Relationship between vibrational states of O-Ni systems and their superficial structure on (100) face of nickel single crystal

High resolution energy loss spectra of 4 eV electrons reflected in the specular direction from Ni(100) surface clean or covered by the ordered structures obtained in the different stages of the metal oxidation, are analysed with reference to LEED patterns. At room temperature, the successive p(2 × 2) and c(2 × 2) structures associated with the chemisorption of oxygen have been observed without modification of the energy loss spectra, in respect of the clean nickel surface. Surface phonons are known to occur in the case of the c(2 × 2)S ordered layer and their absence in the case of Ni-O corresponding system is discussed. After short exposures to oxygen between 200 to 500° C, the surface exhibits a so called “intermediate oxide”. It is identified by its hexagonal unit mesh (～5 Å) with two equivalent orientations along the [100] and [110] directions of the substrate and its vibrational spectra characterized by a loss peak at ? 112.5 meV (± 2.5 meV). Subsequent exposures to oxygen lead to the formation of the (100) face of NiO (in epitaxy on the Ni(100) face) accurately identified by its LEED pattern. The obtained typical multiple loss spectra with spacing 67.5 meV (± 15 meV) reveal a scattering of low energy electrons by long wavelength optical phonons associated to the oxide. The characteristic energy loss (67.5 meV) is in relative good agreement with the energy of the Fuchs-Kliewer surface phonon calculated from the optical constants of the nickel oxide.     

Oxidation of nickel studied by UV and X-ray photoelectron spectroscopy

The nature of argon-ion bombarded nickel surfaces (polycrystalline, and (111), (110) and (100) single crystals) and their subsequent interaction with oxygen at ordinary temperatures have been studied using X-ray and UV photoelectron spectroscopy, including angular variation measurements and the determination of work function changes, in combination in the same apparatus. Variations between the HeI spectra of the four clean substrates were taken to confirm the presence of substantial order within the depth sampled by UPS. The four surfaces exhibited similar but not identical behaviour during oxidation, resembling that reported by other workers from studies of both annealed single crystals and evaporated polycrystalline films. The initial process was deduced to be essentially dissociative chemisorption: no evidence supporting a previous suggestion of associative adsorption at low coverages was found. Oxygen commenced to penetrate below the surface of all samples before oxygen equivalent to a monolayer had been taken up (~10 L exposure) and further substantial uptake followed resulting in the formation of a stable film (~18 Å) of nickel oxide by ~100 L exposure. This oxide layer was not stoichiometric nickel(II) oxide: it was characterized by the presence of two distinct O 1s signals, the relative intensities of which depended on the crystallographic nature of the surface. It is tentatively suggested that the oxygen signal with the higher BE be associated with Ni^III. Comparison of the X-ray and UV spectra suggests that the oxide film is very non-uniform in thickness, some Ni metal remaining very close to the surface.     

Hydrogen chemisorption on Pd(100) studied with he scattering

He scattering from the clean Pd(100) surface yields extremely weak diffraction beams relative to the specular, corresponding to a very small maximum corrugation amplitude of ~ 0.04 Å. Hydrogen adsorption at a temperature of 110 K leads to the formation of a c(2 × 2) ordered phase at a coverage of 0.5 monolayers and a (1 × 1) phase at saturation coverage. The maximum corrugation amplitude of the c(2 × 2)H is ~0.13 Å; surface charge density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens to the topmost Pd layer d_n ? 0.65–0.70 Å corresponding to a H-Pd bonding distance of ~ 2.05 Å in the fourfold hollow sites. The result that the maximum corrugation amplitude of the (1 × 1) hydrogen phase, with ~ 0.025 Å, even smaller than that of the clean surface may indicate a movement of the hydrogens closer to the topmost metal layer, when the coverage is increased from 0.5 monolayers to saturation.     

Low-energy electron diffraction,Auger electron spectroscopy,and thermal desorption studies of chemisorbed CO and O2 on the (111) and stepped [6(111) × (100)] iridium surfaces

The adsorption of CO, O₂, and H₂O was studied on both the (111) and [6(111) × (100)] crystal faces of iridium. The techniques used were LEED, AES, and thermal desorption. Marked differences were found in surface structures and heats of adsorption on these crystal faces. Oxygen is adsorbed in a single bonding state on the (111) face. On the stepped iridium surface an additional bonding state with a higher heat of adsorption was detected which can be attributed to oxygen adsorbed at steps. On both (111) and stepped iridium crystal faces the adsorption of oxygen at room temperature produced a (2 × 1) surface structure. Two surface structures were found for CO adsorbed on Ir(111); a (√3 × √3)R30° at an exposure of 1.5–2.5 L and a (2√3 × 2√3)R30° at higher coverage. No indication for ordering of adsorbed CO was found on the Ir(S)-[6(111) × (100)] surface. No significant differences in thermal desorption spectra of CO were found on these two faces. H₂O is not adsorbed at 300 K on either iridium crystal face. The reaction of CO with O₂ was studied on Ir(111) and the results are discussed. The influence of steps on the adsorption behaviour of CO and O₂ on iridium and the correlation with the results found previously on the same platinum crystal faces are discussed.     

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.     

A photoemission study of the interaction of Ni(100), (110) and (111) surfaces with oxygen

Photoelectron spectroscopic studies of the oxidation of Ni(111), Ni(100) and Ni(110) surfaces show that the oxidation process proceeds at 295 and 485 K in two distinct steps: a fast dissociative chemisorption of oxygen followed by oxide nucleation and lateral oxide growth to a limiting coverage of 3 NiO layers. The oxygen concentration in the 295 K saturated oxygen layer on Ni(111) was confirmed by ¹⁶O(d,p) ¹⁷O nuclear microanalysis. At 295 and 485 K the oxide growth rates are in the order Ni(110) > Ni(111) > Ni(100). At 77 K the oxygen uptake proceeds at the same rate on all three surfaces and shows a continually decreasing sticking coefficient to saturation at ~2.1 layers (based upon NiO). An O 1sb.e. = 529.7 eV is associated with NiO, and O ls b.e.'s of ~531.5 and 531.3 eV can be associated, respectively, with defect oxide (Ni₂O₃) or (in the presence of H₂O) with an NiO(H) species. The binding energies (Ni 2p, O 1s) of this NiO(H) species are similar to those for Ni(OH)₂. Defect oxides are produced by oxidation at 485 K, or by oxidation of damaged films (e.g. from Ar⁺ sputtering) and evaporated films. Wet oxidation (or exposure to air) of clean nickel surfaces and oxides, and exposure of thick oxide to hydrogen at high temperature results in an O 1s b.e. ~531.3 eV species. Nuclear microanalysis ²H(³He,p) ⁴He indicates the presence of protonated species in the latter samples. Oxidation at 77 K yields O 1s b.e.'s of 529.7 and ~531 eV; the nature of the high b.e. species is not known. Both clean and oxidised nickel surfaces show a low reactivity towards H₂O; clean nickel surfaces are ~10³ times less reactive to H₂O than to oxygen.     

Structure of the Ag on Si(111) 7 × 7 interface by means of surface exafs

Polarization dependent surface extended X-ray absorption fine structure (SEXAFS) measurements are used to determine the structure of the Ag on Si(111)7 × 7 system at the early stages (< 3 monolayers (ML)) of interface formation. At room temperature (RT) Ag is found to initially (< 0.5 ML) chemisorb in the threefold hollow site, approximately 0.7 Å above the outermost Si layer with an average Ag-Si distance of 2.48±0.05 Å. Above monolayer coverage the SEXAFS spectrum is dominated by the Ag-Ag distance indicating Ag island formation on the surface. Upon heating (200 ?T? 600°C) a (√3 ×√3)R30° LEED pattern is observed. At the lowest coverage ( < 0.7 ML) this pattern is determined to arise from Ag atoms which are embedded in the threefold hollows, ~ 0.7 Å below the first and above the second Si layer, with a Ag-Si distance of 2.48 ± 0.04 Å. At higher coverage ($\text{\$}\text{?}$1 ML) Ag clusters are found to grow on this interface with the same Ag-Ag distance as in Ag metal. Our results are discussed in the context of previous experimental and theoretical results.     

Mechanism of the transport of nickel along a Si(111) surface in the presence of adsorbed cobalt atoms

The mechanism of the transport of nickel along a Si(111) surface in the presence of adsorbed cobalt atoms is established by LEED and Auger electronic spectroscopy. The mechanism consists of diffusion of nickel atoms through the bulk and segregation of the atoms on the surface during annealing. This mechanism of nickel transport plays the governing role at temperatures below 700°C, where nickel transport along clean silicon surfaces is not observed. It is found that the nickel segregation factor is what determines the lowest temperature at which nickel transport is observed on clean silicon surfaces. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 6, 423–425 (25 March 1999)     

The interference of an electron beam with the surface reaction between oxygen and germanium

Electron beam assisted adsorption and desorption of oxygen was studied by Auger electron spectroscopy (AES). Beam assisted adsorption was observed on clean as well as on oxidized surfaces. After an oxygen exposure of 1000 × 10^?7 Torr min and continuous irradiation with beam voltage of 1.5 kV and beam current density 2 microA mm^?2, the oxygen 510 eV signal amplitude from the point of beam impact was 2.5 times greater than the signal from the non-irradiated region. The Ge 89 eV signal showed a corresponding decrease. Enhanced adsorption occurred at beam energies as low as 16.5 eV. After irradiation, the oxidized surface was not carbon contaminated. Following an oxygen exposure of 30 min at 0.1 Torr and 550°C and subsequent additional beam assisted exposure of 1000 × 10^?7 Torr min, the maximum oxide thickness was about 18 Å. Beam assisted desorption did not occur from thin oxygen layers (0–510 eV signal strength less than 5 units, calculated oxide thickness about 6 Å), but occurred from thick oxides and stopped after the signal amplitude had decreased to 5 units. Based on these results, a model for the structure of the oxygen layer covering the Ge(111) surface is proposed. Mechanisms for adsorption and desorption are discussed. The implications of beam assisted adsorption and desorption on electron beam operated surface measurements (LEED, AES, ELS, APS etc.) are stressed.     

Direct reaction of adsorbed molecular oxygen with hydrazine on a clean Pt(111) surface

The oxidation of hydrazine on the clean Pt(111) surface has been investigated by temperature-programmed reaction spectroscopy (TPRS) in the temperature range 130–800 K. Direct reaction of molecular oxygen is observed on the Pt(111) surface for the first time, as indicated by the desorption of nitrogen beginning at 130 K with a maximum rate at 145 K, below the molecular oxygen dissociation temperature. Direct reaction of hydrazine with adsorbed molecular oxygen results in the formation of water and nitrogen. With excess hydrazine, all surface oxygen is reacted, forming water. When only adsorbed atomic oxygen is present, the low-temperature nitrogen yield decreases by a factor of 3 and the peak nitrogen desorption temperature increases to 170 K. No high-temperature (450–650 K) nitrogen desorption characteristic of nitrogen atom recombination is seen, indicating that during oxidation the nitrogen-nitrogen bond in hydrazine remains intact, as observed previously for hydrazine decomposition on the Pt(111) surface and hydrazine oxidation on rhodium. Two water desorption peaks are observed, characteristic of desorption-limited (175 K) and reaction-limited (200 K) water evolution from the Pt(111) surface. For low coverages of hydrazine, only the reaction-limited water desorption is observed, previously attributed to water formed from adsorbed hydroxyl groups. When excess hydrazine is adsorbed, the usual hydrazine decomposition products, H₂, N₂ and NH₃, are also observed. No nitrogen oxide species (NO, NO₂ and N₂O) were observed in these experiments, even when excess oxygen was available on the surface.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Vibrational excitations and structure of oxygen on Fe(110)

Vibrational spectra of oxygen adsorbed on a clean Fe(110) surface at 300 K have been measured by high resolution electron energy loss spectroscopy (EELS). In the exposure range up to 6 L a single loss around 500 cm^-1 is observed which is interpreted to be due to the stretching vibration of atomic oxygen adsorbed at a 2-fold long-bridge site. Above exposures of 6 L a second loss around 400 cm^-1 appears which is attributed to the formation of a disordered oxide layer. Subsequent heating of the sample leads to the observation of a (5×12) LEED pattern which is explained by a mixed oxygen-iron surface structure which is nearly identical to the (111) face of bulk FeO. A weak loss around 910 cm^-1 appears after oxygen exposures at elevated sample temperatures. This loss is attributed to the formation of bulk oxide.     

Comment on “oscillatory indirect interaction between adsorbed atoms — Non-asymptotic behavior in tight-binding models at realistic parameters” by K.H. Lau and W. Kohn

A sensitive infrared reflectance accessory suitable for the study of surface films on medium size single crystals is described. Oxide films formed on the (100), (110) and (111) crystal faces of aluminum in air, at room temperature, display nearly identical behavior with films approximately 10 Å thick absorbing as a single band near 940 cm^?. After 10⁴ sec at 570 °C, in oxygen, films formed on these crystals begin to display differences in band characteristics and growth kinetics. Between 10⁴ and 4 × 10⁴ sec the rates of growth on the (110) and (111) crystal faces are much greater than on the (100) face. Beyond 4 × 10⁴ sec the growth rate on the (100) face increases while the (110) and (111) growth rates approach zero. Limiting thicknesses reached after 4 × 10⁵ sec approach 3.4 × 10², 2.1 × 10² and 2.2 × 10² Å for (100), (110) and (111) faces, respectively. Oxide compositional differences were reflected by the number and form of the infrared bands after 10⁴ sec of oxidation. After 5 × 10⁴ sec the (100) face oxide was composed of two and possibly three oxide species as evidenced by several bands. Differences in bandwidth and frequency were observed between the (110) and (111) oxide films. The significance of such differences is discussed.     

Electron stimulated oxidation of GaAs,studied by quantitative auger electron spectroscopy

The oxidation properties of the clean polar (111) faces of GaAs, prepared by Ar⁺ bombardment and proper annealing, are investigated. Considering the adsorbed layer as a continuum and using empirical values for the escape depth of the Auger electrons from literature, the coverage of oxygen on these faces is quantitatively determined. For a coverage of up to 10% of a monolayer the sticking coefficients are about 10^?3 for the ($\text{111}$) As face and about 10^?4 for the (111) Ga face, respectively. They decrease rapidly with increasing coverage. The oxidation is strongly stimulated by electron irradiation causing dissociation of the oxygen which is originally adsorbed in molecular form. In this way a compact oxide layer is formed which shows As depletion as a result of sublimation of As₄O₆ and a chemical shift of the Ga Auger peaks is observed. The cross section for the O₂ dissociation is calculated to be 1.8–2.5 Ų depending on electron energy.     

Adsorption of carbon monoxide,oxygen, hydrogen,nitrogen, ethylene and benzene on an iridium (110) surface; Correlation with other iridium crystal faces

The interaction of CO, O₂, H₂, N₂, C₂H₄ and C₆H₆ with an Ir(110) surface has been studied using LEED, Auger electron spectroscopy and flash desorption mass spectroscopy. Adsorption of oxygen at 30°C produces a (1× 2) structure, while a c(2 × 2) structure is formed at 400°C. Two peaks have been detected in the thermal desorption spectrum of oxygen following adsorption at 30°C. The heat of adsorption of hydrogen is slightly higher on Ir(110) than on Ir(111). Adsorption of carbon monoxide at 30°C produces a (2 × 1) surface structure. The main CO desorption peak is found around 230, while two other desorption peaks are observed around 340 and 160°C. At exposures between 250 and 500°C carbon monoxide adsorption yields a c(2 × 2) structure and a desorption peak around 600°C. Carbon monoxide is adsorbed on an Ir(110) surface partly covered with oxygen or carbon in a new binding state with a significantly higher desorption temperature than on the clean surface. Adsorption of nitrogen could not be detected on either clean or on carbon covered Ir(110) surfaces. The hydrocarbon molecules do not form ordered surface structures on Ir(110). The thermal desorption spectra obtained after adsorption of C₆H₆ or C₂H₄ are similar to those reported previously for Ir(111) consisting mostly of hydrogen. Heating the (110) surface above 700°C in the presence of C₆H₆ or C₂H₄ results in the formation of an ordered carbonaceous overlayer with (1 × 1) structure. The results are compared with those obtained previously on the Ir(111) and Ir(755) or stepped [6(111) × (100)] surfaces. The CO adsorption results are discussed in relation to data on similar surfaces of other Group VIII metals.     

Surface crystallography of W(110) and W(110) p (2×1)-O: The position of W atoms

The position of W atoms in the surface layers of clean W (110) and W (110) p (2 × 1)-O is studied using Constant-Momentum-Transfer Averaging of LEED intensities. It is shown that the clean surface is not relaxed to an uncertainty of < 0.06 Å. Analysis of superstructure beam intensity averages from W(110) p (2 × 1)-O indicates that oxygen does not reconstruct W(110) at these coverages and below 1000 ° K. An upper limit of 0.05 Å can be put on the out-of-plane displacement of W atoms by the oxygen. Substrate beam averages from W (110) p (2 × 1)-O verify the non-reconstruction. The use of CMTA for adsorbed-layer crystallography in general is briefly discussed.     

The bonding of oxygen to Al(111)

Oxidation of the Al(111) surface is a two-stage process in which the formation of an ordered oxygen overlayer precedes the creation of a bulk-like amorphous oxide. An electronic structure calculation is reported here for the clean and oxygen-covered Al(111) surface and for bulk A1₂O₃. The calculation uses an atomic-orbital basis and the metal surface is modelled by an infinite two-dimensional crystal, containing seven layers of aluminium atoms. Oxygen atoms occupy three-fold sites, with an Al-O separation of 1.9 Å. The oxygen 2p resonance in the (1 × 1) chemisorbed overlayer is about 3 eV wide, compared to 1.9 eV for an equivalent isolated layer of oxygen atoms unhybridized with the metal. The valence band of A1₂O₃ is about 1.5 eV wider than the chemisorbed oxygen resonance, but in both cases most of the states are concentrated in the top 1.5 eV of the band. The results are related to available ultraviolet photoemission spectra, including the recent angular-resolved spectra of Martinson and Flodström.     

Structural transformations on an oxidized Ag(111) surface

The structure of the Ag(111) surface after the adsorption of molecular oxygen at a temperature of 300 K is studied by low-temperature scanning tunneling microscopy. It is established that local surface oxide is formed at the first stage of adsorption. The subsequent adsorption of O results in the appearance of new objects with a size of 3–8 Å and a height of 1.0–1.5 Å on the Ag(111) surface, which form quasi-ordered structures with increasing degree of coating. The heating of the system obtained up to 500 K leads to a structural transition resulting in the formation of single islands of the p (4×4) phase on the surface. Surface structures are identified by a simulation based on the density functional theory.     

Oxydation de la face (111) d'un monocristal de chrome

Interaction of oxygen with (111) oriented chromium single crystal surface was studied by electron diffraction (LEED and RHEED). From the clean surface, oxygen adsorption induces a $\text{Cr}\text{(111)(}\text{3}\text{×}\text{3}\text{)}\text{R}\text{30°}$ -O structure. No change in the geometry of the LEED pattern occurs with additional oxygen exposure or heating, although the RHEED study denotes the nucleation of rhombohedral oxide on the surface. The orientation relationships with the substrate are determined and compared to those found in the case of (110) chromium surface.     

Passivation of Si (111) surfaces by ordered metal layers

The properties of √3 × √3 ordered gold and silver monolayers on a Si(111) substrate have been investigated by Auger, low energy electron diffraction and photo-emission analysis. It has been found that oxygen adsorption on these surfaces is considerably weaker than on clean Si surfaces. This new result clearly emphasizes the correlation between the oxidation properties of Si atoms and their local environment. A comparison is made with previous results concerning Au-Si amorphous metallic alloys where gold atoms act as a catalyst for the oxidation.