Monte Carlo Simulation of Properties of Monolayers and Metal Islands Adsorbed on Metallic (111) Surfaces Mariana I. Rojas, Mario G. Del Po ´polo, and Ezequiel P. M. Leiva* Unidad de Matema ´ tica y Fı ´sica, Facultad de Ciencias Quı ´micas, INFIQC, Universidad Nacional de Co ´ rdoba, 5000 Co ´ rdoba, Argentina Received October 28, 2003. In Final Form: January 30, 2004 To obtain the surface stress changes Δσ due to the adsorption of metal monolayers onto metallic surfaces, a new model derived from thermodynamic considerations is presented. Such a model is based on continuum Monte Carlo simulations with embedded atom method potentials in the canonical ensemble, and it is extended to consider the behavior on different islands adsorbed onto (111) substrate surfaces. Homoepitaxial and heteroepitaxial systems are studied. Pseudomorphic growth is not observed for small metal islands with considerable positive misfit with the substrate. Instead, the islands become compressed upon increase of their size. A simple model is proposed to interpolate between the misfits of atoms in small islands and the pseudomorphic behavior of the monolayer. 1. Introduction The formation of metal islands on a metal surface represents an important stage in the growth of a phase in the case of homoepitaxy and in the appearance of a new one in the case of heteroepitaxy. In a preliminary study, 1 we have developed a Monte Carlo simulation model that allows the properties of small metal islands on metallic surfaces to be studied. Within these studies, we have shown that metal islands adsorbed onto (111) surfaces acquire hexagonal-like equilibrium shapes and may be subject to considerable stress as a result of a balance between the interactions of the atoms in the monolayer with each other and their interaction with the substrate. In the particular case of the heteroatomic Ag/Pt(111) system, we found for all the island sizes considered an outward relaxation of the atoms located at the edge. In addition, we did not find pseudomorphic growth but rather expanded structures which became progressively com- pressed as the size of the islands increased. On the other hand, for the homoepitaxial systems considered, the islands presented inward relaxation at the edges as expected from bond order conservation analysis. The estimation of the border formation energy was important to understand the thermal stability of the islands because island disintegration is expected to take place when this quantity is of the order of kT. For Ag islands on Pt(111), we calculated a surprisingly low edge energy, in agreement with the experimental finding of Ro ¨der et al. 2 that Ag islands on Pt(111) disintegrate at relatively low temper- atures. On the other hand, Ag on Ag(111) and Cu on Cu- (111) presented larger edge formation energies, of the order of 0.2-0.3 eV, which were close to the values found experimentally by Giesen et al. 3 A second and more appealing outcome of these simula- tions was the fact that, for the adsorption of Ag on Pt- (111), the nearest neighbor distance, d nn , in the center of the island, was a function of its size. For small islands, d nn presented a value close to the Ag-Ag distance on a Ag(111) surface, approaching the distance between Pt atoms on the Pt(111) surface when the island size increased. This observation was consistent with the fact that a considerably stressed (1 × 1) structure was experimentally observed at full coverage. 4 In this respect, the evolution of the surface stress during the deposition of a foreign metal on a metallic surface is a topic of general interest. The surface stress affects the structure of the interface, being the leading force of reconstruction and the source of different kinds of surface defects. Structural changes occurring during the first stage of deposition, which are associated with large changes in the surface stress, affect the shape and the kinetics of growth of subsequent layers in multilayered deposits. As a typical example of the relation between surface stress and structure in heteroepitaxy, the work of Bromann et al. 5 can be mentioned. They observed that Ag islands, below a critical size of 200 Å, grow coherently strained on Pt- (111), while larger islands relieve strain through the introduction of misfit dislocations. Upon completion of the first monolayer, the dislocations disappear and the Ag film adopts a pseudomorphic structure. These authors’ experimental findings were complemented by effective- medium theory calculations to analyze the formation of dislocations, found to occur for islands with a pseudo- morphic core (diameter above 50 Å). In the present work, we present a model to obtain the change of surface stress by continuum canonical Monte Carlo simulations using the embedded atom method (EAM) potentials. 6-7 The model is tested for homoepitaxial and heteroepitaxial monolayers and is extended to con- sider islands whose sizes vary between 12 and 156 atoms. The homoepitaxial system studied is Ag on Ag(111), and the heteroepitaxial ones are Ag/Pt(111), Au/Pt(111), and Ag/Pd(111). According to the theoretical results of Bro- mann et al. 5 for the first of these systems, our island sizes are expected to be below the limit for the formation of dislocations. The magnitude of the stress is found to increase with the number of atoms in the island, something which is associated with the concomitant structural changes: * Corresponding author. Fax, 54-351-4344972; e-mail, eleiva@fcq.unc.edu.ar. (1) Rojas, M. I.; Amilibia, G. E.; Del Po ´polo, M. G.; Leiva, E. P. M. Surf. Sci. 2002, 499, L135-L140. (2) Ro ¨der, H.; Brune, H.; Kern, K. Phys. Rev. Lett. 1994, 73, 2143. (3) Giesen, M.; Steimer, C.; Ibach, H. Surf. Sci. 2001, 471, 80. (4) Ibach, H. Surf. Sci. Rep. 1997, 29, 193-263. (5) Bromann, K.; Brune, H.; Giovannini, M.; Kern, K. Surf. Sci. 1997, 388, L1107-L1114. (6) Daw, M. S.; Baskes, M. I. Phys. Rev. B 1983, 50, 1285. (7) Foiles, S. M.; Baskes, M. I.; Daw, M. S. Phys. Rev. B 1986, 33, 7983. 4279 Langmuir 2004, 20, 4279-4288 10.1021/la036021z CCC: $27.50 © 2004 American Chemical Society Published on Web 04/09/2004