CO adsorption on Rh, Pd and Ag atoms deposited on the MgO surface: a comparative ab initio study

The adsorption properties of CO molecules adsorbed on Rh, Pd, and Ag atoms supported on various sites of the MgO surface have been studied by means of a density functional cluster model approach. The metal atoms are stabilized with different binding energies on the regular and morphological defect sites of the surface. Among others we considered oxide anions, neutral and charged anion vacancies (F centers) located at terraces, steps, edges, and corners. CO is used as a probe molecule to characterize where the metal atoms are located. This is done by analyzing how the metal-CO binding energy and the C-O stretching frequency change as function of the substrate site where the metal atom is bound.     

Theoretical study on adsorption of Au and hydrated Au cations on clean Si(1 1 1) surface

To model the adsorption of Au⁺ cation in aqueous solution on the semiconductor surface, the interactions of Au⁺ and hydrated Au⁺ cations with clean Si(1 1 1) surface were investigated by using hybrid density functional theory (B3LYP) and Møller-Plesset second-order perturbation (MP2) methods. Si(1 1 1) surface was described with Si₇H₁₁, Si₁₁H₁₇ and Si₂₂H₂₁ clusters. The effect of the basis set superposition error (BSSE) was taken into account by applying the counterpoise correction. The calculated results indicated that the binding energies between hydrated Au⁺ cations and clean Si(1 1 1) surface are large, suggesting a strong interaction between hydrated Au⁺ cations and the semiconductor surface. The bonding nature of the chemical adsorption of Au⁺ to Si surface can be classified as partial covalent as well as ionic bonding. As the number of water molecules increases, the water molecules form hydrogen bond network with one another and only one water molecule binds directly to the Au⁺ cation. The Au⁺ cation in aqueous solution will safely attach to the clean Si(1 1 1) surface.     

Sulphur on rutile(1 1 0): A theoretical study

Reactions of elementary sulphur with the rutile (1 1 0) surface are of interest for catalytic desulphurization and have recently been studied experimentally. Here a complete mechanism for the sulphurization of the surface in UHV experiments has been derived from the results of first principles molecular dynamics simulations. It provides the dissociation of S₂ molecules, the reaction of S atoms with oxygen atoms in the surface, the insertion of sulphur atoms into surface vacancies and the formation of SO_x species. Possible elementary steps for these processes are discussed in the paper on the basis of the calculations.     

Epitaxial growth of bcc-FexCo100-x thin films on MgO(1 1 0) single-crystal substrates

Fe_xCo_100-x (x=100, 65, 50 at%) epitaxial thin films were prepared on MgO(1 1 0) single-crystal substrates heated at 300 °C by ultra-high vacuum molecular beam epitaxy. The film structure and the growth mechanism are discussed. FeCo(2 1 1) films with bcc structure grow epitaxially on MgO(1 1 0) substrates with two types of variants whose orientations are rotated around the film normal by 180° each other for all compositions. Fe_xCo_100-x film growth follows the Volmer Weber mode. X-ray diffraction analysis indicates the out-of-plane and the in-plane lattice spacings are in agreement with the values of respective bulk Fe_xCo_100-x crystals with very small errors less than ±0.4%, suggesting the strains in the films are very small. High-resolution cross-sectional transmission electron microscopy shows that periodical misfit dislocations are preferentially introduced in the film at the Fe₅₀Co₅₀/MgO interface along the MgO[1 1¯ 0] direction. The presence of such periodical dislocations decreases the large lattice mismatch of about −17% existing at the FeCo/MgO interface along the MgO[1 1¯ 0] direction.     

Defect-induced dimer pinning on the Si(0 0 1) surface

Room-temperature STM images frequently show regions of antisymmetric dimer ordering surrounding certain types of defect on the Si(0 0 1) surface. While it has been generally believed that any defect asymmetric with respect to the dimer row would induce this dimer pinning effect, recent experimental results have shown that this is not the case. We present a model, based on a nearest-neighbour Ising treatment of the surface, which allows the extent of pinning caused by a dimer to be predicted from ab initio calculations. We use this model to predict the pinning extent for three phosphorus-containing structures important in a proposed silicon-based quantum computer fabrication scheme, and identify one of these asymmetric features as causing no appreciable pinning. In addition, we use ab initio calculations to identify the effects governing the interaction between neighbouring dimers.     

Theoretical study of the electronic structure of the Si3N4(0 0 0 1) surface

Density functional theory has been applied to a study of the electronic structure of the ideally-terminated, relaxed and H-saturated (0 0 0 1) surfaces of β-Si₃N₄ and to that of the bulk material. For the bulk, the lattice constants and atom positions and the valence band density of states are all in good agreement with experimental results. A band gap of 6.7 eV is found which is in fair accord with the experimental value of 5.1-5.3 eV for H-free Si₃N₄. Using a two-dimensionally-periodic slab model, a π-bonding interaction is found between threefold-coordinated Si and twofold-coordinated N atoms in the surface plane leading to π and π* surface-state bands in the gap. A surface-state band derived from s-orbitals is also found in the gap between the upper and lower parts of the valence band. Relaxation results in displacements of surface and first-underlayer atoms and to a stronger π-bonding interaction which increases the π-π* gap. The relaxed surface shows no occupied surface states above the valence band maximum, in agreement with recent photoemission data for a thin Si₃N₄ film. The π* band, however, remains well below the conduction band minimum (but well above the Fermi level). Adsorbing H at all dangling-bond sites on the ideally-terminated surface and then relaxing the surface and first underlayer leads to smaller, but still finite, displacements in comparison to the clean relaxed surface. This surface is more stable, by about 3.67 eV per H, than the clean relaxed surface.     

Ab initio study of the fcc-WC(1 0 0) surface and its interaction with cobalt monolayers

The WC(1 0 0) surface has been studied by using ab initio methods of the density functional theory and pseudopotentials. Calculations have shown that surface and undersurface atoms move from their bulk positions. Namely, carbon atoms moved outward, while tungsten atoms moved inward. Five geometric cases for Co/WC(1 0 0) system were compared: (A) Co atoms are above C atoms; (B) Co atoms are above W atoms; (C) Co atoms are in the four-fold sites above WC pairs; (D and E) Co atoms are above the W-W-C and C-C-W three-fold sites, respectively - and the (A) case has been found to be energetically preferable. In all cases, Co layers have been found to be ferromagnetic. The densities of states for the bulk fcc-WC, the WC(1 0 0) surface, and the WC/Co system were compared.     

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.     

Magnetic and binding properties of metal-divacancy complexes at MgO (0 0 1) surface: DFT calculations

We have analyzed the magnetic and binding properties of Ni, Cr, Mo, and Pt metals deposited on the defect free and defect containing surfaces of MgO by means of density functional theory calculations and embedded cluster model. Clusters of moderate sizes with no border anions, to avoid artificial polarization effects, were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. Spin quenching occurs for Cr and Mo complexes at the defect free (terrace) surface, and Cr, Mo, and Pt complexes at the defect containing “pit” divacancy surface. The binding energies of the metals are significantly enhanced on the cationic vacancy end of the divacancy. The adsorption energies of the low spin states of spin quenched complexes are always greater than those of the high spin states. The metal-support interactions stabilize the low spin states of the adsorbed metals with respect to the isolated metals, but the effect is not always enough to quench the spin. The encountered variations in magnetic properties of free metals and of metal complexes are correlated with the energy gaps of the frontier orbitals. Spin contamination affect the adsorbate-substrate distances, Mulliken charges, Mulliken spin densities, natural charge, natural orbital population, and provide rationalization for the reported magnetic and binding properties. The electrostatic potential energy curves provide clearer understanding of the nature of magnetic and binding interactions. The magnetic and binding properties of a single metal atom adsorbed on a particular surface result from a competition between Hund's rule for the adsorbed metal, and the formation of a chemical bond at the interface.     

Theoretical study on the temperature-induced structural transition of the Si(1 1 3) surface

The temperature-induced structural transition of the Si(1 1 3) surface is investigated by ab initio calculations. In this study, it is found that the room-temperature phase and the high-temperature phase have the 3 × 2 interstitial structure and the 3 × 1 interstitial structure, respectively. The existence of the 3 × 2 and 3 × 1 interstitial structures is supported by the analysis of scanning tunneling microscopy (STM) images and the calculation of surface core level shifts using final state pseudopotential theory. The theoretical STM images of interstitial structures are in good agreement with the STM images suggested by experiments. The analysis of STM images provides an insight into the characteristics of domain boundaries observed frequently in experiments. It is also found that the domain boundary can be formed by local 3 × 1 interstitial structures on the 3 × 2 interstitial surface. The theoretical analysis of the surface core level shifts reveals that the surface core levels in experiment originate from the interstitial structures. The lowest values in the surface core level shifts are found to be associated with the 2p core level shifts of the interstitial atoms.     

Trends in adsorption of open-shell atoms and small molecular fragments on the Ag(1 1 1) surface

The chemisorption of various atoms (C, N, O, Cl) and molecular fragments (OH, NH, CH, NH₂, CH₂) on the Ag(1 1 1) surface has been studied by employing the embedded cluster and multireference single- and double-excitation configuration interaction (MRD-CI) methods. Ground and excited states of the cluster-adsorbate systems have been computed and molecular orbitals (MOs) as well as electronic charge density distributions and Mulliken populations have been analyzed in order to extract general trends in chemisorption properties for different adsorbates. It has been found that the adsorbate-surface bond is energetically most favorable when a maximum of two electrons of the metal are shared with a given adsorbate. As a result atomic/molecular fragments with less than six valence electrons (N, CH, C) retain some open shells upon adsorption, whereas oxygen as well as chlorine isovalent species form a singlet ground state on the surface. All species considered except for Cl have mainly covalent bonding character to the surface, with an electronic charge of up to 1.0 transferred to the adsorbate from the silver cluster. It has been shown that the ionicity of the bond is strongly correlated with the electron affinity of the adsorbed species. Binding energies, equilibrium geometry and adsorbate location on the cluster have been computed and compared with available experimental data. In addition, the characteristic properties of chemisorption on Ag(1 1 1) and Pt(1 1 1) surfaces have been compared.     

A DFT + U description of oxygen vacancies at the TiO2 rutile (1 1 0) surface

Experimental observations indicate that removing bridging oxygen atoms from the TiO₂ rutile (1 1 0) surface produces a localised state approximately 0.7 eV below the conduction band. The corresponding excess electron density is thought to localise on the pair of Ti atoms neighbouring the vacancy; formally giving two Ti³⁺ sites. We consider the electronic structure and geometry of the oxygen deficient TiO₂ rutile (1 1 0) surface using both gradient-corrected density functional theory (GGA DFT) and DFT corrected for on-site Coulomb interactions (GGA + U) to allow a direct comparison of the two methods. We show that GGA fails to predict the experimentally observed electronic structure, in agreement with previous uncorrected DFT calculations on this system. Introducing the +U term encourages localisation of the excess electronic charge, with the qualitative distribution depending on the value of U. For low values of U (?4.0 eV) the charge localises in the sub-surface layers occupied in the GGA solution at arbitrary Ti sites, whereas higher values of U (?4.2 eV) predict strong localisation with the excess electronic charge mainly on the two Ti atoms neighbouring the vacancy. The precise charge distribution for these larger U values is found to differ from that predicted by previous hybrid-DFT calculations.     

Relative energies of surface and defect states: ab initio calculations for the MgO (001) surface

We present the results of calculations of the energy levels of defects at the (001) surface of MgO relative to the top of the valence band and values of defect ionisation potentials and electron affinities. The calculations were made using an embedded cluster method in which a cluster of several tens of ions treated quantum mechanically is embedded in a finite array of polarisable and point ions modelling the crystalline potential and the classical polarisation of the host lattice. The calculated ionisation potential of the ideal surface, which fixes the position of the top of the valence band with respect to the vacuum level, is about 6.7 eV. This value is used as a reference for positioning the energy levels of three charge states of a surface anion vacancy, which are also calculated as ionisation energies with respect to the vacuum level. The surface and vacancy electron affinities are calculated using the same method. As a prototype low-coordinated surface site, we have considered a cube corner. Our calculations predict the splitting of the corner states from the top of the surface valence band by about 1.0 eV. Both unrelaxed and relaxed holes are strongly localised at the corner oxygen ion. The ionisation energies and electron affinities of the corner anion vacancy are calculated. The electrons in the F and F⁺ centres at the corner are shown to be significantly delocalised over surrounding Mg ions.     

Molecular precursor-mediated methanol dissociation on Si(1 1 1)7 × 7: ab initio study

We present an ab initio study of methanol interaction with the Si(1 1 1)7 × 7 surface using a Si(1 1 1)4 × 2 model. The study of the methanol dissociation on Si(1 1 1)4 × 2 shows that pair dissociation on adatom-restatom dangling bonds is largely favoured, in agreement with the experimental observations. The “center” type adatom is slightly more reactive than the “corner” type one, although the difference is weak. Similar behaviour is observed in both adatom types. Our results for a direct CH₃OH dissociation favouring a basic cleavage (adsorption of OH and CH₃ fragments) rather than an acidic one (adsorption of H and OCH₃ fragments), we are finally led to take a kinetic effect into consideration to reconcile theory with experiment. We show that the presence of molecular precursor states is possible. Different orientations with respect to the silicon dangling bonds of these molecular precursors are investigated. However, the corresponding energies are very close and, considering their relative energies, it is finally difficult to discriminate between acidic and basic cleavages.     

First-principles studies on initial growth of Ni on MgO(0 0 1) surface

To elucidate the initial growth of metal on oxide surface, we studied adsorption of small nickel clusters, Ni_n (n = 1-5), on MgO(0 0 1) surface using first-principles method based on density-functional theory. It was found that the preferential adsorption site for an isolated Ni atom is directly above the surface oxygen atom. A strong covalent bond with partial ionic character is formed between the Ni adatom and the surface oxygen atom. Various structures were considered for the Ni_n isomers and 3D structures were found to be energetically more stable than 2D structures for clusters of more than two atoms. For the 2D clusters, metal-metal bonds prevail over metal-substrate bonds with increasing Ni coverage. The calculated work function and ionization energy were found to vary with Ni coverage which is attributed to the change of the surface dipole moment upon metal adsorption, while the evolution of Schottky barrier height at the initial growth stage is dominated by the adatom-induced gap states.     

Kinetic Monte Carlo simulations of Pd deposition and island growth on MgO(1 0 0)

The deposition and ripening of Pd atoms on the MgO(1 0 0) surface are modeled using kinetic Monte Carlo simulations. The density of Pd islands is obtained by simulating the deposition of 0.1 ML in 3 min. Two sets of kinetic parameters are tested and compared with experiment over a 200-800 K temperature range. One model is based upon parameters obtained by fitting rate equations to experimental data and assuming the Pd monomer is the only diffusing species. The other is based upon transition rates obtained from density functional theory calculations which show that small Pd clusters are also mobile. In both models, oxygen vacancy defects on the MgO surface provide strong traps for Pd monomers and serve as nucleation sites for islands. Kinetic Monte Carlo simulations show that both models reproduce the experimentally observed island density versus temperature, despite large differences in the energetics and different diffusion mechanisms. The low temperature Pd island formation at defects is attributed to fast monomer diffusion to defects in the rate-equation-based model, whereas in the DFT-based model, small clusters form already on terraces and diffuse to defects. In the DFT-based model, the strong dimer and trimer binding energies at charged oxygen vacancy defects prevent island ripening below the experimentally observed onset temperature of 600 K.     

A comparative analysis of electron spectroscopy and first-principles studies on Cu(Pd) adsorption on MgO

Ultrathin MgO films were grown on a W(1 1 0) substrate while metastable impact electron (MIES) and photoelectron (UPS) spectra were measured in situ; apart from the valence band emission, no additional spectral features were detected. The oxide surface was exposed to metal atoms (Cu, Pd) at RT. A comparison with the DOS extracted from first-principles DFT calculations shows that the metal-induced intensity developing above the top of the O 2p valence band in the UP spectra under Cu(Pd) exposure is caused by Cu 3d (Pd 4d) emission. The emission seen in the MIES spectra is attributed to the ionization of Cu 3d and 4s states of adsorbed neutral Cu atoms in an Auger process, Auger neutralization, involving two electrons from the surface, at least one of them from the metal adsorbate. The shape of the MIES spectra suggests metallic island growth even at the lowest studied exposures, which is supported by the first-principles calculations.     

Non-contact AFM images of a C60 molecule adsorbed on the Si(0 0 1) surface: An ab initio method

A theoretical non-contact atomic force microscope (nc-AFM) image is simulated for an individual C₆₀ molecule adsorbed on the clean Si(0 0 1) surface. The ability to identify the intramolecular features of the molecule though nc-AFM imaging would allow many of the different configurations the molecule may take when adsorbed, to be identified. This would be an important preliminary step in the manipulation of the molecule along the surface, as the initial configuration that the molecule is in will affect the periodicity of the tip trajectory during the manipulation event (see D.L. Keeling et al., Phys. Rev. Lett. 94 (2005) #146104). Presented in this paper are simulated images and force curves above important sites for key stable configurations of the C₆₀ molecule. It is shown that in principle it is possible to distinguish between the different configurations of the molecule using nc-AFM. An ab initio DFT method was used to accurately describe the chemical interaction between the adsorbate and the Si tip, which has a single dangling bond at its apex.     

Ge adsorption on SiC(0 0 0 1): An ab initio study

In this work we have performed an ab initio total energy investigation of the Ge adsorption process on the Si-terminated SiC(0 0 0 1)- and (3 × 3) surfaces. We find that Ge adatoms lying on the topmost sites of the and (3 × 3) surfaces represent the energetically more stable configurations at the initial stage of the Ge adsorption on the SiC(0 0 0 1) surface. The Si → Ge substitutional adsorption processes have been examined as a function of the Si and Ge chemical potentials. Increasing the Ge coverage, we verify that the formation of Ge wetting layer on the surface, and Ge nanocluster on the (3 × 3) surface are the energetically more stable configurations, in accordance with recent experimental findings.