AVS 45th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS1-WeA

Paper SS1-WeA7
Molecular Dynamics Studies of Interlayer Mass Transport and Dendritic-to-Compact Morphological Transitions during Submonolayer Growth on Pt(111) Surfaces

Wednesday, November 4, 1998, 4:00 pm, Room 308

Session: Electromigration and Surface Transport
Presenter: V. Chirita, Linköping University, Sweden
Authors: V. Chirita, Linköping University, Sweden
E.P. Münger, Linköping University, Sweden
J.-E. Sundgren, Linköping University, Sweden
J.E. Greene, University of Illinois, Urbana
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We use embedded-atom method molecular dynamics simulations to investigate the kinetics of two processes which are critical in achieving the layer-by-layer growth mode: interlayer mass transport and dendritic-to-compact morphological transitions. The former investigation is carried out by following the dynamics of adatoms, vacancies and adatom-vacancy pairs within hexagonal Pt@sub 19@ clusters on Pt(111) at 1000 K, for simulation times totalling ~ 135 ns. The latter study concentrates on the dynamics of Pt dendrites containing up to 25 atoms on the same surface at the same temperature. The mapping of adatoms motion on top of the clusters shows that prior to incorporation, adatoms are trapped near the cluster edge for ~ 80% of the total simulation time. Cluster configurations with central vacancies are found to be quite stable. Adatom incorporation is observed to occur mainly via the two well known mechanisms of hopping and push-out/exchange with edge atoms. However, our simulations for adatom-vacancy pairs within clusters, bring the first direct evidence that both mechanisms can be active in the central region of the cluster, i.e. monovacancies are filled by adatom hopping or via exchanges with one of the atoms adjacent to the vacancy. We carry out minimum energy path calculations to examine the energetics of the two mechanisms. The results show that activation barriers are comparable to the corresponding interlayer mass transport mechanisms observed at the outer edge of the cluster. We have also followed the dynamics of Y-shaped Pt dendrites for simulations times in excess of 100 ns. The results show that, in agreement with previously proposed models, the dendritic-to-compact morphological transition proceeds via diffusion around branch corners,as well as edge diffusion. Our simulations reveal however, the presence of a new diffusion mechanism, in which the entire corner of a dendritic branch translates to adjacent terrace sites via a concerted motion of the atoms forming the corner. The process has an activation barrier similar to that of two-fold coordinated atoms diffusing around the corner.