Electronic Structure of the c(2x2)O/Cu(001) System

The locally self-consistent real space multiple scattering technique has been applied to calculate the electronic structure and chemical binding for the c(2x2)O/Cu(001) system, as a function of $d_{O-Cu1}$ -- the height of oxygen above the fourfold hollow sites. It is found that the chemical binding between oxygen and copper has a mixed ionic-covalent character for all plausible values of $d_{O-Cu1}$. Furthermore, the electron charge transfer from Cu to O depends strongly on $d_{O-Cu1}$ and is traced to the variation of the long-range electrostatic part of the potential. A competition between the hybridization of Cu1-$d_{xz}$ with O-$p_x,p_y$ and Cu1-$d_{x^2-y^2}$ with O-$p_z$ states controls modification of the electronic structure when oxygen atoms approach the Cu(001) surface. The anisotropy of the oxygen valence electron charge density is found to be strongly and non-monotonically dependent on $d_{O-Cu1}$.Comment: 14 pages, 7 figures, 1 tabl

The crossover from collective motion to periphery diffusion for 2D adatom-islands on Cu(111)

The diffusion of two dimensional adatom islands (up to 100 atoms) on Cu(111) has been studied, using the self-learning Kinetic Monte Carlo (SLKMC) method [1]. A variety of multiple- and single-atom processes are revealed in the simulations, and the size dependence of the diffusion coefficients and effective diffusion barriers are calculated for each. From the tabulated frequencies of events found in the simulation, we show a crossover from diffusion due to the collective motion of the island to a regime in which the island diffuses through periphery-dominated mass transport. This crossover occurs for island sizes between 13 and 19 atoms. For islands containing 19 to 100 atoms the scaling exponent is 1.5, which is in good agreement with previous work. The diffusion of islands containing 2 to 13 atoms can be explained primarily on the basis of a linear increase of the barrier for the collective motion with the size of the island

Island Size Selectivity and island-shape analysis during 2D Island Coarsening of Ag/Ag (111) Surface

In our earlier study of Ag island coarsening on Ag(111) surface using kinetic Monte Carlo (KMC) simulations we found that during early stages coarsening proceeds as a sequence of selected island sizes resulting in peaks and valleys in the island-size distribution and that this selectivity is independent of initial conditions and dictated instead by the relative energetics of edge-atom diffusion and detachment/attachment processes and by the large activation barrier for kink detachment. In this paper we present a detailed analysis of the shapes of various island sizes observed during these KMC simulations and show that selectivity is due to the formation of kinetically stable island shapes which survive longer than non-selected sizes, which decay into nearby selected sizes. The stable shapes have a closed-shell structure - one in which every atom on the periphery having at least three nearest neighbors. Our KMC simulations were carried out using a very large database of processes identified by each atom's unique local environment, the activation barriers of which were calculated using semi-empirical interaction potentials based on the embedded-atom method.Comment: 17 pages, 11 figure

SLKMC-II study of self-diffusion of small Ni clusters on Ni (111) surface

We studied self-diffusion of small 2D Ni islands (consisting of up to 10 atoms) on Ni (111) surface using a self-learning kinetic Monte Carlo (SLKMC-II) method with an improved pattern-recognition scheme that allows inclusion of both fcc and hcp sites in the simulations. In an SLKMC simulation, a database holds information about the local neighborhood of an atom and associated processes that is accumulated on-the-fly as the simulation proceeds. In this study, these diffusion processes were identified using the drag method, and their activation barriers calculated using a semi-empirical interaction potential based on the embedded-atom method. Although a variety of concerted, multi-atom and single-atom processes were automatically revealed in our simulations, we found that these small islands diffuse primarily via concerted diffusion processes. We report diffusion coefficients for each island size at various tepmratures, the effective energy barrier for islands of each size and the processes most responsible for diffusion of islands of various sizes, including concerted and multi-atom processes that are not accessible under SLKMC-I or in short time-scale MD simulations

Atomistic studies of thin film growth

We present here a summary of some recent techniques used for atomistic studies of thin film growth and morphological evolution. Specific attention is given to a new kinetic Monte Carlo technique in which the usage of unique labeling schemes of the environment of the diffusing entity allows the development of a closed data base of 49 single atom diffusion processes for periphery motion. The activation energy barriers and diffusion paths are calculated using reliable manybody interatomic potentials. The application of the technique to the diffusion of 2-dimensional Cu clusters on Cu(111) shows interesting trends in the diffusion rate and in the frequencies of the microscopic mechanisms which are responsible for the motion of the clusters, as a function of cluster size and temperature. The results are compared with those obtained from yet another novel kinetic Monte Carlo technique in which an open data base of the energetics and diffusion paths of microscopic processes is continuously updated as needed. Comparisons are made with experimental data where available

A comparative study of CO adsorption on flat, stepped and kinked Au surfaces using density functional theory

Our ab initio calculations of CO adsorption energies on low miller index (111), (100), stepped (211), and kinked (532) gold surfaces show a strong dependence on local coordination with a reduction in Au atom coordination leading to higher binding energies. We find trends in adsorption energies to be similar to those reported in experiments and calculations for other metal surfaces. The (532) surface provides insights into these trends because of the availability of a large number of kink sites which naturally have the lowest coordination (6). We also find that, for all surfaces, an increase in CO coverage triggers a decrease in the adsorption energy. Changes in the work-function upon CO adsorption, as well as the frequencies of the CO vibrational modes are calculated, and their coverage dependence is reported.Comment: 18 pages, 4 figure

Self-learning Kinetic Monte-Carlo method: application to Cu(111)

We present a novel way of performing kinetic Monte Carlo simulations which does not require an {\it a priori} list of diffusion processes and their associated energetics and reaction rates. Rather, at any time during the simulation, energetics for all possible (single or multi-atom) processes, within a specific interaction range, are either computed accurately using a saddle point search procedure, or retrieved from a database in which previously encountered processes are stored. This self-learning procedure enhances the speed of the simulations along with a substantial gain in reliability because of the inclusion of many-particle processes. Accompanying results from the application of the method to the case of two-dimensional Cu adatom-cluster diffusion and coalescence on Cu(111) with detailed statistics of involved atomistic processes and contributing diffusion coefficients attest to the suitability of the method for the purpose.Comment: 18 pages, 9 figure