EPR properties of Au atoms adsorbed on various sites of the MgO(1 0 0) surface from relativistic DFT calculations

Using all electron fully relativistic DFT calculations we have computed the EPR properties of Au atoms bound to various sites of the MgO surface. Changes in g-tensor and hyperfine coupling constants provide a way to identify the gold adsorption site and to map the surface morphology by comparison of measured and calculated EPR spectra. We found a strong reduction of the isotropic hyperfine coupling constant, a_iso(Au), for adsorbed gold compared to the free atom; this reduction, which is about 45% for terrace sites, is more pronounced when Au interacts with low-coordinated sites like steps, edges and corners where it is about 60%. The reduction of a_iso(Au) is accompanied by a corresponding increase of the superhyperfine interaction with the surface oxygen sites, as measured by a_iso(¹⁷O). Large anisotropies in the g-tensor are computed for all sites.     

DFT study of the water/MgO(1 0 0) interface in acidic and basic media

Periodic-DFT has been used to study the acid-base properties of MgO(1 0 0) layers interconnected with an ice filling. The insertion of HCl by substitution of one water molecule shows that all the protons are adsorbed on the surface oxygen atoms forming hydroxyl groups; the Cl⁻ ions are weakly bound to surface cations, generating a monolayer with the adsorbed water molecules. For the insertion of NaOH also by substitution of one water molecule, it is found that OH⁻ ions remain in solution close to the Na⁺ ions, forming ion pairs or chains. Depending on the number of water molecules present, the sodium ions are bi-, tri- or tetra-coordinated. Finally, some aspects concerning corrosion have been highlighted.     

CO interaction with Au atoms adsorbed on terrace, edge and corner sites of the MgO(1 0 0) surface. Electronic structure and vibrational analysis from DFT

The interaction of CO with Au atoms adsorbed on terrace and low-coordinates sites (edge and corner) of the MgO(1 0 0) surface was studied using the density functional theory (DFT) in combination with embedded cluster models. Surface anionic (O²⁻) and neutral oxygen vacancy (F_s) sites were considered. In all the cases, the CO stretching frequencies are shifted with respect to free CO with values between −232 and −358 cm⁻¹. In particular, the values for Au on F_s at edge and corner are shifted to higher stretching frequencies by 100 and 59 cm⁻¹, respectively, with respect to the value on a perfect MgO(1 0 0) surface. This result is in agreement with recent scanning tunneling microscopy and infrared spectroscopy experiments where a corresponding shift of 70 cm⁻¹ was observed by comparing the measurements on perfect and O-deficient MgO(1 0 0) surfaces. However, these results are different than expected because Au atoms on F_s centers are negatively charged and, therefore, according to the generally accepted scheme the CO frequency should be red-shifted with respect to the adsorption on anionic five-coordinated site where the Au atom is essentially neutral. The following picture emerges from the present results: the single occupied HOMO(α) of Au atom on F_s at low-coordinated sites consists in two lobes extended sideward the Au atom. For symmetry reasons, this MO overlaps efficiently with the 2π^∗ MO of CO. This bonding contribution to the Au-CO link is counteracted by a Pauli repulsion between the 5σ MO of CO and more internal orbitals (the HOMO-1(α) and the HOMO(β)) centered on Au. In consequence, CO is forced to vibrate against a region with a high electron density. This is the so-called “wall effect” which by itself contributes to higher CO frequency values.     

Adsorption of SO2 on Li atoms deposited on MgO (1 0 0) surface: DFT calculations

The adsorption of sulfur dioxide molecule (SO₂) on Li atom deposited on the surfaces of metal oxide MgO (1 0 0) on both anionic and defect (F_s-center) sites located on various geometrical defects (terrace, edge and corner) has been studied using density functional theory (DFT) in combination with embedded cluster model. The adsorption energy (E_ads) of SO₂ molecule (S-atom down as well as O-atom down) in different positions on both of O⁻² and F_s sites is considered. The spin density (SD) distribution due to the presence of Li atom is discussed. The geometrical optimizations have been done for the additive materials and MgO substrate surfaces (terrace, edge and corner). The oxygen vacancy formation energies have been evaluated for MgO substrate surfaces. The ionization potential (IP) for defect free and defect containing of the MgO surfaces has been calculated. The adsorption properties of SO₂ are analyzed in terms of the E_ads, the electron donation (basicity), the elongation of S-O bond length and the atomic charges on adsorbed materials. The presence of the Li atom increases the catalytic effect of the anionic O⁻² site of MgO substrate surfaces (converted from physisorption to chemisorption). On the other hand, the presence of the Li atom decreases the catalytic effect of the F_s-site of MgO substrate surfaces. Generally, the SO₂ molecule is strongly adsorbed (chemisorption) on the MgO substrate surfaces containing F_s-center.     

Formation of Ag2, Au2 and AgAu particles on MgO(1 0 0): DFT study on the role of support-induced charge transfer in metal-metal interactions

The formation of Ag₂, Au₂ and AgAu particles oriented perpendicularly to the MgO(1 0 0) surface was studied using the density functional theory. While the support induces a slight enhancement of the Ag-Ag bond (by 0.3-0.4 eV), the Au-Au bond is strongly enhanced (by 0.8-1.1 eV). Concerning the bimetallic particle, the Ag-Au bond stabilization depends on the relative position of each atom. Thus, in general terms, the strength of the metal-metal bond is determined by the nature of the terminal atom; the bond is stronger in Au-terminal particles. The partial electronic charge transfer to the terminal Au atom and its ability to polarize this charge are responsible for this energetic stabilization.     

D2 layers on MgO(0 0 1): Simulation study

In response to recent helium atom scattering (HAS) and neutron scattering results, Monte Carlo simulations and perturbation theory calculations have been performed for D₂ on MgO(0 0 1). Monte Carlo simulations predict that D₂ molecules form a series of interesting structures, p(2 × 2) → p(4 × 2) → p(6 × 2), with coverages Θ = 0.5, 0.75, 0.83 respectively, and followed by a formation of a top layer of p(6 × 2) unit cell symmetry. The three types of mono-layers are stable up to 13 K, whereas the top layer still exists up to 10 K. This is in partial agreement with the neutron scattering and HAS results that report c(2 × 2), c(4 × 2) and c(6 × 2); they agree in terms of coverage and stability, but disagree in terms of symmetry. A quantum mechanical examination of the D₂ molecules’ rotational motion shows the molecular axes are azimuthally delocalized and hence the simulated structures are c-type rather than p-type. These calculations also indicate that ortho-D₂ and helicoptering para-D₂ prefer cationic sites, while cartwheeling para-D₂ prefers anionic sites.     

Metal film growth on regular and defective MgO(0 0 1) surface: A comparative ab initio simulation and thermodynamic study

In order to understand the difference in metallic film growth modes on perfect and defective oxide substrates, we have combined ab initio B3LYP periodic calculations on the slab models of the corresponding Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. For a defectless magnesia surface, we confirm the experimentally observed submonolayer growth of 3D metallic islands (Ag possesses a higher trend than Cu). Formation of F_s centers (neutral O vacancies) on the substrate markedly enhances metal atom adsorption as compared to physisorption over regular sites on a defect-free substrate. For the first time, we predict that the presence of these surface defects (beginning with concentrations of 5% for Cu and 22% for Ag) can stimulate the growth of uniform 2D metallic sublayers.     

FS and FS(OH) defect centers at the MgO(1 0 0) surface: cluster and periodic calculations

We present a model of a new paramagnetic defect center which results from the interaction of atomic hydrogen with the MgO(1 0 0) surface. DFT calculations have been performed using periodic supercells and embedded cluster models where long-range polarization effects are included explicitly. The H atom promotes the creation of an oxygen vacancy (F center) by formation of the F_S⁺(OH⁻) defect where an hydroxyl group is adsorbed near an electron trapped in an oxygen vacancy. This new center has some characteristics similar to those of the classical F_S⁺ centers but a smaller formation energy; furthermore, being globally neutral, it can be treated also with supercell methods.     

Adsorption of NO on Au atoms and dimers supported on MgO(1 0 0): DFT studies

The adsorption of NO on single gold atoms and Au₂ dimers deposited on regular O²⁻ sites and neutral oxygen vacancies (F_s sites) of the MgO(1 0 0) surface have been studied by means of DFT calculations. For Au₁/MgO the adsorption of NO is stronger when the Au atom is supported on an anionic site than when it is on a F_s site, with adsorption binding energies of 1.1 and 0.5 eV, respectively. In the first case the spin density is mainly concentrated on the metal atom and protruding from the surface. In such a way, an active site against radicals such as NO is generated. On the F_s site, the presence of the vacancy delocalizes the spin into the substrate, weakening its coupling with NO. For Au₂/MgO, as this system has a closed-shell configuration, the NO molecules bonds weakly with Au₂. Regarding the N–O stretching frequencies, a very strong shift of 340–400 cm⁻¹ to lower frequencies is observed for Au₁/MgO in comparison with free NO.     

First-principles calculations of C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0)

Density functional theory calculations are performed to investigate the C diffusion through the surface and subsurface of Ag/Ni(1 0 0) and reconstructed Ag/Ni(1 0 0). The calculated geometric parameters indicate the center of doped Ag is located above the Ni(1 0 0) surface owing to the size mismatch. The C binding on the alloy surface is substantially weakened, arising from the less attractive interaction between C and Ag atoms, while in the subsurface, the C adsorption is promoted as the Ag coverage is increased. The effect of substitutional Ag on the adsorption property of Ni(1 0 0) is rather short-range, which agrees well with the analysis of the projected density of states. Seven pathways are constructed to explore the C diffusion behavior on the bimetallic surface. Along the most kinetically favorable pathway, a C atom hops between two fourfold hollow sites via an adjacent octahedral site in the subsurface of reconstructed Ag/Ni(1 0 0). The “clock” reconstruction which tends to improve the surface mobility, is more favorable on the alloy surface because the c(2 × 2) symmetry is inherently broken by the Ag impurity. As a consequence, the local lattice strain induced by the C transport is effectively relieved by the Ag-enhanced surface mobility and the C diffusion barrier is lowered from 1.16 to 0.76 eV.     

Disulphide and metal sulphide formation on the reconstructed (0 0 1) surface of chalcopyrite: A DFT study

The bulk and (0 0 1) surface of chalcopyrite have been investigated using Plane Waves Density Functional calculations. The band structure and the optimized lattice parameters are in good agreement with previously published results. The relaxed S-terminated (0 0 1) surface led to the formation of disulphide, S₂²⁻, upon the reduction of Fe(III) to Fe(II). The relaxed M-terminated (0 0 1) surface led to the formation of metal sulphides and metal-metal bonds. The calculated Fe-Fe, Fe-Cu and Cu-Cu bond lengths are close to the typical bond distances found in the metal. Löwdin population analysis, density of states and electron localization function have been used to understand the electronic structure of the chalcopyrite surfaces. Implications to the chalcopyrite surface reactivity are briefly discussed.     

Adsorption and dissociation of ammonia on Au(1 1 1) surface: A density functional theory study

Periodic density functional theory (DFT) calculations using plane waves had been performed to systematically investigate the stable adsorption amine and its dehydrogenated reaction on Au(1 1 1) surface. The equilibrium configuration including on top, bridge, and hollow (fcc and hcp) sites had been determined by relaxation of the system. The adsorption both NH₃ on top site and NH₂ on bridge site is favorable on Au(1 1 1) surface, while the adsorption of NH on hollow (fcc) site is preferred. The adsorbates are adsorbed on the gold surface with the interaction between p orbital of adsorbate and the d orbital of gold atoms. The interaction between adsorbate and gold slab is more evident on the first layer than on any others. Furthermore, the dissociation reaction of NH₃ on clean gold surface, as well as on the pre-covered oxygen atom and pre-covered hydroxyl group surface had been investigated. The results show that the dehydrogenated reaction energy barrier on the pre-covered oxygen gold surface is lower. The adsorbed O can promote the dehydrogenation of amine. Additionally, OH as the product of the NH₃ dissociation reaction participates in continuous dehydrogenation reaction, and the reaction energy barrier is the lowest (22.77 kJ/mol). The results indicated that OH_ads play a key role in the dehydrogenated reaction on Au(1 1 1) surface.     

Magnetic ordering of Vn/Mo(0 0 1) systems: Ab-initio calculations

A density-functional theory (DFT) study is performed using a full-potential linearized-augmented-plane-waves (FP-LAPW) method to investigate the magnetic structure of vanadium-molybdenum systems (V_n/Mo(0 0 1), n = 1, 2). The topmost V layers relax inward in both systems with a larger contraction in V₂/Mo(0 0 1) system. A p(1 × 1) in-plane ferromagnetic ordering with appreciable magnetic moments is obtained on V overlayers, which is found to be the ground state in both systems. The layers below the surface exhibit induced magnetism with antiferromagnetic interlayer coupling.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Oxygen induced surface structure of Mo(1 1 0) studied by scanning tunneling microscopy

The oxygen induced surface structures formed on Mo(1 1 0) by oxygen exposure at 1300 K in UHV has been studied by scanning tunneling microscopy (STM). Two kinds of oxygen-adsorbed surface structures are observed. One consists of one-dimensional rows running along or directions at substrate molybdenum lattices, and another shows more complex structure including discrete arrangement of large protrusions and zig-zag alignments of small protrusions. This complex structure is probably a further oxygen-adsorbed structure than the well-known p(2 × 2) structure of 0.3 ML coverage. On the basis of STM image, an atomic model is proposed, where adsorbed oxygen atoms occupy both long-bridge and the quasi-threefold sites of molybdenum lattice (0.4 ML coverage). This structure is presumed to be a transient state during site-conversion with increase of oxygen exposure.     

Four-fold magnetic anisotropy in a Co film on MgO(0 0 1)

The development of devices based on magnetic tunnel junctions has raised new interests on the structural and magnetic properties of the interface Co/MgO. In this context, we have grown ultrathin Co films (≤30 Å) by molecular-beam epitaxy on MgO(0 0 1) substrates kept at different temperatures (T_S). Their structural and magnetic properties were correlated and discussed in the context of distinct magnetic anisotropies for Co phases reported in the literature. The sample characterization has been done by reflection high energy electron diffraction, magneto-optical Kerr effect and ferromagnetic resonance. The main focus of the work is on a sample deposited at T_S=25 °C, as its particular way of growth has enabled a bct Co structure to settle on the substrate, where it is not normally obtained without specific seed layers. This sample presented the best crystallinity, softer magnetic properties and a four-fold in-plane magnetic anisotropy with Co〈1 1 0〉 easy directions. Concerning the samples prepared at T_S=200 and 500° C, they show fcc and polycrystalline structures, respectively and more intricate magnetic anisotropy patterns.     

Analysis of energy and force of Pt adatom on Pt (0 0 1) surface by MAEAM

The energy and perpendicular force of a Pt adatom on Pt (0 0 1) surface have been calculated by MAEAM. With increasing the distance of the adatom from the surface, the energy and force maps can be classified into four regions: repulsive region, transformed region, strongly attractive region and weakly attractive region. In repulsive region, the maximum (minimum) values of the energy and repulsive force appear on the top (hole) of the first layer atoms of Pt (0 0 1) surface due to stronger pair-potential interaction. In other regions, the energy and force maps are more complicated than those in repulsive region due to the effects of the many body interactions and nonspherical distribution of the electrons of the atoms in crystal. The most stable position is 0.1664 nm above the hole of the first layer atoms for a Pt adatom on Pt (0 0 1) surface.     

Structure and magnetism of the V/Ta(0 0 1) surface: A DFT calculation

Calculations using the density functional theory, norm-conserving pseudopotentials and the generalized gradient approximation are performed for vanadium (V) and tantalum (Ta) clean surfaces, and the V/Ta(0 0 1) system. Vanadium and Ta(0 0 1) clean surfaces are found to be nonmagnetic with large inward relaxations of 9.81% and 11.19%, respectively, in good agreement with experiment and other calculations. However, we obtained an appreciable magnetic moment of 2.24 μB for the V monolayer in the V/Ta(0 0 1) system. The inward relaxation is reduced to 6.01% for the V overlayer on Ta(0 0 1) as a result of its spin polarization. An induced magnetic moment of 0.41 μB is obtained on the Ta interfacial layer, which is found to be anti-ferromagnetically coupled with the V overlayer and the Ta layers below.     

Coadsorption of CO and O on Ir(1 0 0): First principles calculations

Density functional theory (DFT) calculations with generalized gradient approximation (GGA) are performed to investigate the structural, electronic and energetic properties of Ir(1 0 0)-(mCO+nO), m=1, 2; n=1, 2 and 3, coadsorption systems. The stability of coadsorbed oxygen is not affected by the increase of CO coverage from 0.25 ML to 0.50 ML. However, the stability of CO increases by increasing O coverage from 0.25 ML to 0.50 ML and 0.75 ML as a result of the rumpling of the topmost Ir layer.     

Optical anisotropy at the W(1 1 0) surface

We report ab initio calculations of the anisotropic dielectric function of tungsten (1 1 0) surface using the linear muffin-tin-orbital method. The calculated anisotropy in the optical spectrum, for polarization of light parallel to the surface, exhibits three dominant broad structures at 3.00, 4.01 and 5.34 eV successively positive, negative and then positive. The first peak is clearly assigned to p → d interband transitions in surface atomic sites whereas the two others have their origin in interband transitions in bulk like atoms. Our results, including the interlayer relaxation effect on the surface optical response, are compared to recent reflectance anisotropy measurements.