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.     

Quantitative auger electron spectroscopy analysis of AgPd and NiPd alloys

The relationship between peak amplitude in derivative Auger spectra and alloy composition was investigated for AgPd and NiPd alloys. Measurements were performed on polished and sputter etched samples as well as on samples fractured in vacuo. For clean alloy surfaces and in the absence of sputtering, a linear relationship between peak amplitude of a given alloy component and atomic concentration was observed. Relative Auger peak amplitudes measured on sputter cleaned surfaces were different from those measured on fractured surfaces. Results were interpreted in terms of a simple model considering surface enrichment of the alloy component with the lower relative sputtering yield.     

Preferred sputtering on binary alloy surfaces of the AlPdSi system

Auger spectroscopy has been used to investigate the behavior of preferred sputtering on surfaces of homogeneous AlPd, SiPd and AlSi alloy films. These combinations of alloys were chosen for studying the effects of mass and bonding differences on preferred sputtering. Experiments have been carried out using a 1 keV Ar ion beam over a range of alloy compositions. Our results can be summarized as follows: (a) The preferred sputtering of these binary alloys cannot be predicted according to the sputter yields of individual elements, e.g. both Al and Si have been observed to be removed preferentially relative to Pd although pure Pd has a higher sputter yield, (b) In the alloys studied, mass difference appears to dominate over bonding difference in controlling the preferred sputtering behavior since the extent of preferred sputtering of Al and Si relative to Pd is about the same. This observation is interpreted on the basis of the binary alloy sputtering theory formulated by Andersen and Sigmund. (c) Judging from the composition change of the sputtered surface, there is no evidence for formation of compounds with specific compositions as a result of preferred sputtering in the AlPd and SiPd alloys investigated.     

Formation and characterization of Au/Pd surface alloys on Pd(1 1 1)

The formation of alloys by adsorbing gold on a Pd(1 1 1) single crystal substrate and subsequently annealing to various temperatures is studied in an ultrahigh vacuum by means of Auger and X-ray photoelectron spectroscopy. The nature of the alloy surface is probed by CO chemisorption using temperature-programmed desorption and reflection-absorption infrared spectroscopy. It is found that gold grows in a layer-by-layer fashion on Pd(1 1 1) at 300 K, and starts to diffuse into the bulk after annealing to above ∼600 K. Alloy formation results in a ∼0.5 eV binding energy decrease of the Au 4f XPS signals and a binding energy increase of the Pd 3d features of ∼0.8 eV, consistent with results obtained for the bulk alloy. The experimentally measured CO desorption activation energies and vibrational frequencies do not correlate well with the surface sites expected from the bulk alloy composition but are more consistent with significant preferential segregation of gold to the alloy surface.     

Quantitative Auger electron spectroscopy analysis with co-evaporated AuCu films

A method for the quantitative Auger electron spectroscopy (AES) analysis by using a co-evaporation technique is extended to the AuCu system following the previous work. The calibration curves for lower Auger energy have peaks at 60 eV for Cu and at 69 eV for Au, and for higher Auger energy peaks at 239 eV for Au and at 920 eV for Cu. It is found that a simple linear relation does not exist in the results for AES measurements and the bulk analysis by atomic absorption spectroscopy (AAS) because of the back-scattering effect and the overlap of the spectra at lower energies in the Au-Cu system. It is also found that the adsorption of oxygen caused by electron beam bombardment has a significant influence on the AES results. The calibration curves obtained after a correction for oxygen adsorption are successfully applied to the determination of the composition at the surface of a sputtered AuCu alloy.     

Backscattering correction for quantitative Auger analysis: II. Verifications of the backscattering factors through quantification by AES

The validity and utility of the backscattering correction factors obtained from Monte Carlo calculations for quantitative analysis by Auger electron spectroscopy (AES) were examined through practical quantification of surface concentrations of binary alloys. Quantifications were attempted, first, to access the surface composition of a sputter-deposited NiPt layer, which is probably the most appropriate test-sample with known surface composition for surface analysis. The quantification by AES has led to the result that the surface composition of the layer agrees well with the bulk composition of the sputtered NiPt alloy, as expected. The composition of a sputtered AuCu alloy surface was, then, examined according to the same correction procedure as for the NiPt layer, leading to the confirmation that no preferential sputtering is observed for AuCu alloys by AES as Färber et al. reported.     

The adsorption of acetic acid on clean and oxygen-covered Au/Pd(100) alloy surfaces

The adsorption of acetic acid is studied on clean and oxygen-covered Au/Pd(100) alloys as a function of gold content by temperature-programmed desorption and reflection–absorption infrared spectroscopy. Au/Pd(100) forms ordered alloys such that, for gold coverages above ~ 0.5 monolayers, only isolated palladium atoms surrounded by gold nearest neighbors are present. Predominantly molecular acetic acid forms on Au/Pd(100) alloy surfaces for gold coverages greater than ~ 0.56 ML, and desorbs with an activation energy of ~ 59 kJ/mol. Heating this surface also forms some η¹-acetate species which decompose to form CO and hydrogen. On alloy surfaces with palladium–palladium bridge sites, η¹-acetate species initially form, but rapidly convert into η²-species. They thermally decompose to form CO and hydrogen, with a small portion rehydrogenating to form acetic acid between 280 and 321 K depending on gold coverage. The presence of oxygen on both Pd(100) and Au/Pd(100) alloys facilitates acetate dehydrogenation so that only η²-acetate species form on these surfaces. The presence of oxygen also serves to stabilize the acetate species.     

Electron excited Auger line shape study of ion-beam-mixed Pd–Au alloys

The electronic structure of the ion-beam-mixed Pd–Au alloys have been studied using valence band spectra of XPS and electron excited CVV–Auger spectra. To show the relationship between the electronic structure changes and the Auger spectral line shape, the data of the self-convolution of the partially weighted valence band spectra was compared with the Auger spectra of Pd–Au alloys. The Pd–Au alloy is one of the systems which both atomic and band-like contributions are evident in the Auger spectral line shape. Since the self-convolution of PDOS’s relates to the band-like part of Auger spectra, in Pd–Au alloys, the band-like structure in the Auger line shape can be classified by the self-convolution of the partially weighted valence band spectra. Finally, we found that the increase in peak size at ∼80 eV with the increase in Pd content is due to the band-like contribution in the Au N_6,7VV Auger line shape.     

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.     

Surface composition of PdAu and PdAg catalysts by auger electron spectroscopy

Auger electron spectroscopy (AES) has been employed to examine the metal surface composition of PdAu and PdAg alloys as microspheres and as alumina-supported crystallites. For the PdAu system the observed PdAu ratios at the surface correspond closely to those of the bulk both for the microspheres and crystallites. However in the case of supported PdAg, the surface exhibits silver-enrichment relative to the bulk. By means of the regular solution monolayer model the results are interpreted theoretically and the binding energies between the dissimilar metal atoms are computed.     

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.     

ISS measurement of surface composition of AuCu alloys by simultaneous ion bombardments with Ar+ and He + ions

Surface composition of Au-Cu(43 at%) alloy under 1.5–5 keV argon ion bombardment has been investigated by ion scattering spectroscopy (ISS). In this experiment, we adopted a specific technique to use mixed He⁺ and Ar⁺ ions as primary beam in order to perform sputtering (Ar⁺) and ISS measurement (He⁺) simultaneously. The outermost atom layer of Au-Cu alloys under Ar⁺ ion bombardment is Au-rich leading to the conclusion that Ar⁺ ion bombardment of AuCu alloys causes the preferential sputtering of Cu atoms, resulting in a Au-rich outermost atom layer and a depletion layer of Au atoms beneath the outermost atom layer due to ion-beam-enhanced surface segregation. This result explains the experimental results obtained by AES as well.     

Angular distributions of Au and Cu atoms sputtered from Au-Cu alloys by keV Ar+ ion bombardment

Angular distributions of Au and Cu atoms sputtered from Au-Cu alloys under 3 keV AR⁺ ion bombardment were measured to understand the preferential sputtering. The surface composition of sputter-deposited Au-Cu films on substrates mounted at different ejection angles was analyzed by Auger electron spectroscopy and electron probe microanalysis. Although the result indicated that the proportion of sputtered Cu atoms to the Au atoms in the Au-Cu alloy depends on the ejection angle, marked enhancement of the lighter component in the direction normal to the surface has not been observed in spite of the larger mass ratio of the constituent atoms of the Au-Cu alloy.     

The adsorption and reaction of vinyl acetate on Au/Pd(100) alloy surfaces

The surface chemistry of vinyl acetate monomer (VAM) is studied on Au/Pd(100) alloys as a function of alloy composition using temperature-programmed desorption and reflection–adsorption infrared spectroscopy. VAM adsorbs weakly on isolated palladium sites on the alloy with a heat of adsorption of ~ 55 kJ/mol, with the plane of the VAM adsorbed close to parallel to the surface. The majority of the VAM adsorbed on isolated sites desorbs molecularly with only a small portion decomposing. At lower gold coverages (below ~ 0.5 ML of gold), where palladium–palladium bridge sites are present, VAM binds to the surface in a distorted geometry via a rehybridized vinyl group. A larger proportion of this VAM decomposes and this reaction is initiated by CO bond scission in the VAM to form adsorbed acetate and vinyl species. The implication of this surface chemistry for VAM synthesis on Au/Pd(100) alloys is discussed.     

Auger study of preferred sputtering on binary alloy surfaces

Auger Electron Spectroscopy has been used to investigate the preferred sputtering behavior on homogeneous Cu/Ni alloy surfaces. Measurements were made on a range of alloy compositions with Ar⁺ sputter ions of 0.5 to 2 keV energy. A kinetic model has been formulated to describe the time variation of the surface composition during sputtering. Based on this model, we were able to determine the individual sputter yields for Cu and Ni atoms in the alloy and the depth of the surface layer where the composition is altered by sputtering. The sputter yields were found to be relatively independent of the alloy composition but increased almost linearly with energy. The depth of the altered layer was comparable to the Auger sampling depth with its value increasing from 10 Å to more than 20 Å when ion energy increased from 0.5 to 2 keV.     

The surface composition of the silver—Gold system by Auger electron spectroscopy

The surface composition of polycrystalline AgAu alloy foils is studied by Auger electron spectroscopy. The intensities of principal Auger peaks over a wide range of electron energies are obtained as a function of alloy bulk composition for the clean equilibrated surfaces. Their ratios are compared with ratios expected for a surface showing no surface segregation, and for a surface obeying the regular solution monolayer model that predicts silver surface segregation. The experimental results give evidence for enrichment of the surface with silver, but to an extent less than predicted by the regular solution model.     

Determination of the surface composition of palladium-nickel alloy film catalysts using Auger electron spectroscopy

Palladium-nickel films evaporated in UHV on room temperature substrates form alloys of good bulk homogeneity as indicated by X-ray diffraction. The average composition of the outermost 3 to 5 atom layers has been determined from the intensities of the high energy 848 eV nickel and 330 eV palladium Auger electrons. This average composition is in close agreement with the bulk composition determined by X-ray diffraction, X-ray fluorescence and atomic absorption spectrophotometry. If the nickel concentration is determined from the intensity of the low energy 61 eV nickel Auger electrons, when the analysis refers more critically to the first 1 to 3 atom layers, then a surface enrichment of palladium is indicated for all alloy compositions. From the decrease in the relative intensities of the low energy and high energy nickel Auger electrons with increasing palladium concentration it may be deduced that the enrichment of palladium in the first atom layer is higher than in the second and third layers and that a complete monolayer of palladium is formed for bulk concentrations of 65 at% or more. The experimental observations are in qualitative agreement with theoretical predictions of surface composition from bulk thermodynamic data. The palladium-nickel alloys form a range of surface compositions which can be controlled by changing the bulk composition and which are useful for studying catalytic activity as a function of composition. The alloy films are stable under electron irradiation in the AES analysis in UHV but air exposed films analysed in a residual pressure of 1.3 microPa water vapour show a decrease in palladium surface concentration on irradiation indicating a diffusion of nickel to the surface to form an overlayer of nickel oxide.     

Leed and Aes study of the Au/AuPb2 interface

The adsorption of lead on gold at room temperature in UHV conditions has been studied by LEED and AES. We review some of the data obtained on the Au(100), (111), and (110) faces, published elsewhere, and we give some new experimental results on the stepped Au(S) [n(100) × (111)] (with n = 3, 4, 5, and 6) faces. On all these faces, as lead is deposited on the gold substrate it first forms a monolayer of lead, then a compound AuPb₂. Using the LEED and Auger data we give a model of the epitaxy with a layer-by-layer growth mechanism. We propose a model which involves a transition alloy wich forms at the interface Au/AuPb₂. This model is in agreement with the LEED diagrams observed before the one corresponding to bulk AuPb₂. In the case of the epitaxy of lead on gold (100), we calculate the Auger peak-to-peak ] heights of the gold (72 eV) and lead (93 eV) transitions versus coverage. We obtain good agreement with the experimental data, assuming that the first and last layers of the alloy are lead monolayers and diffusion of lead in gold as well as gold in lead.     

High temperature coadsorption study of zirconium and oxygen on the W(100) crystal face

The coadsorption of zirconium and oxygen on W(100) has been studied by Auger electron spectroscopy, low energy electron diffraction, mass spectroscopy, ion sputtering, and work function measurement techniques. Adsorption of zirconium onto W(100) followed by heating in an oxygen partial pressure produces rapid diffusion of a ZrO complex into the bulk and the formation of a tungsten oxide layer. Heating in vacuum causes desorption of the tungsten oxide and segregation of the ZrO complex to the surface. The activation energy for the ZrO bulk-to-surface diffusion is 30 ± 2 kcal/mole. Upon heating in vacuum at 2000 K the composite surface exhibits predominantly a (1 × 1) LEED structure with a room temperature field emission retarding potential work function of 2.67 ± 0.05 eV. The Richardson work function for this unusually thermally stable surface is 2.56 ± 0.05 eV with a pre-exponential of 6 ± 2. The effects of carbon and nitrogen contamination on this low work function ZrOW composite surface are discussed and a structural model for the surface is presented.     

Adsorption of carbon monoxide Au/Pd(1 0 0) alloys in ultrahigh vacuum: Identification of adsorption sites

The adsorption of carbon monoxide is studied on Au/Pd(1 0 0) alloys by means of reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). The alloy was formed by adsorbing a four-monolayer thick gold film on a Pd(1 0 0) substrate and by heating to various temperatures to form alloys with a range of palladium coverages. The alloy was characterized using X-ray photoelectron spectroscopy and the composition of the outermost layer measured using low-energy ion scattering spectroscopy. CO adsorbs on palladium bridge sites only for palladium coverages greater than 0.5 monolayers (ML) suggesting that next-nearest neighbor sites are preferentially populated by palladium atoms. CO adsorbs on atop palladium sites and desorbs at ∼350 K corresponding to a desorption activation energy of ∼117 kJ/mol. However, at lower palladium coverages, these sites are not occupied and CO desorption states are detected 170 and 112 K corresponding to desorption activation energies of ∼53 kJ/mol and ∼35 kJ/mol, respectively, for these states. It is suggested that these states are due to a restructuring of the surface to form low-coordination gold sites that obscure the atop palladium site.