Ghyslain Boisvert

SELF-DIFFUSION OF ADATOMS, DIMERS, AND VACANCIES ON CU(100)

We use ab initio static relaxation methods and semiempirical molecular-dynamics simulations to investigate the energetics and dynamics of the diffusion of adatoms, dimers, and vacancies on Cu(100). It is found that the dynamical energy barriers for diffusion are well approximated by the static, 0 K barriers and that prefactors do not depend sensitively on the species undergoing diffusion. The ab initio barriers are observed to be significantly lower when calculated within the generalized-gradient approximation (GGA) rather than in the local-density approximation (LDA). Our calculations predict that surface diffusion should proceed primarily via the diffusion of vacancies. Adatoms are found to migrate most easily via a jump mechanism. This is the case, also, of dimers, even though the corresponding barrier is slightly larger than it is for adatoms. We observe, further, that dimers diffuse more readily than they can dissociate. Our results are discussed in the context of recent submonolayer growth experiments of Cu(100).

Island morphology and adatom self-diffusion on Pt(111)

The results of a density-functional-theory study of the formation energies of (100)- and (111)-faceted steps on the Pt(111) surface, as well as of the barrier for diffusion of an adatom on the flat surface, are presented. The step formation energies are found to be in a ratio of 0.88 in favor of the (111)-faceted step, in excellent agreement with experiment; the equilibrium shape of islands should, therefore, clearly be nonhexagonal. The origin of the difference between the two steps is discussed in terms of the release of stress at the surface through relaxation. For the diffusion barrier, we also find relaxation to be important, leading to a

SURFACE DIFFUSION COEFFICIENTS BY THERMODYNAMIC INTEGRATION : CU ON CU(100)

20%

DIFFUSION OF PT DIMERS ON PT(111)

decrease of its energy. The value we obtain, 0.33 eV, however, remains higher than available experimental data; possible reasons for this discrepancy are discussed. We find the ratio of step formation energies and the diffusion barrier to be the same whether using the local-density approximation or the generalized-gradient approximation for the exchange-and-correlation energy.

Single-Atom Diffusion on Silver and Gold Surfaces

The rate of diffusion of a Cu adatom on the Cu(100) surface is calculated using thermodynamic integration within the transition state theory. The results are found to be in excellent agreement with the essentially exact values from molecular-dynamics simulations. The activation energy and related entropy are shown to be effectively independent of temperature, thus establishing the validity of the Arrhenius law over a wide range of temperatures. Our study demonstrates the equivalence of diffusion rates calculated using thermodynamic integration within the transition state theory and direct molecular-dynamics simulations.

Many-body nature of the Meyer-Neldel compensation law for diffusion.

We report the results of a density-functional study of the diffusion of Pt dimers on the (111) surface of Pt. The calculated activation energy of

Energetics of diffusion on the (100) and (111) surfaces of Ag, Au, and Ir from first principles

0.37 \mathrm{eV}

Self-diffusion on low-index metallic surfaces: Ag and Au (100) and (111).

is in exact agreement with the recent experiment of Kyuno et al. [Surf. Sci. 397, 191 (1998)]. Our calculations establish that the dimers are mobile at temperatures of interest for adatom diffusion, and thus contribute to mass transport. The diffusion path for dimers is found to consist of a sequence of concerted two-atom jumps (plus local reorientations).

Activation energy for the decay of two-dimensional islands on Cu(100)

We present the results of a detailed Molecular-dynamics study of single-atom diffusion on the surfaces Ag (100) and (111), and Au (111), using the embedded-atom method to describe the interactions between the atoms. We find that diffusion is Arrhenius-like up to temperatures corresponding to a large fraction of the activation energy. We demonstrate, in addition, that an excellent description of the rate of diffusion is provided by a simple transition-state theory, together with parameters that derive directly from the static potential-energy surface. The Model predicts very accurately the activation energies, while the prefactor for diffusion is obtained within a factor of 2, a discrepancy we attribute to the neglect, in the Model, of the details of the structure of the surface. At higher temperatures, diffusion becomes clearly non-Arrhenius, and the model fails.