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Cu–Al surface
Applied Surface Science 254 (2007) 490–493 www.elsevier.com/locate/apsusc
XPS and LEED study of Pd and Au growth on alumina/Cu–Al surface Slavomı´r Nemsˇa´k a,*, Toma´sˇ Ska´la b, Jirˇ´ı Libra a, Petr Hanysˇ a, Karel Masˇek a, Michiko Yoshitake c, Vladimı´r Matolı´n a a
Charles University, Faculty of Mathematics and Physics, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic b Sincrotrone Elettra, Strada Statale 14, km 163.5, 34012 Basovizza-Trieste, Italy c National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan Received 15 March 2007; received in revised form 14 June 2007; accepted 14 June 2007 Available online 19 June 2007
Abstract Metal–insulator–metal system was prepared using the single-crystalline Cu–9at.% Al(1 1 1) support. Oxidation of the substrate under wellcontrolled conditions at elevated temperature leads to the formation of well-ordered aluminium oxide layer. The Pd–Au topmost layer was prepared by a step-by-step deposition of both metals afterwards on the oxide layer at room temperature. Low energy electron diffraction (LEED) measurement did not confirm epitaxial growth of the metal overlayer and gave only a rise of diffuse background after each deposition step. The growth of Pd–Au overlayer exhibited Stranski–Krastanov mode influenced by intermetallic interaction between those metals. No binding energy shifts were visible for the core-level photoelectron peaks of the substrate and the oxide using X-ray photoelectron spectroscopy (XPS). In contrast, the binding energy shifts of Pd 3d and Au 4f photoelectron levels in both directions were observed during all depositions. Bimetallic interactions between the metals as well as size effects are further discussed. # 2007 Elsevier B.V. All rights reserved. Keywords: XPS; LEED; Palladium; Gold; Alumina; Alloy
1. Introduction Alumina is one of the most used ceramics nowadays. Due to its extraordinary mechanical, chemical, optical and electrical properties it is very popular in many fields of science and industry. Ultra-thin alumina films are being prepared on metallic substrates containing aluminium using controlled oxidation [1–3]. Prepared thin films, besides superior electrical and chemical resistivity, are very suitable for investigation by photoelectron spectroscopy. Due to its very low thickness, the film does not suffer from charging effects and it is a perfect candidate for model catalysts support [2,4,5]. Noble metals, like palladium and gold, are widely used in various catalytic reactions nowadays. Their main purpose lies in the low temperature total oxidation of the dangerous exhaust gases. Palladium is known as a material suitable for the adsorption and/or dissociation of CO as well as for the transformation of other hydrocarbons [6,7]. Bimetallic systems
* Corresponding author. Tel.: +420 221912313; fax: +420 283072297. E-mail address: [email protected] (S. Nemsˇa´k). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.019
have attracted the attention due to their unique properties, which can differ from both original compounds. Such bimetallic systems are prepared in order to manufacture catalyst with higher efficiency, better selectivity or longer catalyst life. Moreover, the addition of second metal can favour the reduction of the first one or increase the dispersion of a metal which has tendency to form large particles [8]. The usage of Pd–Au systems covers the total oxidation of volatile organic compounds as byproduct of combustion [9], direct production of hydrogen peroxide [10], liquid-phase oxidation of glycerol [11] and many other reactions. Moreover, it was found that the support plays an important role in the improvement of the catalytic properties. Therefore, the study of Pd–Au growth on ultra-thin alumina layer can bring important results and comparisons with the growth on bulk alumina. 2. Experimental All measurements were performed at Materials Science Beamline, Synchrotron Elettra in an ultra high vacuum chamber with the background pressure of 1 108 Pa. The sample was
S. Nemsˇa´k et al. / Applied Surface Science 254 (2007) 490–493
purchased from Surface Physical Laboratory Inc. and was cut and mechanically polished with accuracy better than 18 with respect to (1 1 1) surface plane. Cleaning of the sample was carried out by repeated subsequent heating at 770 K and Ar+ sputtering (1000 eV, 10 min, 30 mA/cm2). Cleanliness of the sample was checked by XPS and the well-ordered surface structure confirmed by LEED observation. X-ray source used for the XPS measurements was manufactured by Specs GmbH. and was operated with Al Ka line. Hemispherical electron analyzer Phoibos 150 made by the same manufacturer was operated in fixed analyzer transmission mode with resolution 500 meV. The sample was held at elevated temperature of 910 K during whole process of the oxidation. The sample was exposed to the oxygen at the partial pressure of 8 106 Pa for 18.5 h, which corresponds to the oxygen exposure of 4000 L. Au and Pd were alternatively deposited in 24 steps afterwards with LEED and XPS measurements performed after each step. The deposition rates were 0.055 and 0.075 nm/min for Au and Pd, respectively. 3. Results and discussion After sample cleaning, the oxidation process as described in Section 2 took place. The formation of epitaxial well-ordered ultra-thin alumina layer was confirmed by LEED measurement. LEED patterns before and after the oxidation are shown in Fig. 1 and both of them are in well agreement with previous results [12,13]. The diffractograms in Fig. 1(a and b) were measured with primary electron energy of 115 and 69 eV, respectively. Therefore, their size had to be transformed in order to match the corresponding of diffraction spots. pffiffiffi pffiffidistances ffi Clean substrate exhibit ( 3 3)R308 surface reconstruction due to the segregation of Al atoms onto the substrate surface [12]. pffiffiUltra-thin ffi pffiffiffi oxide layer is present on the surface in form of (7= 3 7= 3)R308 reconstruction. From the evaluation of attenuated Cu 2p3/2 XPS peak intensity we estimated the oxide layer thickness to be 0.8 nm. Previously published results show both the larger and lower thickness of the film. For the thin film growth the oxidation procedure is crucial which has to be done with the strictly controlled parameters (oxygen pressure,
491
Fig. 2. Relative intensity of XPS Cu 2p3/2, and Au 4f + Pd 3d peaks as a function of deposition time. Solid line represents result of calculated model for Stranski–Krastanov growth mode.
sample temperature). Other factor is the morphology of the substrate surface. The substrates with different roughness bring different results of the layer thickness. Generally, it can be stated that the more flat and well-ordered surface is, the thicker and less disordered oxide film can be prepared. Variation of the thickness can be explained by different sample preparation treatment (polishing) or quality of the manufacture. Maximum value of the oxide layer thickness was reported to be 3.5 nm [14]. The step-by-step deposition of gold and palladium leads to the increase of the diffuse background intensity of LEED pattern and no diffraction spots belonging to the metals were observed. Therefore, it can be assumed, that metallic overlayer exhibits non-epitaxial type of growth. The amount of deposited palladium after every gold deposition was chosen to make metallic overlayer stoichiometry approximately Pd2Au3 after every Pd deposition. Overall, 12 gold and 12 palladium deposition steps were made in alternate manner. The first deposited metal was gold, final metallic layer thickness was 1.7 nm according to XPS measurement. The evolution of Cu 2p3/2, Au 4f and Pd 3d XPS peak intensity as a function of metals’ deposition time is shown in Fig. 2. XPS measurement was performed after each deposition step, XPS intensity is normalized. Intensity of Pd 3d and Au 4f
Fig. 1. LEED pattern of the clean substrate (a) and the oxide layer after finishing the oxidation process (b).
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peaks are added together with respect to their cross-sections and analyzer transmission function. Due to different deposition rate of palladium and gold, the resulting deposition time in Fig. 2 is not simple sum of deposition times and is corrected to be proportional to metals amount. In addition to experimental points, Fig. 2 shows also the result of the fitting theoretical model for Stranski–Krastanov growth mode of Pd–Au layer. Three models of metallic overlayer growth were evaluated. The first one is Frank–Van der Merwe growth mode, or layerby-layer growth, in which new layer starts to grow only after finishing the growth of the previous one. Volmer–Weber (or 3D) growth mode is approximated by the growth of the cuboid shaped clusters. Our approximation uses the condition of the equal height of all clusters. Minimal height is one monolayer. Stranski–Krastanov growth mode is simulated as a combination of the previous two models—growth of cuboid islands on the surface of the first metallic overlayer. We used a simple calculation supposing exponential damping of the photoelectron signal with depth. By applying these models to the experimental data and fitting the parameters, Stranski– Krastanov growth mode brings the best agreement with the experimental data, closely followed by Volmer–Weber growth mode. However, the results of the 3D growth mode model show initial growth of the metallic clusters one monolayer high and de facto covering the whole substrate with first monolayer. Therefore, we conclude that metallic overlayer grows in Stranski–Krastanov mode. The usual growth mode for palladium and gold on ultra-thin alumina layer is 3D growth mode. It is shown in the case of palladium [15] and gold [16] clusters growth on thin alumina layer prepared on NiAl (1 1 0) single-crystal. Volmer–Weber growth mode is expected if the surface energy of the substrate is lower than that of the deposit, which in our case is fulfilled. Therefore, we conclude that bimetallic interaction between Pd and Au influences the growth mode. The results of work function (WF) measurement are shown in Fig. 3. By applying the biasing voltage of 20 V to the sample one can observe the secondary electrons cut-off. The changes in energetic shifts of secondary electron cut-off correspond to the changes in the WF. However, this method is not suitable to determine absolute value of the WF because it is
Fig. 3. Work function evaluated after each experimental step. Nominal layer thickness is determined by evaluation of XPS data. The first deposited metal was gold.
very difficult to determine exact position of the Fermi edge from the XPS measurements. Therefore, we used measurement performed on the clean Cu–Al sample as the p reference. ffiffiffi pffiffiffi WF value of clean Cu–9 at.% Al (1 1 1) with ( 3 3)R308 surface reconstruction was assumed to be approximately 4.8 eV, as resulted from our UPS measurements on clean Cu– Al. The value was confirmed by synchrotron radiation photoelectron spectroscopy measurement performed with photon primary energy of 106 eV. All WF measurements were performed with analyzer passing energy of 0.5 eV and at the normal emission of photoelectrons with respect to the surface. After the oxidation, WF decreased by about 0.7 eV. Previously measured WF by Kelvin probe (KP) method brought very similar results, however, the UPS measurement values differ [17]. The variation of the WF can be explained by a different sample conditions, or by different procedure for determining the WF. We have used the method presented in comparative study of KP, UPS and low intensity X-ray photoemission spectroscopy [18]. The results in Ref. [18] show, that these methods gives the same value of the WF unless sample is unstable under the presence of ultra-violet or X-ray radiation. In our case, presence of Al-enriched surface, which can be strongly influenced by sample preparation, determines the value of the WF as well as the resulting oxide layer thickness. Stepwise deposition of the metals induced a rise of the WF. After the deposition time of 6–8 min, which according to the results of the XPS intensity evaluation (Fig. 2) meant finishing the first monolayer, the rate of WF increase has attenuated and the value of the WF began to stabilize. The nominal thickness of the layer was approximately 0.3 nm at that time. From the changing of WF evolution behavior one can assume that at this point the formation of the first atomic layer was finished and the surface was fully covered with the Pd and Au atoms, which encourages our suggestion of Stranski–Krastanov growth mode. However, the WF is still below the bulk values of the polycrystalline Pd and Au (5.12 and 5.1 eV, respectively; [19]). Further deposition of the metals leads to the rise of WF value and its stabilization close to the value of 5.1 eV. Fig. 4 shows the evolution of the Au 4f7/2 and Pd 3d5/2 XPS peak position after each deposition step. After the deposition of very small amount of Au, the binding energy (BE) is higher compared to the bulk value (83.9 eV; [20]). This effect is in
Fig. 4. The position of XPS Au 4f7/2 and Pd 3d5/2 peak after each deposition step.
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agreement with BE shift of the dispersed phase. After the deposition of palladium, Pd 3d5/2 peak is also shifted towards higher BE if compared to the bulk value (335.1 eV; [20]). After further deposition of metals, the BE of both Pd 3d and Au 4f peaks decreases. It is in agreement with our assumption of growing size of Pd–Au metallic objects and covering the surface with metallic overlayer. However, even after finishing the first monolayer and until the end of experiment, the BE of both evaluated peaks keeps decreasing, and therefore, energy shifts cannot be explained only by the size effect. In addition, BE changes to the values below the bulk values of metals. Explanation can be found in bimetallic interaction between Pd and Au atoms as proposed before in discussion of metallic layer growth mode. Pd and Au form the solid solution for the entire concentration range [21]. Furthermore, the shift of the binding energy towards lower energies has been reported for Pd–Au alloys [22]. The BE variation can be explained by the electron charge transfer between Pd and Au atoms as well as by the changes in the valence band of metals. The bimetallic interaction plays role also in changes of WF (Fig. 3).
Czech Republic. It was also supported by the Grant Agency of Charles University in Prague under the grants no. 205/2006/BFYZ/MFF and 243/2005/B-FYZ/MFF.
4. Conclusions
[10]
The oxidation procedure leads to the formation of epitaxial and well-ordered ultra-thin alumina film of thickness 0.8 nm. The photoelectric WF decreases by 0.7 eV after finishing the oxide layer. The metallic overlayer 1.7 nm thick consisted of Pd and Au does not grow epitaxially according to the LEED measurement. Bimetallic interaction between Au and Pd atoms results in Stranski–Krastanov mode of metallic layer growth. Palladium and gold forms alloy according to the XPS measurement of Pd 3d5/2 and Au 4f7/2 peaks, which both exhibit shift towards lower BE. WF of the sample after the final deposition step stabilizes at WF value close to the value of bulk polycrystalline Au and Pd.
References [1] [2] [3] [4]
[5]
[6] [7] [8] [9]
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements This work is a part of the research program no. MSM 0021620834 that is financed by the Ministry of Education of the
[21] [22]
C.T. Campbell, Surf. Sci. Rep. 27 (1997) 1. C.R. Henry, Surf. Sci. Rep. 31 (1998) 231. R. Franchy, Surf. Sci. Rep. 38 (2000) 195. Sh. Shaikhutdinov, M. Heemeier, J. Hoffmann, I. Meusel, B. Richter, M. Ba¨umer, H. Kuhlenbeck, J. Libuda, H.-J. Freund, R. Oldman, et al. Surf. Sci. 501 (2002) 270. S. Andersson, P.A. Bru¨hwiler, A. Sandell, M. Frank, J. Libuda, A. Giertz, B. Brena, A.J. Maxwell, M. Ba¨umer, H.-J. Freund, et al. Surf. Sci. 442 (1999) L964. T. Sekiba, S. Kimura, H. Yamamoto, A. Okada, Catal. Today 22 (1994) 113. A.L. Boehman, S. Niksa, Appl. Catal. B, Environ. 8 (1996) 41. G. Riahi, D. Guillemot, M. Polisset-Thfoin, A.A. Khodadadi, J. Fraissard, Catal. Today 72 (2002) 115. M. Hosseini, S. Siffert, H.L. Tidahy, R. Cousin, J.-F. Lamonier, A. Aboukais, A. Vantomme, M. Roussel, B.-L. Su, Catal. Today 122 (2007) 391. J.K. Edwards, B.E. Solsona, P. Landon, A.F. Carley, A. Herzing, C.J. Kiely, G.J. Hutchings, J. Catal. 236 (2005) 69. L. Prati, A. Villa, F. Porta, D. Wang, D. Su, Catal. Today 122 (2007) 386. M. Yoshitake, S. Bera, Y. Yamauchi, Surf. Interf. Anal. 35 (2003) 824. S. Nemsˇa´k, M. Yoshitake, K. Masˇek, Surf. Sci. 600 (2006) 4357. Y. Yamauchi, M. Yoshitake, W. Song, Jpn. J. Appl. Phys. 42 (2005) 4721. M. Ba¨umer, M. Frank, M. Heemeier, R. Ku¨hnemuth, S. Stempel, H.-J. Freund, Surf. Sci. 454 (2000) 957. C. Winkler, A. Carew, R. Raval, J. Ledieu, R. McGrath, Surf. Rev. Lett. 8 (2001) 693. W. Song, M. Yoshitake, Surf. Interf. Anal. 38 (2006) 793. M.M. Beerbom, B. La¨gel, A.J. Cascio, B.V. Doran, R. Schlaf, J. Electr. Spectr. 152 (2006) 12. R.C. Weast, CRC Handbook of Chemistry and Physics, 58th ed., CRC Press West Palm Beach, Florida, 1977. N. Ikeo, Y. Iijima, N. Nimura, M. Sigematsu, T. Tazawa, S. Matsumoto, K. Kojima, Y. Nagasawa, Handbook of X-ray Photoelectron Spectroscopy, JEOL, Tokyo, 1991. H. Okamoto, T.B. Massalski, Bull. Alloy Phase Diagr. 6 (1985) 229. P.A.P. Nascente, S.G.C. de Castro, R. Landers, G.G. Kleiman, Phys. Rev. B 43 (1991) 4659.
XPS and LEED study of Pd and Au growth on alumina/Cu–Al surface Slavomı´r Nemsˇa´k a,*, Toma´sˇ Ska´la b, Jirˇ´ı Libra a, Petr Hanysˇ a, Karel Masˇek a, Michiko Yoshitake c, Vladimı´r Matolı´n a a
Charles University, Faculty of Mathematics and Physics, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic b Sincrotrone Elettra, Strada Statale 14, km 163.5, 34012 Basovizza-Trieste, Italy c National Institute for Materials Science, 3-13 Sakura, Tsukuba 305-0003, Japan Received 15 March 2007; received in revised form 14 June 2007; accepted 14 June 2007 Available online 19 June 2007
Abstract Metal–insulator–metal system was prepared using the single-crystalline Cu–9at.% Al(1 1 1) support. Oxidation of the substrate under wellcontrolled conditions at elevated temperature leads to the formation of well-ordered aluminium oxide layer. The Pd–Au topmost layer was prepared by a step-by-step deposition of both metals afterwards on the oxide layer at room temperature. Low energy electron diffraction (LEED) measurement did not confirm epitaxial growth of the metal overlayer and gave only a rise of diffuse background after each deposition step. The growth of Pd–Au overlayer exhibited Stranski–Krastanov mode influenced by intermetallic interaction between those metals. No binding energy shifts were visible for the core-level photoelectron peaks of the substrate and the oxide using X-ray photoelectron spectroscopy (XPS). In contrast, the binding energy shifts of Pd 3d and Au 4f photoelectron levels in both directions were observed during all depositions. Bimetallic interactions between the metals as well as size effects are further discussed. # 2007 Elsevier B.V. All rights reserved. Keywords: XPS; LEED; Palladium; Gold; Alumina; Alloy
1. Introduction Alumina is one of the most used ceramics nowadays. Due to its extraordinary mechanical, chemical, optical and electrical properties it is very popular in many fields of science and industry. Ultra-thin alumina films are being prepared on metallic substrates containing aluminium using controlled oxidation [1–3]. Prepared thin films, besides superior electrical and chemical resistivity, are very suitable for investigation by photoelectron spectroscopy. Due to its very low thickness, the film does not suffer from charging effects and it is a perfect candidate for model catalysts support [2,4,5]. Noble metals, like palladium and gold, are widely used in various catalytic reactions nowadays. Their main purpose lies in the low temperature total oxidation of the dangerous exhaust gases. Palladium is known as a material suitable for the adsorption and/or dissociation of CO as well as for the transformation of other hydrocarbons [6,7]. Bimetallic systems
* Corresponding author. Tel.: +420 221912313; fax: +420 283072297. E-mail address: [email protected] (S. Nemsˇa´k). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.06.019
have attracted the attention due to their unique properties, which can differ from both original compounds. Such bimetallic systems are prepared in order to manufacture catalyst with higher efficiency, better selectivity or longer catalyst life. Moreover, the addition of second metal can favour the reduction of the first one or increase the dispersion of a metal which has tendency to form large particles [8]. The usage of Pd–Au systems covers the total oxidation of volatile organic compounds as byproduct of combustion [9], direct production of hydrogen peroxide [10], liquid-phase oxidation of glycerol [11] and many other reactions. Moreover, it was found that the support plays an important role in the improvement of the catalytic properties. Therefore, the study of Pd–Au growth on ultra-thin alumina layer can bring important results and comparisons with the growth on bulk alumina. 2. Experimental All measurements were performed at Materials Science Beamline, Synchrotron Elettra in an ultra high vacuum chamber with the background pressure of 1 108 Pa. The sample was
S. Nemsˇa´k et al. / Applied Surface Science 254 (2007) 490–493
purchased from Surface Physical Laboratory Inc. and was cut and mechanically polished with accuracy better than 18 with respect to (1 1 1) surface plane. Cleaning of the sample was carried out by repeated subsequent heating at 770 K and Ar+ sputtering (1000 eV, 10 min, 30 mA/cm2). Cleanliness of the sample was checked by XPS and the well-ordered surface structure confirmed by LEED observation. X-ray source used for the XPS measurements was manufactured by Specs GmbH. and was operated with Al Ka line. Hemispherical electron analyzer Phoibos 150 made by the same manufacturer was operated in fixed analyzer transmission mode with resolution 500 meV. The sample was held at elevated temperature of 910 K during whole process of the oxidation. The sample was exposed to the oxygen at the partial pressure of 8 106 Pa for 18.5 h, which corresponds to the oxygen exposure of 4000 L. Au and Pd were alternatively deposited in 24 steps afterwards with LEED and XPS measurements performed after each step. The deposition rates were 0.055 and 0.075 nm/min for Au and Pd, respectively. 3. Results and discussion After sample cleaning, the oxidation process as described in Section 2 took place. The formation of epitaxial well-ordered ultra-thin alumina layer was confirmed by LEED measurement. LEED patterns before and after the oxidation are shown in Fig. 1 and both of them are in well agreement with previous results [12,13]. The diffractograms in Fig. 1(a and b) were measured with primary electron energy of 115 and 69 eV, respectively. Therefore, their size had to be transformed in order to match the corresponding of diffraction spots. pffiffiffi pffiffidistances ffi Clean substrate exhibit ( 3 3)R308 surface reconstruction due to the segregation of Al atoms onto the substrate surface [12]. pffiffiUltra-thin ffi pffiffiffi oxide layer is present on the surface in form of (7= 3 7= 3)R308 reconstruction. From the evaluation of attenuated Cu 2p3/2 XPS peak intensity we estimated the oxide layer thickness to be 0.8 nm. Previously published results show both the larger and lower thickness of the film. For the thin film growth the oxidation procedure is crucial which has to be done with the strictly controlled parameters (oxygen pressure,
491
Fig. 2. Relative intensity of XPS Cu 2p3/2, and Au 4f + Pd 3d peaks as a function of deposition time. Solid line represents result of calculated model for Stranski–Krastanov growth mode.
sample temperature). Other factor is the morphology of the substrate surface. The substrates with different roughness bring different results of the layer thickness. Generally, it can be stated that the more flat and well-ordered surface is, the thicker and less disordered oxide film can be prepared. Variation of the thickness can be explained by different sample preparation treatment (polishing) or quality of the manufacture. Maximum value of the oxide layer thickness was reported to be 3.5 nm [14]. The step-by-step deposition of gold and palladium leads to the increase of the diffuse background intensity of LEED pattern and no diffraction spots belonging to the metals were observed. Therefore, it can be assumed, that metallic overlayer exhibits non-epitaxial type of growth. The amount of deposited palladium after every gold deposition was chosen to make metallic overlayer stoichiometry approximately Pd2Au3 after every Pd deposition. Overall, 12 gold and 12 palladium deposition steps were made in alternate manner. The first deposited metal was gold, final metallic layer thickness was 1.7 nm according to XPS measurement. The evolution of Cu 2p3/2, Au 4f and Pd 3d XPS peak intensity as a function of metals’ deposition time is shown in Fig. 2. XPS measurement was performed after each deposition step, XPS intensity is normalized. Intensity of Pd 3d and Au 4f
Fig. 1. LEED pattern of the clean substrate (a) and the oxide layer after finishing the oxidation process (b).
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peaks are added together with respect to their cross-sections and analyzer transmission function. Due to different deposition rate of palladium and gold, the resulting deposition time in Fig. 2 is not simple sum of deposition times and is corrected to be proportional to metals amount. In addition to experimental points, Fig. 2 shows also the result of the fitting theoretical model for Stranski–Krastanov growth mode of Pd–Au layer. Three models of metallic overlayer growth were evaluated. The first one is Frank–Van der Merwe growth mode, or layerby-layer growth, in which new layer starts to grow only after finishing the growth of the previous one. Volmer–Weber (or 3D) growth mode is approximated by the growth of the cuboid shaped clusters. Our approximation uses the condition of the equal height of all clusters. Minimal height is one monolayer. Stranski–Krastanov growth mode is simulated as a combination of the previous two models—growth of cuboid islands on the surface of the first metallic overlayer. We used a simple calculation supposing exponential damping of the photoelectron signal with depth. By applying these models to the experimental data and fitting the parameters, Stranski– Krastanov growth mode brings the best agreement with the experimental data, closely followed by Volmer–Weber growth mode. However, the results of the 3D growth mode model show initial growth of the metallic clusters one monolayer high and de facto covering the whole substrate with first monolayer. Therefore, we conclude that metallic overlayer grows in Stranski–Krastanov mode. The usual growth mode for palladium and gold on ultra-thin alumina layer is 3D growth mode. It is shown in the case of palladium [15] and gold [16] clusters growth on thin alumina layer prepared on NiAl (1 1 0) single-crystal. Volmer–Weber growth mode is expected if the surface energy of the substrate is lower than that of the deposit, which in our case is fulfilled. Therefore, we conclude that bimetallic interaction between Pd and Au influences the growth mode. The results of work function (WF) measurement are shown in Fig. 3. By applying the biasing voltage of 20 V to the sample one can observe the secondary electrons cut-off. The changes in energetic shifts of secondary electron cut-off correspond to the changes in the WF. However, this method is not suitable to determine absolute value of the WF because it is
Fig. 3. Work function evaluated after each experimental step. Nominal layer thickness is determined by evaluation of XPS data. The first deposited metal was gold.
very difficult to determine exact position of the Fermi edge from the XPS measurements. Therefore, we used measurement performed on the clean Cu–Al sample as the p reference. ffiffiffi pffiffiffi WF value of clean Cu–9 at.% Al (1 1 1) with ( 3 3)R308 surface reconstruction was assumed to be approximately 4.8 eV, as resulted from our UPS measurements on clean Cu– Al. The value was confirmed by synchrotron radiation photoelectron spectroscopy measurement performed with photon primary energy of 106 eV. All WF measurements were performed with analyzer passing energy of 0.5 eV and at the normal emission of photoelectrons with respect to the surface. After the oxidation, WF decreased by about 0.7 eV. Previously measured WF by Kelvin probe (KP) method brought very similar results, however, the UPS measurement values differ [17]. The variation of the WF can be explained by a different sample conditions, or by different procedure for determining the WF. We have used the method presented in comparative study of KP, UPS and low intensity X-ray photoemission spectroscopy [18]. The results in Ref. [18] show, that these methods gives the same value of the WF unless sample is unstable under the presence of ultra-violet or X-ray radiation. In our case, presence of Al-enriched surface, which can be strongly influenced by sample preparation, determines the value of the WF as well as the resulting oxide layer thickness. Stepwise deposition of the metals induced a rise of the WF. After the deposition time of 6–8 min, which according to the results of the XPS intensity evaluation (Fig. 2) meant finishing the first monolayer, the rate of WF increase has attenuated and the value of the WF began to stabilize. The nominal thickness of the layer was approximately 0.3 nm at that time. From the changing of WF evolution behavior one can assume that at this point the formation of the first atomic layer was finished and the surface was fully covered with the Pd and Au atoms, which encourages our suggestion of Stranski–Krastanov growth mode. However, the WF is still below the bulk values of the polycrystalline Pd and Au (5.12 and 5.1 eV, respectively; [19]). Further deposition of the metals leads to the rise of WF value and its stabilization close to the value of 5.1 eV. Fig. 4 shows the evolution of the Au 4f7/2 and Pd 3d5/2 XPS peak position after each deposition step. After the deposition of very small amount of Au, the binding energy (BE) is higher compared to the bulk value (83.9 eV; [20]). This effect is in
Fig. 4. The position of XPS Au 4f7/2 and Pd 3d5/2 peak after each deposition step.
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agreement with BE shift of the dispersed phase. After the deposition of palladium, Pd 3d5/2 peak is also shifted towards higher BE if compared to the bulk value (335.1 eV; [20]). After further deposition of metals, the BE of both Pd 3d and Au 4f peaks decreases. It is in agreement with our assumption of growing size of Pd–Au metallic objects and covering the surface with metallic overlayer. However, even after finishing the first monolayer and until the end of experiment, the BE of both evaluated peaks keeps decreasing, and therefore, energy shifts cannot be explained only by the size effect. In addition, BE changes to the values below the bulk values of metals. Explanation can be found in bimetallic interaction between Pd and Au atoms as proposed before in discussion of metallic layer growth mode. Pd and Au form the solid solution for the entire concentration range [21]. Furthermore, the shift of the binding energy towards lower energies has been reported for Pd–Au alloys [22]. The BE variation can be explained by the electron charge transfer between Pd and Au atoms as well as by the changes in the valence band of metals. The bimetallic interaction plays role also in changes of WF (Fig. 3).
Czech Republic. It was also supported by the Grant Agency of Charles University in Prague under the grants no. 205/2006/BFYZ/MFF and 243/2005/B-FYZ/MFF.
4. Conclusions
[10]
The oxidation procedure leads to the formation of epitaxial and well-ordered ultra-thin alumina film of thickness 0.8 nm. The photoelectric WF decreases by 0.7 eV after finishing the oxide layer. The metallic overlayer 1.7 nm thick consisted of Pd and Au does not grow epitaxially according to the LEED measurement. Bimetallic interaction between Au and Pd atoms results in Stranski–Krastanov mode of metallic layer growth. Palladium and gold forms alloy according to the XPS measurement of Pd 3d5/2 and Au 4f7/2 peaks, which both exhibit shift towards lower BE. WF of the sample after the final deposition step stabilizes at WF value close to the value of bulk polycrystalline Au and Pd.
References [1] [2] [3] [4]
[5]
[6] [7] [8] [9]
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Acknowledgements This work is a part of the research program no. MSM 0021620834 that is financed by the Ministry of Education of the
[21] [22]
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