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Comparing the band structure of Ag(1 1 1) monolayers on Ni(1 1 1) and Ni (0 0 1)
Solid State Communications, Vol. 55, No. 12, pp. 1101-1104, 1985. Printed in Great Britain.
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
COMPARING THE BAND STRUCTURE OF Ag(1 1 1) MONOLAYERS ON Ni(1 1 1) AND Ni(0 0 1) A.P. Shapiro, A.L. Wachs, T. Miller and T.-C. Chiang Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, IL 61801, USA
(Received 4 September 1984 by G. Burns) Ag(1 1 1) monolayers prepared on two substrates, Ni(1 1 1) and Ni(00 1), were studied with angle-resolved photoemission; their two-dimensional band dispersions were found to be identical within experimental uncertainties. Comparing the present results with those for Ag/Cu(00 1), the major difference is just a shift of 0.32 eV in all the binding energies. Thus the band topology of Ag overlayers in these systems is quite insensitive to the electronic and atomic structures of the substrates.
THE ELECTRONIC PROPERTIES of thin metallic overlayers are currently a topic of considerable interest [ 1 - 3 ] . These systems play an important role in many technologies such as catalytic surfaces, metal contacts, and metal coatings. Many systematic applications can be advanced if there is a fundamental understanding of how the properties of these systems can be related to or modified by the effects of overlayer-substrate interactions and changes in physical dimensions (or atomic configurations). This is a challenging problem because different competing effects have to be distinguished and identified. It is also technically difficult to prepare suitable simple model systems for a systematic study of these effects. In this paper, we report the comparison of the experimental electronic band structure of Ag overlayers prepared on two different substrates, Ni(1 1 1 ) a n d Ni(0 0 1). On both substrates, smooth and well-ordered Ag(1 1 1)monolayers can be obtained withno detectable atomic mixing [ 4 - 6 ] . Thus the usual problems of atomic clustering and alloying occuring in many other overlayer systems do not occur here. The system Ag(1 1 1)/Ni(1 1 1) exhibits no lattice match except for an overall orientational epitaxial relationship, while the other system Ag(1 1 1 ) / N i ( 0 0 1 ) i s nearly lattice matched in one direction [ 4 - 6 ] . The chemisorption bonds are quite strong in both cases, as no appreciable thermal desorption occurs for temperatures over 500°C. Given the facts of strong bonding and different epitaxial relationships, the two-dimensional band dispersions of monolayer Ag(1 1 1) are nevertheless found to be the same on both substrates within experimental uncertainties. There are only two previous detailed studies of the band dispersions of well-ordered and atomically abrupt metal monolayers on metal substrates. One system is Ag/Pd(00 1) in which Ag forms a (1 x 1) overlayer
and, therefore, is not directly comparable to our results [1]. The other system is Ag/Cu(001) in which Ag forms a slightly distorted (1 1 1) overlayer [2]. Considering that there are large differences in the d-band electronic structure between Cu and Ni, it is interesting to compare the band dispersions of monolayer Ag(1 1 1) on Ni and Cu. We find that the band dispersions in these two systems are very similar except for an overall relative shift in the binding energies of 0.32 eV. This demonstrates that the band topology of monolayer Ag(1 1 1) is quite insensitive to the detailed electronic properties of the substrate. We will explain below that the shift is a necessary consequence of the interfacial electronic properties. The experiment was performed at the Synchrotron Radiation Center of the University of Wisconsin Madison. The angle-resolved photoemission spectra were measured using a hemispherical analyzer with a full angular acceptance of 3 ° and a Seya-Namioka monochromator for dispersing the synchrotron radiation. Overall instrumental resolution was about 0.16 eV at 22 eV photon energy. The Ni samples were prepared by sputtering and annealing in the usual way; the cleanliness was checked with Auger spectroscopy. The Ag was deposited by evaporation with a rate typically about 0.05 A/s determined using a quartz oscillator (estimated accuracy about 20%). Both Ni substrates were heated to near 150°C during evaporation and then briefly annealed at 300°C for Ni(1 1 1) or at 450°C for Ni(00 1). The samples were examined with high-energy electron diffraction. Ag(1 1 1) grows on Ni(1 1 1) in nearly parallel epitaxy, that is, Ag [ 1 ]- 0] II Ni [ 1 ]- 0], etc., while Ag(1 1 1) grows on Ni(001) in large domains in two orthogonal orientations: Ag[1 TO] II Ni[1 1 0] or Ag[1 T0] II Ni[1 ]-0] with about equal populations due to the 4-fold symmetry of the substrate. These
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Fig. 1. Angle-resolved photoemission spectra from 1.2 A (1/2 monolayer) of Ag on Ni(00 1) taken with a photon energy h u = 22eV and kij along Ni[1 1 0]. The polar emission angles 0 are indicated. Features which are due to the presence of the Ag overlayer are marked with arrows. The initial energies are referred to the Fermi edge ( E F = 0). The inset shows a schematic drawing of the Brillouin zone of monolayer Ag(1 1 1). observations are in agreement with earlier studies [4, 5]. Large flat monolayer-thick Ag(1 11) islands are formed at coverages below a monolayer [6]; consequently, the photoemission intensities of Ag-derived features are proportional to coverage without changes in spectral shapes. Close to and beyond one monolayer coverages, extra features associated with Ag atoms in the second layer appear in the spectra. To avoid this undesirable situation, all data reported here are for 1.2 + 20%A (about 1/2 monolayer) equivalent Ag coverage. The two-dimensional Brillouin zone of monolayer Ag(11 1) is shown in the inset of Fig. 1, where we use the corresponding notations for a thick Ag(1 11) film to denote the crystallographic directions. Thus Ag[1 ]-0] is along the F/( direction, etc. To determine the two-dimensional band dispersions along both I-'K, and PM, the photoemission spectra were taken using p-polarized light with a fixed photon energy hu = 22 eV and an incident angle of 60 ° in the plane containing the surface normal (Ni[1 11] or N i [ 0 0 1 ] ) and N i [ l l 0 ] . The polar emission angle 0 was varied to
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Vol. 55, No. 12
change kll (the component of photoelectron wavevector k parallel to the surface). For Ag/Ni(1 1 1), kll was chosen to be along either Ni[1 TO] or Ni[1 2 1], corresponding to scanning along PK or rM, respectively. For Ag/Ni(001), kit was chosen along either Ni[1 TO] or Nil1 10] ; because of the existence of domains in two orientations, both PK and I'M directions were probed simultaneously in each case. Thus the two scans for Ag/Ni(001) should produce identical peak positions but not necessarily the same peak intensities. The set of spectra for 1.2A of Ag on Ni(00 1) with kll along Ni[1 TO] is shown in Fig. 1. The other three sets of spectra are qualitatively very similar and not shown here. By comparing these spectra with those obtained from clean Ni substrates, the peaks derived from Ag can be easily identified; they are marked by arrows in Fig. 1. Appreciable dispersions as a function of 0 (or kll ) are evident. As the system is two-dimensional, the Agderived peaks in normal emission show no dispersions for hu = 16-32eV. Following standard procedures and assuming direct transitions without umklapp, the two-dimensional band dispersions for the Ag(11 1) overlayers have been determined from the photoemission spectra. The results are displayed in Fig. 2 in a manner to facilitate comparison between the two systems Ag/Ni(1 1 1) and Ag/Ni(O 0 1). The data points are typically uncertain by about -+ 0.1-0.2eV; unresolved peaks are indicated only by the average peak positions. The crosses in Figs. 2(a) and (b) represent the dispersions along the FK direction, while the crosses in Figs. 2(c) and (d) represent the dispersions along FM. The cirlces in Fig. 2, corresponding to data points taken from Ag/ Ni(0 01), represent a mixture of dispersions along both PM and FK. The relative intensities of unresolved peaks influence the assigned overall peak positions; thus the data points for the two scans in different directions for Ag/Ni(00 1) do not match exactly in regions of overlapping peaks. Based on the cross comparisons shown in Fig. 2, the band dispersions for both Ag/Ni(1 1 l) and Ag/Ni(001) are the same within experimental uncertainties and resolutions. Six distinct bands can be identified in Fig. 2, corresponding to five d bands and one sp band of Ag. Tobin et al. [2] have studied the band dispersions of an Ag monolayer on Cu(00 1). Ag/Cu(00 1) forms a c(10 x 2) structure which is a Ag(1 1 1) monolayer compressed and expanded in two orthogonal directions, each by about 2% [7]; thus the areal density of Ag atoms is essentially unchanged by the strain. Figure 3 shows their data points (squares) for the band dispersions along FM, obtained from an Ag(1 1 1) monolayer with a single domain orientation on a slightly misaligned substrate; the associated binding energy scale is on the
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Fig. 2. Comparisons of the two-dimensional dispersion relations from 1/2 monolayer Ag overlayers on Ni(0 0 1) and Ni(1 1 1) for kll in the indicated directions. The magnitudes of klE at the points/~ and .~r are indicated. In all cases, hv = 22 eV. The energy zero is the Fermi edge (EF = 0). right-hand side. For comparison, our data points (crosses) for Ag/Ni(1 1 1) with kll along the same ~.~r direction are also shown; the associated binding energy scale is on the left-hand side. The two energy scales are relatively displaced by 0.32eV. With this shift, the two sets of data agree remarkably well within the regions of overlap. The solid curves are smooth curves connecting our data points; they represent the experimental dispersions of bands 1-5. The dashed curve for the dispersion of band 6 is taken from the data for Ag/Ni(001) [8] (the band dispersions near r are isotropic to terms proportional to k~). As indicated by the dashed curve for the proposed dispersion of band 5, bands 4 and 5 show a small spin-orbit splitting of about 0.25 eV at r , which is also present in Tobin's data. The peaks derived from bands 4 and 5 dominate the normal-emission spectra in all cases studied here and also in Tobin's work using hp = 21.22eV; therefore, the two sets of data can be easily associated. The upper part of band 6 has mainly sp character; the remainder and the other bands generally have mostly d character [2]. The two data points of Tobin marked by question marks in Fig. 3 are not seen correspondingly in all our spectra. We believe that these data points are derived from an indirect transition
Fig. 3. A comparison of the two-dimensional dispersion relations from a 1/2 monolayer of Ag on Ni(1 1 1) (data indicated by crosses; energy scale on the left-hand side) to the two-dimensional dispersion relations from a nearmonolayer coverage of Ag on Cu(0 0 1) (data indicated by squares; energy scal_eon the right-hand side). In both cases kll is along the FM direction of the Ag overlayer. The smooth solid and dashed curves are proposed band dispersions of bands 1-6. associated with band 5. This transition can be followed continuously as a function of increasing Ag coverage on Cu(O0 1) [2], and is seen to evolve into an indirect transition for bulk Ag(1 1 1) [9]. The intensities of indirect transitions depend on sample configurations, and can be quite different for Ag/Ni and Ag/Cu. In the following we explain the relative shift in binding energy of 0.32 eV. The Ag 4dbands are relatively tightly bound; therefore, their energy positions are closely tied to those of Ag core levels [10, 11]. The problem is then to determine the core level binding energy shift of Ag, A s, for Ag/Ni relative to Ag/Cu. No such data exist; however, reasonable theoretical models are available [10-13]. Following [11], we have calculated the shift of Ag core level binding energies An, for Ag impurities in Ni relative to Ag impurities in Cu. An = - - 0 . 6 +0.2eV, where the uncertainty is estimated from a comparison between theory and experiment for a large number of systems. This A n is dominated by initial-state shift. Typically, A n = A s + 0 . 2 e V following an empirical rule [11]. The final estimate of As is --0.6-+ 0.4eV. This accounts
1104
Ag(111) MONOLAYERS ON Ni(111) AND Ni(001)
qualitatively for the observed effect; the Ag 4d binding energies for Ag/Ni are smaller by 0.32 eV than for Ag/Cu. This shift can be related to the difference in bonding energies of Ag on Ni and Cu. To summarize, we have demonstrated that Ag(1 1 1) monolayers prepared on Ni(001), Ni(1 1 1), and Cu(0 0 t) show very similar electronic band dispersions despite large differences in the electronic and atomic structures of the substrates. This is perhaps somewhat surprising because the overlayer-substrate bonding is strong. Recent theoretical calculations for other metal overlayers indicate that the two-dimensional band dispersions are in general modified substantially from those of corresponding unsupported monolayers by the overlayer-substrate interaction [1] (although an earlier calculation indicated otherwise [14] ). Our results show that the differences in the overlayer-substrate interactions among the three systems must be relatively small; therefore, only an overall shift in binding energies of 0.32 eV is observed between Ag/Ni and Ag/Cu while the band topology is the same. This raises the question of the role and relative importance ofs,p, and d electrons in the bonding and coupling between the overlayer and the substrate, which can only be answered by more research. Acknowledgements - This material is based upon work supported by the National Science Foundation under Grant No. DMR-8311281. Some of the equipment used for this research was obtained with grants from the National Science Foundation (Grant No. DCR-8352083), the IBM Research Center, The Research Corporation, and the General Motors Research Laboratories. The Synchrotron Radiation Center of the University of Wisconsin - Madison is supported by the National Science Foundation under Contract No. DMR-80-20164. We acknowledge the use of central facilities of the Materials Research Laboratory of the University of Illinois, which is supported by the Department of Energy, Division of
Vol. 55, No. 12
Materials Sciences, under Contract No. DE-AC0276ERO1198, and by the National Science Foundation. REFERENCES 1. 2.
3. 4. 5. 6.
7. 8. 9. 10. 11. 12. 13. 14.
T.W. Capehart et al., J. Vac. Sci. Technol. A1, 1214(1983). J.G. Tobin, S.W. Robey, L.E. Klebanoff & D.A. Shirley, Phys. Rev. B28, 6169 (1983); J.G. Tobin, Ph.D. Thesis, University of California- Berkeley (1983). M.L. Shek, P.M. Stefan, I. Lindau & W.E. Spicer, Phys. Rev. B27, 7277 (1983). L.G. Feinstein, E. Blanc & D. Dufayard, Surf. Sci. 19,269 (1970). D.C. Jackson, T.E. Gallon & A. Chambers, Surf. Sci. 36,381 (1973). A.P. Shapiro, A.L. Wachs, T. Miller & T.-C. Chiang, (unpublished). In a recent experiment, we measured the photoemission intensity of CO chemisorbed on Ag-covered Ni(1 1 1) and Ni(00 1). Since CO sticks to Ni but not to Ag, the photoemission intensity of CO indicates the amount of surface area of Ni which is not covered by Ag. With a series of such measurements for increasing Ag coverage, we determined that a submonolayer amount of Ag on either Ni surface does not form thick islands. P.W. Palmberg & T.N. Rhodin, J. Chem. Phys. 49, 134 (1968). The peak derived from band 6 for Ag/Ni(1 1 1) in the photoemission spectra was masked by a nearby intense Ni-derived peak. A.L. Wachs, T. Miller & T.-C. Chiang, Phys. Rev. B29, 2286 (1984). P.H. Citrin & G.K. Wertheim, Phys. Rev. 27, 3176 (1983). M.G. Mason, Phys. Rev. B27,748 (1983). B. Johansson and N. M~rtensson, Phys. Rev. B21, 4427 (1980). P. Steiner & S. Htifner, Solid State Commun. 37, 279 (1981). I. Abbatietal.,Phys. Rev. Lett. 40,469(1978).
0038-1098/85 $3.00 + .00 Pergamon Press Ltd.
COMPARING THE BAND STRUCTURE OF Ag(1 1 1) MONOLAYERS ON Ni(1 1 1) AND Ni(0 0 1) A.P. Shapiro, A.L. Wachs, T. Miller and T.-C. Chiang Department of Physics and Materials Research Laboratory, University of Illinois at Urbana-Champaign, 1110 W. Green Street, Urbana, IL 61801, USA
(Received 4 September 1984 by G. Burns) Ag(1 1 1) monolayers prepared on two substrates, Ni(1 1 1) and Ni(00 1), were studied with angle-resolved photoemission; their two-dimensional band dispersions were found to be identical within experimental uncertainties. Comparing the present results with those for Ag/Cu(00 1), the major difference is just a shift of 0.32 eV in all the binding energies. Thus the band topology of Ag overlayers in these systems is quite insensitive to the electronic and atomic structures of the substrates.
THE ELECTRONIC PROPERTIES of thin metallic overlayers are currently a topic of considerable interest [ 1 - 3 ] . These systems play an important role in many technologies such as catalytic surfaces, metal contacts, and metal coatings. Many systematic applications can be advanced if there is a fundamental understanding of how the properties of these systems can be related to or modified by the effects of overlayer-substrate interactions and changes in physical dimensions (or atomic configurations). This is a challenging problem because different competing effects have to be distinguished and identified. It is also technically difficult to prepare suitable simple model systems for a systematic study of these effects. In this paper, we report the comparison of the experimental electronic band structure of Ag overlayers prepared on two different substrates, Ni(1 1 1 ) a n d Ni(0 0 1). On both substrates, smooth and well-ordered Ag(1 1 1)monolayers can be obtained withno detectable atomic mixing [ 4 - 6 ] . Thus the usual problems of atomic clustering and alloying occuring in many other overlayer systems do not occur here. The system Ag(1 1 1)/Ni(1 1 1) exhibits no lattice match except for an overall orientational epitaxial relationship, while the other system Ag(1 1 1 ) / N i ( 0 0 1 ) i s nearly lattice matched in one direction [ 4 - 6 ] . The chemisorption bonds are quite strong in both cases, as no appreciable thermal desorption occurs for temperatures over 500°C. Given the facts of strong bonding and different epitaxial relationships, the two-dimensional band dispersions of monolayer Ag(1 1 1) are nevertheless found to be the same on both substrates within experimental uncertainties. There are only two previous detailed studies of the band dispersions of well-ordered and atomically abrupt metal monolayers on metal substrates. One system is Ag/Pd(00 1) in which Ag forms a (1 x 1) overlayer
and, therefore, is not directly comparable to our results [1]. The other system is Ag/Cu(001) in which Ag forms a slightly distorted (1 1 1) overlayer [2]. Considering that there are large differences in the d-band electronic structure between Cu and Ni, it is interesting to compare the band dispersions of monolayer Ag(1 1 1) on Ni and Cu. We find that the band dispersions in these two systems are very similar except for an overall relative shift in the binding energies of 0.32 eV. This demonstrates that the band topology of monolayer Ag(1 1 1) is quite insensitive to the detailed electronic properties of the substrate. We will explain below that the shift is a necessary consequence of the interfacial electronic properties. The experiment was performed at the Synchrotron Radiation Center of the University of Wisconsin Madison. The angle-resolved photoemission spectra were measured using a hemispherical analyzer with a full angular acceptance of 3 ° and a Seya-Namioka monochromator for dispersing the synchrotron radiation. Overall instrumental resolution was about 0.16 eV at 22 eV photon energy. The Ni samples were prepared by sputtering and annealing in the usual way; the cleanliness was checked with Auger spectroscopy. The Ag was deposited by evaporation with a rate typically about 0.05 A/s determined using a quartz oscillator (estimated accuracy about 20%). Both Ni substrates were heated to near 150°C during evaporation and then briefly annealed at 300°C for Ni(1 1 1) or at 450°C for Ni(00 1). The samples were examined with high-energy electron diffraction. Ag(1 1 1) grows on Ni(1 1 1) in nearly parallel epitaxy, that is, Ag [ 1 ]- 0] II Ni [ 1 ]- 0], etc., while Ag(1 1 1) grows on Ni(001) in large domains in two orthogonal orientations: Ag[1 TO] II Ni[1 1 0] or Ag[1 T0] II Ni[1 ]-0] with about equal populations due to the 4-fold symmetry of the substrate. These
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Fig. 1. Angle-resolved photoemission spectra from 1.2 A (1/2 monolayer) of Ag on Ni(00 1) taken with a photon energy h u = 22eV and kij along Ni[1 1 0]. The polar emission angles 0 are indicated. Features which are due to the presence of the Ag overlayer are marked with arrows. The initial energies are referred to the Fermi edge ( E F = 0). The inset shows a schematic drawing of the Brillouin zone of monolayer Ag(1 1 1). observations are in agreement with earlier studies [4, 5]. Large flat monolayer-thick Ag(1 11) islands are formed at coverages below a monolayer [6]; consequently, the photoemission intensities of Ag-derived features are proportional to coverage without changes in spectral shapes. Close to and beyond one monolayer coverages, extra features associated with Ag atoms in the second layer appear in the spectra. To avoid this undesirable situation, all data reported here are for 1.2 + 20%A (about 1/2 monolayer) equivalent Ag coverage. The two-dimensional Brillouin zone of monolayer Ag(11 1) is shown in the inset of Fig. 1, where we use the corresponding notations for a thick Ag(1 11) film to denote the crystallographic directions. Thus Ag[1 ]-0] is along the F/( direction, etc. To determine the two-dimensional band dispersions along both I-'K, and PM, the photoemission spectra were taken using p-polarized light with a fixed photon energy hu = 22 eV and an incident angle of 60 ° in the plane containing the surface normal (Ni[1 11] or N i [ 0 0 1 ] ) and N i [ l l 0 ] . The polar emission angle 0 was varied to
l)
AND Ni(001)
Vol. 55, No. 12
change kll (the component of photoelectron wavevector k parallel to the surface). For Ag/Ni(1 1 1), kll was chosen to be along either Ni[1 TO] or Ni[1 2 1], corresponding to scanning along PK or rM, respectively. For Ag/Ni(001), kit was chosen along either Ni[1 TO] or Nil1 10] ; because of the existence of domains in two orientations, both PK and I'M directions were probed simultaneously in each case. Thus the two scans for Ag/Ni(001) should produce identical peak positions but not necessarily the same peak intensities. The set of spectra for 1.2A of Ag on Ni(00 1) with kll along Ni[1 TO] is shown in Fig. 1. The other three sets of spectra are qualitatively very similar and not shown here. By comparing these spectra with those obtained from clean Ni substrates, the peaks derived from Ag can be easily identified; they are marked by arrows in Fig. 1. Appreciable dispersions as a function of 0 (or kll ) are evident. As the system is two-dimensional, the Agderived peaks in normal emission show no dispersions for hu = 16-32eV. Following standard procedures and assuming direct transitions without umklapp, the two-dimensional band dispersions for the Ag(11 1) overlayers have been determined from the photoemission spectra. The results are displayed in Fig. 2 in a manner to facilitate comparison between the two systems Ag/Ni(1 1 1) and Ag/Ni(O 0 1). The data points are typically uncertain by about -+ 0.1-0.2eV; unresolved peaks are indicated only by the average peak positions. The crosses in Figs. 2(a) and (b) represent the dispersions along the FK direction, while the crosses in Figs. 2(c) and (d) represent the dispersions along FM. The cirlces in Fig. 2, corresponding to data points taken from Ag/ Ni(0 01), represent a mixture of dispersions along both PM and FK. The relative intensities of unresolved peaks influence the assigned overall peak positions; thus the data points for the two scans in different directions for Ag/Ni(00 1) do not match exactly in regions of overlapping peaks. Based on the cross comparisons shown in Fig. 2, the band dispersions for both Ag/Ni(1 1 l) and Ag/Ni(001) are the same within experimental uncertainties and resolutions. Six distinct bands can be identified in Fig. 2, corresponding to five d bands and one sp band of Ag. Tobin et al. [2] have studied the band dispersions of an Ag monolayer on Cu(00 1). Ag/Cu(00 1) forms a c(10 x 2) structure which is a Ag(1 1 1) monolayer compressed and expanded in two orthogonal directions, each by about 2% [7]; thus the areal density of Ag atoms is essentially unchanged by the strain. Figure 3 shows their data points (squares) for the band dispersions along FM, obtained from an Ag(1 1 1) monolayer with a single domain orientation on a slightly misaligned substrate; the associated binding energy scale is on the
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kll(A- )
k,(A- )
Fig. 2. Comparisons of the two-dimensional dispersion relations from 1/2 monolayer Ag overlayers on Ni(0 0 1) and Ni(1 1 1) for kll in the indicated directions. The magnitudes of klE at the points/~ and .~r are indicated. In all cases, hv = 22 eV. The energy zero is the Fermi edge (EF = 0). right-hand side. For comparison, our data points (crosses) for Ag/Ni(1 1 1) with kll along the same ~.~r direction are also shown; the associated binding energy scale is on the left-hand side. The two energy scales are relatively displaced by 0.32eV. With this shift, the two sets of data agree remarkably well within the regions of overlap. The solid curves are smooth curves connecting our data points; they represent the experimental dispersions of bands 1-5. The dashed curve for the dispersion of band 6 is taken from the data for Ag/Ni(001) [8] (the band dispersions near r are isotropic to terms proportional to k~). As indicated by the dashed curve for the proposed dispersion of band 5, bands 4 and 5 show a small spin-orbit splitting of about 0.25 eV at r , which is also present in Tobin's data. The peaks derived from bands 4 and 5 dominate the normal-emission spectra in all cases studied here and also in Tobin's work using hp = 21.22eV; therefore, the two sets of data can be easily associated. The upper part of band 6 has mainly sp character; the remainder and the other bands generally have mostly d character [2]. The two data points of Tobin marked by question marks in Fig. 3 are not seen correspondingly in all our spectra. We believe that these data points are derived from an indirect transition
Fig. 3. A comparison of the two-dimensional dispersion relations from a 1/2 monolayer of Ag on Ni(1 1 1) (data indicated by crosses; energy scale on the left-hand side) to the two-dimensional dispersion relations from a nearmonolayer coverage of Ag on Cu(0 0 1) (data indicated by squares; energy scal_eon the right-hand side). In both cases kll is along the FM direction of the Ag overlayer. The smooth solid and dashed curves are proposed band dispersions of bands 1-6. associated with band 5. This transition can be followed continuously as a function of increasing Ag coverage on Cu(O0 1) [2], and is seen to evolve into an indirect transition for bulk Ag(1 1 1) [9]. The intensities of indirect transitions depend on sample configurations, and can be quite different for Ag/Ni and Ag/Cu. In the following we explain the relative shift in binding energy of 0.32 eV. The Ag 4dbands are relatively tightly bound; therefore, their energy positions are closely tied to those of Ag core levels [10, 11]. The problem is then to determine the core level binding energy shift of Ag, A s, for Ag/Ni relative to Ag/Cu. No such data exist; however, reasonable theoretical models are available [10-13]. Following [11], we have calculated the shift of Ag core level binding energies An, for Ag impurities in Ni relative to Ag impurities in Cu. An = - - 0 . 6 +0.2eV, where the uncertainty is estimated from a comparison between theory and experiment for a large number of systems. This A n is dominated by initial-state shift. Typically, A n = A s + 0 . 2 e V following an empirical rule [11]. The final estimate of As is --0.6-+ 0.4eV. This accounts
1104
Ag(111) MONOLAYERS ON Ni(111) AND Ni(001)
qualitatively for the observed effect; the Ag 4d binding energies for Ag/Ni are smaller by 0.32 eV than for Ag/Cu. This shift can be related to the difference in bonding energies of Ag on Ni and Cu. To summarize, we have demonstrated that Ag(1 1 1) monolayers prepared on Ni(001), Ni(1 1 1), and Cu(0 0 t) show very similar electronic band dispersions despite large differences in the electronic and atomic structures of the substrates. This is perhaps somewhat surprising because the overlayer-substrate bonding is strong. Recent theoretical calculations for other metal overlayers indicate that the two-dimensional band dispersions are in general modified substantially from those of corresponding unsupported monolayers by the overlayer-substrate interaction [1] (although an earlier calculation indicated otherwise [14] ). Our results show that the differences in the overlayer-substrate interactions among the three systems must be relatively small; therefore, only an overall shift in binding energies of 0.32 eV is observed between Ag/Ni and Ag/Cu while the band topology is the same. This raises the question of the role and relative importance ofs,p, and d electrons in the bonding and coupling between the overlayer and the substrate, which can only be answered by more research. Acknowledgements - This material is based upon work supported by the National Science Foundation under Grant No. DMR-8311281. Some of the equipment used for this research was obtained with grants from the National Science Foundation (Grant No. DCR-8352083), the IBM Research Center, The Research Corporation, and the General Motors Research Laboratories. The Synchrotron Radiation Center of the University of Wisconsin - Madison is supported by the National Science Foundation under Contract No. DMR-80-20164. We acknowledge the use of central facilities of the Materials Research Laboratory of the University of Illinois, which is supported by the Department of Energy, Division of
Vol. 55, No. 12
Materials Sciences, under Contract No. DE-AC0276ERO1198, and by the National Science Foundation. REFERENCES 1. 2.
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T.W. Capehart et al., J. Vac. Sci. Technol. A1, 1214(1983). J.G. Tobin, S.W. Robey, L.E. Klebanoff & D.A. Shirley, Phys. Rev. B28, 6169 (1983); J.G. Tobin, Ph.D. Thesis, University of California- Berkeley (1983). M.L. Shek, P.M. Stefan, I. Lindau & W.E. Spicer, Phys. Rev. B27, 7277 (1983). L.G. Feinstein, E. Blanc & D. Dufayard, Surf. Sci. 19,269 (1970). D.C. Jackson, T.E. Gallon & A. Chambers, Surf. Sci. 36,381 (1973). A.P. Shapiro, A.L. Wachs, T. Miller & T.-C. Chiang, (unpublished). In a recent experiment, we measured the photoemission intensity of CO chemisorbed on Ag-covered Ni(1 1 1) and Ni(00 1). Since CO sticks to Ni but not to Ag, the photoemission intensity of CO indicates the amount of surface area of Ni which is not covered by Ag. With a series of such measurements for increasing Ag coverage, we determined that a submonolayer amount of Ag on either Ni surface does not form thick islands. P.W. Palmberg & T.N. Rhodin, J. Chem. Phys. 49, 134 (1968). The peak derived from band 6 for Ag/Ni(1 1 1) in the photoemission spectra was masked by a nearby intense Ni-derived peak. A.L. Wachs, T. Miller & T.-C. Chiang, Phys. Rev. B29, 2286 (1984). P.H. Citrin & G.K. Wertheim, Phys. Rev. 27, 3176 (1983). M.G. Mason, Phys. Rev. B27,748 (1983). B. Johansson and N. M~rtensson, Phys. Rev. B21, 4427 (1980). P. Steiner & S. Htifner, Solid State Commun. 37, 279 (1981). I. Abbatietal.,Phys. Rev. Lett. 40,469(1978).