Theoretical study of oxygen chemisorption on (111) and (100) silicon surfaces

The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especially for the (111) surface.     

Nature of versatile chemisorption on TiC(111) and TiN(111) surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR’s) at the Fermi level and X2p-derived SR’s deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR’s in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.     

Pyridine chemisorption on silicon and germanium (111) surfaces

The chemisorption of pyridine molecules on cleaved Si(111) and Ge(111) surfaces was investigated by ultraviolet photoemission spectroscopy with synchrotron radiation. Evidence was found that the chemisorption process strongly affects all the three highest occupied molecular levels, i.e. the n-level and the two π-levels la₂ and 2b₂. This result was used to rule out a chemisorption geometry with the aromatic ring parallel to the surface. In the most likely chemisorption geometry the ring is tilted with respect to the substrate and the n-orbital plays a leading role in the formation of chemisorption bonds.     

The process of oxygen chemisorption on the Si(111) surface

The chemisorption of oxygen on the Si(111) surface has been studied by the ASED-MO method. Three steps of the initial oxidation process have been proposed. The first step is molecular oxygen chemisorption. The second step is that of dissociated oxygen chemisorption in which the atomic short bridge site (between the first layer and second layer silicon atoms) can be occupied only after the saturation of the dangling bonds of the surface silicon with oxygen. The third step is the diffusion of atomic oxygen from the short bridge positions into the bulk of silicon to form an SiO₂ film. For molecular chemisorption, both the peroxy vertical and peroxy bridge models are possible although the peroxy vertical model is the more stable. The dissociated atomic oxygen can chemisorb for both the on-top and the short bridge models. Our results can explain, and are consistent with, most experimental results.     

Vibration spectroscopy of water on stepped gold surfaces

The vibration spectrum of H₂O (ice) adsorbed at low temperatures on Au(1 0 0), Au(1 11 1), and Au(1 1 5) is studied using electron energy loss spectroscopy. On the Au(1 0 0) surface, the spectra show the presence of the typical H-bonded network of water molecules for all coverages from the submonolayer into the multilayer range. The absence of a non-H-bonded OH-stretching mode is indicative for the “H-down bilayer”. On stepped surfaces, on the other hand, a considerable fraction of the H-atoms remains in the non-H-bonded state; surprisingly even in the multilayer range, and even after annealing. The fraction of non-H-bonded hydrogen atoms scales with the step density. Spectral features of water adsorbed at step-sites are isolated after annealing a surface exposed to small doses of H₂O. The results are discussed in the context of recent theoretical studies as well as in conceivable relation to the experimentally found reduction of the Helmholtz-capacitance on stepped Au(1 1 n) electrodes.     

Study of chemisorption on copper low-index and stepped surfaces

Adsorption of CHCl₃, O₂, and hydrocarbons has been studied on Cu(111) and stepped surfaces using LEED, AES, and UPS at room temperature. We find that ordered Cl overlayers form upon Cu(111), Cu[3(111) × (100)], and Cu[5(111) × (100)] surfaces upon exposure to CHCl₃. Exposure to O₂ results in rearrangement of the Cu[5(111) × (100)] surface to hill-and-valley regions with large (111) areas, whereas Cu[2(111) × (100)] is stable for the same exposure. The photoemission spectra show new energy levels due to C1 above and below the Cu d band region and a small splitting of the halogen p orbitals. Effects consistent with interaction with the Cu d band are observed. Similar effects are observed with oxygen adsorption. The initial rate of Cl or O₂ chemisorption as measured by photoemission is proportional to the density of steps on these surfaces. Apparently, structural effects play an important role in chemisorption on metals (such as copper) with low density of states at the Fermi energy.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

First-principles study of the water structure on flat and stepped gold surfaces

The geometric structure and electronic properties of flat and stepped gold–water interfaces have been addressed by periodic density functional theory (DFT) calculations. This work was motivated by a recent electron energy loss spectroscopy study [H. Ibach, Surf. Sci. 604 (2010) 377] indicating that the structure of a water layer on stepped Au(511) differs significantly from the one on Au(100). Based on ab initio molecular dynamics simulations, the measured spectra have been reproduced and linked to the geometric arrangement of the water molecules. Furthermore, we find a strong polarization of the water layers which contributes to the water-induced work function change of the substrate.     

The adsorption of oxygen on the stepped Pt(S)-[9(111) × (111)] face

The adsorption of oxygen on the stepped Pt(S)-[9(111) × (111)) face has been studied by flash desorption, LEED and AES. On adsorbing oxygen the (1 × 1) LEED pattern of the clean face was transformed into a (2 × 2) pattern. A lower limit of the initial sticking coefficient of 0.06 and a saturation coverage of approximately 0.5 monolayer were determined. The flash desorption spectra exhibited two not completely resolved desorption maxima. From the peak temperatures the activation energies of desorption were estimated to be 41 and 49 kcal/mole. Under the same experimental conditions some experiments were done on a smooth (111) Pt face. However, the results did not differ significantly from those obtained on the stepped surface. In addition on the smooth (111) face the adsorption of oxygen activated in a high frequency discharge was studied. Oxidation was not observed beyond the chemisorption layer which is formed from molecular oxygen.     

Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.     

Formation of strontium atomic chains on the singular and stepped Si(111) surfaces

The surface structures 3 × 2, 5 × 2, 7 × 2, and 9 × 2 formed on the Si(111) plane during adsorption of submonolayer strontium have been investigated using scanning tunneling microscopy. The experimental data obtained are in agreement with the models of surface reconstructions proposed earlier for the structures induced by the adsorption of divalent metal atoms. An important constituent of these structures is provided by strontium atomic chains in the 〈1$ \bar 1 $ \bar 1 0〉 direction. Three types of domains of surface structures are formed on the Si(111) plane, which correspond to the rotational symmetry C ₃ of the surface under consideration. The reduction of the symmetry of the substrate to C ₁ with the use of the stepped Si(7 7 10) surface makes it possible to form strontium atomic chains, which are oriented with respect to the substrate in a similar manner.     

The behaviour of water at stepped surfaces of single crystal gold electrodes

New information concerning the structure of the metal-electrolyte interface is obtained by comparing the electronic work function of metallic single crystals in vacuum and the potential of zero charge (pzc) at the metal-electrolyte interface. The close parallelism of these two properties is notable in comparing our results for the pzc of gold single crystals of a number of orientations located along the sides of the stereographic triangle and those for the work function of copper by Peralta et al. [24]. This comparison shows that the presence of an adsorbed water layer at the metal-electrolyte interface does not perturb to any large extent the atomic arrangement of the outermost metallic surface with respect to the structure assumed to be displayed in vacuum. In the case of stepped gold surfaces near the (111) face, a comparison between the work function measured by Besocke et al. [23] and our results for pzc allows us to conclude that the metal-solvent interaction depends on the nature of the monoatomic step whether it corresponds to a quaternary site in the case of the (100) step structure or to a ternary site for the (111) step structure.     

The chemisorption of Co on Rh(111)

The adsorption of CO on Rh(111) has been studied by thermal desorption mass spectrometry and low-energy electron diffraction (LEED). At temperatures below 180 K, CO adsorbs via a mobile precursor mechanism with sticking coefficient near unity. The activation energy for first-order CO desorption is 31.6 kcal/mole (ν_d = 10^13.6s^?1) in the limit of zero coverage.As CO coverage increases, a (√3 ×√3)R30^u overlayer is produced and then destroyed with subsequent formation of an overlayer yielding a (2 × 2) LEED pattern in the full coverage limit. These LEED observations allow the absolute assignment of the full CO coverage as 0.75 CO molecules per surface Rh atom. The limiting LEED behavior suggests that at full CO coverage two CO binding states are present together.     

The adsorption of oxygen on silicon (111) surfaces. II

At a wavelength of 546 nm the change of the ellipsometric angles δΔ (relative phase change) and δψ (relative amplitude change) has been studied on clean cleaved silicon (111) surfaces during adsorption of oxygen. δψ increases linearly with exposure up to a saturation value. The saturation dose, i.e. also the sticking coefficient for the corresponding process depends exponentially on the mean step density of the surface. δΔ is nearly independent of step density. The interpretation in terms of a macroscopic theory (Drude model) gives evidence of two adsorption processes, the formation of a monolayer of oxygen controlled by step density and a subsequent further oxidation process.     

Density functional theory study of the adsorption of oxygen atoms on gold (111), (100) and (211) surfaces

The interaction of an oxygen atom with various gold surfaces was examined computationally using density functional theory. The relative binding energies for a range of possible adatom locations on each surface were obtained. The results demonstrated the relative importance of low-coordinated gold atoms to bind oxygen for a particular surface and a preference for three-fold adatom coordination over the two-fold and single-coordination sites. Pseudo-potential energy curves were obtained by following the calculated energy as a function of surface position. These general results provide a reference for the interaction of oxygen atoms with gold nanoparticles that may project faces similar to the surfaces studied here.     

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO₄ tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO₄ tetrahedra are readily formed even at room temperature. Nevertheless, the SiO₂ thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ～400°C), silicon diffusion towards the surface is promoted and a thicker SiO₂ layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.