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Sulfur-metal complexes, containing only a few atoms, can open new, highly efficient pathways for transport of metal atoms on surfaces. For example, they can accelerate changes in the shape and size of morphological features, such as two-dimensional nanoclusters, over time. In this study we perform STM under conditions that are designed to specifically isolate such complexes. We find a new, unexpected S-Cu complex on the Cu(111) surface, which we identify as Cu2S3. We propose that Cu2S3 enhances mass transport in this system, which contradicts a previous proposal based on Cu3S3. We analyze bonding within these Cu-S complexes, identifying a principle for stabilization of sulfur complexes on coinage metal surfaces. [ABSTRACT FROM AUTHOR]

An accurate description of oxygen dissociation pathways and kinetics for various local adlayer environments is key for an understanding not just of the coverage dependence of oxygen sticking, but also of reactive steady states in oxidation reactions. Density functional theory analysis for M (100) surfaces with M = Pd, Rh, and Ni, where O prefers the fourfold hollow adsorption site, does not support the traditional Brundle-Behm-Barker picture of dissociative adsorption onto second-nearest-neighbor hollow sites with an additional blocking constraint. Rather adsorption via neighboring vicinal bridge sites dominates, although other pathways can be active. The same conclusion also applies for M = Pt and Ir, where oxygen prefers the bridge adsorption site. Statistical mechanical analysis is performed based on kinetic Monte Carlo simulation of a multisite lattice-gas model consistent with our revised picture of adsorption. This analysis determines the coverage and temperature dependence of sticking for a realistic treatment of the oxygen adlayer structure. [ABSTRACT FROM AUTHOR]

Schloegl's second model on a (d ≥ 2)-dimensional hypercubic lattice involves: (i) spontaneous annihilation of particles with rate p and (ii) autocatalytic creation of particles at vacant sites at a rate proportional to the number of suitable pairs of neighboring particles. This model provides a prototype for nonequilibrium discontinuous phase transitions. However, it also exhibits nontrivial generic two-phase coexistence: Stable populated and vacuum states coexist for a finite range, pf(d) < p < pe(d), spanned by the orientation-dependent stationary points for planar interfaces separating these states. Analysis of interface dynamics from kinetic Monte Carlo simulation and from discrete reaction-diffusion equations (dRDEs) obtained from truncation of the exact master equation, reveals that pe(f) ∼ 0.211 3765 + ce(f),/d as d →∞, where Δc = ce -- Cf &thkap; 0.014. A metastable populated state persists above pe(d) up to a spinodal p = ps(d), which has a well-defined limit ps(d →∞) = ¼. The dRDEs display artificial propagation failure, absent in the stochastic model due to fluctuations. This feature is amplified for increasing d, thus complicating our analysis. [ABSTRACT FROM AUTHOR]

We consider Schloegl models (or contact processes) where particles on a square grid annihilate at a rate p and are created at a rate of kn=n(n-1)/[N(N-1)] at empty sites with n particles in a neighborhood ΩN of size N. Simulation reveals a discontinuous transition between populated and vacuum states, but equistable p=peq determined by the stationarity of planar interfaces between these states depends on the interface orientation and on ΩN. The behavior for large ΩN follows from continuum equations. These also depend on the interface orientation and on ΩN shape, but a unique peq=0.211 376 320 4 emerges imposing a Gibbs phase rule. [ABSTRACT FROM AUTHOR]

For diffusion of two-dimensional homoepitaxial clusters of N atoms on metal (100) surfaces mediated by edge atom hopping, macroscale continuum theory suggests that the diffusion coefficient scales like DN ~N -β with β = 3/2. However, we find quite different and diverse behavior in multiple size regimes. These include: (i) facile diffusion for small sizes N < 9 ; (ii) slow nucleation-mediated diffusion with small β < 1 for "perfect" sizes N = Np = L2 or L(L+1), for L = 3, 4, ... having unique ground-state shapes, for moderate sizes 9 ≤ N ≤ O (102); the same also applies for N = Np+3,Np+4, ... (iii) facile diffusion but with large β > 2 for N = Np+1 andNp+ 2 also for moderate sizes 9 ≤ N ≤ O (102); (iv) merging of the above distinct branches and subsequent anomalous scaling with 1 ≲ β < 3/2, reflecting the quasifacetted structure of clusters, for larger N = O (102) to N = O (103) ; (v) classic scaling with β = 3/2 for very large N = O (103) and above. The specified size ranges apply for typical model parameters. We focus on the moderate size regime where we show that diffusivity cycles quasiperiodically from the slowest branch forNp+ 3 (notNp) to the fastest branch forNp+ 1. Behavior is quantified by kinetic Monte Carlo simulation of an appropriate stochastic lattice-gas model. However, precise analysis must account for a strong enhancement of diffusivity for short time increments due to back correlation in the cluster motion. Further understanding of this enhancement, of anomalous size scaling behavior, and of the merging of various branches, is facilitated by combinatorial analysis of the number of the ground-state and low-lying excited state cluster configurations, and also of kink populations. [ABSTRACT FROM AUTHOR]