Observations on the d-band width of AuAg and AuCu alloys

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Solid State Communications, Vol. 80, No. 1, PP. 29-32, 1991. Printed in Great Britain.

0038-1098/9153.00+ .00 Pergamon Press plc

OBSERVATIONS ON THE d-BAND WIDTH OF Au-Ag AND Au-Cu ALLOYS T.K. Sham, A. Bzowski, M. Kuhn and C.C. Tyson Department of Chemistry, University of Western Ontario, London, N6A 5B7, Canada (Received 15 May 1991 by G. Burns) The d-band widths of a series of Au-Ag and Au-Cu alloys have been studied with photoemission spectroscopy. The main concern is whether or not the Au d band width is maintained in these alloys. It is found that in concentrate alloys (>50% atomic Au), the alloy d-band width narrows little in both Ag and Cu alloys relative to that of pure Au. At 25% (and less) atomic Au concentration however, the d band widths of the Au alloys in the two hosts are significantly different. While the d-band width of Cu3Au (ordered and disordered) remains the same as that of the pure Au, the d-band width of Ag3Au (disordered) narrows by - l e V . The implication of this observation is discussed in terms of dilution and chemical effects.

The electronic structure of Au alloys has recently become an interesting subject for photoernission studies. This is partly because of the availability of intense synchrotron source in the VUV region and partly because of the interest in whether or not the d partial densities of states of the constituents spread over the entire d band of alloys of overlapping d bands such as Au-Ag and Au-Cu. Photoemission with variable photon energy enables, under favorable circumstances, the partial separation of the partial densities of states of overlapping d-band components by virtue of cross-section differences. Thus this technique may shed some light on the nature of the alloy d band, as has been shown in the case of Cu3Au. 1,2

A related issue which has drawn relatively little attention in the Cu3Au problem is the alloy d-band width which is the same as that of pure Au. It was suggested in a previous analysis6 that Cu 3d - Au 5d mixing in Cu3Au is important in maintaining the width of the d band and that Ag-Au alloys, of which the d bands do have substantial overlap, behave like Cu3Au in that the Au 5d band narrows little upon dilution in silver. In this communication, we report new findings on the d-band widths of Au-Ag and Au-Cu alloys. We find that the Au d band width is nearly maintained only in concentrate Ag-Au alloys (>50% atomic Au); but not in dilute Ag-Au alloys such as Ago.75Auo.25 (henceforth denoted Ag3Au ) where the d band does not behave like Cu3Au. However, alloy d band narrowing in dilute Au-Cu alloys with composition comparable to those of Ag alloys is relatively insignificant. The difference in the d band widths of these alloys are reported here and those of Ag3Au and Cu3Au in particular are discussed in some details.

Recent discussions of this issue have focused on the valence band of Cu3Au(001 ). Eberhardt et. al. reported the Cu3Au(001 ) spectrum which exhibits two distinct regions: an intense band at 2 -4 eV and a doublet at 4 - 8 eV binding energy. On the basis of a d-d repulsion model23, they attributed these features to respectively the well-separated Cu and Au d partial densities of states of the alloy d bands. This notion is supported by a study of g~olycrystalline Cu3Au alloys (ordered and disordered)" which provides further evidence for Cu 3d - Au 5d repulsion. This view is also borne out in the calculation of Davenport et. al However, DiCenzo et. al.5,6 using an analysis based on photoelectron cross-section and electron escape depth, suggested that the Au 5d partial densities of states are more likely to extend over the entire region of the alloy d band. More recent experiment and theoretical results 7,s show that the best description is that the majority of the Au and Cu d partial densities of states do separate but the Au d band also spreads across the entire d band albeit to a much lesser extend than in pure Au.

Photoemission experiments were carried out at the Synchrotron Radiation Centre, University of Wisconsin-Madison using the Canadian grasshopper beamline and a UHV chamber equipped with a Leybold analyser. Fig. 1 shows the angle integrated spectra of the valence band of polycrystalline samples of Cu3Au (disordered), Ag3Au (disordered) and corresponding elements recorded at 60 eV photon energy and mixed polarization. Representative Ag3Au and Cu3Au spectra obtained at various photon energies between 50 and 100 eV are shown in figs. 2 and 3 respectively. The XPS (AI Ka x-ray ) valence band spectra of a series of Ag-Au alloys are shown in fig. 4. From fig. 1, several interesting features are observed. First, both the Cu and Ag d bands overlap 29

30

THE d-BAND WIDTH OF Au-Ag AND Au-Cu ALLOYS l

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entirely with the Au d band in energy; and while the C u d band overlaps with the "d5/2" portion of the Au d band, the Ag d band overlaps with both components of the Au d band. Second, the centroid of the Au d band is less tightly bound than that of the Ag d band as the result of sisnificant spin-orbit coupling in Au. Finally, the perhaps most interesting feature is seen in the d band widths that is that while the d-band width of Cu3Au (5.4 eV, fig. 1) narrows little relative to that of the pure Au, the AgaAu d band width (4.5 eV) narrows significantly (these widths are measured between the points of inflection of the top and the bottom of the observed d band). This last observation is particularly surprising at first glance since it is commonly believed that Au-Ag d orbitals interact strongly and if d-d interaction of overlapping d bands is important in maintaining the Au d-band width in Cu3Au , then the same is expected in Ag3Au. This is not observed however. A trend of d-band narrowing upon dilution of Au in Ag is also found in the XPS spectra (fig. 4). Before we attempt to explain the discrepancy, We look at the photon energy dependent d band intensity and its effect on the experimental widths. It is apparent from figs. 2 and 3 that although there are noticeable intensity changes, the alloy d band width changes little. The intensity variations in figs. 2 and 3 can be explained in terms of experimental d band

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31

THE d-BAND WIDTH OF Au-Ag AND Au-Cu ALLOYS

intensity of the pure metals i°, based on which we can infer from fig. 2 that the major All d states concentrated at higher bindin~ energies li since the Ag/Au intensity ratio increases by a factor of 1.8 from 50 to 100 eV photon energy, and from fig. 3 that the majority of the Au and C u d components are separated.i'2'7'8 In the case that the host metal does not have a d band or d bands do not overlap, dilution is the main mechanism for Au d band narrowing. For example, in a nearest neighbor tight binding scheme for a random alloy, the band width varies as the square root of the number of like nearest neighbors. 15 This is often modified by d rehybridization with sp and neighor/ng atomic states. The effect on the valence band spectrum is that relative to pure Au, the Au d band component in alloys narrows and the centroid of which shifts away from the Fermi level with the "d5/2" component being more sensitive to dilution than the "d3/2" ~omponent which moves relatively little in most cases. 12"I4 This is accompanied by a reduction of the apparent "d5/3" "d3/2" separation (2.7 eV in pure Au to somewhere between 2.7 and 1.5 eV of atomic Au) and a positive core level shift.

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It is not clear at present at what Au concentrations d band nan-owing becomes significant in Au-Ag and Au-Cu alloys. Experimentally, no sj£nificant narrowing is observed for both Ag and Cu alloys at > 50% atomic Au. The alloy d band in Ag3Au is about 1 eV narrower than that in Cn3Au. Noticeable d-band narrowing continues in more dilute Ag-Au alloys (fig. 4) but not in Cu-Au alloys where no siotmificant width narrowing occurs even at 5 % atomic Au and the narrowing is less significant (-0.5 eV). It thus appears that the apparently strong d-d interaction does not maintain the Au d band width in Ag3Au. Let us first consider the bandwidth of the constituents on the basis of their sublattice in the alloy. Assuming, to the first approximation, a fcc lattice with same lattice constant for both Cu3Au and Ag3Au and the band width is determined by an ordered sublattice of Au alone, then the essential features of the alloy d bands such as the apparent splitting and the width can be described as

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The situation is different in Ag-Au and Au-Cu alloys where both constituents have d bands that overlap in energy. Here, d hybridization may be significant. To have the same band width as in pure Au, Au must have Au and host nearest neighbors which interact significantly with Au so that Au-Au interaction can occur through mixing with host atom orbitals. Thus states and virtual orbitals are formed through which the Au d band electrons can tunnel out at above the same rate as in pure Au. It is therefore not surprising that the Au d-band width is maintained in concentrate Au-Ag and Au-Cu alloys (>50% atomic Au) where greater than half of the nearest neighbours are Au.

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Fig. 4 XPS valence band spectra of Ag-Au alloys at 1486.6 eV photon energy. where aso is the spin orbit splitting term and aband is the intrinsic band term which can be estimated from band structure calculations or experiment. 17 Similar consideration can also be made for the host d bands by leaving out the Au atoms in the lattice. The results of such a calculation are shown in fig. 1 as pairs of arrow markers centered at approximately the centroid of the d bands, the separation of which is the band width of the constituent d components based on dilution. Is It can be seen from fig. 1 that if we accept the d-d repulsion model in Au-Cu alloys, this approach explains the data very well except in the case of Ag3Au where the Au component with the usual doublet appearance cannot be revealed and the overall width is much wider than predicted based on intrinsic dilution alone indicating the importance of Ag-Au d-d mixing. We now switch on the interaction between the d orbitais of Au atoms with the d band of the host in a dilute alloy. This modifies the dilution effect. Here we consider the interaction between Au orbitals and the d band of the host. Conventional wisdom suggests that in general, chemically active orbitals are those closer to the Fermi level. Previous results of a series of An alloys 12"14 and recent studies of surface alloying 16 show that the "ds/2" orbital in Au is indeed more sensitive to alloying than the "d312" orbital. Thus, the Au d orbitals will interact with C'u d band like a two-level system, leading to the formation of bonding and antibonding states of primarily Au and Cu d characters respectively (figs. 1 and 2). In the case of Ag3Au, the Au 5d orbitals and the Ag 4d

32

THE d-BAND WIDTH OF Au-Ag AND Au-Cu ALLOYS

band also overlap in energy and should interact like a two level system also. The result of this interaction however does not seem to produce an alloy d band comparable in width to that of Au although the upper bound of the Au d band components are wider than that expected on dilution considerations alone. In the sprit of d-d repulsion the centroid of the Au d component may be regarded as being pushed up slightly towards the Fermi level relative to pure Au. What emerges from these observations is a picture in which dilution as well as the relative energy position of Au 5d5/2, Au 5d3/2, Cu 3d and Ag 4d relative to the Fermi level plays a subtle but crucial role in the d band width of dilute Au alloys. The AuAu d interaction is the dominant factor and the net effect of increasing number of host-atom nearest neighbors (dilution) in Ag alloy then leads to the significant reduction of Au-Au interaction and d band narrowing at 25% Au composition resulting in alloy d band narrowing without any significant d-d repulsion. In Cu-Au alloys, although narrowing of the main component of both Cu and Au partial densities of

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states occurs also, the alloy d band which is determined by the split d components from the constituents happens to have about the same width as that of Au. Acknowledgement: We are indebted to W. Kunnmann of Brookhaven National Laboratory for the preparation of the alloy samples and to R.E. Watson for insightful discussions. Synchrotron radiation experiments were carried out at the Synchrotron Radiation Center, University of Wisconsin-Madison, which is supported by U.S. National Science Foundation. X-ray photoelectron experiments were carded out at Surface Science Western which is supported partially by The Natural Science and Engineering Research Council (NSERC) of Canada. Research carded out at The University of Western Ontario is supported by the Ontario Centre for Materials Research (OCMR) and NSERC. Assistance of J.M. Chen, ICH. Tan and P. Kristof in obtaining some of the measurements is gratefully acknowledged.

References band intensity ratio of Ag/Au increases by a factor of 1. W. Eberhardt, S.C. Wu, R. Garrett, D. Sonderick, 1.8 from 50 eV to 100 eV. Based on these results, and F. Jona, Phys. Rev. B. 31, 8285(1985). and the assumption that the d band intensity of the 2. M. Kuhn, T.K. Sham, J.M. Chen and K.H. Tan, elements in the alloy is essentially the same as in the Solid State Comm. 75, 861(1990) pure metal, we can infer from fig. 2, that the partial 3. V.L Murrozzi, A.R. Williams and J.F. Janak, densities of state of Ag, of which the intensity should Phys. Rev. B. 10, 4856(1974). increase as photon energy increases, are concentrated 4. J.W. Davenport, R.E. Watson and M.Weinert, in the 4 to 7 eV binding energy region. Phys. Rev. B 37, 9985(1988) and J.W. Davenport, Phys. 12. R.M. Friedman, J. Hudis, M.L Perlman and R.E. Rev. B. 38, 7442(1988). Watson, Phys. Rev. B 8, 2433 (1973). 5. S.B. DiCenzo, P.H. Citrin, E.H. Hartford, Jr and 13. T.K. Sham, M.L. Perlman and R.E. Watson, Phys. G.K. Wertheim, Phys. Rev. B 34, 1343(1986). Rev. B 9, 539 (1979). 6. G.IC Wertheim, Phys. Rev. B 36, 4432(1987). 14. G.K. Wertheim, R.L. Cohen, G. Crecrlius and J.H. 7. G.K. Wertheim, LF. Mattheiss and D.N.E. Wernick, Phys. Rev. B 20, 860 (1979). Buchanan, Phys. Rev. B 38,5988 (1989). 15. F. Cyrot-Lackmann, Adv. Phys.16, 393 (1967); J. 8. G.S. Sohal, C. Carbone, E. Kisker, S. Kummacher, Phys. Chem. Solids 29, 1235 (1968). A. Fattah, W. Uelhof, R.C. Alberts and P. Weinberger, 16. M. Kuhn, Z.H. Lu and T.IC Sham, Phys. Rev. B Z. Phys, B 78, 295 (1990). submitted. 9. G.IC Wertheim, C.W. Bates, Jr. J.H. Wemick and 17. see for example A.H. MacDonald, J.M. Daams, D.N.E. Buchanan, Appl. Phys. Lett. 35, 403(1979). S.H. Vosko and D.D. Koelling, Phys. Rev. B 25, 713 10. P.S. Wehner, S.D. Kevan, R.S. Willaims, R.F. (1982). Davis and D.A. Shirely, Chem. Phys. Lett. 57, 18. Calculated from the consideration that the 334(1978). bandwidth varies as the square root of the number of 11. The results of Wehner et. al. (ref. 10) show that like neighbors, see R.E. Watson and M.L. Perlman, in the photon energy region of 50 to 100 eV, Ag has Physica Scripta 21, 527 (1980) and ref. 12. a slightly higher cross-section than Au. The relative d