XPS analysis of palladium oxide layers and particles

The thermal reduction of both oxidised palladium foil and SiO₂/Si(100) supported palladium oxide particles, ranging in size fro 3.5 to 13 nm, was investigated with XPS. Equations were derived for the XPS intensities, measured at normal emission angles, of the particles which consisted of a metallic core and an oxidic skin. By applying these equations on the spectra measured after each reduction step, the particle size and the size of the metallic core were calculated. Measurements on palladium foil showed that the oxide layer thickness decreases linearly with the reduction time up to the last monolayer oxide. The reduction rate of the surface oxide is about eight times lower than the reduction rate of the bulk oxide. The growth of the metallic core in palladium oxide particles appeared to be linearly proportional to the surface area. The reduction rate of the smallest particles was comparable to the reduction rate of the surface oxide of the palladium foil. The larger particles behave identical to the palladium foil.     

XPS evidence for band bending at semiconducting oxide surfaces

The magnitude of the surface potential, directly influenced by defect properties and donor or acceptor chemisorption, has been studied using XPS for p-type nickel oxide surfaces. Above 200 W, the KE shifts are independent of X-ray intensity. A simple explanation of the dependence of the surface potential (V_ch) on band bending (V_s) is proposed with preliminary experimental evidence allowing derivation of values of V_s.     

Chemisorption of pyridine and pyrrole on iron oxide surfaces studied by XPS

Adsorption of pyridine and pyrrole on model iron oxide surfaces has been studied by X-ray photoelectron spectroscopy. At room temperature both molecules adsorb with their molecular identity intact. Upon heating above 320 K pyridine decomposes on the surface whereas pyrrole desorbs intact at 345 K. The bonding of pyridine and pyrrole to the surface at room temperature is studied by monitoring the change in measured N(1s) and C(1s) binding energies between gas-phase molecules and the monolayer phase at room temperature. The results suggest that pyridine bonds via electron donation from the non-bonding nitrogen lone pair to the surface while pyrrole bonds via electron donation from a π-bonding orbital to the surface. The differences in surface bonding are interpreted in the context of the concept of Lewis acids and bases applied to surfaces.     

Combined SIMS,AES, and XPS investigations of tantalum oxide layers

Thick layers of tantalum oxide prepared by thermal and anodic oxidation have been studied by combined SIMS, AES, and XPS during depth profiling by 3keV Ar⁺ ion sputtering. The chemical composition of these films is revealed by the OKLL and O 1s signals and by the “lattice valence” parameter determined from the TaO _n ^± intensities. Thus the anodic film consists of a contamination layer, an oxygen-rich reactive interface and a thick homogeneous oxide layer followed by an interface to the Ta metal. The thermal oxide shows an oxygen concentration decreasing with depth and a broad oxide-metal interface. In both cases, carbon contamination (carbide) prevents the application of the valence model to the clean Ta substrate. The sputtering yield of the oxides was found to be 0.6 Ta₂O₅/ion.     

XPS study on the evaporation of gold submonolayers on carbon surfaces

We investigated the early nucleation stages of evaporated gold submonolayers on different carbon surfaces (pristine HOPG, argon-ion irradiated HOPG and amorphous carbon). Gold core-level and valence band spectra were measured by monochromatised X-ray photoelectron spectroscopy (MXPS). The Au 4f spectra for the lowest coverages (0.1 Å equivalent thickness) on irradiated HOPG and amorphous carbon surprisingly exhibited two well-separated doublets. We attribute this phenomenon to a bimodal particle size distribution caused by gold atom pinning at carbon defect sites. Deposition at elevated temperatures (on irradiated HOPG) opens a possibility to grow particles preferentially on defect sites. The influence of carbon surface defects on the cluster morphology was checked by SEM imaging. These results are interesting for future applications as they help to improve control over metal nanodots growth.     

The acid-base properties of the surface of native zinc oxide layers: An XPS study of adsorption of 1,2-diaminoethane

The acid-base properties of native zinc oxide surfaces have been studied using X-ray photoelectron spectroscopy (XPS). The native layers of zinc oxide have been obtained by ageing mechanically polished pure zinc disks in a glass dryer for 1 month. Such a treatment lead to the formation of an unstable oxide layer and dehydroxylation has been observed during storage in vacuum. By following adsorption in ultrahigh vacuum of 1,2-diaminoethane (DAE) several types of active sites have been evidenced. Zinc cations react with the probe molecule following a Lewis acid/base interaction, while the hydroxyl and the carbonate-like species react following a Brønsted acid/base reaction. Although initial interaction via the Brønsted-like mechanisms is favoured, it has been shown that the resulting complexes are not stable. Under vacuum conditions, the adsorbed DAE molecules either partly desorb or modify their interaction mode with the surface to form additional Lewis-like bonded stable complexes. In addition, a cleaning effect of the molecule has been observed which lead to partial removal of the carbonate-like contamination.     

XPS—Investigation of contamination layers

The ratio (d/Λ) of the average thickness of a contamination layer to the mean free path of the photoelectrons is measured by means of a method utilizing variable take-off geometry. A sample holder, which permits a stepwise variation of the take-off angle from 27.5° to 83.5°, is described. The sample investigated consisted of a thin metal film upon a glass substrate chosen in order to provide a smooth and flat surface upon which a hydrocarbon contamination layer was formed during the measurement. The evaluation of d/Λ was carried out using a graphical procedure suggested by Head¹.     

Structure of ultrathin oxide layers on metal surfaces from grazing scattering of fast atoms

The structure of ultrathin oxide layers grown on metal substrates is investigated by grazing scattering of fast atoms from the film surface. We present three recent experimental techniques which allow us to study the structure of ordered oxide films on metal substrates in detail. (1) A new variant of a triangulation method with fast atoms based on the detection of emitted electrons, (2) rainbow scattering under axial surface channeling conditions, and (3) fast atom diffraction (FAD) for studies on the structure of oxide films. Our examples demonstrate the attractive features of grazing fast atom scattering as a powerful analytical tool in surface physics.     

An infrared study of poisoning of hydroxyl layers on oxide catalysts

Spectra are reported for aluminosilicate and zirconium silicate catalysts under fixed conditions; poisoning does not cause alkali metals to replace the hydrogen in the OH groups, and these continue to function largely unaltered. The uptake of small amounts of water shows some differences between poisoned and unpoisoned catalysts.     

Study of the influence of native oxide layers on atomic force microscopy imaging of semiconductor surfaces

We have investigated the influence of the native oxide layer on semiconductor surfaces on the imaging properties of the atomic force microscope operated under ambient conditions by using epitaxial In_1–x Ga _x As layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (001) oriented InP substrates which have been kept under ambient conditions for two years. The thickness and composition of the native oxide layers were studied with ellipsometry and X-ray photoelectron spectroscopy, respectively. Subsequently, the sample surfaces were imaged by means of atomic force microscopy operated in air which revealed terrace structures separated by monoatomic steps. The obtained data were compared with the surface morphology which can be expected from the MOCVD growth process. The results suggest that an accurate study of semiconductor layer growth by atomic force microscopy in air is possible.     

XPS study of the phase transition in pure zirconium oxide nanocrystallites

Pure zirconium oxide nanocrystallites with diameters 6-140 nm are fabricated from ultrafine metallo-organic complexes by thermal hydrolysis at 120 °C and/or heat treatment at 125-1025 °C. X-ray photoelectron spectroscopy shows that effective ionic valence of Zr decreases with decreasing particle diameter. The size dependence of the ionic valences suggests that the phase transition from cubic to tetragonal occurs at an effective Zr valence of 2.0 near 3 nm in diameter and that the phase transition from tetragonal to monoclinic takes place at a critical size of 25 nm diameter with an effective Zr valence of about 2.6.     

Spectroscopic ellipsometry study of porous silicon-tin oxide nanocomposite layers

The layer-by-layer distribution of components in a porous silicon-tin oxide nanocomposite produced by the following three methods is studied by spectroscopic ellipsometry: chemical vapor deposition, atomic layer deposition, and magnetron sputtering. It is shown that, in the nanocomposites fabricated by these methods, SnO _x penetrates to a depth more than 400 nm and is nonuniformly distributed over the porous layer thickness. The nanocomposite prepared by magnetron sputtering followed by heat treatment has the maximum penetration depth and the maximum uniformity of layer-by-layer SnO _x distribution.     

XPS study of tellurium—niobium and tellurium—tantalum oxide systems

Two series of mixed oxides, tellurium—niobium and tellurium—tantalum, have been investigated by X-ray photoelectron spectroscopy (XPS) to test the quantitative aspects of the method and to evaluate the roles of various correction factors. Elemental abundances were calculated by taking into account the photoelectron cross-sections, the escape depths, the presence of compound oxygen peaks, and the proposed sensitivity factors. Agreement between calculated and theoretical values was found to be satisfactory. Residual systematic differences between surface and bulk compositions are interpreted as indicating embedding of high-melting oxides in lower-melting components, for example Nb₂O₅ in TeO₂·3Nb₂O₅, or binary oxides in TeO₂. Doubt has been cast on the existence of the previously described high-temperature phase 2TeO₂ ·Ta₂ O₅. The XPS and X-ray fluorescence (XRF) results obtained here better conform to the stoichiometry TeO₂ · 2Ta₂O₅ or TeO₂ · 3Ta₂O₅.     

A combined LEED,AES and XPS study of the ZnO {0001} polar surfaces: I. LEED,AES and XPS of contaminated surfaces

The {0001} polar surfaces of ZnO single crystals have first been examined after a chemical treatment involving HCl and H₃PO₄ and a 24 hr bakeout at 250 °C. The impurities detected on the (000$\text{1}$)-O surface with AES were carbon, chlorine, phosphorus and to a lesser extent sulphur. On the (0001)-Zn surface, carbon, chlorine and sulphur were the dominant impurities, while the phosphorus signal was less important. These results were confirmed by XPS measurements on frehsly etched surfaces. The AES spectra were recorded as distribution curves N(E). Averaging, curve-fitting and related numerical techniques were used to obtain high resolution spectra, enabling the identification of the phosphorus L₁-transitions. The etched surfaces were cleaned progressively using argon ion bombardment and ohmic heating. It has been consistently observed that the clean surfaces exhibit primitive (1 × 1) structures. Superstructures such as $\text{(}\text{3}\text{×}\text{3}\text{)}$ on the (000$\text{1}$)-O surface, and $\text{(4}\text{3}\text{× 4}\text{3}\text{)}$ and (3 × 3) on the (0001)-Zn surface, were repeatedly observed at discrete spots of contaminated surfaces. A clear correlation with impurities as observed by AES however could not be found. Facetting was observed after prolonged heating.     

Local triboelectricity on oxide surfaces

In triboelectric phenomena, electric charges are transferred when two materials are touched or rubbed together. We present in this paper a study of this effect performed on metallic oxides at the nanometric scale by an Atomic Force Microscope in the resonant mode. We show that following the electrification processes, positive or negative charges can be deposited. From our experimental data, we conclude that the charge transfer results in an equilibrium final state, the occupied states in the gap being “surface states” with large density and spread under the surface along a characteristic distance of about 100 nm. Received 18 March 1998 and Received in final form 8 July 1998