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 favour of the (111)-faceted step, in excellent agreement with experiment; the equilibrium shape of islands should therefore clearly be non-hexagonal. 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 20% 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.Comment: Submitted to Physical Review B; 11 postscript pages including 4 figures; this and related publications available from web sites at http://www.centrcn.umontreal.ca/~lewis and http://www.fhi-berlin.mpg.de/th/th.htm

Les besoins de formation des éducateurs et infirmières utilisant le plan de services individualisé

Cette étude examine, à l'aide de la méthode élaborée par Herskowitz ( 1977), les besoins de formation de 42 intervenants (19 éducateurs et 23 infirmières) qui utilisent lorsque requis le plan de services individualisé pour la clientèle qui présente des troubles sévères et persistants de santé mentale. Une version adaptée d'un questionnaire développé par Hurteau (1988), et modifié pour un contexte de santé mentale, identifie les besoins de formation des intervenants utilisant le plan, sous cinq dimensions précises et trois types de savoirs particuliers. À la lumière des résultats, certains besoins de formation apparaissent prioritaires, notamment l'animation des rencontres et les modalités d'élaboration du plan de services individualisé. De plus, les intervenants aimeraient obtenir de la formation sur la communautarisation des services offerts aux personnes présentant des besoins en santé mentale.With the help of the method elaborated by Herskowitz (1977), this study examines the training needs of 42 workers (19 educators and 23 nurses) using when needed, the individualized services plan in a context of external services (deinstitutionalization) in mental health and internal services (hospital). A modified version of a questionnaire developed by Hurteau (1988) adapted to the context of mental health, identifies the training needs of workers using the individualized services plan under five specific dimensions and three types of knowledge. In the light of results obtained, certain training needs appear as top priority, especially conducting meetings and methods of elaboration of individualized services plan. Moreover, workers would like to obtain training on community-oriented services offered to people with needs in mental health

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

First-principles calculations using the full-potential linear-muffin-tin-orbital technique have been performed to determine the energy barriers for adatom homodiffusion on the (100) and (111) surfaces of Ag, Au, and Ir. Our results agree very well with the measured energy barriers (when available), i.e., to within 0.03 eV, thereby confirming the adequacy of the theoretical method. On the (111) surfaces, we find that the barriers for Ag and Ir have values that are close to those corresponding to the melting point of the bulk materials, and conclude that ‘‘correlated jumps’’ should be present at high temperatures on these surfaces. For Au(111), on the other hand, the barrier is about twice as large as the melting temperature, and the random-walk model should provide an accurate description of the diffusion process, just as on the (100) surfaces, where the barriers are much larger. Semiempirical models are found to reproduce the first-principles energy barriers within 0.2 eV, which, in some cases, means errors as large as 90 %.Peer reviewe

Surface diffusion coefficients by thermodynamic integration: Cu on Cu(100)

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.Comment: 4 pages (revtex), two figures (postscript

Diffusion of Pt dimers on Pt(111)

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 0.37 eV is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.} \protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations establish that the dimers are mobile at temperatures of interest for adatom diffusion, and thus contribute to mass transport. They also indicate that the diffusion path for dimers consists of a sequence of one-atom and (concerted) two-atom jumps.Comment: Pour pages postscript formatted, including one figure; submitted to Physical Review B; other papers of interest can be found at url http://www.centrcn.umontreal.ca/~lewi

Self-diffusion of adatoms, dimers, and vacancies on Cu(100)

We use ab initio static relaxation methods and semi-empirical 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).Comment: Submitted to the Physical Review B; 15 pages including postscript figures; see also http://www.centrcn.umontreal.ca/~lewi