Monte Carlo simulation for the formation and growth of low dimensionality phases during underpotential deposition of Ag on Au(100)

Adsorption of Metals on (100) Metallic Surfaces: Monte Carlo Simulations and Geometrical Phase Transitions

Adsorption and continuous phase transitions (percolation) of metals on (100) metallic surfaces are studied by means of Monte Carlo simulations and the finite-size scaling theory. The studied systems are Ag/Au(100), Au/Ag(100), Ag/Pt(100), and Pt/Ag(100), and the embedded atom method (EAM) is employed for energy calculations. Pairwise interactions are also considered for comparative purposes. The study of critical exponents reveals that these systems belong to the same universality as random sequential adsorption (RSA). For the four systems studied, and the two kinds of interactions considered, phase diagrams of percolation threshold, θ_c, as a function of temperature are presented. In all cases, and for all temperatures, θ_c is always below the value corresponding to RSA, as expected for attractive interactions, and it tends to that value as T → ∞. At intermediate temperatures, a particular behavior is found for EAM interactions.

Monomolecular adsorption on nanoparticles with repulsive interactions: a Monte Carlo study

In the present work, we study the adsorption of different monomolecular species on nanoparticles with different sizes and geometries using a grand canonical Monte Carlo method. These species are characterized by repulsive lateral interactions between themselves, as takes place in the case of the adsorption of partially charged atoms or molecules. Nanosize effects are analyzed in terms of adsorption on edge and facet sites. The energy minimization in these systems comes out as a complex conjugation of the repulsive lateral interactions between the adsorbates and the attractive interactions of the adsorbates with the nanoparticle. The phenomenon is analyzed as a function of the occurrence of different ordered structures being formed on the surface of the nanoparticle. We find that layers with different structures may coexist on different facets of the nanoparticle. Finally, a discussion of deposition on flat surfaces and in finite systems is given.

Statistical thermodynamics of straight rigid rods with nonadditive lateral interactions: Theory and Monte Carlo simulations

The statistical thermodynamics of straight rigid rods of length k (k-mers) with nonadditive lateral interactions was developed on a generalization in the spirit of the lattice-gas model and the classical Bragg-Williams approximation (BWA) and the quasichemical approximation (QCA). The new theoretical framework is obtained by combining (i) the exact analytical expression for the partition function of noninteracting linear k-mers adsorbed in one dimension and its extension to higher dimensions, and (ii) a generalization of BWA and QCA in which the adsorbate can occupy more than one adsorption site. The traditional assumption of a strictly pairwise additive nearest-neighbors interaction is replaced by a more general one, namely that the bond linking a certain atom with any of its neighbors depends considerably on how many of them are actually present (or absent) on the sites in the first coordination shell of the atom. The coverage and temperature dependence of the Helmholtz free energy, chemical potential, configurational entropy, and differential heat of adsorption are given. The formalism (i) reproduces the classical results for monomers, (ii) leads to the exact statistical thermodynamics of nonadditive interacting k-mers adsorbed in one dimension, and (iii) provides a close approximation for two-dimensional systems, taking into account multisite occupancy and nonadditive lateral interactions. Comparisons with Monte Carlo simulations are performed in order to test the validity of the theoretical model. Significant quantitative differences are shown and discussed. In all cases, the QCA appears to be the more accurate approach.