The surface composition of the gold-palladium binary alloy system

The surface composition of the AuPd alloy system has been determined by Auger electron spectroscopy. Measurements were performed on polished polycrystalline alloy foils. After cleaning, the intensities of the 71 eV and 2024 eV gold Auger transitions, and the intensity of the 330 eV palladium transition were measured, and then converted to atom concentrations in the surface layer. The surfaces of the annealed samples were found to be significantly enriched in gold with respect to the bulk. This result disagrees with the regular solution theory prediction. After extensive sputtering of the AuPd alloys with 1.5 keV Ar⁺ ions, a slight surface enrichment with palladium was found, as predicted by the simple theoretical model for sputtering.     

Segregation at CuAu alloy surfaces

Experimental results for Auger measurements on clean evaporated CuAu alloy films having (111)-orientation are presented. Signals from Auger transitions at 72 eV, 239 eV, and 2024 eV from Au in the alloys were normalized to signals from pure Au references. The experimental data were converted to atomic layer compositions using a model which allowed the first two atomic layers to differ in composition from the bulk and using estimates of the energy dependence of the electron attenuation length derived from published results. Significant enrichment of the first layer with Au was found over the entire range of composition studied.     

The surface composition of the nickel-copper alloy system as determined by Auger electron spectroscopy

The Ni-Cu alloys were prepared by evaporation of the specpure metals in UHV onto a quartz substrate. Spectra were obtained from clean as well as from gas covered surfaces. The Auger signal intensity of a monolayer of both metals was determined for the low energy electrons (102–105 eV) and for the high energy electrons (716–920 eV). The overlapping peaks of Cu and Ni in the low energy region (102–105 eV) were evaluated by comparing them with computer simulated alloy spectra. The results of the sintered alloys are interpreted by means of a model by Gallon and Jackson, using the experimentally determined signal intensity of a monolayer. Several surface enrichment data were used to predict the experimentally observed Auger signal intensities. A clear indication of surface enrichment of Cu was obtained; this is in good agreement with previous conclusions based upon hydrogen adsorption and work function measurements. An explanation is suggested why previous work with AES and CO chemisorption did not reveal any surface enrichment.     

A quantitative analysis of surface segregation and in-depth profile of copper-nickel alloys

By utilizing the difference in escape length of the Auger electrons with different energies, a calculation of the in-depth distribution of atomic concentration at the surface is presented on the basis of Palmberg's physical mechanism. Experimental results on clean surfaces of Cu-Ni alloys over the entire composition region with Auger spectroscopy were performed to make the in-depth profile of surface composition caused by annealing. The alloy composition of the first atomic layer at the surface was plotted against the bulk composition, showing significant enric enrichment of Cu atoms in that layer. The results indicated that the segregation takes place in about four atomic layers at the surface.     

An AES study of surface segregation of AgAu alloys with ion bombardment and annealing

Equilibrium segregation and selective sputtering in the surface of AgAu alloys have been investigated systematically with argon ion bombardment and with annealing by means of AES measurements. Slight enrichment of Ag was observed on the alloy surfaces after the annealing of the alloys at 550°C, while relatively large enrichment of Au was observed on the ion-bombarded surfaces with the use of Au (240 eV) and Ag (300 eV) Auger electrons. With the aid of other Auger electrons with different escape lengths, it was found that the concentration varies with distance from the surface within the sampling depth of the Auger electrons. On the basis of the above facts, the depth profiles were proposed for the annealed and the ion-bombarded surfaces. The uppermost surface layer is enriched more with Ag than the apparent AES observations on both the ion-bombarded and the annealed surfaces. The proposed depth profiles on both the surface layers were compared with previous results by different authors.     

Surface composition and depth concentration profile of platinum/tin alloys from combined X-ray photoelectron and Auger spectroscopic data

Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (X-PS) have been combined to study surface enrichment of two platinum/tin alloys, viz. PtSn and Pt₃Sn, effected by reduction in hydrogen at 550°C. By making use of the different information depths of AES and X-PS for this alloy system, as determined by the corresponding mean free paths of inelastic scattering of the emitted characteristic electrons, a rather detailed picture has been obtained of the composition of the topmost atomic layers in these alloys. It is shown that both alloys exhibit phenomena of surface enrichment and depletion of the underlying layers over distances of the order of 10 Å.     

AES studies of palladium films formed on copper and mica

Thin epitaxial films of palladium were grown on epitaxial copper films and cleaved mica in ultra high vacuum. The growth modes of these films were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), transmission electron microscopy (TEM), and TEM replica techniques. Layer by layer growth of Pd on Cu and mica was observed and inelastic mean free paths of Auger electrons for energies of 60 eV (Cu MMM) and 329 eV (Pd MNN) were calculated. These values were 5.7 and 6.9 Å respectively. The thermal stability of monocrystalline and polycrystalline Pd/Cu bilayer films at 483 K was also investigated by AES and TEM. It was found that Pd agglomerates on the Cu at this temperature to form a Stranski-Krastanov growth morphology. The agglomeration is much more rapid on polycrystalline films, suggesting that high surface diffusivity paths (grain boundaries and possibly other defects) enhance the surface diffusion of Pd on Cu.     

Formation of Ti and TiN ultra-thin films on Si by ion beam sputter deposition

Ultra-thin titanium and titanium nitride films on silicon substrate were obtained by ion beam sputtering of titanium target in vacuum and nitrogen atmosphere, using argon ions with energy of 5 keV and 15 μA target current. Elemental composition and chemical state of obtained films were investigated by X-ray photoelectron spectroscopy with using Mg-Kα X-ray radiation (photon energy 1253.6 eV). It was shown that it is possible to form both ultra-thin titanium films (sputtering in vacuum) and ultra-thin titanium nitride films (sputtering in nitrogen atmosphere) in the same temperature conditions. Photoelectron spectra of samples surface, obtained in different steps of films synthesis, detailed spectra of photoelectron emission from Si 2p, Ti 2p, N 1s core levels and also X-ray photoelectron spectra of Auger electrons emission are presented.     

Adsorption of benzonitrile and alkyl cyanides on evaporated nickel and palladium films studied by XPS

The adsorption states of benzonitrile and alkyl cyanides on evaporated nickel and palladium films have been studied by X-ray photoelectron spectroscopy. Two N 1s peaks are observed for adsorbed benzonitrile on nickel; one is located at the same binding energy as that of condensed benzonitrile (399.8 eV) and the other is located at much lower binding energy (397.9–397.6 eV). The former species is seen only at low temperature (170 K) and adsorbs weakly through nitrogen lone pair electrons. The latter species exists even at temperatures as low as ∼ 170 K but predominantly at room temperature. This species seems to adsorb with rehybridization of the CN triple bond, not to be bound through π electrons of the aromatic ring. On palladium, however, there are three kinds of adsorbed states for benzonitrile, giving three N 1s peaks (399.5, 398.0 and 397.4 eV). Two of them are similar to those on nickel and the other is assigned to the species rehybridized to less extent (nearly to sp² hybridization) giving the N1s peak at 398.0 eV. The other nitriles studied in this work show adsorption behavior similar to benzonitrile on nickel and palladium.     

Effet de l'oxydation en surface et en volume sur les spectres de pertes caractéristiques d'énergie d'électrons dans les couches minces de titane

Transmission and reflection electron energy loss spectra, together with electronic diffraction patterns and Auger spectra have been registered on evaporated titanium thin films. The optical, crystallographic and chemical properties for the bulk and the surface of the films are compared. The 25 eV energy loss observed in the spectra of the low-energy electrons seems to give evidence for the formation of a superficial oxide layer. This assumption is confirmed by the transmission energy loss spectrum of pure rutile which is also presented.     

Interaction of oxygen and hydrogen with Pd-Au alloys: An AES and XPS study

Pd-Au alloys of three different concentrations have been studied with Auger Electron Spectroscopy and Photoelectron Spectroscopy. A quantitative method shows that the decontaminated surfaces possess the same composition as the bulk. Interaction of oxygen up to 600°C with gold rich alloys (>85%) is very weak. It starts around 300°C on the other two alloys and results in a significant palladium surface enrichment. On the 60 at% Pd-Au alloy, at 500–580°C, the surface may be completely covered with PdO. A hydrogen treatment at 350°C leads to a complete reduction of the oxide without reequilibration of the surface.     

A combined XPS/AES study of Cu segregation to the high and low index surfaces of a Cu-Ni alloy

Quantitative information was obtained regarding the equilibrium segregation of Cu to the high index surfaces (cut 5° off the (100) plane in the [001] and [011] directions) and low index surfaces ((100), (111) and (110)) of a Cu: Ni, 5: 95% alloy crystal. Data regarding segregation to the (111) surface of a Cu: Ni, 50: 50% alloy crystal was also obtained. Equilibrium surfaces were obtained by careful annealing in the temperature range 850–920 K to avoid Cu loss by evaporation. The surface composition profile was calculated using data from the combination of the X-ray Photo-electron Spectroscopy and Auger Electron Spectroscopy techniques. Using these techniques, a wide range of electron kinetic energies and associated electron escape depths can be probed, yielding information about the topmost layer concentration and about the concentration profile into the bulk. Extensive Cu segregation was observed for the 5% and 50% Cu alloys. Topmost layer compositions of 85–100% Cu were found for both the high and low index surfaces of the 5% Cu alloy. In the next two or three atomic layers the composition was found to drop swiftly to near bulk Cu levels. For atomic layers deeper than this, some experimental evidence suggested a rise in Cu composition over three or four layers to 8–17% Cu before bulk levels were finally regained. For the 50% Cu alloy sample, a topmost layer composition was found of 95–100% Cu. Bulk levels of Cu were regained in about four atomic layers. These results are discussed in relation to other theoretical and experimental studies of segregation in these Cu-Ni alloys. The significance of the measured composition in relation to an equilibrium surface is also considered.     

Surface core level shifts of Pt(111) measured with Y Mχ radiation (132.3 eV)

An Y anode was prepared in situ under UHV conditions and used for core level spectroscopy of Pt 4f levels of a clean and CO covered Pt(111) crystal. The Y Mχ photons of 132.3 eV energy and 0.45 eV half width were sufficiently surface sensitive in order to produce a large fraction of surface to bulk emission. The background of Auger and secondary electrons in the photoemission spectra was subtracted by use of an analogous spectrum excited by Mo Mχ radiation (193.3 eV). The Pt 4f spectra were fitted to Doniach-Šunjić line shapes and decomposed into surface and bulk contributions. The surface core level shift was found to be −0.37 and 0.63 eV for clean Pt(111) and CO covered Pt(111), respectively. The intensity of the clean Pt(111) surface peaks was zero after CO adsorption, hence all surface Pt atoms are affected by CO bonding. The ratio of bulk to surface peak intensity was 2.3. The half width of the CO related surface peak was twice that of the clean Pt surface peak consistent with the presence of two different adsorbed CO species.     

Interactions of BaO with mixed-metal substrates: W---Ir

Surface studies of BaO on W---Ir alloy substrates of different composition show work functions lower for the alloys than on either pure W or pure Ir. In particular, the lowest work function ( 1.8 eV) appears near the 50% W, 50% Ir concentration. Peak height characterizations and energy shifts of interatomic Auger lines indicate an adsorbate-substrate interaction is responsible for the function lowering. The interaction consists of the O of the adsorbed Ba⁺O⁻ molecule receiving electrons from the W and the Ba from the same molecule contributing electrons to the Ir. The resulting increase in surface dipole (as observed from interatomic O 2p and Ba 6s peak heights) is in very good agreement with the actual work function lowering. If the substrate consists of significant “patches” of W and Ir, then the Ba and O from the same molecule cannot interact with the different substrate components. When this happens, neither is the adsorbate-substrate interaction seen in the interatomic Auger spectra nor is any work function-lowering observed. The minimum in work function near 50% W, 50% Ir is the point where the dipole lowering (i.e., charge transfer) is most efficient because at thatconcentration there is one Ir atom available for each Ba atom and one W atom available for each O atom on the surface.     

Low energy Auger and loss electron spectra from magnesium and its oxide

Data have been obtained from Auger and energy loss processes in clean metallic Mg, Mg during stages of oxidation, and UHV cleaved MgO(100) surfaces. Particular attention has been paid to twenty features below 200 eV in the Auger spectra from these surfaces. A comparison of spectra from the metal, oxidised metal surface, and single crystal MgO has enabled estimates to be made of surface charging effects, and the MgO steady state surface potential is found to be near + 10 V above ground. All the Auger features are given assignments, two of which are interfacial processes involving ionic initial states and metallic final states. Several features in the low energy Auger spectrum are attributed to diffraction of true secondary electrons.     

Angular and energy distributions of low energy electrons from backscattered-conversion electron Mössbauer spectroscopy

Angular and energy distributions of backscattered-low energy resonant and nonresonant electrons from iron films are studied both theoretically and experimentally. The results indicate that relatively few emerging resonant or nonresonant electrons are observed/predicted between 600 eV and 50 eV kinetic energy. A significant fraction of the total electron signal emerges, however, below 50 eV, which is shown to result from low energy Auger cascades, low energy shake-off electrons and secondary electrons attributed to higher energy events. The large number of low energy electrons permits relatively short acquisition times (ca. 1h) and enhanced surface sensitivities. They may be especially useful in future depth-deconvolution efforts.     

An electronspectroscopic study of adsorbate structure

The ultraviolet photoelectron spectra of xenon adsorbed on crystals of (100) nickel and (110) iron have been measured as a function of xenon coverage. It is shown that the total current in the xenon peaks is a measure of the xenon coverage but that the attenuation of the electrons from the metal substrate is dependent on the packing density of xenon on the surface. Previous measurements by Auger electron spectroscopy are compared and it is shown that the mean free path of 16 eV electrons from the d-bands of nickel and iron is 2.75 times that of 60 eV Auger electrons. The data allows the calculation according to a slab model of a mean thickness of a xenon monolayer. This is interpreted as a measure of the packing density of xenon localized on metal lattices of varying dimensions. The UPS spectrum for xenon on (110) iron shows a marked broadening of the xenon peaks as compared to (100) nickel. This is interpreted as due to variation in relaxation energy over adsorbate states on the iron surface arising from xenon atoms in offsite positions.     

An Auger and X-ray photoelectron spectroscopic study of sodium metal and sodium oxide

Photoelectron and Auger electron measurements have been made on polycrystalline films of sodium metal evaporated in ultra high vacuum, and on Na₂O produced by in-situ oxidation by dry oxygen. Most of the spectra were recorded using Mg Kα (1254 eV) radiation but excitation by 5 keV electrons or monochromatized Al Kα (1487 eV) X-rays was used for specific purposes. Core and valence electron binding energies, photoionization cross-sections relative to Na 1s, KLL and KLV Auger energies and transition probabilities are reported. Energy losses in the metal and oxide are discussed and the relative intensities of surface and bulk plasmon losses have been used to calculate mean electron escape depths in the metal. When corrections were made for experimental geometry, escape depths of 10 Å at 180 eV and 31 Å at 1200 eV were obtained. An escape depth of 23 Å at 980 eV was obtained by Na 1s-Na K-Auger intensity correlation and this is consistent with the plasmon data. Data on Auger satellite lines are presented and, in particular, evidence has been obtained which indicates that a high energy satellite should not be attributed to a plasmon gain mechanism. Valence band influences on the KLV Auger spectra are discussed with reference to the XPS spectrum and other sources of valence band information. Unexpected structure was found in the KLV spectra of the metal which, pending thorough interpretation, offsets the sensitivity and resolution advantages which these spectra otherwise offer for valence band studies.     

Electron-stimulated desorption of potassium and cesium atoms from oxidized molybdenum surfaces

The electron-stimulated desorption (ESD) yields and energy distributions for potassium (K) and cesium (Cs) atoms have been measured from K and Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appearance threshold for K and Cs atoms is independent of the molybdenum oxidation state and is close to the oxygen 2s level ionization energy of 25 eV. Additional thresholds for both K and Cs atoms are observed at about 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-covered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Cs atoms consist of single peaks. The most probable kinetic energy of atoms decreases in going from cesium to potassium and with increasing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward very low kinetic energies. The data can be interpreted by means of the Auger stimulated desorption model, in which neutralization of adsorbed alkali-metal ions occurs after filling of holes created by incident electrons in the O 2s, Mo 4s or Mo 4p levels.     

Investigation of titanium and titanium hydride by AES and EELS

Effects of hydriding on the Auger spectrum of titanium are investigated and can clearly be distinguished from the changes induced by oxygen contamination of the surface. New transitions occur about 5 eV below all transitions involving metal valence band electrons. Beam induced decomposition of the hydride could be avoided by proper experimental procedure. The composition of the investigated hydride was inferred from the measured bulk plasmon energy shift.