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Ultrathin metal films on TiO2(110): metal overlayer spreading and surface reactivity
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Surface Science North-Holland
287/288
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(1993) 896-900
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Ultrathin metal films on Ti02( 110): metal overlayer spreading and surface reactivity U. Diebold, J.-M. Pan and T.E. Madey Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers, The State Unklersity of New Jersey, Piscataway, NJ 08855, USA Received
1 September
1992; accepted
for publication
11 October
1992
We have studied the growth of ultrathin (< 50 .&) Cu overlayers on stoichiometric TiO,(llO) surfaces. Using low energy ion scattering, the growth mode of Cu was determined to be Volmer-Weber type (three-dimensional islands) at room temperature; evidence for cluster growth is observed even at 160 K. A comparison with Fe and Cr metal films shows that the spreading of the films correlates with the heat of formation of the metal oxide of the overlayer. Fe and Cr cause a reduction of the Ti4+ surface cations at the metal/metal oxide interface as observed with X-ray photoelectron spectroscopy. No reduction of the TiOz cations takes place upon contact with a Cu overlayer.
1. Introduction
Metal/oxide interfaces are important in many areas of science and technology, among them microelectronics, sensors, ceramic materials and heterogeneous catalysis. Because of the high catalytic activity of transition metals dispersed on a metal oxide support, metal/metal oxide systems have been studied with great interest. One of the crucial properties of a catalyst is the ability of the transition metal to wet the support, since this will affect the dispersion of the catalytically active elements. Additionally, the support material itself may play a role, e.g., the reduced states of the cation of some metal oxides may influence the catalytic reactivity [1,2]. Titanium dioxide is a good candidate for studying this phenomenon, since TiO, is stable in several reduced states. Because of this and the relative ease of handling TiO, experimentally, several metal overlayer studies have been performed on single crystal surfaces [3-131. This study is part of a systematic investigation of a series of ultrathin metal films using the 0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
(rutile) TiO,(llO) surface as a model support material. A more detailed description of growth modes, thermal treatment and geometric structure of various transition metal overlayers is given elsewhere [14-161. The growth behavior of thin metal films is usually categorized in three modes: three-dimensional islanding (“Volmer-Weber”) where the overlayer does not wet the substrate, complete wetting of the first monolayer followed by growth in three-dimensional islands (“StranskiKrastanov”) and growth in a layer-by-layer like fashion (“Franck-van der Merwe”). We have used low energy ion scattering (LEIS), which is very sensitive to the topmost layer of a surface to distinguish between the first and the latter modes. We show that Cu grows in a Volmer-Weber mode with substrate TiO, being obseyed up to very high nominal coverages (> 50 A) in the LEIS spectra. The thermal behavior of the clustering is studied also. The LEIS results for Cu are compared to the trends for the growth of Cr and Fe films. X-ray photoelectron spectroscopy (XPS) was employed to obtain chemical information
B.V. All rights reserved
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
897
about the metal/oxide interface and we show that electronic changes at the interface are closely correlated with the wetting ability of the metal on TiO,(llO).
2. Experimental The experimental details of the apparatus are described elsewhere [16,17]. In brief, it consists of a UHV chamber equipped with a hemispherical analyzer, twin anode X-ray source, a differentially pumped ion gun and LEED/ESDIAD (low energy electron diffraction/ electron stimulated desorption ion angular distributions) optics. The metal films were evaporated from a hot filament loaded with the pure metal. The average thickness of the overlayer was obtained from a quartz crystal thickness monitor, cross calibrated with Rutherford backscattering. The XPS data shown were taken with Al Ka radiation. The LEIS measurements were performed with a 1.5 keV ion beam and a scattering angle of 135”. Either He (for Cu and Fe overlayers) or Ne (for Cr) was used as a probe ion; the better mass resolution of LEIS with Ne ions enables us to distinguish between Cr and substrate Ti. Care has been taken to limit damage by ion bombardment during the LEIS measurements. The (rutile) TiO,(llO) single crystal surface was prepared by sputtering and subsequent annealing in an oxygen atmosphere. This treatment, as described in detail by Pan et al. [18] results in a nearly perfect, stoichiometric surface with no oxygen vacancies observable.
3. Results Fig. 1 shows the attenuation of the substrate Ti (LEIS) signal with increasing Cu coverage at room temperature. Low energy rare gas ions are neutralized with a very high probability during the interaction with the surface; this makes LEIS an extremely surface-sensitive tool. The attenuation of the substrate signal can be taken as an upper limit for the covering of the surface by overlayer atoms. For complete wetting of the substrate by the metal overlayer one would ex-
thickness [A] Fig. 1. The integrated peak area of the titanium LEIS signal versus average film thickness, normalized to the value of a clean, uncovered Ti surface. The full circles show the Ti attenuation with Cu overlayer, and the full line was drawn to guide the eye. The trends for attenuation by Fe (long-dashed line) and Cr (short-dashed line) overlayers are indicated. The data were taken at room temperature.
pect a linear decrease with the signal apgroaching zero at monolayer coverage (N 2 A for a Cu(ll1) overlayer [3,16]). This is very different from the behavior shown in fig. 1. We take the fact that the LEIS substrate signal is visible up to high average coverages as a clear evidence for Volmer-Weber (three-dimensional islands) growth, in disagreement with the StranksiKrastanov growth mode proposed formerly for Cu/TiO,(llO) [3]. A more detailed analysis show! that even a film with an average thickness of 30 A (as determined with the quartz crystal monitor) leaves * 37% of the surface uncovered [16]. Fig. 1 shows the trend of the Ti LEIS signal during Fe and Cr deposition. Clearly the attenuation of the substrate signal with increasing average thickness is much more pronounced for these materials. The average thickness necessary for complete suppression of the substrate signal is lower for Cr than for Fe. A Franck-van der Merwe growth was proposed by Deng et al. 171 for Fe/TiO,(llO), but neither Fe nor Cr films
898
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
show the behavior expected for complete wetting by the first monolayer. However, it is clear that Cr and Fe spread to wet the TiO, substrate more effectively than Cu. A cluster growth process is also observed when Cu was deposited onto a cooled surface (160 K). No substantial deviation from the behavior at room temperature deposition was observed with LEIS. At 160 K, the cluster formation process can be observed within the time scale of the experiment. The top curve of fig. 2 shows the change of the XPS Cu signal with time after the initial deposition. The overlayer signal decreases a few percent, at the same time the 0 1s and Ti 2p3,* substrate signals increase by a similar amount (not shown in fig. 2). This can be explained by a coarsening of the clusters which are formed upon deposition. The self attenuation of the copper photoelectrons within the clusters is increased during coarsening, which leads to a decrease in the Cu photoelectron intensity. The decrease is more pronounced for a surface an-
464
460
456
452
Binding Energy [eV] Fig. 3. XPS Ti2p signals for 2 A (a) Cu, (b) Fe and (c) Cr on TiO,(liO). For comparison, the spectrum for the uncovered surface is shown in each case (dotted lines). Deposition was performed at room temperature.
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Fig. 2. Change of the XPS Cu2p,,, signal versus annealing time at the temperatures indicated. Initial deposition and data collection were performed at room temperature, except for the upper curve where deposition was done at 160 K. The signal was normalized f,” the as-deposited values. The average film thickness was 27 A for annealing at 160 K and 18 A for all others.
nealed to higher temperature following deposition at 300 K (lower curves of fig. 2). The high mobility of Cu at these low temperatures is due to a very weak interaction with the substrate. Fig. 3a shows the XPS Ti2p,,, signal of a clean TiO,(llO) surface aad of a surface with an average Cu thickness of 2 A. The line shape of the Ti signal remains unchanged; only a slight intensity decrease is visible. This behavior is contrasted with the zhange of the Ti 2p,,, peak after deposition of 2 A Fe and Cr, respectively (figs. 3b and 3~). The formal ionic charge for Ti is 4 + for a stoichiometric, well-oxidized surface. The shoulder at lower binding energy indicates the presence of reduced states of Ti; Ti3+ shows a shift in binding energy of - 1.8 eV [8]. The shoulder is even more pronounced for a Cr overlayer, and the broadening might include a contribution from a lower reduced state. The reduction of the TiO, cations is accompanied by oxidation of the Fe and Cr atoms [14,15].
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
Fe
Cr
Fig. 4. Schematic drawing of ultrathin Cu, Fe and Cr films on a TiO,(llO) surface.
4. Discussion
The wetting behavior of a film on a surface is governed by the energy balance of the surface free energy of the constituents and the energy of the interface. The surface free energy of TiO, is very low compared to the transition metals under consideration [193; from this one would predict a strong tendency for Volmer-Weber growth for Cu, Fe and Cr on TiO,. Our results show that considerations based only on the surface free energies of the substrate and the overlayer are not sufficient for these systems. The fact that Cr and Fe spread more effectively than Cu must be due to the strength of the metal/oxide bond at the interface. The different reactivity of the various metals towards the surface oxygen appears to be a decisive factor governing the interaction between the metal overlayers and the TiO, surface. The reactivity itself results from electron transfer from the overlayer metal via the oxygen ion to the substrate metal ion. The larger the heat of formation of the metal oxide of the overlayer metal, namely Cr > Fe > Cu, the more effective is the covering of the surface atoms at the same average film thickness. This is shown schematically in fig. 4. Peden et al. [20] observed a change of the growth mode of ultrathin metal films on an oxi-
899
dized tungsten surface, from complete non-wetting (Volmer-Weber growth) for Cu to a layer growth mode for Cr, and correlated the wetting tendency with the metal-oxygen bond strength of the metal overlayer. Although the details are different, we observe a similar trend on TiO,(llO): Cr spreads better than Fe, and both Cr and Fe spread better than Cu, which does not appear to wet the surface effectively.
5. Conclusion
This work shows the close correlation between the wetting ability of 3d-transition metal overlayers and the formation of reduced Ti states metal/TiO,(llO) interface. The combination of LEIS, which is extremely surface sensitive, and XPS, which provides chemical information, is very useful for studying interface phenomena.
Acknowledgement
This work was supported in part by the National Science Foundation, grant DMR-8907553.
References
111G.L. Haller and D.E. Resasco, Adv. Catal. 36 (1989) 173. 121S.J. Tauster, in: Strong-Metal Support Interactions, Eds. R.T.K. Baker, S.J. Tauster and J.A. Dumesic (American Chemical Society, Washington, 1986) p. 21. [31 P.J. Moller and M.-C. Wu, Surf. Sci. 224 (1989) 265. 141 MC Wu and P.J. MoIIer, Surf. Sci. 224 (1989) 250. 151 H. Onishi, T. Aruga, C. Egawa and Y. Iwasawa, Surf. Sci. 199 (1988) 54. 161 R.J. Lad and L.S. Dake, Proc. Mater. Res. Sot. 238 (1992) 823. 171J. Deng, D. Wang, X. Wei, R. Zhai and H. Wang, Surf. Sci. 249 (1991) 213. 181 G. Rocker and W. Giipel, Surf. Sci. 181 (1987) 530. 191H.R. Sadhegi, Appt. Surf. Sci. 19 (1984) 330. m M.-C. Wu and P.J. Moller, Surf. Sci. 235 (1990) 228. illI Z. Zhang and V.E. Henrich, Surf. Sci. 277 (1992) 263. WI V. Andera, Appl. Surf. Sci. 51 (1991) 1. [131 D. Brugniau, S.D. Parker and G.E. Rhead, This Solid Films 121 (1984) 247. [141 J.-M. Pan, U. Diebold, L. Zhang and T.E. Madey, Surf. Sci., submitted.
900
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
[15] J.-M. Pan and T.E. Madey, J. Vat. Sci. Technol. (1993), in press. [16] U. Diebold, J.-M. Pan and T.E. Madey, Phys. Rev. B 47 (1993). [17] B.L. Maschhoff, J.-M. Pan and T.E. Madey, Surf. Sci. 259 (1991) 190.
[18] J.-M. Pan, U. Diebold, B.L. Maschhoff and T.E. Madey, J. Vat. Sci. Technol. A 10 (1992) 2470. 1191 S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [20] C.H.F. Peden, K.B. Kidd and N.D. Shinn. J. Vat. Sci. Technol. A 9 (1991) 1518.
~~,~::::::i:~::::::::s::jj:.:.: ......... .-‘...:.:i..‘.‘.:.~: . ..... .:.:.z+.:.::::~.:.:+.: ....... .... .... ... .,.,.,,..,.,,,,,,,,,,,, .. . . :.:.:.:.:‘...:.>:.:
.;:;.;::::::::::
Surface Science North-Holland
287/288
::....
. . . . . . . . . . . . . . ;:.:
..,...:.:.:
~
. . . . .. ..;,:.:. .j
surface science ::-..: .A... .,...... ..,.,. ....:.:.::.: ....... : ‘.’..‘.C. .i...:.:.:.:.~ ....~:.:. :.:i:~:i:i::.:::::~:~:~~~,~ ‘y:““:.
(1993) 896-900
.’“““‘.:.::::!:::i:~:::::~..:.:.~~.~ “”““‘:‘.‘...:.:.i...: ........‘....‘i...............~.~.~.~~.~ ‘.‘i.‘........ ““-‘......A ....:.,.,.(_,(, .~: ....~,.,:,) ::,,,,,,,(,
Ultrathin metal films on Ti02( 110): metal overlayer spreading and surface reactivity U. Diebold, J.-M. Pan and T.E. Madey Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers, The State Unklersity of New Jersey, Piscataway, NJ 08855, USA Received
1 September
1992; accepted
for publication
11 October
1992
We have studied the growth of ultrathin (< 50 .&) Cu overlayers on stoichiometric TiO,(llO) surfaces. Using low energy ion scattering, the growth mode of Cu was determined to be Volmer-Weber type (three-dimensional islands) at room temperature; evidence for cluster growth is observed even at 160 K. A comparison with Fe and Cr metal films shows that the spreading of the films correlates with the heat of formation of the metal oxide of the overlayer. Fe and Cr cause a reduction of the Ti4+ surface cations at the metal/metal oxide interface as observed with X-ray photoelectron spectroscopy. No reduction of the TiOz cations takes place upon contact with a Cu overlayer.
1. Introduction
Metal/oxide interfaces are important in many areas of science and technology, among them microelectronics, sensors, ceramic materials and heterogeneous catalysis. Because of the high catalytic activity of transition metals dispersed on a metal oxide support, metal/metal oxide systems have been studied with great interest. One of the crucial properties of a catalyst is the ability of the transition metal to wet the support, since this will affect the dispersion of the catalytically active elements. Additionally, the support material itself may play a role, e.g., the reduced states of the cation of some metal oxides may influence the catalytic reactivity [1,2]. Titanium dioxide is a good candidate for studying this phenomenon, since TiO, is stable in several reduced states. Because of this and the relative ease of handling TiO, experimentally, several metal overlayer studies have been performed on single crystal surfaces [3-131. This study is part of a systematic investigation of a series of ultrathin metal films using the 0039-6028/93/$06.00
0 1993 - Elsevier
Science
Publishers
(rutile) TiO,(llO) surface as a model support material. A more detailed description of growth modes, thermal treatment and geometric structure of various transition metal overlayers is given elsewhere [14-161. The growth behavior of thin metal films is usually categorized in three modes: three-dimensional islanding (“Volmer-Weber”) where the overlayer does not wet the substrate, complete wetting of the first monolayer followed by growth in three-dimensional islands (“StranskiKrastanov”) and growth in a layer-by-layer like fashion (“Franck-van der Merwe”). We have used low energy ion scattering (LEIS), which is very sensitive to the topmost layer of a surface to distinguish between the first and the latter modes. We show that Cu grows in a Volmer-Weber mode with substrate TiO, being obseyed up to very high nominal coverages (> 50 A) in the LEIS spectra. The thermal behavior of the clustering is studied also. The LEIS results for Cu are compared to the trends for the growth of Cr and Fe films. X-ray photoelectron spectroscopy (XPS) was employed to obtain chemical information
B.V. All rights reserved
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
897
about the metal/oxide interface and we show that electronic changes at the interface are closely correlated with the wetting ability of the metal on TiO,(llO).
2. Experimental The experimental details of the apparatus are described elsewhere [16,17]. In brief, it consists of a UHV chamber equipped with a hemispherical analyzer, twin anode X-ray source, a differentially pumped ion gun and LEED/ESDIAD (low energy electron diffraction/ electron stimulated desorption ion angular distributions) optics. The metal films were evaporated from a hot filament loaded with the pure metal. The average thickness of the overlayer was obtained from a quartz crystal thickness monitor, cross calibrated with Rutherford backscattering. The XPS data shown were taken with Al Ka radiation. The LEIS measurements were performed with a 1.5 keV ion beam and a scattering angle of 135”. Either He (for Cu and Fe overlayers) or Ne (for Cr) was used as a probe ion; the better mass resolution of LEIS with Ne ions enables us to distinguish between Cr and substrate Ti. Care has been taken to limit damage by ion bombardment during the LEIS measurements. The (rutile) TiO,(llO) single crystal surface was prepared by sputtering and subsequent annealing in an oxygen atmosphere. This treatment, as described in detail by Pan et al. [18] results in a nearly perfect, stoichiometric surface with no oxygen vacancies observable.
3. Results Fig. 1 shows the attenuation of the substrate Ti (LEIS) signal with increasing Cu coverage at room temperature. Low energy rare gas ions are neutralized with a very high probability during the interaction with the surface; this makes LEIS an extremely surface-sensitive tool. The attenuation of the substrate signal can be taken as an upper limit for the covering of the surface by overlayer atoms. For complete wetting of the substrate by the metal overlayer one would ex-
thickness [A] Fig. 1. The integrated peak area of the titanium LEIS signal versus average film thickness, normalized to the value of a clean, uncovered Ti surface. The full circles show the Ti attenuation with Cu overlayer, and the full line was drawn to guide the eye. The trends for attenuation by Fe (long-dashed line) and Cr (short-dashed line) overlayers are indicated. The data were taken at room temperature.
pect a linear decrease with the signal apgroaching zero at monolayer coverage (N 2 A for a Cu(ll1) overlayer [3,16]). This is very different from the behavior shown in fig. 1. We take the fact that the LEIS substrate signal is visible up to high average coverages as a clear evidence for Volmer-Weber (three-dimensional islands) growth, in disagreement with the StranksiKrastanov growth mode proposed formerly for Cu/TiO,(llO) [3]. A more detailed analysis show! that even a film with an average thickness of 30 A (as determined with the quartz crystal monitor) leaves * 37% of the surface uncovered [16]. Fig. 1 shows the trend of the Ti LEIS signal during Fe and Cr deposition. Clearly the attenuation of the substrate signal with increasing average thickness is much more pronounced for these materials. The average thickness necessary for complete suppression of the substrate signal is lower for Cr than for Fe. A Franck-van der Merwe growth was proposed by Deng et al. 171 for Fe/TiO,(llO), but neither Fe nor Cr films
898
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
show the behavior expected for complete wetting by the first monolayer. However, it is clear that Cr and Fe spread to wet the TiO, substrate more effectively than Cu. A cluster growth process is also observed when Cu was deposited onto a cooled surface (160 K). No substantial deviation from the behavior at room temperature deposition was observed with LEIS. At 160 K, the cluster formation process can be observed within the time scale of the experiment. The top curve of fig. 2 shows the change of the XPS Cu signal with time after the initial deposition. The overlayer signal decreases a few percent, at the same time the 0 1s and Ti 2p3,* substrate signals increase by a similar amount (not shown in fig. 2). This can be explained by a coarsening of the clusters which are formed upon deposition. The self attenuation of the copper photoelectrons within the clusters is increased during coarsening, which leads to a decrease in the Cu photoelectron intensity. The decrease is more pronounced for a surface an-
464
460
456
452
Binding Energy [eV] Fig. 3. XPS Ti2p signals for 2 A (a) Cu, (b) Fe and (c) Cr on TiO,(liO). For comparison, the spectrum for the uncovered surface is shown in each case (dotted lines). Deposition was performed at room temperature.
1.0
n
570K
0.7
, 0
a
2
I
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”
”
I
20 minutes
”
g
I
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”
”
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Fig. 2. Change of the XPS Cu2p,,, signal versus annealing time at the temperatures indicated. Initial deposition and data collection were performed at room temperature, except for the upper curve where deposition was done at 160 K. The signal was normalized f,” the as-deposited values. The average film thickness was 27 A for annealing at 160 K and 18 A for all others.
nealed to higher temperature following deposition at 300 K (lower curves of fig. 2). The high mobility of Cu at these low temperatures is due to a very weak interaction with the substrate. Fig. 3a shows the XPS Ti2p,,, signal of a clean TiO,(llO) surface aad of a surface with an average Cu thickness of 2 A. The line shape of the Ti signal remains unchanged; only a slight intensity decrease is visible. This behavior is contrasted with the zhange of the Ti 2p,,, peak after deposition of 2 A Fe and Cr, respectively (figs. 3b and 3~). The formal ionic charge for Ti is 4 + for a stoichiometric, well-oxidized surface. The shoulder at lower binding energy indicates the presence of reduced states of Ti; Ti3+ shows a shift in binding energy of - 1.8 eV [8]. The shoulder is even more pronounced for a Cr overlayer, and the broadening might include a contribution from a lower reduced state. The reduction of the TiO, cations is accompanied by oxidation of the Fe and Cr atoms [14,15].
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
Fe
Cr
Fig. 4. Schematic drawing of ultrathin Cu, Fe and Cr films on a TiO,(llO) surface.
4. Discussion
The wetting behavior of a film on a surface is governed by the energy balance of the surface free energy of the constituents and the energy of the interface. The surface free energy of TiO, is very low compared to the transition metals under consideration [193; from this one would predict a strong tendency for Volmer-Weber growth for Cu, Fe and Cr on TiO,. Our results show that considerations based only on the surface free energies of the substrate and the overlayer are not sufficient for these systems. The fact that Cr and Fe spread more effectively than Cu must be due to the strength of the metal/oxide bond at the interface. The different reactivity of the various metals towards the surface oxygen appears to be a decisive factor governing the interaction between the metal overlayers and the TiO, surface. The reactivity itself results from electron transfer from the overlayer metal via the oxygen ion to the substrate metal ion. The larger the heat of formation of the metal oxide of the overlayer metal, namely Cr > Fe > Cu, the more effective is the covering of the surface atoms at the same average film thickness. This is shown schematically in fig. 4. Peden et al. [20] observed a change of the growth mode of ultrathin metal films on an oxi-
899
dized tungsten surface, from complete non-wetting (Volmer-Weber growth) for Cu to a layer growth mode for Cr, and correlated the wetting tendency with the metal-oxygen bond strength of the metal overlayer. Although the details are different, we observe a similar trend on TiO,(llO): Cr spreads better than Fe, and both Cr and Fe spread better than Cu, which does not appear to wet the surface effectively.
5. Conclusion
This work shows the close correlation between the wetting ability of 3d-transition metal overlayers and the formation of reduced Ti states metal/TiO,(llO) interface. The combination of LEIS, which is extremely surface sensitive, and XPS, which provides chemical information, is very useful for studying interface phenomena.
Acknowledgement
This work was supported in part by the National Science Foundation, grant DMR-8907553.
References
111G.L. Haller and D.E. Resasco, Adv. Catal. 36 (1989) 173. 121S.J. Tauster, in: Strong-Metal Support Interactions, Eds. R.T.K. Baker, S.J. Tauster and J.A. Dumesic (American Chemical Society, Washington, 1986) p. 21. [31 P.J. Moller and M.-C. Wu, Surf. Sci. 224 (1989) 265. 141 MC Wu and P.J. MoIIer, Surf. Sci. 224 (1989) 250. 151 H. Onishi, T. Aruga, C. Egawa and Y. Iwasawa, Surf. Sci. 199 (1988) 54. 161 R.J. Lad and L.S. Dake, Proc. Mater. Res. Sot. 238 (1992) 823. 171J. Deng, D. Wang, X. Wei, R. Zhai and H. Wang, Surf. Sci. 249 (1991) 213. 181 G. Rocker and W. Giipel, Surf. Sci. 181 (1987) 530. 191H.R. Sadhegi, Appt. Surf. Sci. 19 (1984) 330. m M.-C. Wu and P.J. Moller, Surf. Sci. 235 (1990) 228. illI Z. Zhang and V.E. Henrich, Surf. Sci. 277 (1992) 263. WI V. Andera, Appl. Surf. Sci. 51 (1991) 1. [131 D. Brugniau, S.D. Parker and G.E. Rhead, This Solid Films 121 (1984) 247. [141 J.-M. Pan, U. Diebold, L. Zhang and T.E. Madey, Surf. Sci., submitted.
900
U. Diebold et al. / Ultrathin metal films on TiO,(llO)
[15] J.-M. Pan and T.E. Madey, J. Vat. Sci. Technol. (1993), in press. [16] U. Diebold, J.-M. Pan and T.E. Madey, Phys. Rev. B 47 (1993). [17] B.L. Maschhoff, J.-M. Pan and T.E. Madey, Surf. Sci. 259 (1991) 190.
[18] J.-M. Pan, U. Diebold, B.L. Maschhoff and T.E. Madey, J. Vat. Sci. Technol. A 10 (1992) 2470. 1191 S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547. [20] C.H.F. Peden, K.B. Kidd and N.D. Shinn. J. Vat. Sci. Technol. A 9 (1991) 1518.