Coverage-dependent absorption of atomic hydrogen into the sub-surface of Cu(1 1 1) studied by density-functional-theory calculations

With density-functional-theory calculations, we have studied coverage-dependent absorption of H atoms into the sub-surface below a face-centered-cubic (fcc) hollow site of Cu(1 1 1). Both frozen and relaxed surface lattices were considered when the atomic H migrated from the surface to the sub-surface. The potential energy curve for the absorbing H shows that the surface site is in general favored over the sub-surface site, and this trend varies little with the H coverage (0.11-0.67 ML). If the hexagonal-close-packed (hcp) hollow sites immediately vicinal to the absorbing H are pre-adsorbed with other H atoms, the surface adsorption potential is greatly increased, because of the repulsive H-H interaction, to a value near, or even greater than, the sub-surface absorption potential; when two or three H atoms (on the hcp sites) are beside the absorbing H, the energy barrier for the sub-surface absorption is decreased, whereas that for diffusion from the sub-surface to the surface is enhanced. These results indicate that, on an H-saturated Cu(1 1 1) surface (0.67 ML), the sub-surface sites below the fcc sites with two or three neighboring H atoms can trap the sub-surface H.     

Adsorption and reaction of methanethiol on the Ru(0 0 0 1)-p(2 × 2)O surface: A TPD and XPS study

Methanethiol adsorbed on Ru(0 0 0 1)-p(2 × 2)O has been studied by TPD and XPS. The dissociation of methanethiol to methylthiolate and hydrogen at 90 K is evidenced by the observation of hydroxyl and water. The saturation coverage of methylthiolate is ∼0.15 ML, measured by both XPS and TPD. A detailed analysis suggests that only the hcp-hollow sites have been occupied. Upon annealing the surface, water and hydroxyl desorb from the surface at ∼210 K. Methylthiolate decomposes to methyl radical and atomic sulphur via C-S cleavage between 350 and 450 K. Some methyl radicals (0.05 ML) have been transferred to Ru atoms before they decompose to carbon and hydrogen. The rest of methyl radicals desorb as gaseous phase. No evidence for the transfer of methyl radical to surface oxygen has been found.     

Density functional theory study of selenium adsorption on Fe(1 1 0)

The adsorption of atomic Se on a Fe(1 1 0) surface is examined using the density functional theory (DFT). Selenium is adsorbed in high-symmetry adsorption sites: the -short and long-bridge, and atop sites at 1/2, 1/4, and 1 monolayer (ML) coverages. The long bridge (LB) site is found to be the most stable, followed by the short bridge (SB) and top sites (T). The following overlayer structures were examined, p(2 × 2), c(2 × 2), and p(1 × 1), which correspond to 1/4 ML, 1/2 ML, and 1 ML respectively. Adsorption energy is −5.23 eV at 1/4 ML. Se adsorption results in surface reconstruction, being more extensive for adsorption in the long bridge site at 1/2 ML, with vertical displacements between +8.63 and −6.69% -with regard to the original Fe position-, affecting the 1st and 2nd neighbours. The largest displacement in x or y-directions was determined to be 0.011, 0.030, and 0.021 Å for atop and bridge sites. Comparisons between Se-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the Se. At the long bridge site, the presence of Se causes a decrease in the surface Fe d-orbital density of states between 4 and 5 eV below Fermi level. The density of states present a contribution of Se states at −3.1 eV and −12.9 eV. stabilized after adsorption. The Fe-Fe overlap population decrease and a Fe-Se bond are formed at the expense of the metallic bond.     

Coadsorption of CN and O on Cu (1 0 0) surface: A density functional study

The adsorption of cyanide (CN) or oxygen atom, as well as the coadsorption of CN + O on Cu (1 0 0) surface is studied by using density functional theory (DFT) and the cluster model method. Cu₁₄ cluster is used to simulate the surface. Perpendicular and parallel bonding geometries of CN adsorbed on Cu (1 0 0) surface are considered, respectively. The present calculations show that the CN may be absorbed on top and bridge sites by carbon atom of cyanide (C-down), and C-down on top site is the most favorable. The adsorbed C-N stretch frequencies compared with that of the gaseous CN species are all red-shifted, except the C-down on top site. The charge transfer from the surface to the CN species leads to an increase in work function for the Cu surface. The oxygen atom adsorbed on the four-fold hollow site of Cu (1 0 0) is the most favorable, and is consistent with the experimental study. The coadsorption of O at a four-fold hollow site tends to block adsorption of CN at the nearby sites. If O coverage increases, the CN may be adsorbed on the top and bridges sites with the C-down model. The reaction CN + O → OCN on the Cu (1 0 0) is predicted to be exothermic, and formed OCN species may be stably absorbed on the Cu (1 0 0).     

Perpendicular magnetic anisotropy of V/Co(0 0 1)

Based on the full-potential linearized augmented plane wave (FLAPW) calculations, various magnetic properties of ultra thin face centered cubic (fcc) Co(0 0 1) film and V adsorbed systems on Co(0 0 1) surface are explored. It was found that the V film grown on fcc Co(0 0 1) surface has large induced magnetic moment and the direction of magnetization is antiparallel to that of Co atom in the submonolayer coverage. Very interestingly, we found that the surface alloy and 0.5 ML adsorbed V/Co(0 0 1) systems have perpendicular magnetocrystalline anisotropy and the magnitude of anisotropy energy in 0.5 ML V on fcc Co(0 0 1) surface is greatly larger than that of surface alloy, while we observed in-plane magnetization in pure fcc Co(0 0 1) film. It was found that the spin-orbit interaction through spin-flip process cannot be ignored, therefore the simple relation with orbital anisotropy is not applicable in the interpretation of magnetocrystalline anisotropy.     

Self-assembly of 2,6-dimethylpyridine on Cu(1 1 0) directed by weak hydrogen bonding

Sequential stages of formation of a self-assembled monolayer of flat-lying 2,6-dimethylpyridine molecules on a single crystal Cu(1 1 0) surface have been observed by low-temperature scanning tunneling microscopy (LT-STM). At an adsorption temperature of 10 K, all of the molecules are randomly distributed at low coverage upon adsorption. The isolated molecules align their molecular axes parallel to the 〈0 0 1〉 azimuth of the Cu lattice. The nitrogen atom in the molecule is located at the four-fold hollow site. Upon annealing to 100 K, the molecules associate to form head-to-head dimers. The dimer units involve a pair of weak hydrogen bonds between methyl group-hydrogen atoms and N moieties on adjacent molecules, forming a core structure for further growth. In a later stage of self-assembly, single head-to-tail weak hydrogen bonds between ring C-H bonds and N moieties form in chains on the periphery of the central cores, leading to larger domains with a c(6 × 2) overlayer structure.     

Asymmetric adsorption-site of potassium atoms in the (3 × 2)-p2mg structure formed on Cu(0 0 1)

The adsorption of K atoms on Cu(0 0 1) has been studied by low-energy electron diffraction (LEED) at room temperature (RT) and 130 K. At RT, a (3 × 2)-p2mg LEED pattern with single-domain was observed at coverage of 0.33, whereas the orthogonal two-domain was found at 130 K. At 130 K, a c(4 × 2) pattern with orthogonal two-domain was observed at coverage 0.25. Both the (3 × 2)-p2mg and c(4 × 2) structures have been determined by a tensor LEED analysis. It is demonstrated that K atoms are adsorbed on surface fourfold hollow sites in the c(4 × 2), while in the (3 × 2) structure two K atoms in the unit cell are located at an asymmetric site with a glide-reflection-symmetry. The asymmetric site is at near the midpoint between the exact hollow site and bridge-site but slightly close to the hollow site. A rumpling of 0.07 Å in the first Cu layer was confirmed, which might stabilize K atoms at the asymmetric site. Surface structures appearing in a coverage range 0.25-0.33 are discussed in terms of the occupation of the asymmetric site with increase of coverage.     

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) surfaces

Adsorption of carbon monoxide on Pd(3 1 1) and (2 1 1) stepped surfaces has been investigated by the extended London-Eyring-Polyani-Sato (LEPS) method constructed using a 5-parameter Morse potential. The calculated results show that there exist common characteristics of CO adsorption on the two surfaces. At low coverage, CO occupies threefold hollow site of the (1 1 1) terrace and is tilted with respect to the surface normal. Among the threefold hollow sites on the (1 1 1) terrace, the nearer the site is to the step, the greater is the influence of the step. The twofold bridge site on the (1 0 0) step is also a stable adsorption site at high coverage. Because of the different lengths of the (1 1 1) terraces, the (3 1 1) and (2 1 1) stepped surfaces have different characteristics. A number of new sites are exposed on the boundary regions, including the fourfold hollow site (H₄) of the (3 1 1) surface and the fivefold hollow site (H₅) of the (2 1 1) surface. At high coverage, CO resides in the H₅ site of the (2 1 1) surface, but the H₄ site of the (3 1 1) surface is not a stable adsorption site. This study further shows that the on-top site on the (1 0 0) step of Pd(3 1 1) is a stable adsorption site, but the same type of site on Pd(2 1 1) is not.     

A density-functional study on the atomic geometry and adsorption of the Cu(1 0 0) c(2 × 2)/N Surface

In this work we have performed total-energy calculations on the geometric structure and adsorption properties of Cu(1 0 0) c(2 × 2)/N surface by using the density-functional theory and the projector-augmented wave method. It is concluded that nitrogen atom was adsorbed on a FFH site with a vertical distance of 0.2 Å towards from surface Cu layer. The bond length of the shortest Cu-N bonding is calculated to be 1.83 Å. Geometry optimization calculations exclude out the possibilities of adsorbate induced reconstruction mode suggested by Driver and Woodruff and the atop structural model. The calculated workfunction for this absorbate-adsorbent system is 4.63 eV which is quite close to that of a clean Cu(1 0 0) surface. The total-energy calculations showed that the average adsorption energy per nitrogen in the case of Cu(1 0 0) c(2 × 2)-N is about 4.88 eV with respect to an isolated N atom. The absorption of nitrogen on Cu(1 0 0) surface yields the hybridization between surface Cu atoms and N, and generates the localized surface states at −1.0 eV relative to Fermi energy E_F. The stretch mode of the adsorbed nitrogen at FFH site is about 30.8 meV. The present study provides a strong criterion to account for the local surface geometry in Cu(1 0 0) c(2 × 2)/N surface.     

A study of Ga layers on Si(1 0 0)-(2 × 1) by SR-PES: Influence of adsorbed water

Results for deposition and thermal annealing of gallium on the Si(1 0 0)-(2 × 1) surface achieved by synchrotron radiation photoelectron spectroscopy (SR-PES) and low energy electron diffraction (LEED) are presented. In addition to deposition of Ga on a clean surface, the influence of water adsorption on the arrangement of gallium atoms was also studied. The results on Ga deposition at a higher temperature (490 °C) are consistent with a Ga ad-dimer model showing equivalent bond arrangement of all Ga atoms for coverages up to 0.5 ML. The deposition onto a surface with adsorbed water at room temperature led to a disordered gallium growth. In this case gallium atoms bind to silicon dimers already binding fragments of adsorbed water. A subsequent annealing of these layers leads to a surface structure similar to the Ga-(2 × 2), however, it is less ordered, probably due to the presence of silicon oxides formed from water fragments.     

Adsorption and dissociation of O2 on CuCl(1 1 1) surface: A density functional theory study

The adsorption and dissociation of O₂ on CuCl(1 1 1) surface have been systematically studied by the density functional theory (DFT) slab calculations. Different kinds of possible modes of atomic O and molecular O₂ adsorbed on CuCl(1 1 1) surface and possible dissociation pathways are identified, and the optimized geometry, adsorption energy, vibrational frequency and Mulliken charge are obtained. The calculated results show that the favorable adsorption occurs at hollow site for O atom, and molecular O₂ lying flatly on the surface with one O atom binding with top Cu atom is the most stable adsorption configuration. The O-O stretching vibrational frequencies are significantly red-shifted, and the charges transferred from CuCl to oxygen. Upon O₂ adsorption, the oxygen species adsorbed on CuCl(1 1 1) surface mainly shows the characteristic of the superoxo (O₂⁻), which primarily contributes to improving the catalytic activity of CuCl, meanwhile, a small quantity of O₂ dissociation into atomic O also occur, which need to overcome very large activation barrier. Our results can provide some microscopic information for the catalytic mechanism of DMC synthesis over CuCl catalyst from oxidative carbonylation of methanol.     

Electrochemical scanning tunneling microscopy examination of the structures of benzenethiol molecules adsorbed on Au(1 0 0) and Pt(1 0 0) electrodes

In situ electrochemical scanning tunneling microscopy (STM) has been used to examine the structures of benzenethiol adlayers on Au(1 0 0) and Pt(1 0 0) electrodes in 0.1 M HClO₄, revealing the formation of well-ordered adlattices of Au(1 0 0)-(√2 × √5) between 0.2 and 0.9 V and Pt(1 0 0)-(√2 × √2)R45° between 0 and 0.5 V (versus reversible hydrogen electrode), respectively. The coverage of Au(1 0 0)-(√2 × √5) is 0.33, which is identical to those observed for upright alkanethiol admolecules on Au(1 1 1). In comparison, the coverage of Pt(1 0 0)-(√2 × √2)R45° - benzenethiol is 0.5, much higher than those of thiol molecules on gold surfaces. This result suggests that benzenethiol admolecules on Pt(1 0 0) could stand even more upright than those on Au(1 0 0). All benzenethiol admolecules were imaged by the STM as protrusions with equal corrugation heights, suggesting identical molecular registries on Au(1 0 0) and Pt(1 0 0) electrodes, respectively. Modulation of the potential of a benzenethiol-coated Au(1 0 0) electrode resulted in irreversible desorption of admolecules at E ? 0.1 V (vs. reversible hydrogen electrode) and oxidation of admolecules at E ? 0.9 V. In contrast, benzenethiol admolecule was not desorbed from Pt(1 0 0) at potentials as negative as the onset of hydrogen evolution. Raising the potential rendered deposition of more benzenethiol molecules before oxidation of admolecules commenced at E > 0.9 V.     

Olefin bonding mechanism and growth to (1 0 0)(2 × 1):H diamond

The bonding and growth mechanism of photochemically attached olefin molecules to (1 0 0)(2 × 1):H diamond is characterized using atomic force (AFM) and scanning tunneling microscopy (STM) experiments in combination with molecular orbital calculations. To identify growth schemas, diamond surfaces after 10, 40 and 90 min of photo-chemically stimulated growth have been characterized. These data show clearly island formation which is discussed taking into account a growth model from silicon. The island growth shows no directional properties which are attributed to arrangement and geometrical properties of hydrogen terminated carbon bonds at the surface of (1 0 0) oriented (2 × 1) reconstructed diamond.     

Coverage effects on phenol adsorption on Si(1 0 0)2 × 1 as: A first principle calculation

We investigated the adsorption of a 6-dimers Si(1 0 0)2 × 1 surface as a function of coverage and adsorption type (molecular/dissociative) by first principle calculations. In particular, we performed calculations on models with 2, 3, 4 and 6 phenol molecules, corresponding to coverage Θ = 0.34, 0.5, 0.67 and 1. We found that total adsorption energy, when at least one phenol is in a molecular state is lower than the sum of the corresponding singly adsorbed molecules. The dissociative adsorption of multiple molecules, both in parallel and switched configuration is most favoured for a coverage Θ = 0.34 (2.6 eV per adsorbed molecule). This values decreases to 2.0 eV and remains constant till the coverage 1 is reached.The energy barrier for the molecular-to-dissociated transition of a phenol molecule, in presence of another dissociatively adsorbed molecule is ∼0.008 eV and it is similar to the value in case of single adsorption. Possible hydrogen displacements were also considered.     

Sulphur overlayers on the Au(1 1 0) surface: LEED and TPD study

The adsorption and desorption of sulphur on the clean reconstructed Au(1 1 0)-(1 × 2) surface has been studied by low energy electron diffraction, Auger electron spectroscopy and temperature programmed desorption. The results obtained show a complex behaviour of the S/Au(1 1 0) system during sulphur desorption at different temperatures. Two structures of the stable ordered sulphur overlayer on the Au(1 1 0) surface, p(4 × 2) and c(4 × 4), were found after annealing the S/Au(1 1 0) system at 630 K and 463 K, respectively. The corresponding sulphur coverage for these overlayers was estimated by AES signal intensity analysis of the Au NOO and S LMM Auger lines to be equal to 0.13 ML and 0.2 ML, respectively. Both sulphur structures appear after removing an excess of sulphur, which mainly desorbs at 358 K as determined from TPD spectra. Furthermore, it was not possible to produce the lower coverage p(4 × 2) sulphur structure by annealing the c(4 × 4) surface. In the case of the p(4 × 2) S overlayer on the Au(1 1 0)-(1 × 2) surface it is proposed that the sulphur is attached to “missing row” sites only. The c(4 × 4) S overlayer arises via desorption of S₂ molecules that are formed on the surface due to mobility of sulphur atoms after a prolonged anneal.     

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.     

Epitaxial growth of Bi thin films on Ge(1 1 1)

We investigated Bi thin film growth on Ge(1 1 1) by using low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). In the submonolayer regime, adsorbed Bi atoms form patches of the (2×1) structure. However, the structure does not grow to a long-range order. Following the formation of a (1×1) monolayer (ML) film, two-dimensional (1 1 0)-orientated Bi islands grow. The film orientation changes from (1 1 0) to (1 1 1) at 6-10 ML. The (1 1 0)-oriented Bi film shows a six-domain LEED pattern with missing spots, associated with a glide-line symmetry. The hexagonal (1 1 1) film at 14 ML has a lattice constant 2% smaller than bulk Bi(1 1 1).     

Temperature dependent quantum well state on Cu(1 1 0)(2 × 1) striped surface studied by angle resolved photoelectron spectroscopy

We have investigated the temperature dependence of angle resolved photoelectron spectroscopy for the lateral quantum well states (QWS) on the striped Cu(1 1 0)(2 × 1)O surface. For the striped surface with oxygen coverage of 0.25 ML, we have successfully observed two discrete levels along a perpendicular direction to the stripes in the surface Brillouin zone, which is generated by quantization of the Shockley surface state. We have found that the relative photoelectron intensity of the two discrete levels depends on the temperature. The photoelectron intensity tends to concentrate on the ground level of the QWS with decreasing temperature. Our investigation indicates that the electron population of each quantum well level depends on the temperature.     

STM study of Bi-on-Cu(1 0 0)

Scanning tunneling microscopy (STM) has been used to study the various possible structures of adsorbed Bi on the Cu(1 0 0) surface, after equilibration at a temperature of 520 K. All of the structures previously identified by X-ray diffraction (lattice gas, c(2 × 2), c(9√2 × √2)R45°, and p(10 × 10), in order of increasing Bi-coverage) were found to be present on a single sample produced by diffusing Bi onto the Cu(1 0 0) surface from a 3-d source. By investigating the possible coexistence of various pairs of phases, it was demonstrated that the c(2 × 2) phase transforms to the c(9√2 × √2)R45° phase by a first order transition, whereas the transition from c(9√2 × √2)R45° to p(10 × 10) is continuous. In addition, the structure of surface steps was studied as a function of Bi-coverage. The results showed that the presence of Bi changes the nature of the step-step interactions at the Cu(1 0 0) surface from repulsive to attractive. The attractive step-step interactions transform any small deviations from the nominal (1 0 0) orientation of the Cu substrate into (3 1 0) microfacets. When compared with the known equilibrium crystal shape (ECS) of Bi-saturated Cu, the observed microfaceting may imply that the ECS of Cu-Bi alloys is temperature dependent.     

Comparison of S, Pt, and Hf adsorption on NiAl(1 1 0)

First-principles periodic slab density-functional theory (DFT) calculations with a plane-wave basis are used to predict the properties of S, Pt, and Hf adsorption on NiAl(1 1 0). Stable adsorption sites are identified, and adsorbate binding energies and structures are predicted. We find that while S adsorbs in a threefold site, the metals prefer to adsorb in the Ni-Ni twofold bridge site. The latter finding is consistent with scanning tunneling microscopy experiments for adsorption of various transition metals on NiAl(1 1 0) by Ho and coworkers. S is predicted to easily diffuse between threefold sites. We find that Pt and Hf both induce significant changes in the local surface structure, changing twofold bridge sites into fourfold coordination sites by drawing next-nearest-neighbor atoms nearly equidistant with the nearest-neighbor atoms. We find Pt favors interaction with Al slightly more than Ni, while Hf shows a particularly strong affinity for Ni compared to Al. We also predict that Hf may diffuse one-dimensionally along Ni rows with a barrier of ≈0.6 eV.