behavior. Finally, the remarkably accurate performance of the adaptive quantum thermal bath approach is documented and motivated. Published under an exclusive license by AIP Publishing.

Electronic properties of materials are dominated by quantum effects, but nuclei, being much heavier, are usually treated as classical particles. This approximation, although tremendously convenient, is not always valid, even in close to ambient pressure and temperature conditions, especially when light nuclei such as hydrogen are involved. Zero point energy and proton tunneling can be relevant. Isotopic effects, obtained by replacing hydrogen with deuterium, are observed experimentally and are a clear indication of Nuclear Quantum Effects (NQE) since mean values obtained through classical statistical physics do not depend on mass. Introducing NQEs into simulations at an acceptable computational cost raises fundamental questions and yields subtle and unexpected results.

International audience; Transport of hydrogen in hydrous minerals under high pressure is a key step for the water cycle within the Earth interior. Brucite Mg(OH) 2 is one of the simplest minerals containing hydroxyl groups and is believed to decompose under the geological condition of the deep Earth's mantle. In the present study, we investigate the proton diffusion in brucite under high pressure, which results from a complex interplay between two processes: the O-H reorientations motion around the c axis and O-H covalent bond dissociations. First-principle path-integral molecular dynamics simulations reveal that the increasing pressure tends to lock the former motion, while, in contrast, it activates the latter which is mainly triggered by nuclear quantum effects. These two competing effects therefore give rise to a pressure sweet spot for proton diffusion within the mineral. In brucite Mg(OH) 2 , proton diffusion reaches a maximum for pressures close to 70GPa, while the structurally similar portlandite Ca(OH) 2 never shows proton diffusion within the pressure range and time scale that we explored. We analyze the different behavior of brucite and portlandite, which might constitute two prototypes for other minerals with same structure. Hydroxide minerals play an important role in several problems in geology, surface science or for industrial applications. Among them, brucite Mg(OH) 2 can be formed at the interface between periclase MgO and water at ambient conditions 1-3. The trigonal brucite structure consists of alternating layers along the c axis that terminate with hydroxyls (Fig. 1). This structure is common to other hydroxides of divalent metals, such as Ca(OH) 2 , Ni(OH) 2 and Cd(OH) 2. Portlandite Ca(OH) 2 is the main component of cements and concretes, which motivated a large number of investigations about its elastic properties. Because of their anisotropic structure, brucite iso-structural minerals are much more compressible along the c axis than in the other two directions, parallel to the stacks. Mg(OH) 2 can also act as a water vector in subduction zones, through complex processes that take place within the Earth interior 4,5. Therefore, the behavior of brucite and brucite-like minerals at very high pressure has been widely investigated. X-ray diffraction of Mg(OH) 2 up to 78 GPa showed that the c a / ratio decreases steadily from ambient pressure up to about 25 GPa and then stays almost constant 6. Those results suggest that the properties of brucite at very high pressure, and in particular the nature of the inter-layer bonding, could differ significantly from ambient conditions. Moreover, the hydroxyl groups that are parallel to the c axis at ambient conditions slant in three equivalent positions as the inter-layer distance shrinks even under moderate pressure 7 or when decreasing temperature 8. Besides reducing the global symmetry from P m 3 1 down to P3, the slanted OH groups can significantly alter several physical properties of brucites. Firstly, it could allow for the formation of hydrogen bonds between the layers 9 , which eventually reinforce under further compression and modify the compressibility along c 10. Secondly, when the protons form a non null θ angle with the c axis, they cannot arrange in a static ordered structure. Such proton disorder, which is closely related to proton frustration 11,12 , has also been invoked as the reason for the pressure-induced hydrogen sublattice amorphization in brucites 7,13. The existence of a quasi two-dimensional proton liquid in those extreme conditions can be conjectured, but the properties of the whole structure, if stable, have so far escaped a precise characterization. In particular, the occurrence of proton hopping is plausible, but whether this process results in a long-range diffusion is a totally open question. From the theoretical viewpoint, the previous observations call for a dynamical treatment of the proton arrangement within the brucite structure at high pressure. Moreover, in such conditions nuclear quantum effects, that is, all the properties that go beyond a purely classical description of ion dynamics 14 , such as zero-point

International audience; Methane hydrate was recently shown, both experimentally and through simulations, to be stable up to the remarkably high pressure of 150 GPa. A new methane hydrate high-pressure (MH-IV) phase, reminiscent of ice at ambient pressure, was described for pressures above approximately 40 GPa. We disentangle here the main contributions to the relative stability of the lower pressure, denoted MH-III and the high-pressure MH-IV structures. Through several simulation techniques, including metadynamics and Path Integral Molecular Dynamics for nuclear quantum effects, we analyze the phase transition mechanism, which implies hydrogen bond breaking and reforming, as well as methane reordering. The transition pathway is far from trivial and the quantum delocalization of the hydrogen nuclei plays a significant role.

Two aspects of the contribution of grazing incidence fast atom diffraction (GIFAD) to molecular beam epitaxy (MBE) are reviewed here: the ability of GIFAD to provide \emph{in-situ} a precise description of the atomic-scale surface topology, and its ability to follow larger-scale changes in surface roughness during layer-by-layer growth. Recent experimental and theoretical results obtained for the He atom beam incident along the highly corrugated $[ 1\bar{1}0 ]$ direction of the $\beta_{2}$(2$\times$4) reconstructed GaAs(001) surface are summarized and complemented by the measurements and calculations for the beam incidence along the weakly corrugated [010] direction where a periodicity twice smaller as expected is observed. The combination of the experiment, quantum scattering matrix calculations, and semiclassical analysis allows in this case to reveal structural characteristics of the surface. For the in situ measurements of GIFAD during molecular beam epitaxy of GaAs on GaAs surface we analyse the change in elastic and inelastic contributions in the scattered beam, and the variation of the diffraction pattern in polar angle scattering. This analysis outlines the robustness, the simplicity and the richness of the GIFAD as a technique to monitor the layer-by-layer epitaxial growth.

The quantum thermal bath (QTB) has been presented as an alternative to path-integral-based methods to introduce nuclear quantum effects in molecular dynamics simulations. The method has proved to be efficient, yielding accurate results for various systems. However, the QTB method is prone to zero-point energy leakage (ZPEL) in highly anharmonic systems. This is a well-known problem in methods based on classical trajectories where part of the energy of the high-frequency modes is transferred to the low-frequency modes leading to a wrong energy distribution. In some cases, the ZPEL can have dramatic consequences on the properties of the system. Thus, we investigate the ZPEL by testing the QTB method on selected systems with increasing complexity in order to study the conditions and the parameters that influence the leakage. We also analyze the consequences of the ZPEL on the structural and vibrational properties of the system. We find that the leakage is particularly dependent on the damping coefficient and that increasing its value can reduce and, in some cases, completely remove the ZPEL. When using sufficiently high values for the damping coefficient, the expected energy distribution among the vibrational modes is ensured. In this case, the QTB method gives very encouraging results. In particular, the structural properties are well-reproduced. The dynamical properties should be regarded with caution although valuable information can still be extracted from the vibrational spectrum, even for large values of the damping term.

Gas hydrates consist of hydrogen-bonded water frameworks enclosing guest gas molecules and have been the focus of intense research for almost 40 y, both for their fundamental role in the understanding of hydrophobic interactions and for gas storage and energy-related applications. The stable structure of methane hydrate above 2 GPa, where CH4 molecules are located within H2O or D2O channels, is referred to as methane hydrate III (MH-III). The stability limit of MH-III and the existence of a new high-pressure phase above 40 to 50 GPa, although recently conjectured, remain unsolved to date. We report evidence for a further high-pressure, room-temperature phase of the CH4-D2O hydrate, based on Raman spectroscopy in diamond anvil cell and ab initio molecular dynamics simulations including nuclear quantum effects. Our results reveal that a methane hydrate IV (MH-IV) structure, where the D2O network is isomorphic with ice Ih, forms at similar to 40 GPa and remains stable up to 150 GPa at least. Our proposed MH-IV structure is fully consistent with previous unresolved X-ray diffraction patterns at 55 GPa [T. Tanaka et al., J. Chem. Phys. 139, 104701 (2013)]. The MH-III -> MH-IV transition mechanism, as suggested by the simulations, is complex. The MH-IV structure, where methane molecules intercalate the tetrahedral network of hexagonal ice, represents the highest-pressure gas hydrate known up to now. Repulsive interactions between methane and water dominate at the very high pressure probed here and the tetrahedral topology outperforms other possible arrangements in terms of space filling.

International audience; The configurations associated with the dissociative adsorption of water on a variety of low-coordinated sites of MgO(100) surfaces, including corners, steps, MgO vacancies and kinks on steps have been studied and assigned by combining infrared spectroscopy and ab initio calculations. Three kinds of MgO powders were examined, powders of very high specific surface area prepared by chemical vapor synthesis and well-defined cubic smoke particles obtained by combustion either in 20:80 or 60:40 O2:Ar mixtures, the latter one involving less defects and smaller particles. It appears that an imperative requirement to obtain a precise characterization of the reactive behavior of defects is to keep the samples in ultra-high vacuum conditions and to control the water partial pressure finely.

International audience; The Wigner thermal density is a function of considerable interest in the area of approximate (linearized or semiclassical) quantum dynamics where it is employed to generate initial conditions for the propagation of appropriate sets of classical trajectories. In this paper, we propose an original approach to compute the Wigner density, based on a generalized Langevin equation. The stochastic dynamics is non-trivial in that it contains a coordinate-dependent friction coefficient and a generalized force that couples momenta and coordinates. These quantities are, in general, not known analytically and have to be estimated via auxiliary calculations. The performance of the new sampling scheme is tested on standard model systems with highly non classical features such as relevant zero point energy effects, correlation between momenta and coordinates, and negative parts of the Wigner density. In its current brute force implementation, the algorithm, whose convergence can be systematically checked, is accurate and has only limited overhead compared to schemes with similar characteristics. We briefly discuss potential ways to further improve its numerical efficiency.

Recent results have demonstrated an exceptionally high permittivity in the range 200–330 K in crystalline titanium oxide Rb2Ti2O5. In this article, the possibility of a structural transition giving rise to ferroelectricity is carefully inspected. In particular, X-ray diffraction, high-resolution transmission electron microscopy and Raman spectroscopy are performed. The crystal structure is shown to remain invariant and centrosymmetric at all temperatures between 90 K and 450 K. The stability of the C2/m structure is confirmed by density functional theory calculations. These important findings allow the existence of a conventional ferroelectric phase transition to be ruled out as a possible mechanism for the colossal permittivity and polarization observed in this material.

International audience; We conducted ab initio molecular dynamics simulations of the d phase of the hydrous mineral aluminium oxide hydroxide (AlOOH) at ambient temperature and high pressure. Nuclear quantum effects were included through a Langevin dynamics in a bath of quantum harmonic oscillators. We confirm that under increasing pressure d-AlOOH undergoes a phase transition from a P2 1 nm structure with asymmetric and disordered O-H bonds to a stiffer Pnnm phase with symmetric hydrogen bonds, which should be stable within the pressure and temperature ranges typical for the Earth's mantle. The transition is initially triggered by proton tunneling, which makes the mean proton position to coincide with the midpoint of the O-O distance, at pressures as low as 10 GPa. However, only at much larger pressures, around 30 GPa as previously found by other calculations, the Pnnm phase with symmetric hydrogen bonds is stable from the classical point of view. The transition is also characterized through the analysis of the H-O stretching modes, which soften considerably and fade out around 10 GPa in the P2 1 nm structure, when thermal and nuclear quantum effects are taken into account in the simulations. At variance, the harmonic picture is not adequate to describe the highly anharmonic effective potential that is seen by the protons at the transition. Finally, we propose that the picture of a dynamical transition to the high-symmetry and proton-centered Pnnm phase, which is brought about by the onset of proton tunneling, could be confirmed by quasi-elastic neutron scattering and vibrational spectroscopy under pressure.

International audience; First-principle molecular dynamics simulations of methane hydrate MH-III, including the quantum properties of the hydrogen nuclei, were carried out at pressures in the 5-65 GPa range, in order to observe the H-bond symmetrization at high pressure and at room temperature. According to our simulations, the symmetrization transition takes place around 40 GPa and is little dependent on isotope substitution. We find that, consistently with the rather complex crystal structure of MH-III, the transition is much more convoluted in the hydrate than in ices VII and X. In methane hydrate, due to the presence of non equivalent O ions with distinct O-O distances, the dissimilar H bonds symmetrize in a pressure domain rather than at a single critical pressure.

International audience; It is generally accepted that nuclear quantum effects (NQEs) trigger the transition to the nonmolecular form of ice under increasing pressure. This picture is challenged in salty ice, where Raman scattering measurements up to 130 GPa of molecular ice VII containing NaCl or LiCl impurities show that the transition pressure to the symmetric phase ice X is shifted up by about 30 GPa, even at small salt concentrations. We address the question of how the inclusion of salt induces the drastic reduction of NQEs by selectively including NQEs in ab initio calculations of ice in the presence of distinct ionic impurities. We quantitatively show that this is mainly a consequence of the electric field generated by the ions. We propose a simple model that is able to capture the essence of this phenomenon, generalizing this picture to other charged defects and for any concentration. This result is potentially generalizable to most "dirty" ices in which the electric field due to the doping is much more significant than local lattice distortions.

International audience; The adsorption of 3-ethynyl-trifluoro-toluene (ETFT) on Si(100)-2 x 1 surface in ultra high vacuum is studied in the low coverage regime, through a joint experimental and theoretical approach. The STM images of both filled and empty states revealed few distinct adsorption configurations. On the basis of Density Functional Theory (DFT) calculations the STM images were simulated and three main adsorption configurations were identified, with a predominance of di-sigma bonded species that leave the benzene ring unreacted. A discussion of the reactivity of the reconstructed silicon surface towards benzene derivatives is proposed by comparing the adsorption of ETFT close related molecules, like styrene and phenylacetylene. (C) 2010 Elsevier B.V. All rights reserved.

International audience; The dissociative adsorption of water vapor on MgO smokes is studied by infrared spectroscopy and first-principles calculations. The MgO smokes are synthesized in controlled conditions so to display a particularly high concentration of defect-free step edges (4-fold coordinated `'4C'' sites). Exposure to water vapor results in a set of twinned infrared bands at nu(A) = 3480 and nu(B) = 3710 cm(-1)-the former having never been observed before-with an intensity ratio I(nu(A))/I(nu(B)) approximate to 2. Simulated configurations include water adsorption, for various coverages, on terraces of distinct orientations, at step edges and corners. It is shown that OH stretching frequencies can be robustly correlated with OH bond lengths. The twinned (nu(A),alpha nu(B)) bands are assigned the hydrogen-bonded (O(s)(4C)-H, Mgs(4C)-OH) moiety adsorbed at fully decorated monatomic < 100 > step edges on (100) terraces. The coordination number of adsorbed OH species is shown not to be sufficient to characterize the physicochemical properties of wet MgO smokes and it is suggested that the hydrated surface can be better described in terms of (OH, MgO) complexes.

International audience; We report a density-functional theory (DFT)-based study of the interface of bulk water with a prototypical oxide surface, MgO(001), and focus our study on the often-overlooked surface electric field. In particular, we observe that the bare MgO(001) surface, although charge-neutral and defectless, has an intense electric field on the Å scale. The MgO(001) surface covered with 1 water monolayer (1 ML) is investigated via a supercell accounting for the experimentally-observed (2 × 3) reconstruction, stable at ambient temperature, and in which two out of six water molecules are dissociated. This 1 ML-hydrated surface is also found to have a high, albeit short-ranged, normal component of the field. Finally, the oxide/water interface is studied via room-temperature ab initio molecular dynamics (AIMD) using 34 H2O molecules between two MgO(001) surfaces. To our best knowledge this is the first AIMD study of the MgO(001)/liquid water interface in which all atoms are treated using DFT and including several layers above the first adsorbed layer. We observe that the surface electric field, averaged over the AIMD trajectories, is still very strong on the fully-wet surface, peaking at about 3 V Å−1. Even in the presence of bulk-like water, the structure of the first layer in contact with the surface remains similar to the (2 × 3)-reconstructed ice ad-layer on MgO(001). Moreover, we observe proton exchange within the first layer, and between the first and second layers – indeed, the O–O distances close to the surface are found to be distributed towards shorter distances, a property which has been shown to directly promote proton transfer.

A grazing incidence fast atom diffraction (GIFAD or FAD) setup, installed on a molecular beam epitaxy chamber, has been used to characterize the ${\ensuremath{\beta}}_{2}(2\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of a GaAs(001) surface at $530{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ under an ${\mathrm{As}}_{4}$ overpressure. Using a 400-eV ${}^{4}\mathrm{He}$ beam, high-resolution diffraction patterns with up to eighty well-resolved diffraction orders are observed simultaneously, providing a detailed fingerprint of the surface structure. Experimental diffraction data are in good agreement with results from quantum scattering calculations based on an ab initio projectile-surface interaction potential. Along with exact calculations, we show that a straightforward semiclassical analysis allows the features of the diffraction chart to be linked to the main characteristics of the surface reconstruction topography. Our results demonstrate that GIFAD is a technique suitable for measuring in situ the subtle details of complex surface reconstructions. We have performed measurements at very small incidence angles, where the kinetic energy of the projectile motion perpendicular to the surface can be reduced to less than 1 meV. This allowed the depth of the attractive van der Waals potential well to be estimated as $\ensuremath{-}8.7$ meV in very good agreement with results reported in literature.

A theoretical investigation of the properties of the Si3C4, Si4C3, and Si4C4 clusters is reported. Systematic explorations of the potential energy surfaces of the three clusters are performed using a combination of ab initio molecular dynamics and local energy minimizations using density functional theory. A large number of isomers with a large variety of geometries has been found. The geometries, energies, and vibrational frequencies yielded are discussed. Furthermore, a quantitative analysis of the interatomic distances, angles, and coordination numbers observed, as well as the conclusions on the bonding properties, are presented. The cluster properties are then compared to those of solid SiC and of the smaller Si–C clusters (with size up to 6) obtained in a previous study. Analysis of our results and comparison with bulk properties show that even clusters as small as Si3C4, Si4C3, and Si4C4 exhibit properties similar to those of the amorphous bulk, in particular as for the structures and bonds formed by C atoms.

First-principles DFT calculations were performed to study molecular MgO stoichiometric flat clusters on the perfect MgO(0 0 1) surface. It is shown that a model description of their energetics could be provided within a very simple independent defect approximation with surface, step, corners and kinks. The formation energies of the first three defects are derived, respectively: E su =0.50 eV/MgO, E st =0.88 eV/MgO, E 4c =5.79 eV. The model applies even for very small cluster sizes ( n ⩾4). For large clusters kink and corner energy contributions tend to be negligible, reflecting the tendency towards a constant binding energy of a single molecule to large clusters independently of their geometry.

Various (1 x 1) terminations of the SrTiO 3 (110) polar surface are considered and their electronic and structural properties computed through first-principles calculations. The relative stability of terminations that differ by stoichiometry is compared by taking into account the influence of the chemical environment. In contrast with the metallic stoichiometric SrTiO termination, the nonstoichiometric ones are insulating and their stability comparable to that of the SrTiO 3 (100) surfaces. We thus argue that, among the (1 x 1) unreconstructed faces, some nonstoichiometric (110) orientations may be obtained in suitable O-poor environments.

We discuss several aspects of electron redistributions in low-dimensional oxides: surfaces, ultra-thin films or clusters. We rely upon a combination of first principles density functional calculations, Bader analysis and an analytical model, the bond electron transfer model, that we derive and which is valid whatever the covalent strength of the anion–cation bonding. We show that, despite the fact that nearly bulk-like charges are found on low-coordinated sites, there usually exists a strong increase of the covalent character of the oxygen–cation bonds, which is reflected in experimentally measurable quantities. On polar orientations, we discuss the efficiency of various processes yielding a cancellation of the macroscopic dipole moment.

First-principles calculations based on the density functional theory of the adsorption and migration energies of Mg atom and MgO molecule on a flat MgO (0 0 1) surface are reported and discussed. The isolated neutral Mg adatom binds quite loosely to the surface (E ads ≃0.46 eV ) and has to overcome a barrier of about 0.30 eV in between two stable O adsorption sites. The MgO molecule forms iono-covalent bonds with the surface, with an adsorption energy of 2.5±0.1 eV . At variance with the isolated Mg adatom, the surface diffusion of the MgO molecule may imply a sequence of elementary rotation step around its ends. The computed barriers for molecule, 90° rotation around its O and Mg atoms are 0.35 and 0.46 eV, respectively.

International audience; The phase transition from ice VII to ice X under extreme pressures is an example where quantum proton delocalization coexists with classical thermal fluctuations. We investigate this transition, including quantum effects on the nuclear motion through adapted Langevin dynamics. This approach, which allows us to follow the semiclassical trajectories of protons, provides excellent agreement with experimental vibrational spectra indicating a transition pressure of about 65 GPa. Furthermore, we map the full dynamical problem onto a pressure-dependent, one-dimensional mean-field potential for the proton. By solving exactly the corresponding Schr¨odinger equation, we disentangle tunneling and quantum delocalization from classical thermal effects and identify the transition through the topological changes of the proton ground state and its susceptibility. The process is dominated by quantum effects even at ambient temperature and can be considered to be a paradigmatic case of a quantum-driven phase transition.

International audience; The adsorption of tetraethoxysilane (TEOS, Si[OC2H5](4)) on the Si(001)-2 x 1 surface at 300 K is studied through a joint experimental and theoretical approach, combining scanning tunneling microscopy (STM) and synchrotron radiation X-ray photoelectron spectroscopy (XPS) with first-principles simulations within the density functional theory (DFT). XPS shows that all Si-O bonds within the TEOS molecules are broken upon adsorption, releasing one Si atom per dissociated molecule, while the ethoxy (-OC2H5) groups form new Si-O bonds with surface Si dimers. A comparison between experimental STM images and DFT adsorption configurations shows that the four ethoxy groups bind to two second-neighbor silicon dimers within the same row, while the released silicon atom is captured as a monomer on an adjacent silicon dimer row. Additionally, the surface displays alternate ethoxy- and Si adatom-covered rows as TEOS coverage increases. This patterning, which spontaneously forms upon TEOS adsorption, can be used as a template for the nanofabrication of one-dimensional self-organized structures on Si(001)-2 x 1.

Electronic ground-state properties and quasiparticle energies of the charge-transfer insulating oxides SrO and MgO have been computed. Structural properties calculations have been performed within DFT–LDA, while for the electronic excitations an efficient DFT–GW scheme, founded on the use of a model dielectric function, has been used. The comparison of our results with available theoretical and experimental data turns out to be satisfactory either for the ground state or for the excitation spectrum.

Relying on the results obtained by a total energy, semi-empirical Hartree–Fock method, we discuss polarity effects at the (111) and (110) surfaces of SrTiO3. For these orientations, we consider some prototypical (1×1) configurations, which differ by their surface composition and the coordination number of the surface atoms. We argue that the compensation for these polar orientations is achieved through anomalous fillings of surface states, which, in principle, should be detectable by surface spectroscopies. The compensation does not imply a partial filling, so that all these surfaces keep their insulating character. The thermodynamic stability of the different configurations is also analyzed, and we conclude that, consistently with the experimental observations, these surfaces are relatively stable, especially when compared with polar faces of more ionic oxides, such as MgO(111). The efficiency of SrTiO3 for screening the strong electrostatic perturbation associated to the polar orientations is invoked to explain such a behavior.

Density functional calculations are performed on small neutral and negatively charged TinO2n clusters, with n=1–3. The relative stability of the various isomers results from a subtle competition between ionic and covalent effects in the Ti–O bonding, and is affected by the overall charge state of the cluster. Electron affinities and optical excitation gaps, calculated through total-energy differences, are discussed and compared with recent anion photoelectron data. The excess electron of the negatively charged clusters is mostly localized on titanium with the weakest electrostatic potential, which permits a simple interpretation of the computed electron affinities based on the chemical shifts of the deep Ti levels. The trends obtained for both the electron affinities and the excitation gaps as a function of n help the identification of the most stable isomers consistent with the experimental conditions.