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Valence band studies of Ag and Pd codeposited on Ru(0 0 0 1)
Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 102–106
Valence band studies of Ag and Pd codeposited on Ru(0 0 0 1) L. Bech a , Z. Li b , J. Onsgaard a,∗ a
Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark b Institute of Storage Ring Facilities, University of Aarhus, DK-8000 Aarhus C, Denmark Available online 28 November 2006
Abstract Codeposition of Ag and Pd on the Ru(0 0 0 1) surface has been studied by photoelectron spectroscopy (PES) by recording of valence band (VB) and 3d5/2 core electron energy distribution curves (EDCs). Two- and three-dimensional total film coverages in the temperature range 300–765 K have been characterized. At submonolayer total coverage, a flat, monolayer (ML) thick film with some degree of mixing is formed. A stable surface alloy, with Ag–Pd hybridization states ∼2.5 eV binding energy (BE), is found when Pd is deposited after Ag. A 1 nm thick Ag52 Pd48 film formed by deposition of Pd on a Ag-precovered surface at room temperature exhibits a substantial interdiffusion of Pd and Ag. A stable concentration profile is reached between 550 and 660 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Silver; Palladium; Ruthenium; Surface alloy; Valence bands; Photoelectron spectroscopy
1. Introduction Studies of the electronic and geometrical structures of ultrathin bimetallic films are motivated by the need to obtain an understanding of their alloying conditions in two- and threedimensions, electronics in the nanometer range, sensor potential and catalytic properties. In the present work, the bimetallic samples are produced by codepositing Ag and Pd on a Ru(0 0 0 1) substrate where no interdiffusion between the components of the overlayer and the substrates takes place. Hereby, ultrathin films and surface alloys can be produced. In an earlier report, the core level shifts for two- and three-dimensional bimetallic Pdx Cu1−x and Pdx Ag1−x alloys on Ru(0 0 0 1) were investigated [1]. It was found that a two monolayer (ML) thick Pd overlayer, deposited at 550 K, on Ru(0 0 0 1) precovered with 2 ML Ag exhibits a hexagonal LEED pattern. The valence band (VB) of Ag overlayers on Ru(0 0 0 1) has been investigated earlier by Bzowski et al. [2]. From the valence band changes observed between bulk Ag and Ru-supported Ag film of varying thickness, which include Ag 4d band narrowing with decreasing Ag coverage as well as lack of significant Ag 4dband centroid shifts, they concluded that Ag–Ru d-hybridization is not important.
∗
Corresponding author. Tel.: +45 51255758. E-mail address: [email protected] (J. Onsgaard).
0368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.11.036
Pd and Ag are completely miscible, and the PdAg alloys form fcc lattices, which generally are believed not to posses any short-range order in the bulk [3, and references therein]. There have though been a few results pointing towards a shortrange ordering in the concentration range near Pd60 Ag40 [4]. Also, theoretical calculations point at a tendency towards shortrange order at low temperatures for AgPd alloy compositions Pd50 Ag50 into the Ll1 structure [5]. To our knowledge, no clear evidence for ordering in the PdAg alloy system has been found. The lattice parameter of the PdAg alloy increases almost linearly from the pure Pd value to the pure Ag value when the atomic concentration is varied [3]. The surface energy of Ag (1.1 J/m2 ) is much less than of Pd (2.0 J/m2 ) and Ru (3.4 J/m2 ) [6], and surface segregation is by far driven by the large surface energy difference that strongly favours surface enrichment in Ag. In the case of AgPd alloy (1 1 1) surfaces, a large Ag segregation was e.g. demonstrated by Wouda et al. [4] and by Kuyers and Ponec [7]. Ag and Pd(1 1 1) layers can be grown on each other at room temperature without interdiffusion and with no hybridization of signification between the layers; interdiffusion between the layers initiates at annealing or deposition temperatures around 500 K [8,9]. Concerning the energy distribution of the d states of the pure fcc Pd and Ag metals, the nearly filled Pd d-band is wide, extends above the Fermi level and exhibits a high density of unoccupied states at the Fermi level, whereas the completely filled Ag d-band is more narrow and well-separated from the Fermi level, above which some unoccupied states with 5d character
L. Bech et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 102–106
exist as a result of spd rehybridization of the Ag valence band [10–12]. With respect to the charge redistribution in the AgPd alloys when compared to the pure metals, summarizing crucely over results found in the literature, there is general agreement that the Pd 4d-band is being increasingly filled on the cost of non-d Pd charge and charge at Ag site [4–5,11–15]. Changes that are more pronounced at the respective sites the higher the number of unlike nearest neighbours [5]. There is little agreement concerning the changes in the charge at the Ag site. Lu et al. [5] demonstrated that compression of the fcc lattice of pure Ag to hold the lattice constant of the 50:50 AgPd alloy causes a shift in the bonding Ag states to higher BEs leading to a broadening of the local DOS (LDOS), whereas the similar expansion of pure Pd shifts the bonding Pd states to lower BEs and thus narrows the LDOS. Furthermore, following the effects of alloying, beyond the volume effects, when the number of unlike nearest neighbours increases, the effects listed below are expected [5]: (i) Both the Ag and Pd bands tend to narrow as an effect of the fewer matching states to which electrons can tunnel. (ii) Band repulsion between non-matching Ag and Pd states tends to shift the Ag states towards higher binding energy (BE) and the Pd states towards lower BE, the shifts being most significant when the coupling occurs between energetically close-lying states. (iii) Hybridization between Ag and Pd states causes the formation of a low-BE Ag tail and a high-BE Pd tail of hybridization states. Support for these results is found in a series of experimental investigations of the d-band changes in AgPd alloys [12,14,16]. The average Ag and Pd LDOS calculated by Abrikosov et al. [10] exhibit composition-dependent changes that are consistent with the result by Lu et al. [5]. This paper is organized as follows: in Section 2, the experimental conditions are given, Section 3 deals with results and discussion of VB measurements of coadsorped Ag and Pd on Ru(0 0 0 1), subdivided in Sections 3.1 and 3.2 according to the total coverages, 0.83 and 4 ML, respectively. The conclusion is presented in Section 4.
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order to prevent CO from sticking to Ru and Pd. The evaporation sources, with Ag and Pd, were calibrated by using the photoelectron intensities, BE-shifts and FWHM, as explained below. Low-energy electron diffraction was used to ensure the homogeneity and structure of the thin films. The photon beam incidence angle was 50◦ relative to the crystal surface and the spectra were collected with the electron emission parallel to normal of the surface. Highly resolved 3d5/2 core level spectra of Ru, Pd and Ag were recorded with photon energies of 350, 400 and 437 eV, respectively, and a total instrumental resolution of better than 200, 250 and 300 meV, respectively. The analysis and fitting of the core level spectra of Pd 3d5/2 and Ag 3d5/2 were based on a Doniach–Sunjic line shape convoluted with a Gaussian distribution function [17,18]. 3. Results and discussion 3.1. Valence band spectra of 0.40 ML Pd/0.43 ML Ag/Ru(0 0 0 1) In the following, the question is addressed whether surface alloying in one layer takes place when approximately one-half a monolayer of Pd is adsorbed on the Ru(0 0 0 1) surface precovered with a comparable coverage of Ag. In order to assess the coverages of the two components, Ag and Pd, individual growth curves were measured by recording the intensities of the Ag and Pd 3d5/2 core electrons as a function of the metal deposition time. As an example, the growth of Ag on Ru(0 0 0 1) at 550 K is shown in Fig. 1 with Ag 3d5/2 photoelectron spectroscopy (PES) spectra representative for different coverages. The deposition rate was 0.12 ML/min. A plot of the intensity evolution with coverage exhibits a linear increase until a kink appears, after which the linear increase holds a slope of only 20% of the initial slope. This kink is assigned to the onset of growth of the second layer and the corresponding coverage is denoted 1 ML. The uncertainty in the coverage assignment is estimated to be around ±5%. For the submonolayer growth curve, the Ag 3d5/2 BE is constant 367.91 eV in good agreement with the Mg K␣ XPS
2. Experimental The experiments were performed in an UHV chamber (base pressure < 5 × 10−10 T) attached to the SGM1/Scienta beamline at the storage ring ASTRID at Aarhus University. The beamline consists of a spherical grating monochromator and a surface science end-station equipped with a hemispherical Scienta SES200 electron energy analyzer and a channelplate as a detector. Cleaning of the Ru(0 0 0 1) sample was carried out by cycles of Ar+ sputtering followed by annealing to 1300 K. Overlayers of Ag were prepared by evaporation from an electron bombarded crucible and Pd was evaporated from a tip of a thin Pd wire heated by direct electron bombardment. Typically, the depositions were carried out with the Ru substrate kept at 550 K in
Fig. 1. Ag 3d5/2 PES spectra for a growth sequence of Ag on Ru(0 0 0 1) at 550 K. The deposition rate was 0.12 ML/min and the photon energy was 437 eV.
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results presented by Rodriguez[19], who found the BE to be 367.90 eV at coverages between 0.3 and 1.0 ML. The constant BE reflects that the Ag adatoms are nucleated in large islands. Based on growth curves the shift in the Ag 3d5/2 peak, when going from monolayer thick films to a 2 ML Ag coverage, is estimated to be 0.14 ± 0.10 eV. The core level shifts of Ag 3d5/2 have been discussed and compared to theoretical results in Ref. [1]. In the case of Pd, the observed Pd 3d5/2 BE and FWHM evolution during Pd growth gives a simple method for coverage calibration, Ref. [20]. Both are constant at initial Pd coverages carried out at 550 K. With increasing exposure, the Pd 3d5/2 line starts to broaden and shifts towards lower BE. These effects are explained by the appearance of a new component at lower BE, which was ascribed to Pd atoms in the second layer. Accordingly, the onset of the FWHM increase with Pd exposure is ascribed to the onset of second layer growth, and the corresponding Pd coverage was appointed to 1 ML [20]. Also, for the Pd/Ru(0 0 0 1) interface, a good agreement between theory and experiments was obtained [1]. The evolution of the valence band spectra of a AgPd film with a total coverage of 0.83 ML and the composition Ag52 Pd48 was followed during preparation and annealing of the film, see the upper part of Fig. 2. The valence band was probed by the use of a 40 eV photon energy and a total instrumental resolution of 0.1 eV. All of the spectra were recorded in normal emission geometry and with an acceptance angle of ±9◦ . The VB PES spectra from below are recorded for a clean Ru(0 0 0 1) surface, after deposition of 0.43 ML Ag at 550 K, after further deposition of 0.40 ML Pd at the same temperature. Then follows, as the fourth spectrum from below, the spectrum recorded after subsequent annealing for 2 min at 660 K and finally, the spectrum obtained after annealing for 2 min at 765 K. The VB of pure Ru(0 0 0 1) has been treated extensively in literature, i.e. [21, and references therein], and the present spectrum shows the same characteristics. Several changes in the VB are observed when 0.43 ML Ag is present on the surface. The dominant 2.5 eV peak from the Ru substrate is reduced and new features related to the Ag layer evolve at 0.15 eV and between 4 and 8 eV. Even at this Ag coverage, two peaks at 4.85 and 6.30 eV, respectively, dominate the spectrum. They originate from levels with mainly Ag 4d5/2 and 4d3/2 character. In order to emphasize the spectral changes, a difference spectrum is shown in the lower part of Fig. 2, curve (i). The spectrum is obtained by subtraction of the substrate spectrum from the 0.43 ML Ag spectrum scaled down by a factor of 0.79. This factor corresponds to the relative intensity of the substrate signal when attenuated by a 2D overlayer of 0.43 ML as derived using an attenuation length of 1.5 ML. It should be stressed that only an unaltered Ru valence band contribution is eliminated this way. Besides the Ag 4d levels, a peak at the Fermi level is significant, peaking at 0.15 eV that is assigned to a Ag/Ru(0 0 0 1) interface state. The present results corrobate the general trends of the Ag 4d band narrowing, split reduction, shift in the Ag 4d3/2 feature towards lower BE and lack of shift of the Ag 4d5/2 feature upon decreasing Ag coverage previously demonstrated and explained by Bzowski et al. in their comprehensive study of Ag/Ru(0 0 0 1) valence
Fig. 2. Upper figure: Valence band PES spectra of the 0.40 ML Pd/0.43 ML Ag/Ru(0 0 0 1) interface obtained with a photon energy of 40 eV. From below, the spectra are recorded from a clean Ru(0 0 0 1) surface, after deposition of 0.43 ML Ag and further deposition of 0.40 ML Pd, both at a temperature of 550 K. Subsequently, annealing at 660 and 765 K result in the upper two spectra. The dashed lines represent the preceding spectrum. Lower figure: Difference spectra obtained from the spectra presented in the upper part of the figure. Spectra (i), (ii) and (iii)* are obtained from the Ag/Ru, Pd/Ag/Ru and the Pd/Ru spectra, respectively, by subtraction of the pure Ru(0 0 0 1) substrate spectrum after suitable scaling. The dashed curve, number 2 spectrum from below, represents the sum spectrum (i) + (iii)* ; see text for details.
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bands [2]. The effects were attributed to the fewer noble-metal neighbours relative to bulk metals. Deposition of 0.40 ML Pd on the 0.43 ML Ag/Ru(0 0 0 1) interface at 550 K induces significant characteristic changes in the valence band spectrum as demonstrated by the third, full drawn spectrum in Fig. 2, upper part. For comparison, the preceding spectrum is displayed with a dashed line. It is clearly seen that the Ag 4d band component is significantly narrowed at its high-BE side when Pd is co-adsorbed. As judged on the basis of these two spectra, the Ag 4d high-BE part shifts approximately 0.5 eV towards lower BE due to the interaction with the Pd. The bimetallic interaction causes a change in the relative intensity of the high-BE part and the low-BE part of the Ag 4d band. The net effect on the Ag 4d band component intensity is a 19% reduction. It is illustrative to analyze the other difference spectra displayed in the lower part of Fig. 2. By subtraction of the pure Ru(0 0 0 1) substrate spectrum, scaled down by a factor of 0.80, from the VB spectrum of the Pd(0.40 ML)/Ru(0 0 0 1) interface, the upper difference spectrum, (iii)* , in Fig. 2 is obtained. It clearly shows that the influence of Pd is confined to the low-BE regime. The spectrum denoted by (ii) is obtained from the Pd(0.4 ML)/Ag(0.43 ML)/Ru(0 0 0 1) spectrum by subtraction of the pure Ru substrate spectrum. The third spectrum from below, (ii) − (i), is obtained by subtraction of the difference spectra depicted below. The marked difference between the expected pure Pd overlayer contribution, (iii)* , and the changes induced by the Pd adsorption at the Ag/Ru(0 0 0 1) interface, (ii) − (i), demonstrates a significant modification of the Pd states when co-adsorbed with Ag. The electrons that for pure Pd submonolayers populate the ∼1.2 eV interface state characteristic for the Pd/Ru(0 0 0 1) interface shift to higher BE as a consequence of the coadsorption of Ag. The shift demonstrates that the Pd–Ru interaction is modified by the co-adsorbed Ag. In order to highlight the VB changes induced by the adsorption of Pd on the Ag/Ru(0 0 0 1) interface, the difference spectrum (ii) is shown together with the sum spectrum (i) + (iii)* , which is depicted as the dashed curve. At around 7 eV, a significant Ag-related intensity is observed when the Ag-deposit is not allowed to interact with Pd, whereas the overlayer-induced additional intensity at ∼7 eV is reduced to a background level at the Pd/Ag/Ru interface. With respect to the 0.15 eV feature, its intensity is clearly reduced by the deposition of Pd. That may be caused by a Pdinduced modification in the Ag–Ru interaction, with the loosely bound electrons of the interface state participating in the AgPd bond formation. Nearly no changes are induced in the valence band spectra when the interface is annealed at 660 and 765 K, indicating that the alloying mechanisms and the overall electronic structure in the supported overlayer do not change substantially. 3.2. Valence band spectra of 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1) Valence band spectra recorded in connection with the investigations of the 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1)
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Fig. 3. Valence band PES spectra of the 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1) interface obtained with a photon energy of 40 eV. Starting from below with the spectrum from pure Ru(0 0 0 1), Ag/Ru, Pd/Ag/Ru, and subsequently the spectra after annealing at increasing temperatures. The upper curve with features at 8 and 11 eV BE shows accumulation of CO on Pd/Ru after a period of 45 min.
interface are presented in Fig. 3. Along with some of the spectra, the spectrum shown just below is reproduced as a dashed curve, which serves to emphasize the changes induced by the latter manipulation. At the 2.05 ML Ag coverage, the Ru substrate contribution is no longer recognizable, illustrating the surface sensitivity of the measurements along with complete covering of the substrate with Ag. The dominating features at 4.85 and 6.30 eV are attributed to levels with mainly Ag 4d5/2 and 4d3/2 character, respectively. Both peaks have a shoulder at around 4.4 and 7 eV, respectively. A measure for the band width of the total Ag 4d complex is the energy separation between the inflection points of the steeply increasing intensity at the band edges. It is estimated to 3.2 eV. Between the steep onset of the Ag 4d5/2 peak at 4 eV and the Fermi level, the intensity buckles. The state at 0.15 eV is assigned as an interface state with d-like electrons induced at the Ag adatoms as a response to the adsorbate–substrate interaction, suggesting hybridization between the Ag and Ru valence bands. Addition of 1.8 ML Pd introduces significant intensity and structure in the BE region below 3.6 eV that is attributed to mainly reflect the Pd 4d band. The Ag 4d band low-BE component shifts to even lower BE, whereas the less dominant high-BE peak turns into a shoulder-like structure centered around lower BE. Also a significant narrowing at the high-BE side of the Ag 4d band is observed. These changes resemble the effects generally expected when Ag alloys with Pd. Similar effects do in contrast not appear when Pd layers grow without interdiffusion on Ag(1 1 1), in which case, the Ag-related states hardly change [22]. Accordingly, the changes in the Ag 4d
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band component support that interdiffusion between the Pd and Ag layers has started already at the 300 K Pd deposition temperature. The changes involve a considerable, around 75%, intensity reduction consistent with attenuation of the Ag-related signal of regions covered by Pd and a minor effect related to AgPd interactions. The Pd exposure causes the position of the dominant low-BE Ag 4d band peak to shift to lower BE. From a bottom up perspective, the changes induced in the Ag 4d band region by the Pd exposure resemble the effects observed when Pd is codeposited and alloys with Ag at Ru(0 0 0 1) at the submonolayer coverage. The similarities include narrowing and relative enhancement of the high-BE part of the Ag 4d5/2 peak towards lower BE. The Pd-induced intensity observed below 3.6 eV upon the Pd exposure differs on the other hand considerably from the submonolayer coverage case. In the former case, the Pd intensity proceeds all the way to the Fermi edge. This difference correlates with the trend observed for pure Pd when the film thickness exceeds one monolayer. This supports that the differences observed in the respective valence band spectra recorded at the 4 ML AgPd film with respect to the Ag and Pd 4d bands observed in the relevant pure metal overlayers are related to alloying between Ag and Pd. Annealing up to 550 K is clearly observed to enhance the signal in the region dominated by the Ag 4d band consistent with the upwards transport of Ag atoms and concomitant downwards transport of Pd realized from the core level investigations presented in Ref. [1]. Further annealing at 660 K enhances the Ag 4d band region further whereas the intensity changes within the Pd 4d band region are scarcer. Finally, the removal of Ag by annealing at 870 K causes a substantial enhancement in the Pd 4d band region. The highest degree of mixing between Ag and Pd is observed after annealing at 435 K. Upon annealing at higher temperatures, the continued upwards–downwards AgPd exchange tends to separate the film into a strongly Ag-enriched surface region and Pd-enriched interfacial region. Ag desorption has started at 765 K and completes upon annealing at 870 K. 4. Conclusion The analysis of the present results concerning the 0.40 ML Pd/0.43 ML Ag/Ru(0 0 0 1) interface leads to the following conclusions: • The deposited metals form monolayer thick AgPd islands with a high degree of mixing and alloying between Ag and Pd. • The formed surface alloy is stable up to ∼765 K where Ag starts to desorb. • The alloy-induced modifications of the states associated with Ag resemble the effects found in bulk alloys of Ag and Pd and they seem little affected by the substrate.
• Upon codeposition of Pd and Ag, a significant narrowing of the high-BE side of the Ag 4d band takes place. Intensity enhancement at the high-BE part of the Ag 4d band component on the cost of intensity at its low-BE part is observed. • A strong effect of the Ag codeposit on the Pd-related states is evident from the difference between the overlayer contributions induced in the 0–4 eV BE region by Pd adsorption at the Ag-precovered Ru when compared to the contributions expected for a pure Pd deposit. Concerning the 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1) interface, the main conclusions are: • Interdiffusion and alloying between Ag and Pd take place. • As the interface is gradually annealed to 550 K, Ag atoms move upward and Pd atoms downward in the film and a rather stable concentration profile is reached. Ag desorption has started at 765 K and completes upon annealing at 870 K. • The negative shifts in the high-BE edge of the Ag 4d band is a good indicator for the degree of mixing between Ag and Pd in thin films. References [1] W. Olofsson, L. Bech, T.H. Andersen, Z. Li, S.V. Hoffmann, B. Johansson, I.A. Abrikosov, J. Onsgaard, Phys. Rev. B 72 (2005) 75444. [2] A. Bzowski, T.K. Sham, R.E. Watson, M. Weinert, Phys. Rev. B 51 (1995) 9979. [3] O.M. Løvvik, R.A. Olsen, J. Alloys Compd. 330–332 (2002) 332. [4] P.T. Wouda, M. Schmid, B.E. Nieuwenhuys, P. Varga, Surf. Sci. 417 (1998) 292. [5] Z.W. Lu, S.-H. Wei, A. Zunger, Phys. Rev. B 44 (1991) 10470. [6] L.Z. Mezey, J. Giber, Jpn. J. Appl. Phys. 21 (1982) 1565. [7] F.J. Kuyers, V. Ponec, J. Catal. 60 (1979) 100. [8] J.A. Rodriguez, D.W. Goodman, Science 257 (1992) 897. [9] U. Bardi, Rep. Prog. Phys. 57 (1994) 939. [10] I.A. Abrikosov, W. Olovsson, B. Johansson, Phys. Rev. Lett. 87 (2001) 176403. [11] T.M. Grehk, W. Drube, G. Materlik, J.E. Hansen, T.K. Sham, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 241. [12] I. Coulthard, T.K. Sharm, Phys. Rev. Lett. 77 (1996) 4824. [13] K. Kokko, R. Laihia, M. Alatalo, P.T. Salo, M.P.J. Punkkinen, I.J. V¨ayrynen, W. Hergert, D. K¨odderitzsch, Phys. Rev. B60 (1999) 4659. [14] W. Drube, T.K. Sham, A. Kravtsova, A.V. Soldatov, Phys. Rev. B67 (2003) 35122. [15] P.F. Barbieri, A. De Siervo, M.F. Carazzolle, R. Landers, G.G. Kleiman, J. Electron Spectrosc. Relat. Phenom. 135 (2004) 113. [16] P. Weightman, P.T. Andrews, J. Phys. C: Solid State Phys. 13 (1980) L821. [17] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [18] D. Adams, The program FitXPS (2001) unpublished. [19] J.A. Rodriguez, Surf. Sci. 296 (1993) 149. [20] T.H. Andersen, Z. Li, S.V. Hoffmann, L. Bech, J. Onsgaard, J. Phys.: Condens. Matter 14 (2002) 7853. [21] M. Lindroos, P. Hofmann, D. Menzel, Phys. Rev. B 33 (1986) 6798. [22] G.C. Smith, C. Norris, C. Binns, H.A. Padmore, J. Phys. C 15 (1982) 6481.
Valence band studies of Ag and Pd codeposited on Ru(0 0 0 1) L. Bech a , Z. Li b , J. Onsgaard a,∗ a
Department of Physics and Nanotechnology, Aalborg University, DK-9220 Aalborg East, Denmark b Institute of Storage Ring Facilities, University of Aarhus, DK-8000 Aarhus C, Denmark Available online 28 November 2006
Abstract Codeposition of Ag and Pd on the Ru(0 0 0 1) surface has been studied by photoelectron spectroscopy (PES) by recording of valence band (VB) and 3d5/2 core electron energy distribution curves (EDCs). Two- and three-dimensional total film coverages in the temperature range 300–765 K have been characterized. At submonolayer total coverage, a flat, monolayer (ML) thick film with some degree of mixing is formed. A stable surface alloy, with Ag–Pd hybridization states ∼2.5 eV binding energy (BE), is found when Pd is deposited after Ag. A 1 nm thick Ag52 Pd48 film formed by deposition of Pd on a Ag-precovered surface at room temperature exhibits a substantial interdiffusion of Pd and Ag. A stable concentration profile is reached between 550 and 660 K. © 2006 Elsevier B.V. All rights reserved. Keywords: Silver; Palladium; Ruthenium; Surface alloy; Valence bands; Photoelectron spectroscopy
1. Introduction Studies of the electronic and geometrical structures of ultrathin bimetallic films are motivated by the need to obtain an understanding of their alloying conditions in two- and threedimensions, electronics in the nanometer range, sensor potential and catalytic properties. In the present work, the bimetallic samples are produced by codepositing Ag and Pd on a Ru(0 0 0 1) substrate where no interdiffusion between the components of the overlayer and the substrates takes place. Hereby, ultrathin films and surface alloys can be produced. In an earlier report, the core level shifts for two- and three-dimensional bimetallic Pdx Cu1−x and Pdx Ag1−x alloys on Ru(0 0 0 1) were investigated [1]. It was found that a two monolayer (ML) thick Pd overlayer, deposited at 550 K, on Ru(0 0 0 1) precovered with 2 ML Ag exhibits a hexagonal LEED pattern. The valence band (VB) of Ag overlayers on Ru(0 0 0 1) has been investigated earlier by Bzowski et al. [2]. From the valence band changes observed between bulk Ag and Ru-supported Ag film of varying thickness, which include Ag 4d band narrowing with decreasing Ag coverage as well as lack of significant Ag 4dband centroid shifts, they concluded that Ag–Ru d-hybridization is not important.
∗
Corresponding author. Tel.: +45 51255758. E-mail address: [email protected] (J. Onsgaard).
0368-2048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2006.11.036
Pd and Ag are completely miscible, and the PdAg alloys form fcc lattices, which generally are believed not to posses any short-range order in the bulk [3, and references therein]. There have though been a few results pointing towards a shortrange ordering in the concentration range near Pd60 Ag40 [4]. Also, theoretical calculations point at a tendency towards shortrange order at low temperatures for AgPd alloy compositions Pd50 Ag50 into the Ll1 structure [5]. To our knowledge, no clear evidence for ordering in the PdAg alloy system has been found. The lattice parameter of the PdAg alloy increases almost linearly from the pure Pd value to the pure Ag value when the atomic concentration is varied [3]. The surface energy of Ag (1.1 J/m2 ) is much less than of Pd (2.0 J/m2 ) and Ru (3.4 J/m2 ) [6], and surface segregation is by far driven by the large surface energy difference that strongly favours surface enrichment in Ag. In the case of AgPd alloy (1 1 1) surfaces, a large Ag segregation was e.g. demonstrated by Wouda et al. [4] and by Kuyers and Ponec [7]. Ag and Pd(1 1 1) layers can be grown on each other at room temperature without interdiffusion and with no hybridization of signification between the layers; interdiffusion between the layers initiates at annealing or deposition temperatures around 500 K [8,9]. Concerning the energy distribution of the d states of the pure fcc Pd and Ag metals, the nearly filled Pd d-band is wide, extends above the Fermi level and exhibits a high density of unoccupied states at the Fermi level, whereas the completely filled Ag d-band is more narrow and well-separated from the Fermi level, above which some unoccupied states with 5d character
L. Bech et al. / Journal of Electron Spectroscopy and Related Phenomena 156–158 (2007) 102–106
exist as a result of spd rehybridization of the Ag valence band [10–12]. With respect to the charge redistribution in the AgPd alloys when compared to the pure metals, summarizing crucely over results found in the literature, there is general agreement that the Pd 4d-band is being increasingly filled on the cost of non-d Pd charge and charge at Ag site [4–5,11–15]. Changes that are more pronounced at the respective sites the higher the number of unlike nearest neighbours [5]. There is little agreement concerning the changes in the charge at the Ag site. Lu et al. [5] demonstrated that compression of the fcc lattice of pure Ag to hold the lattice constant of the 50:50 AgPd alloy causes a shift in the bonding Ag states to higher BEs leading to a broadening of the local DOS (LDOS), whereas the similar expansion of pure Pd shifts the bonding Pd states to lower BEs and thus narrows the LDOS. Furthermore, following the effects of alloying, beyond the volume effects, when the number of unlike nearest neighbours increases, the effects listed below are expected [5]: (i) Both the Ag and Pd bands tend to narrow as an effect of the fewer matching states to which electrons can tunnel. (ii) Band repulsion between non-matching Ag and Pd states tends to shift the Ag states towards higher binding energy (BE) and the Pd states towards lower BE, the shifts being most significant when the coupling occurs between energetically close-lying states. (iii) Hybridization between Ag and Pd states causes the formation of a low-BE Ag tail and a high-BE Pd tail of hybridization states. Support for these results is found in a series of experimental investigations of the d-band changes in AgPd alloys [12,14,16]. The average Ag and Pd LDOS calculated by Abrikosov et al. [10] exhibit composition-dependent changes that are consistent with the result by Lu et al. [5]. This paper is organized as follows: in Section 2, the experimental conditions are given, Section 3 deals with results and discussion of VB measurements of coadsorped Ag and Pd on Ru(0 0 0 1), subdivided in Sections 3.1 and 3.2 according to the total coverages, 0.83 and 4 ML, respectively. The conclusion is presented in Section 4.
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order to prevent CO from sticking to Ru and Pd. The evaporation sources, with Ag and Pd, were calibrated by using the photoelectron intensities, BE-shifts and FWHM, as explained below. Low-energy electron diffraction was used to ensure the homogeneity and structure of the thin films. The photon beam incidence angle was 50◦ relative to the crystal surface and the spectra were collected with the electron emission parallel to normal of the surface. Highly resolved 3d5/2 core level spectra of Ru, Pd and Ag were recorded with photon energies of 350, 400 and 437 eV, respectively, and a total instrumental resolution of better than 200, 250 and 300 meV, respectively. The analysis and fitting of the core level spectra of Pd 3d5/2 and Ag 3d5/2 were based on a Doniach–Sunjic line shape convoluted with a Gaussian distribution function [17,18]. 3. Results and discussion 3.1. Valence band spectra of 0.40 ML Pd/0.43 ML Ag/Ru(0 0 0 1) In the following, the question is addressed whether surface alloying in one layer takes place when approximately one-half a monolayer of Pd is adsorbed on the Ru(0 0 0 1) surface precovered with a comparable coverage of Ag. In order to assess the coverages of the two components, Ag and Pd, individual growth curves were measured by recording the intensities of the Ag and Pd 3d5/2 core electrons as a function of the metal deposition time. As an example, the growth of Ag on Ru(0 0 0 1) at 550 K is shown in Fig. 1 with Ag 3d5/2 photoelectron spectroscopy (PES) spectra representative for different coverages. The deposition rate was 0.12 ML/min. A plot of the intensity evolution with coverage exhibits a linear increase until a kink appears, after which the linear increase holds a slope of only 20% of the initial slope. This kink is assigned to the onset of growth of the second layer and the corresponding coverage is denoted 1 ML. The uncertainty in the coverage assignment is estimated to be around ±5%. For the submonolayer growth curve, the Ag 3d5/2 BE is constant 367.91 eV in good agreement with the Mg K␣ XPS
2. Experimental The experiments were performed in an UHV chamber (base pressure < 5 × 10−10 T) attached to the SGM1/Scienta beamline at the storage ring ASTRID at Aarhus University. The beamline consists of a spherical grating monochromator and a surface science end-station equipped with a hemispherical Scienta SES200 electron energy analyzer and a channelplate as a detector. Cleaning of the Ru(0 0 0 1) sample was carried out by cycles of Ar+ sputtering followed by annealing to 1300 K. Overlayers of Ag were prepared by evaporation from an electron bombarded crucible and Pd was evaporated from a tip of a thin Pd wire heated by direct electron bombardment. Typically, the depositions were carried out with the Ru substrate kept at 550 K in
Fig. 1. Ag 3d5/2 PES spectra for a growth sequence of Ag on Ru(0 0 0 1) at 550 K. The deposition rate was 0.12 ML/min and the photon energy was 437 eV.
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results presented by Rodriguez[19], who found the BE to be 367.90 eV at coverages between 0.3 and 1.0 ML. The constant BE reflects that the Ag adatoms are nucleated in large islands. Based on growth curves the shift in the Ag 3d5/2 peak, when going from monolayer thick films to a 2 ML Ag coverage, is estimated to be 0.14 ± 0.10 eV. The core level shifts of Ag 3d5/2 have been discussed and compared to theoretical results in Ref. [1]. In the case of Pd, the observed Pd 3d5/2 BE and FWHM evolution during Pd growth gives a simple method for coverage calibration, Ref. [20]. Both are constant at initial Pd coverages carried out at 550 K. With increasing exposure, the Pd 3d5/2 line starts to broaden and shifts towards lower BE. These effects are explained by the appearance of a new component at lower BE, which was ascribed to Pd atoms in the second layer. Accordingly, the onset of the FWHM increase with Pd exposure is ascribed to the onset of second layer growth, and the corresponding Pd coverage was appointed to 1 ML [20]. Also, for the Pd/Ru(0 0 0 1) interface, a good agreement between theory and experiments was obtained [1]. The evolution of the valence band spectra of a AgPd film with a total coverage of 0.83 ML and the composition Ag52 Pd48 was followed during preparation and annealing of the film, see the upper part of Fig. 2. The valence band was probed by the use of a 40 eV photon energy and a total instrumental resolution of 0.1 eV. All of the spectra were recorded in normal emission geometry and with an acceptance angle of ±9◦ . The VB PES spectra from below are recorded for a clean Ru(0 0 0 1) surface, after deposition of 0.43 ML Ag at 550 K, after further deposition of 0.40 ML Pd at the same temperature. Then follows, as the fourth spectrum from below, the spectrum recorded after subsequent annealing for 2 min at 660 K and finally, the spectrum obtained after annealing for 2 min at 765 K. The VB of pure Ru(0 0 0 1) has been treated extensively in literature, i.e. [21, and references therein], and the present spectrum shows the same characteristics. Several changes in the VB are observed when 0.43 ML Ag is present on the surface. The dominant 2.5 eV peak from the Ru substrate is reduced and new features related to the Ag layer evolve at 0.15 eV and between 4 and 8 eV. Even at this Ag coverage, two peaks at 4.85 and 6.30 eV, respectively, dominate the spectrum. They originate from levels with mainly Ag 4d5/2 and 4d3/2 character. In order to emphasize the spectral changes, a difference spectrum is shown in the lower part of Fig. 2, curve (i). The spectrum is obtained by subtraction of the substrate spectrum from the 0.43 ML Ag spectrum scaled down by a factor of 0.79. This factor corresponds to the relative intensity of the substrate signal when attenuated by a 2D overlayer of 0.43 ML as derived using an attenuation length of 1.5 ML. It should be stressed that only an unaltered Ru valence band contribution is eliminated this way. Besides the Ag 4d levels, a peak at the Fermi level is significant, peaking at 0.15 eV that is assigned to a Ag/Ru(0 0 0 1) interface state. The present results corrobate the general trends of the Ag 4d band narrowing, split reduction, shift in the Ag 4d3/2 feature towards lower BE and lack of shift of the Ag 4d5/2 feature upon decreasing Ag coverage previously demonstrated and explained by Bzowski et al. in their comprehensive study of Ag/Ru(0 0 0 1) valence
Fig. 2. Upper figure: Valence band PES spectra of the 0.40 ML Pd/0.43 ML Ag/Ru(0 0 0 1) interface obtained with a photon energy of 40 eV. From below, the spectra are recorded from a clean Ru(0 0 0 1) surface, after deposition of 0.43 ML Ag and further deposition of 0.40 ML Pd, both at a temperature of 550 K. Subsequently, annealing at 660 and 765 K result in the upper two spectra. The dashed lines represent the preceding spectrum. Lower figure: Difference spectra obtained from the spectra presented in the upper part of the figure. Spectra (i), (ii) and (iii)* are obtained from the Ag/Ru, Pd/Ag/Ru and the Pd/Ru spectra, respectively, by subtraction of the pure Ru(0 0 0 1) substrate spectrum after suitable scaling. The dashed curve, number 2 spectrum from below, represents the sum spectrum (i) + (iii)* ; see text for details.
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bands [2]. The effects were attributed to the fewer noble-metal neighbours relative to bulk metals. Deposition of 0.40 ML Pd on the 0.43 ML Ag/Ru(0 0 0 1) interface at 550 K induces significant characteristic changes in the valence band spectrum as demonstrated by the third, full drawn spectrum in Fig. 2, upper part. For comparison, the preceding spectrum is displayed with a dashed line. It is clearly seen that the Ag 4d band component is significantly narrowed at its high-BE side when Pd is co-adsorbed. As judged on the basis of these two spectra, the Ag 4d high-BE part shifts approximately 0.5 eV towards lower BE due to the interaction with the Pd. The bimetallic interaction causes a change in the relative intensity of the high-BE part and the low-BE part of the Ag 4d band. The net effect on the Ag 4d band component intensity is a 19% reduction. It is illustrative to analyze the other difference spectra displayed in the lower part of Fig. 2. By subtraction of the pure Ru(0 0 0 1) substrate spectrum, scaled down by a factor of 0.80, from the VB spectrum of the Pd(0.40 ML)/Ru(0 0 0 1) interface, the upper difference spectrum, (iii)* , in Fig. 2 is obtained. It clearly shows that the influence of Pd is confined to the low-BE regime. The spectrum denoted by (ii) is obtained from the Pd(0.4 ML)/Ag(0.43 ML)/Ru(0 0 0 1) spectrum by subtraction of the pure Ru substrate spectrum. The third spectrum from below, (ii) − (i), is obtained by subtraction of the difference spectra depicted below. The marked difference between the expected pure Pd overlayer contribution, (iii)* , and the changes induced by the Pd adsorption at the Ag/Ru(0 0 0 1) interface, (ii) − (i), demonstrates a significant modification of the Pd states when co-adsorbed with Ag. The electrons that for pure Pd submonolayers populate the ∼1.2 eV interface state characteristic for the Pd/Ru(0 0 0 1) interface shift to higher BE as a consequence of the coadsorption of Ag. The shift demonstrates that the Pd–Ru interaction is modified by the co-adsorbed Ag. In order to highlight the VB changes induced by the adsorption of Pd on the Ag/Ru(0 0 0 1) interface, the difference spectrum (ii) is shown together with the sum spectrum (i) + (iii)* , which is depicted as the dashed curve. At around 7 eV, a significant Ag-related intensity is observed when the Ag-deposit is not allowed to interact with Pd, whereas the overlayer-induced additional intensity at ∼7 eV is reduced to a background level at the Pd/Ag/Ru interface. With respect to the 0.15 eV feature, its intensity is clearly reduced by the deposition of Pd. That may be caused by a Pdinduced modification in the Ag–Ru interaction, with the loosely bound electrons of the interface state participating in the AgPd bond formation. Nearly no changes are induced in the valence band spectra when the interface is annealed at 660 and 765 K, indicating that the alloying mechanisms and the overall electronic structure in the supported overlayer do not change substantially. 3.2. Valence band spectra of 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1) Valence band spectra recorded in connection with the investigations of the 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1)
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Fig. 3. Valence band PES spectra of the 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1) interface obtained with a photon energy of 40 eV. Starting from below with the spectrum from pure Ru(0 0 0 1), Ag/Ru, Pd/Ag/Ru, and subsequently the spectra after annealing at increasing temperatures. The upper curve with features at 8 and 11 eV BE shows accumulation of CO on Pd/Ru after a period of 45 min.
interface are presented in Fig. 3. Along with some of the spectra, the spectrum shown just below is reproduced as a dashed curve, which serves to emphasize the changes induced by the latter manipulation. At the 2.05 ML Ag coverage, the Ru substrate contribution is no longer recognizable, illustrating the surface sensitivity of the measurements along with complete covering of the substrate with Ag. The dominating features at 4.85 and 6.30 eV are attributed to levels with mainly Ag 4d5/2 and 4d3/2 character, respectively. Both peaks have a shoulder at around 4.4 and 7 eV, respectively. A measure for the band width of the total Ag 4d complex is the energy separation between the inflection points of the steeply increasing intensity at the band edges. It is estimated to 3.2 eV. Between the steep onset of the Ag 4d5/2 peak at 4 eV and the Fermi level, the intensity buckles. The state at 0.15 eV is assigned as an interface state with d-like electrons induced at the Ag adatoms as a response to the adsorbate–substrate interaction, suggesting hybridization between the Ag and Ru valence bands. Addition of 1.8 ML Pd introduces significant intensity and structure in the BE region below 3.6 eV that is attributed to mainly reflect the Pd 4d band. The Ag 4d band low-BE component shifts to even lower BE, whereas the less dominant high-BE peak turns into a shoulder-like structure centered around lower BE. Also a significant narrowing at the high-BE side of the Ag 4d band is observed. These changes resemble the effects generally expected when Ag alloys with Pd. Similar effects do in contrast not appear when Pd layers grow without interdiffusion on Ag(1 1 1), in which case, the Ag-related states hardly change [22]. Accordingly, the changes in the Ag 4d
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band component support that interdiffusion between the Pd and Ag layers has started already at the 300 K Pd deposition temperature. The changes involve a considerable, around 75%, intensity reduction consistent with attenuation of the Ag-related signal of regions covered by Pd and a minor effect related to AgPd interactions. The Pd exposure causes the position of the dominant low-BE Ag 4d band peak to shift to lower BE. From a bottom up perspective, the changes induced in the Ag 4d band region by the Pd exposure resemble the effects observed when Pd is codeposited and alloys with Ag at Ru(0 0 0 1) at the submonolayer coverage. The similarities include narrowing and relative enhancement of the high-BE part of the Ag 4d5/2 peak towards lower BE. The Pd-induced intensity observed below 3.6 eV upon the Pd exposure differs on the other hand considerably from the submonolayer coverage case. In the former case, the Pd intensity proceeds all the way to the Fermi edge. This difference correlates with the trend observed for pure Pd when the film thickness exceeds one monolayer. This supports that the differences observed in the respective valence band spectra recorded at the 4 ML AgPd film with respect to the Ag and Pd 4d bands observed in the relevant pure metal overlayers are related to alloying between Ag and Pd. Annealing up to 550 K is clearly observed to enhance the signal in the region dominated by the Ag 4d band consistent with the upwards transport of Ag atoms and concomitant downwards transport of Pd realized from the core level investigations presented in Ref. [1]. Further annealing at 660 K enhances the Ag 4d band region further whereas the intensity changes within the Pd 4d band region are scarcer. Finally, the removal of Ag by annealing at 870 K causes a substantial enhancement in the Pd 4d band region. The highest degree of mixing between Ag and Pd is observed after annealing at 435 K. Upon annealing at higher temperatures, the continued upwards–downwards AgPd exchange tends to separate the film into a strongly Ag-enriched surface region and Pd-enriched interfacial region. Ag desorption has started at 765 K and completes upon annealing at 870 K. 4. Conclusion The analysis of the present results concerning the 0.40 ML Pd/0.43 ML Ag/Ru(0 0 0 1) interface leads to the following conclusions: • The deposited metals form monolayer thick AgPd islands with a high degree of mixing and alloying between Ag and Pd. • The formed surface alloy is stable up to ∼765 K where Ag starts to desorb. • The alloy-induced modifications of the states associated with Ag resemble the effects found in bulk alloys of Ag and Pd and they seem little affected by the substrate.
• Upon codeposition of Pd and Ag, a significant narrowing of the high-BE side of the Ag 4d band takes place. Intensity enhancement at the high-BE part of the Ag 4d band component on the cost of intensity at its low-BE part is observed. • A strong effect of the Ag codeposit on the Pd-related states is evident from the difference between the overlayer contributions induced in the 0–4 eV BE region by Pd adsorption at the Ag-precovered Ru when compared to the contributions expected for a pure Pd deposit. Concerning the 1.8 ML Pd/2.05 ML Ag/0.15 ML Pd/Ru(0 0 0 1) interface, the main conclusions are: • Interdiffusion and alloying between Ag and Pd take place. • As the interface is gradually annealed to 550 K, Ag atoms move upward and Pd atoms downward in the film and a rather stable concentration profile is reached. Ag desorption has started at 765 K and completes upon annealing at 870 K. • The negative shifts in the high-BE edge of the Ag 4d band is a good indicator for the degree of mixing between Ag and Pd in thin films. References [1] W. Olofsson, L. Bech, T.H. Andersen, Z. Li, S.V. Hoffmann, B. Johansson, I.A. Abrikosov, J. Onsgaard, Phys. Rev. B 72 (2005) 75444. [2] A. Bzowski, T.K. Sham, R.E. Watson, M. Weinert, Phys. Rev. B 51 (1995) 9979. [3] O.M. Løvvik, R.A. Olsen, J. Alloys Compd. 330–332 (2002) 332. [4] P.T. Wouda, M. Schmid, B.E. Nieuwenhuys, P. Varga, Surf. Sci. 417 (1998) 292. [5] Z.W. Lu, S.-H. Wei, A. Zunger, Phys. Rev. B 44 (1991) 10470. [6] L.Z. Mezey, J. Giber, Jpn. J. Appl. Phys. 21 (1982) 1565. [7] F.J. Kuyers, V. Ponec, J. Catal. 60 (1979) 100. [8] J.A. Rodriguez, D.W. Goodman, Science 257 (1992) 897. [9] U. Bardi, Rep. Prog. Phys. 57 (1994) 939. [10] I.A. Abrikosov, W. Olovsson, B. Johansson, Phys. Rev. Lett. 87 (2001) 176403. [11] T.M. Grehk, W. Drube, G. Materlik, J.E. Hansen, T.K. Sham, J. Electron Spectrosc. Relat. Phenom. 88–91 (1998) 241. [12] I. Coulthard, T.K. Sharm, Phys. Rev. Lett. 77 (1996) 4824. [13] K. Kokko, R. Laihia, M. Alatalo, P.T. Salo, M.P.J. Punkkinen, I.J. V¨ayrynen, W. Hergert, D. K¨odderitzsch, Phys. Rev. B60 (1999) 4659. [14] W. Drube, T.K. Sham, A. Kravtsova, A.V. Soldatov, Phys. Rev. B67 (2003) 35122. [15] P.F. Barbieri, A. De Siervo, M.F. Carazzolle, R. Landers, G.G. Kleiman, J. Electron Spectrosc. Relat. Phenom. 135 (2004) 113. [16] P. Weightman, P.T. Andrews, J. Phys. C: Solid State Phys. 13 (1980) L821. [17] S. Doniach, M. Sunjic, J. Phys. C 3 (1970) 285. [18] D. Adams, The program FitXPS (2001) unpublished. [19] J.A. Rodriguez, Surf. Sci. 296 (1993) 149. [20] T.H. Andersen, Z. Li, S.V. Hoffmann, L. Bech, J. Onsgaard, J. Phys.: Condens. Matter 14 (2002) 7853. [21] M. Lindroos, P. Hofmann, D. Menzel, Phys. Rev. B 33 (1986) 6798. [22] G.C. Smith, C. Norris, C. Binns, H.A. Padmore, J. Phys. C 15 (1982) 6481.