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Angle dependence and defect production in metal-on-metal cluster deposition on surfaces
COMPUTATIONAL MATERIALS SCIENCE ELSEVIER
Computational Materials Science 10 (1998) 427431
Angle dependence and defect production in metal-on-metal cluster deposition on surfaces C. Fklix a,b, C. Massobrio ‘,* , B. Nacer d, T. Bekkay d a Institut de Physique Experimentale, Ecole Polytechnique Fed&ale, CH-1015 Lausanne, Switzerland b Northwestern University Department of Chemistry Evanston, IL 60208, USA ’ Institut de Physique et de Chimie des Mate’riaux de Strasbourg, 23, rue du Loess, F-67037 Strasbourg, France d Universite’ Cady Ayyad, Faculte’ des Sciences Semlalia, Departement de Physique, LPSCM, Marrakech, Morocco
Abstract We use molecular dynamics to analyze the dependence on the impact angle of the distribution of defects originated by the deposition of a Agts cluster on Pd( 10 0) at initial kinetic energies 0.1,2, 20 and 95 eV. For increasing energy the cluster undergoes a transition from a multi-layered adsorbed structure to a two-dimensional one. Implantation of Ag atoms and promotion of Pd substrate atoms is common to all energies and angles and, for a given initial total kinetic energy, it increases with decreasing impact angle. Copyright 0 1998 Elsevier Science B.V.
1. Introduction The interest of the scientific community in the de-
position and growth of homo- and hetero-structures on well-defined surfaces is triggered by both fundamental (how the individual atomic processes combine to give a very rich variety of structures, and how they grow [ 1,2]) and applied reasons (related to the growth of materials with controlled optical, catalytic, magnetic as well as mechanical properties [3,4]). The idea of using clusters as construction units instead of atoms is already fairly old [5,6]. However, the deposition process is more complicated than for atoms. In particular, contamination and possible fragmentation of the clusters need to be accounted for. If one is interested in using clusters as elementary units for the construc*Corresponding author. Tel.: 333 881 07040; fax: 333 881 07249; e-mail: [email protected].
tion of controlled surface structures, it is necessary to know how a single deposited cluster looks like. In a recent experimental and theoretical effort, the deposition of silver clusters (Agt , Ag7 and Agt9) on Pd( 100) has been investigated [7-l l] via thermal energy atom scattering (TEAS) for impact energies Eimr, equal to 20 and 95 eV. These experiments show that implantation and fragmentation are important in a way directly proportional to the impact energy per atom. Molecular dynamics simulations [ 10,12-l 51 provide an ideal complement to the very sensitive but global measurements of He scattering, which cannot yield direct information on the morphology of the adsorbate/substrate system created by deposition. As part of a simulation study devoted to the collision of clusters on surfaces, we investigate in this paper the effect of an incidence angle on the impact of Agr9 clusters on Pd(lO0) as a function of energy. Even though there is no direct experimental counterpart to
0927-0256/98/$19.00 Copyright 0 1998 Elsevier Science B.V. All rights reserved PII SO927-0256(97)00143-2
428
C. Fdix et al. /Computational
these simulations,
the size of the cluster and the depo-
Materials Science 10 (1998) 427431
Table 1. It is worth noticing that implantation
occurs
sition energies have been chosen to match the one pre-
for any initial deposition
conditions.
For low deposi-
viously used in the experiments
tion energies one atom is implanted,
while for bigger
and simulations.
For
these reasons the results presented here can be highly
impact energies
useful to experimental&s
of a Ag atom in the second substrate layer takes place
tive experimental
willing to optimize innova-
conditions
in the search of peculiar
are based on the embedded
atom
details on the setting up of the calculations we refer the reader to a recent publication devoted to a full description of the simulation conditions [ 171. The simulation slab consists of seven layers, modeling the ( 10 0) surface of Pd and submitted to periodic boundary conditions along the [0 0 l] and [0 IO] directions. Each layer contains 200 Pd atoms. The structure for the Agl9 cluster is a highly distorted six-capped icosahedron selected among several local minima proruns.
The temperature of the Agt9 cluster and the temperature of the Pd( 10 0) surface are initially set to zero. The cluster is directed towards the substrate with an incidence angle 8 = 15”, 30” or 60” in the [00 l] direction and an initial kinetic energy Eimr. For each combination
with increas-
the impacts and only for the higher impact energies.
(EAM) potentials of Foiles et al. [ 161 for Ag and Pd, with cutoff radii & equal to 5.25A. For relevant
energy-angle
character, which diminishes
ing impact energy. Very few vacancies are created by
2. Model and calculations
duced via annealing
Implantation
created by the impact have a clear three-
dimensional
Our simulations
increases.
only for Eimp = 95 eV. At low deposition energies, the adsorbates
surface morphologies.
this number
50 depositions
are pro-
duced for statistical purposes, by varying randomly the initial cluster location with respect to the surface. The evolution of the system takes place in the microcanonical ensemble, with trajectories lasting up to 20 ps. By running simulations at Eimp = 20 eV we have checked that our results are unaffected by the consideration of a different, low (T = 150 K) substrate temperature. The issue of the dependence of the results on substrate temperature, system size and control temperature conditions have been thoroughly addressed in [ 171.
3. Results The distribution of Ag and Pd atoms in the different layers and the number of vacancies are given in
From Table 1 we deduce that the defect production and the concomitant change towards- two-dimensional clusters decrease with increasing incidence angle. It appears that the critical parameter is the energy perpendicular to the surface, while the incident energy in the direction parallel to the surface has a much smaller effect on the defect production. The differences between Eimp = 0.1 or 2eV depositions are small and only the distributions of atoms above and in the adlayer differ slightly. This is not surprising since Eimp is negligible compared to the energy released during the adsorption
process (-
12 eV).
Fig. 1 shows the position of the silver and palladium atoms after a collision at Eimp = 0.1 eV projected onto the surface plane. For this initial energy the cluster remains three-dimensional after the collision. We notice that even at this very low deposition energy a Ag atom is implanted into the Pd( 1 00) substrate while an atom from the substrate is pushed into the adlayer. Under similar conditions
this never happens
for Ag atoms on the same substrate [ 1 l]. As shown recently [ 111, the onset of chemical disorder is a direct result of the initial three-dimensionality
of the cluster.
We believe that this interesting phenomenon can be of high importance for the so-called “softlanding” deposition studies, since the implanted atom can act as a nucleation center and stabilize the deposited particle with respect to diffusion. However, at the same time, it prevents the occurring of a perfect defect-free adsorption of the cluster with no intermixing between the two species. Figs. 2 and 3 show the positions of the atoms at the end of typical depositions with Eimp= 95 eV and impact angles 8 = 30” and 0 = 60”, respectively. Here the cluster is no more three-dimensional. Some
C. F&x et al. /Computational Materials Science 10 (1998) 427431
429
Table 1 Averaged distribution of Ag and Pd atoms in different layers resulting from the deposition of Agl9 with an incident angle 0 Impact energy (eV)
e
0.1
30 60
8.9 8.0
2
30 60
20
95
Ag above adlayer
Ag in adlayer
Ag in 1st layer
Ag in 2nd layer
9.8 9.3
1.0 1.0
0.0 0.0
1.0 1.0
0.0 0.0
6.6 7.6
11.4 10.4
1.0 1.0
0.0 0.0
1.0 1.0
0.0 0.0
15 30 60
3.9 3.4 6.8
10.9 13.1 11.2
4.0 2.5 1.0
0.1 0.0 0.0
4.1 2.6 1.0
0.0 0.1 0.0
15 30 60
0.1 0.0 0.5
7.5 11.4 14.5
7.3 6.3 3.9
4.1 1.3 0.1
12.0 7.8 4.1
0.6 0.2 0.1
0000000000
0000000000 0000000000 0000000000 0000000000 0000000000
0000000000
0 0
0
0 0
0 0
Pd in adlayer
Vacancies
(“1
0 0
0
0
0 0 0 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000
0 0 0
0’0.0 OB6I--B
om
~~@‘L_.~
omooo’mo
0 0
0 0
0
0 0
0
0
.~
WI Fig. 1. Final location of cluster and substrate atoms, for the collision of Agt9 at El, = 0.1 eV and an incidence angle of 30’. Ag atoms are represented with squares while Pd atoms are represented with circles. White symbols refer to atoms in the first layer, black symbols to atoms in the adlayer and dotted contours to atoms above the adlayer. The cluster clearly stays three-dimensional and one Ag atom is implanted.
of the Ag atoms are implanted in a compact shape close to the point of impact, while the rest of them are positioned around the impact. In Fig. 2 one silver atom has escaped the impact point by diffusing along a channel in the [0 1 i] direction and is no more bound to the other atoms of the initial cluster. Anytime a Ag atom is implanted in the substrate, a Pd
atom is promoted to the adlayer, as a consequence we do not observe the creation of interstitial defects. For low deposition energies or small incident angles the ejected Pd atoms are located in the proximity of the implanted Ag atoms. Figs. 2 and 3 show that the multiple collisions mechanism resulting in Pd atoms ejection into the adlayer can produce adatoms or clusters at - 5 sites away from the rest of the adatoms. This is a typical feature occurring in more than 50% of the depositions at 0 = 60”. Atoms, dimers, up to tetramers (see Fig. 3) are observed as result of this mechanism. Interestingly, we never observe two separate Pd adatoms away from the cluster impact region. Pd adatoms leaving the cluster impact region end up forming a small cluster, since the energy to initiate a second multiple collision event is likely to be higher than that required to move a second atom in an already existing cascade. It seems therefore that the system dissipates part of this energy in a long range multiple collision event which leads, if the impact energy parallel to the surface is high enough, to the appearance of isolated Pd adatoms or small clusters in the proximity of the impact of the initial particle. In view of these indications experimentalists could choose to lower the amount of defects produced during the deposition by increasing the incidence angle of the in-coming particles, as the simulations discussed in this paper suggest. Long range collision effects could arise as a limitation. However, since they only appear
430
C. Fdix
et al./Compututional
Materials
Science
10 (1998) 427-431
WI Fig. 2. Final location of cluster and substrate atoms, for the collision ‘of Agt9 at Elmp = 95eV and incidence angles of 30’. Ag atoms are represented with squares while Pd atoms are represented with circles. White symbols refer to atoms in the first layer, black symbols to atoms in the adlayer. The gray square (underneath a black square) represents a silver atom implanted in the second layer.
Fig. 3. Final location of cluster and substrate atoms, for the collision of Agt9 at Eimn = 95 eV and incidence angles of 60”. The collision cascade in the Pd substrate can lead to the creation of a small Pd cluster in the proximity of the Agt9 impact.
these investigations are worth to be pursued on other metal-on-metal combinations of current experimental interest.
at Eimp = 95 eV and are not observed for smaller deposition energies we do not consider them as a serious limitation.
4. Conclusions We
have
simulations
investigated the angle
by
molecular
dependence
dynamics
in the collision
process of Agt9 on Pd( 100). The cluster is threedimensional for low deposition energies but evolves towards a two-dimensional shape with increasing deposition energies. The amount of defect creation by an impact decreases with increasing incidence angle, thereby showing that the impact energy parallel to the surface is not responsible for implantation or interlayer mass transport. However, at 95 eV deposition energy and 60” incidence angle, adatoms or small Pd clusters appear due to a collision cascade in the near proximity of an impact. Due to the previous good agreement recorded between simulated depositions and experimental data, we believe that
References ]I] J.A. Venables, G.D.T. Spiller and M. Hanbticken, Rep. Prog. Phys. 47 (1984) 399. [2] H. Roder, H. Brune, J.P. Bucher and K. Kern, Surf. Sci. 298 (1993) 121. [3] X.P. Xu and D.W. Goodman, Catal. Lett. 24 (1994) 31. [4] H. Haberland, M. Karrais, M. Mall and Y. Thumer, J. Vat. Sci. Technol. A 10 (1992) 3266. [S] T. Takagi, I. Yamada and A. Sasaki, J. Vat. Sci. Technol. 12 (1975) 1128. 161 S.B.D. Cenzo, S.D. Berry and E.H.H. Jr., Phys. Rev. B 38 (1988) 8465. I71 C. Vandoni, C. Felix, R. Monot, J. Buttet and W. Harbich, Chem. Phys. Lett. 229 (1994) 51. L81C. Felix, G. Vandoni, C. Massobrio, R. Monot, .I. Buttet and W. Harbich, to be published. L91K. Bromann et al., Science 274 (1996) 956. 1101Cl. Vandoni, C. Felix and C. Massobrio, Phys. Rev. B 54 (1996) 1553. IlIl B. Nacer, C. Massobrio and C. Felix, to be published. [I21 H. Hsieh, R. Averback, H. Sellers and C. Flynn, Phys. Rev. B 45 (1992) 4417. 1131 F. Karetta and H. Urbassek, J. Appl. Phys. 71 (1992) 5410.
C. Fdlix et al,/Computational [14] P. Stoltze and J.K. Nerskov, Phys. Rev. B 48 (1993) 5607. [15] H.-P. Cheng and U. Landman, J. Phys. Chem. 98 (1994) 3527.
Materials Science 10 (1998) 427-431
431
[16] SM. Foiles, M.I. Baskes and M.S. Daw, Phys. Rev. B 33 (1986) 7983. [17] C. Massobrio, B. Nacer, T. Bekkay, G. Vandoni and C. F&ix, Surf. Sci. (1997), to be published.
Computational Materials Science 10 (1998) 427431
Angle dependence and defect production in metal-on-metal cluster deposition on surfaces C. Fklix a,b, C. Massobrio ‘,* , B. Nacer d, T. Bekkay d a Institut de Physique Experimentale, Ecole Polytechnique Fed&ale, CH-1015 Lausanne, Switzerland b Northwestern University Department of Chemistry Evanston, IL 60208, USA ’ Institut de Physique et de Chimie des Mate’riaux de Strasbourg, 23, rue du Loess, F-67037 Strasbourg, France d Universite’ Cady Ayyad, Faculte’ des Sciences Semlalia, Departement de Physique, LPSCM, Marrakech, Morocco
Abstract We use molecular dynamics to analyze the dependence on the impact angle of the distribution of defects originated by the deposition of a Agts cluster on Pd( 10 0) at initial kinetic energies 0.1,2, 20 and 95 eV. For increasing energy the cluster undergoes a transition from a multi-layered adsorbed structure to a two-dimensional one. Implantation of Ag atoms and promotion of Pd substrate atoms is common to all energies and angles and, for a given initial total kinetic energy, it increases with decreasing impact angle. Copyright 0 1998 Elsevier Science B.V.
1. Introduction The interest of the scientific community in the de-
position and growth of homo- and hetero-structures on well-defined surfaces is triggered by both fundamental (how the individual atomic processes combine to give a very rich variety of structures, and how they grow [ 1,2]) and applied reasons (related to the growth of materials with controlled optical, catalytic, magnetic as well as mechanical properties [3,4]). The idea of using clusters as construction units instead of atoms is already fairly old [5,6]. However, the deposition process is more complicated than for atoms. In particular, contamination and possible fragmentation of the clusters need to be accounted for. If one is interested in using clusters as elementary units for the construc*Corresponding author. Tel.: 333 881 07040; fax: 333 881 07249; e-mail: [email protected].
tion of controlled surface structures, it is necessary to know how a single deposited cluster looks like. In a recent experimental and theoretical effort, the deposition of silver clusters (Agt , Ag7 and Agt9) on Pd( 100) has been investigated [7-l l] via thermal energy atom scattering (TEAS) for impact energies Eimr, equal to 20 and 95 eV. These experiments show that implantation and fragmentation are important in a way directly proportional to the impact energy per atom. Molecular dynamics simulations [ 10,12-l 51 provide an ideal complement to the very sensitive but global measurements of He scattering, which cannot yield direct information on the morphology of the adsorbate/substrate system created by deposition. As part of a simulation study devoted to the collision of clusters on surfaces, we investigate in this paper the effect of an incidence angle on the impact of Agr9 clusters on Pd(lO0) as a function of energy. Even though there is no direct experimental counterpart to
0927-0256/98/$19.00 Copyright 0 1998 Elsevier Science B.V. All rights reserved PII SO927-0256(97)00143-2
428
C. Fdix et al. /Computational
these simulations,
the size of the cluster and the depo-
Materials Science 10 (1998) 427431
Table 1. It is worth noticing that implantation
occurs
sition energies have been chosen to match the one pre-
for any initial deposition
conditions.
For low deposi-
viously used in the experiments
tion energies one atom is implanted,
while for bigger
and simulations.
For
these reasons the results presented here can be highly
impact energies
useful to experimental&s
of a Ag atom in the second substrate layer takes place
tive experimental
willing to optimize innova-
conditions
in the search of peculiar
are based on the embedded
atom
details on the setting up of the calculations we refer the reader to a recent publication devoted to a full description of the simulation conditions [ 171. The simulation slab consists of seven layers, modeling the ( 10 0) surface of Pd and submitted to periodic boundary conditions along the [0 0 l] and [0 IO] directions. Each layer contains 200 Pd atoms. The structure for the Agl9 cluster is a highly distorted six-capped icosahedron selected among several local minima proruns.
The temperature of the Agt9 cluster and the temperature of the Pd( 10 0) surface are initially set to zero. The cluster is directed towards the substrate with an incidence angle 8 = 15”, 30” or 60” in the [00 l] direction and an initial kinetic energy Eimr. For each combination
with increas-
the impacts and only for the higher impact energies.
(EAM) potentials of Foiles et al. [ 161 for Ag and Pd, with cutoff radii & equal to 5.25A. For relevant
energy-angle
character, which diminishes
ing impact energy. Very few vacancies are created by
2. Model and calculations
duced via annealing
Implantation
created by the impact have a clear three-
dimensional
Our simulations
increases.
only for Eimp = 95 eV. At low deposition energies, the adsorbates
surface morphologies.
this number
50 depositions
are pro-
duced for statistical purposes, by varying randomly the initial cluster location with respect to the surface. The evolution of the system takes place in the microcanonical ensemble, with trajectories lasting up to 20 ps. By running simulations at Eimp = 20 eV we have checked that our results are unaffected by the consideration of a different, low (T = 150 K) substrate temperature. The issue of the dependence of the results on substrate temperature, system size and control temperature conditions have been thoroughly addressed in [ 171.
3. Results The distribution of Ag and Pd atoms in the different layers and the number of vacancies are given in
From Table 1 we deduce that the defect production and the concomitant change towards- two-dimensional clusters decrease with increasing incidence angle. It appears that the critical parameter is the energy perpendicular to the surface, while the incident energy in the direction parallel to the surface has a much smaller effect on the defect production. The differences between Eimp = 0.1 or 2eV depositions are small and only the distributions of atoms above and in the adlayer differ slightly. This is not surprising since Eimp is negligible compared to the energy released during the adsorption
process (-
12 eV).
Fig. 1 shows the position of the silver and palladium atoms after a collision at Eimp = 0.1 eV projected onto the surface plane. For this initial energy the cluster remains three-dimensional after the collision. We notice that even at this very low deposition energy a Ag atom is implanted into the Pd( 1 00) substrate while an atom from the substrate is pushed into the adlayer. Under similar conditions
this never happens
for Ag atoms on the same substrate [ 1 l]. As shown recently [ 111, the onset of chemical disorder is a direct result of the initial three-dimensionality
of the cluster.
We believe that this interesting phenomenon can be of high importance for the so-called “softlanding” deposition studies, since the implanted atom can act as a nucleation center and stabilize the deposited particle with respect to diffusion. However, at the same time, it prevents the occurring of a perfect defect-free adsorption of the cluster with no intermixing between the two species. Figs. 2 and 3 show the positions of the atoms at the end of typical depositions with Eimp= 95 eV and impact angles 8 = 30” and 0 = 60”, respectively. Here the cluster is no more three-dimensional. Some
C. F&x et al. /Computational Materials Science 10 (1998) 427431
429
Table 1 Averaged distribution of Ag and Pd atoms in different layers resulting from the deposition of Agl9 with an incident angle 0 Impact energy (eV)
e
0.1
30 60
8.9 8.0
2
30 60
20
95
Ag above adlayer
Ag in adlayer
Ag in 1st layer
Ag in 2nd layer
9.8 9.3
1.0 1.0
0.0 0.0
1.0 1.0
0.0 0.0
6.6 7.6
11.4 10.4
1.0 1.0
0.0 0.0
1.0 1.0
0.0 0.0
15 30 60
3.9 3.4 6.8
10.9 13.1 11.2
4.0 2.5 1.0
0.1 0.0 0.0
4.1 2.6 1.0
0.0 0.1 0.0
15 30 60
0.1 0.0 0.5
7.5 11.4 14.5
7.3 6.3 3.9
4.1 1.3 0.1
12.0 7.8 4.1
0.6 0.2 0.1
0000000000
0000000000 0000000000 0000000000 0000000000 0000000000
0000000000
0 0
0
0 0
0 0
Pd in adlayer
Vacancies
(“1
0 0
0
0
0 0 0 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000 0000000000
0 0 0
0’0.0 OB6I--B
om
~~@‘L_.~
omooo’mo
0 0
0 0
0
0 0
0
0
.~
WI Fig. 1. Final location of cluster and substrate atoms, for the collision of Agt9 at El, = 0.1 eV and an incidence angle of 30’. Ag atoms are represented with squares while Pd atoms are represented with circles. White symbols refer to atoms in the first layer, black symbols to atoms in the adlayer and dotted contours to atoms above the adlayer. The cluster clearly stays three-dimensional and one Ag atom is implanted.
of the Ag atoms are implanted in a compact shape close to the point of impact, while the rest of them are positioned around the impact. In Fig. 2 one silver atom has escaped the impact point by diffusing along a channel in the [0 1 i] direction and is no more bound to the other atoms of the initial cluster. Anytime a Ag atom is implanted in the substrate, a Pd
atom is promoted to the adlayer, as a consequence we do not observe the creation of interstitial defects. For low deposition energies or small incident angles the ejected Pd atoms are located in the proximity of the implanted Ag atoms. Figs. 2 and 3 show that the multiple collisions mechanism resulting in Pd atoms ejection into the adlayer can produce adatoms or clusters at - 5 sites away from the rest of the adatoms. This is a typical feature occurring in more than 50% of the depositions at 0 = 60”. Atoms, dimers, up to tetramers (see Fig. 3) are observed as result of this mechanism. Interestingly, we never observe two separate Pd adatoms away from the cluster impact region. Pd adatoms leaving the cluster impact region end up forming a small cluster, since the energy to initiate a second multiple collision event is likely to be higher than that required to move a second atom in an already existing cascade. It seems therefore that the system dissipates part of this energy in a long range multiple collision event which leads, if the impact energy parallel to the surface is high enough, to the appearance of isolated Pd adatoms or small clusters in the proximity of the impact of the initial particle. In view of these indications experimentalists could choose to lower the amount of defects produced during the deposition by increasing the incidence angle of the in-coming particles, as the simulations discussed in this paper suggest. Long range collision effects could arise as a limitation. However, since they only appear
430
C. Fdix
et al./Compututional
Materials
Science
10 (1998) 427-431
WI Fig. 2. Final location of cluster and substrate atoms, for the collision ‘of Agt9 at Elmp = 95eV and incidence angles of 30’. Ag atoms are represented with squares while Pd atoms are represented with circles. White symbols refer to atoms in the first layer, black symbols to atoms in the adlayer. The gray square (underneath a black square) represents a silver atom implanted in the second layer.
Fig. 3. Final location of cluster and substrate atoms, for the collision of Agt9 at Eimn = 95 eV and incidence angles of 60”. The collision cascade in the Pd substrate can lead to the creation of a small Pd cluster in the proximity of the Agt9 impact.
these investigations are worth to be pursued on other metal-on-metal combinations of current experimental interest.
at Eimp = 95 eV and are not observed for smaller deposition energies we do not consider them as a serious limitation.
4. Conclusions We
have
simulations
investigated the angle
by
molecular
dependence
dynamics
in the collision
process of Agt9 on Pd( 100). The cluster is threedimensional for low deposition energies but evolves towards a two-dimensional shape with increasing deposition energies. The amount of defect creation by an impact decreases with increasing incidence angle, thereby showing that the impact energy parallel to the surface is not responsible for implantation or interlayer mass transport. However, at 95 eV deposition energy and 60” incidence angle, adatoms or small Pd clusters appear due to a collision cascade in the near proximity of an impact. Due to the previous good agreement recorded between simulated depositions and experimental data, we believe that
References ]I] J.A. Venables, G.D.T. Spiller and M. Hanbticken, Rep. Prog. Phys. 47 (1984) 399. [2] H. Roder, H. Brune, J.P. Bucher and K. Kern, Surf. Sci. 298 (1993) 121. [3] X.P. Xu and D.W. Goodman, Catal. Lett. 24 (1994) 31. [4] H. Haberland, M. Karrais, M. Mall and Y. Thumer, J. Vat. Sci. Technol. A 10 (1992) 3266. [S] T. Takagi, I. Yamada and A. Sasaki, J. Vat. Sci. Technol. 12 (1975) 1128. 161 S.B.D. Cenzo, S.D. Berry and E.H.H. Jr., Phys. Rev. B 38 (1988) 8465. I71 C. Vandoni, C. Felix, R. Monot, J. Buttet and W. Harbich, Chem. Phys. Lett. 229 (1994) 51. L81C. Felix, G. Vandoni, C. Massobrio, R. Monot, .I. Buttet and W. Harbich, to be published. L91K. Bromann et al., Science 274 (1996) 956. 1101Cl. Vandoni, C. Felix and C. Massobrio, Phys. Rev. B 54 (1996) 1553. IlIl B. Nacer, C. Massobrio and C. Felix, to be published. [I21 H. Hsieh, R. Averback, H. Sellers and C. Flynn, Phys. Rev. B 45 (1992) 4417. 1131 F. Karetta and H. Urbassek, J. Appl. Phys. 71 (1992) 5410.
C. Fdlix et al,/Computational [14] P. Stoltze and J.K. Nerskov, Phys. Rev. B 48 (1993) 5607. [15] H.-P. Cheng and U. Landman, J. Phys. Chem. 98 (1994) 3527.
Materials Science 10 (1998) 427-431
431
[16] SM. Foiles, M.I. Baskes and M.S. Daw, Phys. Rev. B 33 (1986) 7983. [17] C. Massobrio, B. Nacer, T. Bekkay, G. Vandoni and C. F&ix, Surf. Sci. (1997), to be published.