Diffusion of a single Cu adatom on low-index Cu surfaces with different morphologies (with and without the presence of other Cu adatoms as well as near and over stepped surfaces) is studied using the embedded-atom method and a molecular static simulation. Migration energies of a Cu adatom in the presence of other Cu adatoms which are relevant in computer simulations of island growth are calculated. We have also calculated the formation and migration energies of an adatom and a vacancy in different layers as well as formation energies of steps on various Cu surfaces. Step-step interaction is shown to be repulsive and consistent with elasticity theory. Our calculations predict a lower activation energy for diffusion of a vacancy than of an adatom for all three Cu surfaces.

We present a class of models that describe self-diffusion on FCC(001) metal substrates within a common framework. The models are tested for Cu(001), Ag(001), Au(001), Ni(001) and Pd(001), and found to apply well for all of them. For each of these metals the models can be used to estimate the activation energy of any diffusion process using a few basic parameters which may be obtained from experiments, ab-initio or semiempirical calculations. To demonstrate the approach, the parameters of the models are optimized to describe self-diffusion on the (001) surface, by comparing the energy barriers to a full set of barriers obtained from semi-empirical potentials via the embedded atom method (EAM). It is found that these models with at most four parameters, provide a good description of the full landscape of hopping energy barriers on FCC(001) surfaces. The main features of the diffusion processes revealed by EAM calculations are quantitatively reproducible by the models.

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The diffusion of molecular hydrogen (H2) on a layer of graphene and in the interlayer space between the layers of graphite is studied using molecular dynamics computer simulations. The interatomic interactions were modeled by an Adaptive Intermolecular Reactive Empirical Bond Order (AIREBO) potential. Molecular statics calculations of H2 on graphene indicate binding energies ranging from 41 meV to 54 meV and migration barriers ranging from 3 meV to 12 meV. The potential energy surface of an H2 molecule on graphene, with the full relaxations of molecular hydrogen and carbon atoms is calculated. Barriers for the formation of H2 through the Langmuir-Hinshelwood mechanism are calculated. Molecular dynamics calculations of mean square displacements and average surface lifetimes of H2 on graphene at various temperatures indicate a diffusion barrier of 9.8 meV and a desorption barrier of 28.7 meV. Similar calculations for the diffusion of H2 in the interlayer space between the graphite sheets indicate high and low temperature regimes for the diffusion with barriers of 51.2 meV and 11.5 meV. Our results are compared with those of first principles.

Electromigration-induced flow of islands and voids on the Cu(001) surface is studied at the atomic scale. The basic drift mechanisms are identified using a complete set of energy barriers for adatom hopping on the Cu(001) surface, combined with kinetic Monte Carlo simulations. The energy barriers are calculated by the embedded atom method, and parameterized using a simple model. The dependence of the flow on the temperature, the size of the clusters, and the strength of the applied field is obtained. For both islands and voids it is found that edge diffusion is the dominant mass-transport mechanism. The rate limiting steps are identified. For both islands and voids they involve detachment of atoms from corners into the adjacent edge. The energy barriers for these moves are found to be in good agreement with the activation energy for island/void drift obtained from Arrhenius analysis of the simulation results. The relevance of the results to other FCC(001) metal surfaces and their experimental implications are discussed.

A model that describes self diffusion, island nucleation and film growth on FCC(001) metal substrates is presented. The parameters of the model are optimized to describe Cu diffusion on Cu(001), by comparing activation energy barriers to a full set of barriers obtained from semiempirical potentials via the embedded atom method. It is found that this model (model I), with only three parameters, provides a very good description of the full landscape of hopping energy barriers. These energy barriers are grouped in four main peaks. A reduced model (model II) with only two parameters, is also presented, in which each peak is collapsed into a single energy value. From the results of our simulations, we find that this model still maintains the essential features of diffusion and growth on this model surface. We find that hopping rates along island edges are much higher than for isolated atoms (giving rise to compact island shapes) and that vacancy mobility is higher than adatom mobility. We observe substantial dimer mobility (comparable to the single atom mobility) as well as some mobility of trimers. Mobility of small islands affects the scaling of island density N vs. deposition rate F , N ∼ F γ , as well as the island size distribution. In the asymptotic limit of slow deposition, scaling arguments and rate equations show that γ = i * /(2i * +1) where i * is the size of the largest mobile island. Our Monte Carlo results, obtained for a range of experimentally relevant conditions, show γ = 0.32 ± 0.01 for the EAM, 0.33 ± 0.01 for model I and 0.31 ± 0.01 for model II barriers. These results are lower than the anticipated γ ≥ 0.4 due to dimer (and trimer) mobility. *

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