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Ag(111 ) trilayer by means of electronic spectroscopies
Surface Science 507–510 (2002) 234–239 www.elsevier.com/locate/susc
Investigation of the structure of a Ag/Pd/Ag(1 1 1) trilayer by means of electronic spectroscopies J. Dumont
a,b,*
, J. Ghijsen b, R. Sporken
a
a
b
Laboratoire de Physique des Mat eriaux Electroniques, Facult es Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facult es Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium
Abstract The growth of the Ag/Pd/Ag system has been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. No chemical reaction or interdiffusion was observed between the Pd and Ag layers. The growth of the Pd interlayer follows the Frank Van der Merwe mode but is not pseudomorphic on the Ag(1 1 1) substrate. The growth of the top Ag layer on the Pd interlayer is pseudomorphic and layer by layer but contains around 12% of voids. 2002 Elsevier Science B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Low energy electron diffraction (LEED); Superlattices; Metallic surfaces; Epitaxy; Growth; Silver; Palladium
1. Introduction Since the beginning of the 1980s, metallic multilayers, including the Ag–Pd system, have been widely studied, essentially for their magnetic properties [1]. The interdiffusion-free layer-by-layer growth of Ag on Pd(1 1 1) was first studied by Guglielmacci and Gillet [2] using Auger electron spectroscopy (AES) below 200 C. Above 250 C, alloying by interdiffusion was observed. According to Eisenhut et al. [3], the first Ag monolayer (ML) on Pd(1 1 1) is pseudomorphic. The follow-
*
Corresponding author. Address: Laboratoire de Physique des Materiaux Electroniques, Facultes Universitaires NotreDame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium. Tel.: +32-81-72-54-38; fax: +32-81-72-45-95. E-mail address: [email protected] (J. Dumont).
ing layers contain stacking faults to restore the bulk structure of the Ag film. Burland and Dobson [4] studied the growth of Pd on Ag(1 1 1) by low energy and reflection high energy electron diffraction (LEED and RHEED) and AES. At room temperature, they observed that thin Pd films (around 10 ML) keep the lattice spacing of the Ag(1 1 1) substrate. The Pd films recover the bulk Pd(1 1 1) lattice spacing only after . Annealing around 240 C results in Ag-rich 200 A Pd films. The pseudomorphic growth of Pd(1 1 1) on Ag(1 1 1) at room temperature has been confirmed by the X-ray diffraction (XRD) studies of H€aupl and Wissmann [5] who also mentioned the formation of an alloy above 200 C. Smith et al. [6] described the growth mode and the electronic structure of palladium on the (1 0 0) and (1 1 1) surfaces of silver at room temperature were studied by LEED, AES, angle-resolved photoemission
0039-6028/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 2 5 2 - 9
J. Dumont et al. / Surface Science 507–510 (2002) 234–239
and work function measurements. Among other results, on the (1 1 1) surface they observed a layerby-layer growth of palladium for three MLs after which the LEED pattern is fading out. Interdiffusion in Ag/Pd multilayers annealed at 375 C was first studied by Henein and Hilliard [7]. Then Pienkos et al. and Gladyszewski investigated the interdiffusion in Ag/Pd multilayers at room temperature but at initial stages of ion beam mixing [8,9]. Recently Temst et al. [10] investigated annealed (2 0 0) and (1 1 1) Ag/Pd superlattices using XRD and atomic force microscopy (AFM). Before thermal annealing, they measured a layer thickness fluctuation on the order of 1 ML and a root mean square interface roughness of which is close to the difference in lattice 0.2 A spacing between Ag and Pd. Moreover the lattice parameter of silver is slightly expanded while the one from palladium is slightly contracted to ensure the pseudomorphic growth of the system. Nevertheless, when annealing at 200 C, Ag- and Pd-rich phases appeared due to clustering of the two materials. In conclusion, much is known not only about the growth and the electronic properties of Ag on Pd and vice versa but also on the post-growth structural properties of the Ag/Pd superlattice. This system looks like the ideal superlattice: perfectly layered when grown at room temperature and without any interdiffusion. Nevertheless until now, no in situ study exists of the evolution of the structure and electronic properties of the superlattice during its growth. The aim of this work is to characterize the Ag/Pd/ Ag(1 1 1) system during the growth, using X-ray photoelectron spectroscopy (XPS) and LEED. These combined techniques allow us to follow the evolution of the surface crystallinity and of the chemical interactions at the interfaces.
2. Experimental procedure Ag and Pd were evaporated on a muscovite mica substrate or on a Ag(1 1 1) single crystal. The freshly cleaved muscovite mica was first annealed Ag layer was to 200 C under UHV before a 1800 A deposited at 1 A/s from a resistively heated tung-
235
sten boat. Before deposition the Ag(1 1 1) single crystal was mechanically polished and cleaned under UHV by repeated sputtering (500 eV) and annealing (250 C) cycles until no contamination was detected by XPS and a sharp hexagonal LEED was observed. The Ag and Pd layers were evaporated from 99.99% purity materials. During deposition the substrate was left at room temperature. Ag and Pd /s respectively were evaporated at 0.01 and 0.002 A from 0.25 mm diameter wires rolled around coiled 0.5 mm tungsten wires. We checked the thickness of the layers with a quartz crystal thickness monitor. The pressure was about 2:5 1010 mbar before the evaporations, and about 2 108 mbar (resp. 2 109 mbar) during the growth of the thick Ag layer (resp. the superlattice). A Scienta 300 photoelectron spectrometer was used to record XPS spectra. This instrument uses a monochromatic Al Ka X-ray source (hm ¼ 1486:6 eV) with a rotating anode operated at 16 kV and 400 mA. Photoelectrons were detected by a hemispherical analyzer (150 eV pass energy) and a two-dimensional position-sensitive detector (microchannel plate with CCD camera). The energy resolution obtained in these conditions is 0.3 eV.
3. Results and discussion 3.1. Ag/Pd/Ag/mica The first multilayer was deposited on Ag/mica prepared following the procedure described above. 16 ML of Pd were then grown on the Ag substrate and we observed a weak hexagonal LEED pattern. Thereafter we recorded XPS spectra of the Ag 3d and Pd 3d core levels (Fig. 1) between the steps of the deposition of 15.2 ML of Ag on the Pd16 ML / Ag/mica system. From the spectra, it can be seen that only intensities vary whereas the position and shape of the core levels is unchanged. Hence, alloying at the interface can be ruled out. 3.2. Ag/Pd/Ag(1 1 1) The second Ag/Pd/Ag trilayer was grown on a Ag(1 1 1) single crystal prepared following the
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Fig. 1. XPS spectra of the Ag 3d and Pd 3d core levels between the steps of the deposition of 15.2 ML of Ag on the Pd16 ML /Ag/ mica system.
procedure described above. Fig. 2a shows the spectra of Ag 3d and Pd 3d core levels during the growth of a 9.2 ML thick Pd layer on the Ag(1 1 1) single crystal and Fig. 2b displays the same levels for the growth of 18.4 ML of Ag on the Pd9:2 ML / Ag(1 1 1) system. The satellites observed at 3.7 and 6.1 eV from the Ag 3d and Pd 3d peaks respectively were amplified for better visibility. There is again no noticeable change in the shape or in the position of the peaks during the growth. The satellite near the Ag 3d peak is a loss induced by the excitation of a bulk (at 3.78 eV) and a surface (3.63 eV) plasmon of silver [11,12]. Fig. 2a shows that the deposition of the Pd overlayer gives rise to the attenuation of the loss induced by the plasmon. The bulk plasmon is not attenuated relatively to the parent line by an overlayer because an overlayer attenuates all the photoelectrons equally. By analogy with the findings of Bates et al. [13] who deposited gold on silver and observed the same plasmon loss attenuation by XPS, we conclude that in our case also the major part of the plasmon peak is in fact of surface character and thus fades out when a Pd overlayer is deposited on silver.
Fig. 2. Ag 3d, Pd 3d core level spectra during the growth of a 9.2 ML thick Pd layer on the Ag(1 1 1) single crystal (a) and during the growth of 18.4 ML of Ag on Pd9:2 ML /Ag(1 1 1) (b). The spectra have been scaled to equal height for easy comparison of lineshapes and the satellites at 3.7 and 6.1 eV from the Ag 3d and Pd 3d lines respectively have been amplified, also to the same height, to make them visible.
In the photoemission process, two final states coexist for the metallic palladium: the (mainly) 4p5 4d10 electronic configurations which gives rise
J. Dumont et al. / Surface Science 507–510 (2002) 234–239
237
to Pd 3d peak and the (mainly) 4p5 4d9 which generates the satellite at 6.1 eV from the main peak. This satellite appears when the first Pd layer is completed. During the growth of the Ag top layer, the Ag plasmon loss appears between 2.4 and 3.7 ML, which is probably the required thickness to sustain Ag surface plasmons. The satellite of Pd is not fading out even when covered with 18.4 ML of silver. From the description of this satellite given above, we deduce that in fact, it will be observed as long as the Pd 3d peak itself is observed. In all cases the satellites were always observed at the same energy from the Ag 3d and Pd 3d lines respectively. Fig. 3 displays the valence band spectra obtained during the growth of the Ag/Pd/Ag system. Even though the energy resolution is too low to describe the electronic structure completely, the valence band seems to be simply a linear combination of the bulk valence bands of palladium and silver. We see no hybridization of the bands. This confirms and completes the results of Smith et al. [6]. 3.3. Growth mode In XPS, surface sensitivity can be enhanced by collecting the photoelectrons at small takeoff angle (grazing mode). In contrast, collecting photoelectrons at 90 (normal mode) reduces surface sensitivity. Fig. 4 shows the evolution of the XPS intensities of Ag 3d and Pd 3d in normal and grazing mode during the growth of the system. In normal mode, during the growth of the Pd interlayer, the intensity of the Ag substrate (resp. Pd adsorbate) signal can be fitted with an exponential ed=k (resp. 1 ed=k ) where d is the thickness and k is the photoelectron attenuation length and is set as free parameter. This confirms that the growth of the Pd interlayer is close to the Frank Van der Merwe (FVdM) mode. The values ob 10% for Ag 3d and Pd 3d tained for k are 11.3 A photoelectrons during the growth of the Pd interlayer (Fig. 4). For the growth of the top Ag layer a proper fit of the experimental data could only be achieved ð1Þ ð2Þ using the following functions: IAg ¼ IAg þ IAg þ
Fig. 3. Valence band spectra during the growth of Pd interlayer (a) and the Ag top layer (b).
ð3Þ
IAg and IPd ¼ 1 IAg for the Ag and Pd signals respectively. These functions can be shown to include the following effects as a refinement of a simple layer-by-layer model but only on fraction I0 of the Pd interlayer of thickness d0 :
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Fig. 4. Evolution of the XPS intensities of Ag 3d and Pd 3d in normal (dots along the solid lines) and grazing (dashed lines) mode during the growth of the Ag/Pd/Ag(1 1 1) system. The vertical solid line separates the growth of the Pd layer (on the left) form the growth of the Ag layer. The solid lines are curves fitted based on a modified FVdM model. k is the photoelectron attenuation length. See the text for details.
ð1Þ
• IAg ¼ ð1 I0 Þed0 =k is due to photoelectrons from the lower silver layer through the uncovered part (1 I0 ) of the Pd interlayer. ð2Þ • IAg ¼ I0 ed=k is due to photoelectrons from the lower silver layer through the covered part I0 of the Pd interlayer. ð3Þ • IAg ¼ I0 ð1 expððd d0 Þ=kÞÞ is due to photoelectrons from the upper silver layer. From these equations we extracted I0 and found that voids cover around 12% of the Ag top layer. 10% The values obtained for are 15.8 A for Ag 3d and Pd 3d photoelectrons are in good agreement with those found using Seah and and Dench’s formula [14] (kAg 3d ¼ 17:9 A ). The discrepancies between the kPd 3d ¼ 16:9 A attenuation length obtained for the growth of the Pd interlayer and the Ag top layer are attributed to imprecision in the calibration of the quartz microbalance. In grazing mode, we see that the intensity of the substrate decreases faster than in normal mode and this quick attenuation confirms the layerby-layer growth of the whole Ag/Pd/Ag system. Fig. 5a diplays the sharp hexagonal LEED of the Ag(1 1 1) single crystal surface pattern. During the growth of the Pd layer, the background of the
Fig. 5. LEED patterns of the Ag(1 1 1) single crystal (a) and of the Ag/Pd/Ag system (b).
LEED fades out and the spots nearly disappear after 1.5 ML. Then the hexagonal pattern weakly reappears after 6 ML but even on the 9.2 ML Pd layer the spots are too weak to obtain a good picture. This attenuation of the diffraction pattern was also observed by Smith et al. [6] and Burland and Dobson [4] and explained by a roughening of
J. Dumont et al. / Surface Science 507–510 (2002) 234–239
the surface due to the relaxation of the Pd overlayer to the Pd bulk structure. The presence of some contamination may also play a role in the attenuation. The growth of the Ag layer on the Pd/ Ag system improves the pattern considerably. Fig. 5b shows the LEED image obtained on the 18.4 ML thick Ag layer on Pd/Ag. The tabulated interatomic distance on the Ag(1 1 1) and Pd(1 1 1) surfaces are respectively . Using the LEED pattern ob2.89 and 2.75 A tained on the Ag(1 1 1) single crystal, we deduced for the an interatomic distance of 2:52 0:2 A surface of the Ag(1 1 1)/Pd/Ag system. This means that the top Ag layer surface has an interatomic distance closer to the one of Pd(1 1 1) than to the one of Ag(1 1 1) and that the growth of this Ag layer is pseudomorphic with the Pd interlayer.
4. Conclusion The Ag/Pd/Ag system has been studied by XPS and LEED with muscovite mica or Ag(1 1 1) single crystal as substrates. In both cases no hybridization of the valence bands or chemical reaction or interdiffusion was observed between the Pd and Ag layers. The growth of the Pd layer follows the FVdM mode but is not pseudomorphic on the Ag(1 1 1) substrate. Moreover, the XPS intensities and LEED patterns show that the growth of the Ag top layer on the Pd interlayer is pseudomorphic and layer by layer but contains around 12% of voids.
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Acknowledgements This work was supported by the Belgian Office for Scientific, Technical and Cultural Affairs (PAI 4/10) and by the Belgian National Fund for Scientific Research. JD is supported by the FRIA and JG is supported by the NFSR (Belgium).
References [1] I.K. Schuller, S. Kim, C. Leighton, J. Magn. Magn. Matter 200 (1999) 571. [2] J.M. Guglielmacci, M. Gillet, Thin Solid Films 68 (1980) 407. [3] B. Eisenhut, J. Stober, G. Rangelov, Th. Fauster, Phys. Rev. B 47–19 (1993) 12980. [4] G.N. Burland, P.J. Dobson, Thin Solid Films 75 (1981) 383. [5] K. H€aupl, P. Wissmann, Thin Solid Films 174 (1989) 105. [6] G.C. Smith, C. Norris, C. Binns, H.A. Padmore, J. Phys. C.: Solid State Phys. 15 (1982) 6481. [7] G.E. Henein, J.E. Hilliard, J. Appl. Phys. 54–2 (1983) 728. [8] T. Pienkos, L. Gladyszewski, D. Chocyk, G. Gladyszewski, Mater. Sci. Eng. A 288 (2000) 266. [9] G. Gladyszewski, Mater. Lett. 13 (1992) 287. [10] K. Temst, M.J. Van Bael, C. Van Haesendonck, Y. Bruynseraede, D.G. de Groot, N. Koeman, R. Griessen, Thin Solid Films 342 (1999) 174. [11] J. Geiger, K. Wittmaack, Z. Phys. 195 (1966) 44. [12] J. Daniels, Z. Phys. 203 (1967) 235, and references therein. [13] C.W. Bates Jr., G.K. Wertheim, D.N.E. Buchanan, Phys. Lett. 72A (2) (1979) 178. [14] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2.
Investigation of the structure of a Ag/Pd/Ag(1 1 1) trilayer by means of electronic spectroscopies J. Dumont
a,b,*
, J. Ghijsen b, R. Sporken
a
a
b
Laboratoire de Physique des Mat eriaux Electroniques, Facult es Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium Laboratoire Interdisciplinaire de Spectroscopie Electronique, Facult es Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium
Abstract The growth of the Ag/Pd/Ag system has been studied by X-ray photoelectron spectroscopy and low energy electron diffraction. No chemical reaction or interdiffusion was observed between the Pd and Ag layers. The growth of the Pd interlayer follows the Frank Van der Merwe mode but is not pseudomorphic on the Ag(1 1 1) substrate. The growth of the top Ag layer on the Pd interlayer is pseudomorphic and layer by layer but contains around 12% of voids. 2002 Elsevier Science B.V. All rights reserved. Keywords: X-ray photoelectron spectroscopy; Low energy electron diffraction (LEED); Superlattices; Metallic surfaces; Epitaxy; Growth; Silver; Palladium
1. Introduction Since the beginning of the 1980s, metallic multilayers, including the Ag–Pd system, have been widely studied, essentially for their magnetic properties [1]. The interdiffusion-free layer-by-layer growth of Ag on Pd(1 1 1) was first studied by Guglielmacci and Gillet [2] using Auger electron spectroscopy (AES) below 200 C. Above 250 C, alloying by interdiffusion was observed. According to Eisenhut et al. [3], the first Ag monolayer (ML) on Pd(1 1 1) is pseudomorphic. The follow-
*
Corresponding author. Address: Laboratoire de Physique des Materiaux Electroniques, Facultes Universitaires NotreDame de la Paix, Rue de Bruxelles 61, 5000 Namur, Belgium. Tel.: +32-81-72-54-38; fax: +32-81-72-45-95. E-mail address: [email protected] (J. Dumont).
ing layers contain stacking faults to restore the bulk structure of the Ag film. Burland and Dobson [4] studied the growth of Pd on Ag(1 1 1) by low energy and reflection high energy electron diffraction (LEED and RHEED) and AES. At room temperature, they observed that thin Pd films (around 10 ML) keep the lattice spacing of the Ag(1 1 1) substrate. The Pd films recover the bulk Pd(1 1 1) lattice spacing only after . Annealing around 240 C results in Ag-rich 200 A Pd films. The pseudomorphic growth of Pd(1 1 1) on Ag(1 1 1) at room temperature has been confirmed by the X-ray diffraction (XRD) studies of H€aupl and Wissmann [5] who also mentioned the formation of an alloy above 200 C. Smith et al. [6] described the growth mode and the electronic structure of palladium on the (1 0 0) and (1 1 1) surfaces of silver at room temperature were studied by LEED, AES, angle-resolved photoemission
0039-6028/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 2 ) 0 1 2 5 2 - 9
J. Dumont et al. / Surface Science 507–510 (2002) 234–239
and work function measurements. Among other results, on the (1 1 1) surface they observed a layerby-layer growth of palladium for three MLs after which the LEED pattern is fading out. Interdiffusion in Ag/Pd multilayers annealed at 375 C was first studied by Henein and Hilliard [7]. Then Pienkos et al. and Gladyszewski investigated the interdiffusion in Ag/Pd multilayers at room temperature but at initial stages of ion beam mixing [8,9]. Recently Temst et al. [10] investigated annealed (2 0 0) and (1 1 1) Ag/Pd superlattices using XRD and atomic force microscopy (AFM). Before thermal annealing, they measured a layer thickness fluctuation on the order of 1 ML and a root mean square interface roughness of which is close to the difference in lattice 0.2 A spacing between Ag and Pd. Moreover the lattice parameter of silver is slightly expanded while the one from palladium is slightly contracted to ensure the pseudomorphic growth of the system. Nevertheless, when annealing at 200 C, Ag- and Pd-rich phases appeared due to clustering of the two materials. In conclusion, much is known not only about the growth and the electronic properties of Ag on Pd and vice versa but also on the post-growth structural properties of the Ag/Pd superlattice. This system looks like the ideal superlattice: perfectly layered when grown at room temperature and without any interdiffusion. Nevertheless until now, no in situ study exists of the evolution of the structure and electronic properties of the superlattice during its growth. The aim of this work is to characterize the Ag/Pd/ Ag(1 1 1) system during the growth, using X-ray photoelectron spectroscopy (XPS) and LEED. These combined techniques allow us to follow the evolution of the surface crystallinity and of the chemical interactions at the interfaces.
2. Experimental procedure Ag and Pd were evaporated on a muscovite mica substrate or on a Ag(1 1 1) single crystal. The freshly cleaved muscovite mica was first annealed Ag layer was to 200 C under UHV before a 1800 A deposited at 1 A/s from a resistively heated tung-
235
sten boat. Before deposition the Ag(1 1 1) single crystal was mechanically polished and cleaned under UHV by repeated sputtering (500 eV) and annealing (250 C) cycles until no contamination was detected by XPS and a sharp hexagonal LEED was observed. The Ag and Pd layers were evaporated from 99.99% purity materials. During deposition the substrate was left at room temperature. Ag and Pd /s respectively were evaporated at 0.01 and 0.002 A from 0.25 mm diameter wires rolled around coiled 0.5 mm tungsten wires. We checked the thickness of the layers with a quartz crystal thickness monitor. The pressure was about 2:5 1010 mbar before the evaporations, and about 2 108 mbar (resp. 2 109 mbar) during the growth of the thick Ag layer (resp. the superlattice). A Scienta 300 photoelectron spectrometer was used to record XPS spectra. This instrument uses a monochromatic Al Ka X-ray source (hm ¼ 1486:6 eV) with a rotating anode operated at 16 kV and 400 mA. Photoelectrons were detected by a hemispherical analyzer (150 eV pass energy) and a two-dimensional position-sensitive detector (microchannel plate with CCD camera). The energy resolution obtained in these conditions is 0.3 eV.
3. Results and discussion 3.1. Ag/Pd/Ag/mica The first multilayer was deposited on Ag/mica prepared following the procedure described above. 16 ML of Pd were then grown on the Ag substrate and we observed a weak hexagonal LEED pattern. Thereafter we recorded XPS spectra of the Ag 3d and Pd 3d core levels (Fig. 1) between the steps of the deposition of 15.2 ML of Ag on the Pd16 ML / Ag/mica system. From the spectra, it can be seen that only intensities vary whereas the position and shape of the core levels is unchanged. Hence, alloying at the interface can be ruled out. 3.2. Ag/Pd/Ag(1 1 1) The second Ag/Pd/Ag trilayer was grown on a Ag(1 1 1) single crystal prepared following the
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Fig. 1. XPS spectra of the Ag 3d and Pd 3d core levels between the steps of the deposition of 15.2 ML of Ag on the Pd16 ML /Ag/ mica system.
procedure described above. Fig. 2a shows the spectra of Ag 3d and Pd 3d core levels during the growth of a 9.2 ML thick Pd layer on the Ag(1 1 1) single crystal and Fig. 2b displays the same levels for the growth of 18.4 ML of Ag on the Pd9:2 ML / Ag(1 1 1) system. The satellites observed at 3.7 and 6.1 eV from the Ag 3d and Pd 3d peaks respectively were amplified for better visibility. There is again no noticeable change in the shape or in the position of the peaks during the growth. The satellite near the Ag 3d peak is a loss induced by the excitation of a bulk (at 3.78 eV) and a surface (3.63 eV) plasmon of silver [11,12]. Fig. 2a shows that the deposition of the Pd overlayer gives rise to the attenuation of the loss induced by the plasmon. The bulk plasmon is not attenuated relatively to the parent line by an overlayer because an overlayer attenuates all the photoelectrons equally. By analogy with the findings of Bates et al. [13] who deposited gold on silver and observed the same plasmon loss attenuation by XPS, we conclude that in our case also the major part of the plasmon peak is in fact of surface character and thus fades out when a Pd overlayer is deposited on silver.
Fig. 2. Ag 3d, Pd 3d core level spectra during the growth of a 9.2 ML thick Pd layer on the Ag(1 1 1) single crystal (a) and during the growth of 18.4 ML of Ag on Pd9:2 ML /Ag(1 1 1) (b). The spectra have been scaled to equal height for easy comparison of lineshapes and the satellites at 3.7 and 6.1 eV from the Ag 3d and Pd 3d lines respectively have been amplified, also to the same height, to make them visible.
In the photoemission process, two final states coexist for the metallic palladium: the (mainly) 4p5 4d10 electronic configurations which gives rise
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237
to Pd 3d peak and the (mainly) 4p5 4d9 which generates the satellite at 6.1 eV from the main peak. This satellite appears when the first Pd layer is completed. During the growth of the Ag top layer, the Ag plasmon loss appears between 2.4 and 3.7 ML, which is probably the required thickness to sustain Ag surface plasmons. The satellite of Pd is not fading out even when covered with 18.4 ML of silver. From the description of this satellite given above, we deduce that in fact, it will be observed as long as the Pd 3d peak itself is observed. In all cases the satellites were always observed at the same energy from the Ag 3d and Pd 3d lines respectively. Fig. 3 displays the valence band spectra obtained during the growth of the Ag/Pd/Ag system. Even though the energy resolution is too low to describe the electronic structure completely, the valence band seems to be simply a linear combination of the bulk valence bands of palladium and silver. We see no hybridization of the bands. This confirms and completes the results of Smith et al. [6]. 3.3. Growth mode In XPS, surface sensitivity can be enhanced by collecting the photoelectrons at small takeoff angle (grazing mode). In contrast, collecting photoelectrons at 90 (normal mode) reduces surface sensitivity. Fig. 4 shows the evolution of the XPS intensities of Ag 3d and Pd 3d in normal and grazing mode during the growth of the system. In normal mode, during the growth of the Pd interlayer, the intensity of the Ag substrate (resp. Pd adsorbate) signal can be fitted with an exponential ed=k (resp. 1 ed=k ) where d is the thickness and k is the photoelectron attenuation length and is set as free parameter. This confirms that the growth of the Pd interlayer is close to the Frank Van der Merwe (FVdM) mode. The values ob 10% for Ag 3d and Pd 3d tained for k are 11.3 A photoelectrons during the growth of the Pd interlayer (Fig. 4). For the growth of the top Ag layer a proper fit of the experimental data could only be achieved ð1Þ ð2Þ using the following functions: IAg ¼ IAg þ IAg þ
Fig. 3. Valence band spectra during the growth of Pd interlayer (a) and the Ag top layer (b).
ð3Þ
IAg and IPd ¼ 1 IAg for the Ag and Pd signals respectively. These functions can be shown to include the following effects as a refinement of a simple layer-by-layer model but only on fraction I0 of the Pd interlayer of thickness d0 :
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Fig. 4. Evolution of the XPS intensities of Ag 3d and Pd 3d in normal (dots along the solid lines) and grazing (dashed lines) mode during the growth of the Ag/Pd/Ag(1 1 1) system. The vertical solid line separates the growth of the Pd layer (on the left) form the growth of the Ag layer. The solid lines are curves fitted based on a modified FVdM model. k is the photoelectron attenuation length. See the text for details.
ð1Þ
• IAg ¼ ð1 I0 Þed0 =k is due to photoelectrons from the lower silver layer through the uncovered part (1 I0 ) of the Pd interlayer. ð2Þ • IAg ¼ I0 ed=k is due to photoelectrons from the lower silver layer through the covered part I0 of the Pd interlayer. ð3Þ • IAg ¼ I0 ð1 expððd d0 Þ=kÞÞ is due to photoelectrons from the upper silver layer. From these equations we extracted I0 and found that voids cover around 12% of the Ag top layer. 10% The values obtained for are 15.8 A for Ag 3d and Pd 3d photoelectrons are in good agreement with those found using Seah and and Dench’s formula [14] (kAg 3d ¼ 17:9 A ). The discrepancies between the kPd 3d ¼ 16:9 A attenuation length obtained for the growth of the Pd interlayer and the Ag top layer are attributed to imprecision in the calibration of the quartz microbalance. In grazing mode, we see that the intensity of the substrate decreases faster than in normal mode and this quick attenuation confirms the layerby-layer growth of the whole Ag/Pd/Ag system. Fig. 5a diplays the sharp hexagonal LEED of the Ag(1 1 1) single crystal surface pattern. During the growth of the Pd layer, the background of the
Fig. 5. LEED patterns of the Ag(1 1 1) single crystal (a) and of the Ag/Pd/Ag system (b).
LEED fades out and the spots nearly disappear after 1.5 ML. Then the hexagonal pattern weakly reappears after 6 ML but even on the 9.2 ML Pd layer the spots are too weak to obtain a good picture. This attenuation of the diffraction pattern was also observed by Smith et al. [6] and Burland and Dobson [4] and explained by a roughening of
J. Dumont et al. / Surface Science 507–510 (2002) 234–239
the surface due to the relaxation of the Pd overlayer to the Pd bulk structure. The presence of some contamination may also play a role in the attenuation. The growth of the Ag layer on the Pd/ Ag system improves the pattern considerably. Fig. 5b shows the LEED image obtained on the 18.4 ML thick Ag layer on Pd/Ag. The tabulated interatomic distance on the Ag(1 1 1) and Pd(1 1 1) surfaces are respectively . Using the LEED pattern ob2.89 and 2.75 A tained on the Ag(1 1 1) single crystal, we deduced for the an interatomic distance of 2:52 0:2 A surface of the Ag(1 1 1)/Pd/Ag system. This means that the top Ag layer surface has an interatomic distance closer to the one of Pd(1 1 1) than to the one of Ag(1 1 1) and that the growth of this Ag layer is pseudomorphic with the Pd interlayer.
4. Conclusion The Ag/Pd/Ag system has been studied by XPS and LEED with muscovite mica or Ag(1 1 1) single crystal as substrates. In both cases no hybridization of the valence bands or chemical reaction or interdiffusion was observed between the Pd and Ag layers. The growth of the Pd layer follows the FVdM mode but is not pseudomorphic on the Ag(1 1 1) substrate. Moreover, the XPS intensities and LEED patterns show that the growth of the Ag top layer on the Pd interlayer is pseudomorphic and layer by layer but contains around 12% of voids.
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Acknowledgements This work was supported by the Belgian Office for Scientific, Technical and Cultural Affairs (PAI 4/10) and by the Belgian National Fund for Scientific Research. JD is supported by the FRIA and JG is supported by the NFSR (Belgium).
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