A perturbation theory study of H2 on LiF(0 0 1)

In response to recent helium atom scattering (HAS) results, Monte Carlo (MC) simulations and perturbation theory have been performed for H₂ on LiF(0 0 1). MC simulations predict that H₂ molecules form a series of structures, p(2 × 2), p(8 × 2), p(4 × 2) with coverages Θ = 0.5, 0.625 and 0.75, respectively, that are stable up to 8 K. This is in partial agreement with the HAS results that report c(2 × 2) and c(8 × 2) structures; they agree in terms of coverage and stability, but disagree in terms of symmetry. To reconcile the results of the simulations and experiments, the orientation of the adsorbed H₂ molecules was studied using perturbation theory. These calculations show that the adsorbed H₂ molecules are azimuthally delocalized and that the structures are c-type rather then p-type. The calculations also indicate that p-H₂ and helicoptering o-H₂ prefer cationic sites, while cartwheeling o-H₂ prefers anionic sites.     

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.     

A kinetic Monte Carlo/UBI-QEP model of O2 adsorption on fcc (1 1 1) metal surfaces

The previously developed kinetic Monte Carlo model of molecular oxygen adsorption on fcc (1 0 0) metal surfaces has been extended to fcc (1 1 1) surfaces. The model treats uniformly all elementary steps of the process—O₂ adsorption, dissociation, recombination, desorption, and atomic oxygen hopping—at various coverages and temperatures. The model employs the unity bond index—quadratic exponential potential (UBI-QEP) formalism to calculate coverage-dependent energetics (atomic and molecular binding energies and activation barriers of elementary steps) and a Metropolis-type algorithm including the Arrhenius-type reaction rates to calculate coverage- and temperature-dependent features, particularly the adsorbate distribution over the surface. Optimal values of non-energetic model parameters (the spatial constraint, a travel distance of “hot” atoms, attempt frequencies of elementary steps) have been chosen. Proper modifications of the fcc (1 0 0) model have been made to reflect structural differences in the fcc (1 1 1) surface, in particular the presence of two different hollow sites (fcc and hcp). Detailed simulations were performed for molecular oxygen adsorption on Ni(1 1 1). We found that at very low coverages, only O₂ adsorption and dissociation were effective, while O₂ desorption and O₂ and O diffusion practically did not occur. At a certain O + O₂ coverage, the O₂ dissociation becomes the fastest process with a rate one-two orders of magnitude higher than adsorption. Dissociation continuously slows down due to an increase in the activation energy of dissociation and due to the exhaustion of free sites. The binding energies of both molecular and atomic oxygen decrease with coverage, and this leads to greater mobility of atomic oxygen and more pronounced desorption of molecular oxygen. Saturation is observed when the number of adsorbed molecules becomes approximately equal to the number of desorbed molecules. Simulated coverage dependences of the sticking probability and of the atomic binding energy are in reasonable agreement with experimental data. From comparison with the results of the previous work, it appears that the binding energy profiles for Ni(1 1 1) and Ni(1 0 0) have similar shapes, although at any coverage the absolute values of the oxygen binding energy are higher for the (1 0 0) surface. For metals other than Ni, particularly Pt, the model projections were found to be too parameter-dependent and therefore less certain. In such cases further model developments are needed, and we briefly comment on this situation.     

A Monte Carlo simulation study of Nitrogen on LiF(0 0 1)

The adsorption of N₂ gas on the LiF(0 0 1) surface is studied by canonical Monte Carlo (CMC) computer simulation. These results show that N₂ forms an ordered structure where the molecules are arranged in a unit cell of symmetry at temperatures below 23 K with 50% coverage. The nitrogen molecules are tilted by 53° from the surface normal and have the same azimuthal orientation along diagonals, with diagonals alternating their orientation. Beyond 23 K, the molecules become azimuthally disordered but with residual short-range order. No change in the position of the peak of the polar (tilt) angle distribution was observed above the transition temperature. This transition is purely of the order-disorder type.     

Adsorption dynamics of CO2 on Cu(1 1 0): A molecular beam study

Molecular beam scattering measurements have been conducted to examine the adsorption dynamics of CO₂ on Cu(1 1 0). The initial adsorption probability, S₀, decreases exponentially from 0.43 ± 0.03 to a value close to the detection limit (∼0.03) within the impact energy range of E_i = (0.12-1.30) eV. S₀ is independent of the adsorption temperature, T_s, and the impact angle, α_i, i.e., the adsorption is non-activated and total energy scaling is obeyed. The coverage, Θ, dependent adsorption probability, S(Θ), agrees with precursor-assisted adsorption dynamics (Kisliuk type) above T_s ∼ 91 K. However, below that temperature adsorbate-assisted adsorption (S increases with Θ) has been observed. That effect is most distinct at large E_i and low T_s. The S(Θ) data have been modeled by Monte Carlo simulations. No indications of CO₂ dissociation were obtained from Auger Electron Spectroscopy or the molecular beam scattering data.     

Theoretical study of the structures of MgO(1 0 0)-supported Au clusters

A theoretical method, which combines the first-principle calculations and a canonical Monte Carlo (CMC) simulation, was used to study the structures of Au clusters with sizes of 25-54 atoms supported on the MgO(1 0 0) surface. Based on a potential energy surface (PES) fitted to the first-principle calculations, an effective approach was derived to model the Au-MgO(1 0 0) interaction. The second moment approximation to the tight-binding potential (TB-SMA) was used to model the Au-Au interactions in the CMC simulation. It is found that the Au clusters with sizes of 25-54 atoms supported on the MgO(1 0 0) surface possess an ordered layered fcc epitaxial structure.     

Stress modifications induced by dimer vacancies in the Si(0 0 1) surface: Monte Carlo simulations

By means of Monte Carlo simulations, we investigate the local stress modifications induced by dimer vacancies (DVs) in the Si(0 0 1) subsurface layers. In presence of n isolated compact DVs, the sites located below these defect rows are under clearly compressive stress in the third layer and under more and more tensile stress, as n increases, in the fourth layer. At higher DVs densities, analogous trends are observed, but the stress modifications are then slightly extended between the dimer rows. Applying our results to the Ge penetration in Si(0 0 1), we show how the knowledge of the local stress may allow predictions of a given impurity behaviour in the vicinity of the surface, provided that the impurity-defect and impurity-impurity interactions do not play a major role compared to the local stress modification induced by the presence of DVs.     

A multi-scale Monte Carlo study of oxide structures on the Cu(1 0 0) surface

We present a multi-scale Monte Carlo study of the oxidation of the Cu(1 0 0) surface based on the Bortz-Kalos-Lebowitz model with the equilibrium energetics obtained from ab initio calculations. The radial and island size distribution functions are examined and Cu-O structures are analyzed at different temperatures and coverages. We concentrate on the coverages of 0.3 monolayer O or less, with variable sub-monolayer coverages of Cu. The results show that even though the ab initio calculations yield a higher barrier for O than for Cu adatom diffusion on Cu(1 0 0), the stability of Cu structures causes the O adatoms to be more mobile on the Cu(1 0 0) surface than the Cu adatoms. We are able to reproduce the c(2 × 2)-O domains seen in the experiments. However, we give an alternative explanation based on the repulsive interactions of O that, on one hand, cause the local ordering and, on the other hand, prohibits large well-ordered domains. We also give interpretation on the formation of the R45°-O reconstruction of Cu(1 0 0) above the O coverages of 0.3 monolayer based on the ab initio energetics.     

Study of oscillations and pattern formation in the CO + O2 reaction on Pt(1 0 0) surfaces through dynamic Monte Carlo simulation

Oscillations and pattern formation driven by a surface reconstruction are studied for the catalytic oxidation of CO on Pt(1 0 0) single-crystal surfaces through dynamic Monte Carlo simulations at low pressure and relatively high temperatures conditions. Sustained, modulated, irregular and damped oscillations are observed in our analysis as well as the formation of cellular, target, double spiral, spiral wave and turbulent patterns. The effect and the importance of the hex ? 1 × 1 surface phase transition and partial pressure of the reactants in the gas phase on the behavior of the system are discussed.     

Surface chemistry of NO and NO2 on the Pt(1 1 0)-(1 × 2) surface: A comparative study

The surface chemistry of NO and NO₂ on clean and oxygen-precovered Pt(1 1 0)-(1 × 2) surfaces were investigated by means of high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy (TDS). At room temperature, NO molecularly adsorbs on Pt(1 1 0), forming linear NO(a) and bridged NO(a). Coverage-dependent repulsive interactions within NO(a) drive the reversible transformation between linear and bridged NO(a). Some NO(a) decomposes upon heating, producing both N₂ and N₂O. For NO adsorption on the oxygen-precovered surface, repulsive interactions exist between precovered oxygen adatoms and NO(a), resulting in more NO(a) desorbing from the surface in the form of linear NO(a). Bridged NO(a) experiences stronger repulsive interactions with precovered oxygen than linear NO(a). The desorption activation energy of bridged NO(a) from oxygen-precovered Pt(1 1 0) is lower than that from clean Pt(1 1 0), but the desorption activation energy of linear NO(a) is not affected by the precovered oxygen. NO₂ decomposes on Pt(1 1 0)-(1 × 2) surface at room temperature. The resulted NO(a) (both linear NO(a) and bridged NO(a)) and O(a) repulsively interact each other. Comparing with NO/Pt(1 1 0), more NO(a) desorbs from NO₂/Pt(1 1 0) as linear NO(a), and both linear NO(a) and bridged NO(a) exhibit lower desorption activation energies. The reaction pathways of NO(a) on Pt(1 1 0), desorption or decomposition, are affected by their repulsive interactions with coexisting oxygen adatoms.     

Adsorption of Nd on the Mo(1 1 0) surface

The adsorption of Nd on the Mo(1 1 0) surface has been studied by low energy electron diffraction and Auger electron spectroscopy. It has been found that at low coverages Nd adatoms form a rich amount of dilute (n × 2) commensurate structures, which can be explained as forming zigzag chains oriented along the [1 1 0] direction. Monte Carlo simulations indicate that the formation of the zigzag chain structures is initiated by the indirect lateral interaction between Nd adatoms.     

The investigation of room temperature ferromagnetism in (1 0 0) oriented BaNb2O6 PLD films on LaAlO3 (1 0 0) substrate

(1 0 0) oriented BaNb₂O₆ films have been successfully grown on LaAlO₃ (1 0 0) substrate at 750 °C or 450 °C in vacuum by pulsed laser deposition. The deposited BaNb₂O₆ PLD films exhibit room-temperature ferromagnetism. Ab initio calculations demonstrate that stoichiometric BaNb₂O₆ and that with barium vacancy are nonmagnetic, while oxygen and niobium vacancy can induce magnetism due to the spin-polarization of Nb s electrons and O p electrons respectively. Moreover, ferromagnetic coupling is energetically more favorable when two Nb/O vacancies are located third-nearest-neighbored. The observed room temperature ferromagnetism in BaNb₂O₆ films should be mainly induced by oxygen vacancies introduced during vacuum deposition, with certain contribution by Nb vacancies.     

A computational study of H2 dissociation and CO adsorption on the PtML/WC(0 0 0 1) surface

We studied computationally the relative stability of Pt_ML/WC(0 0 0 1) [pseudomorphic monolayer of Pt(1 1 1) on WC(0 0 0 1)] interfacial structures using a density functional slab model approach. The work of adhesion was calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. The results show that the optimal interfacial structure of Pt_ML/WC(0 0 0 1) is the W-terminated WC(0 0 0 1) with Pt atoms adhesion on the hcp site (W-hcp). The nature of metal/carbide bonding for the W-hcp interfacial geometry was determined on the basis of the partial density of states (PDOS). Adsorption of atomic hydrogen and dissociation of the hydrogen molecule on the W-hcp Pt_ML/WC(0 0 0 1) was investigated and compared to that on Pt(1 1 1). It is found that the most favorable H₂ dissociation channels need similar activation energies of 5.28 and 4.93 kJ/mol on Pt_ML/WC(0 0 0 1) and Pt(1 1 1), respectively, with the release of considerable reaction energies. Furthermore, adsorption of CO on the W-hcp Pt_ML/WC(0 0 0 1) and Pt(1 1 1) was also investigated. The results indicate that Pt_ML/WC(0 0 0 1) is much less susceptible to CO poisoning than Pt(1 1 1), especially at the low coverage of CO.     

Surface diffusion of Pb clusters on Cu(1 1 1) influenced by size dependent cluster-substrate misfit: Monte Carlo tight binding simulation model

The migration of two-dimensional Pb clusters on Cu(1 1 1) surface is studied in the framework of three-dimensional (3D) continuum space tight binding (TB) computational model. Monte Carlo simulations based on many-body TB interactions reveal significant influence of cluster-substrate misfit on diffusion behavior of two-dimensional (2D) clusters. The analysis of pair distribution function (PDF) demonstrates that cluster lattice constant depends on the number of atoms N for 1 < N < 10. The observed effect is more pronounced for heteroepitaxial than homoepitaxial systems. It can be explained in the framework of model accounting for enhanced charge transfer in heteroepitaxy and strong influence of surface potential on cluster atomic arrangement. The variation of lattice constant leads to variable misfit which affects the island migration behavior. The results are discussed in a physical model that implies cluster diffusion with size dependent cluster-substrate misfit.     

Adsorption and dissociation of H2 on the Cu2O(1 1 1) surface: A density functional theory study

Interactions of atomic and molecular hydrogen with perfect and deficient Cu₂O(1 1 1) surfaces have been investigated by density functional theory. Different kinds of possible modes of H and H₂ adsorbed on the Cu₂O(1 1 1) surface and possible dissociation pathways were examined. The calculated results indicate that O_SUF, Cu_CUS and O_vacancy sites are the adsorption active centers for H adsorbed on the Cu₂O(1 1 1) surface, and for H₂ adsorption over perfect surface, Cu_CUS site is the most advantageous position with the side-on type of H₂. For H₂ adsorption over deficient surface, two adsorption models of H₂, H₂ adsorbing perpendicularly over O_vacancy site and H₂ lying flatly over singly-coordinate Cu-Cu short bridge, are typical of non-energy-barrier dissociative adsorption leading to one atomic H completely inserted into the crystal lattice and the other bounded to Cu_CUS atom, suggesting that the dissociative adsorption of H₂ is the main dissociation pathway of H₂ on the Cu₂O(1 1 1) surface. Our calculation result is consistent with that of the experimental observation. Therefore, Cu₂O(1 1 1) surface with oxygen vacancy exhibits a strong chemical reactivity towards the dissociation of H₂.     

Solvent effects for CO and H2 adsorption on Cu2O (1 1 1) surface: A density functional theory study

To investigate solvent effects, CO and H₂ adsorption on Cu₂O (1 1 1) surface in vacuum, liquid paraffin, methanol and water are studied by using density functional theory (DFT) combined with the conductor-like solvent model (COSMO). When H₂ and CO adsorb on Cu_cus of Cu₂O (1 1 1) surface, solvent effects can improve CO and H₂ activation. The H-H bond increases with dielectric constant increasing as H₂ adsorption on O_suf of Cu₂O (1 1 1) surface, and the H-H bond breaks in methanol and water. It is also found that both the structural parameters and Mulliken charges are very sensitive to the COSMO solvent model. In summary, the solvent effects have obvious influence on the clean surface of Cu₂O (1 1 1) and the adsorptive behavior.     

First-principles study of the (0 0 1) surface of cubic SrZrO3

Density functional calculations have been used to investigate the (0 0 1) surface of cubic SrZrO₃ with both SrO and ZrO₂ termination. Surface structure and electronic structure have been obtained. The SrO surface is found to be similar to its counterpart in SrTiO₃, while there are marked differences between the ZrO₂ and TiO₂ terminations in SrZrO₃ and SrTiO₃, respectively, concerning surface relaxation and rumpling. For the ZrO₂-terminated surface of SrZrO₃, the covalency of the interaction between the outmost Zr and the O beneath is enhanced as a result of their bond contraction. The band gap reduction and the presence of the surface states are also discussed in relation with the behavior of the electrostatic potential.     

Epitaxial MoOx nanostructures on TiO2(1 1 0) obtained using thermal decomposition of Mo(CO)6

Stoichiometric and highly-defective TiO₂(1 1 0) surfaces (called as yellow and blue, respectively) were exposed to Mo(CO)₆ vapours in UHV and in a reactive O₂ atmosphere. In the case of yellow-TiO₂, an O₂ reactive atmosphere was necessary to obtain the Mo(CO)₆ decomposition at 450 °C with deposition of MoO_x nanostructures where, according to core level photoemission data, the Mo⁺⁴ state is predominant. In the case of blue-TiO₂ it was possible to obtain Mo deposition both in UHV and in an O₂ atmosphere. A high dose of Mo(CO)₆ in UHV on blue-TiO₂ allowed the deposition of a thick metallic Mo layer. An air treatment of this sample at 580 °C led to the elimination of Mo as MoO₃ and to the formation of a transformed layer of stoichiometry of Ti_(1−x)Mo_xO₂ (where x is close to 0.1) which, according to photoelectron diffraction data, can be described as a substitutional near-surface alloy, where Mo⁺⁴ ions are embedded into the titania lattice. This embedding procedure results in a stabilization of the Mo⁺⁴ ions, which are capable to survive to air exposure for a rather long period of time. After exposure of the blue-TiO₂(1 1 0) substrate to Mo(CO)₆ vapours at 450 °C in an O₂ atmosphere it was possible to obtain a MoO₂ epitaxial ultrathin layer, whose photoelectron diffraction data demonstrate that is pseudomorphic to the substrate.