AVS 46th International Symposium
    Surface Science Division Wednesday Sessions
       Session SS3+NS-WeA

Paper SS3+NS-WeA7
Coalescence Dynamics of Small Pt Clusters on Pt(111) Surfaces: A Molecular Dynamics Study

Wednesday, October 27, 1999, 4:00 pm, Room 604

Session: Islands, Clusters, and Steps
Presenter: V. Chirita, Linköping University, Sweden
Authors: V. Chirita, Linköping University, Sweden
E.P. Münger, Linköping University, Sweden
L. Hultman, Linköping University, Sweden
J.E. Greene, University of Illinois, Champaign-Urbana
Correspondent: Click to Email
The diffusion and coalescence of small clusters are fundamental intralayer mass transport processes, playing a crucial role during the early stages of thin film deposition and crystal growth. We use embedded-atom method molecular dynamics simulations to follow the kinetics characterizing these processes for compact, 2D Pt@sub 5@ and Pt@sub 6@ clusters on Pt(111), at 1000 K. Investigations are carried out for configurations consisting of clusters initially separated by a distance equal to that between second neighbors, in statistically independent runs of 10 ns each. Prior to coalescence, we observe that the intercluster separation distance is reduced via two pathways: net cluster diffusion, involving mechanisms which preserve cluster shape, and repeated cluster reshaping. Cluster diffusion occurs primarily via concerted gliding and reptation, a recently proposed diffusion mechanism for 2D clusters on (111) metallic surfaces. Cluster reshaping involves edge-diffusion and/or concerted dimer/trimer gliding. Our simulations reveal that cluster coalescence is achieved via complex dynamics. Clusters preserving their initial compact shape can coalesce via concerted gliding and form clusters with a high number of intracluster bonds. These larger clusters maximize the number of intracluster bonds in relatively short times, primarily via edge-diffusion. We observe for the first time, that during the fusion process, cluster-cluster interactions can induce cluster translations via sequential atom motion, through fcc/hcp bridge sites, within clusters. For clusters that reshape prior to coalescence, we observe that cluster-cluster interactions induce the transfer of single atoms, from either cluster, to sites neighboring both clusters, and thus create a "bond" between clusters. Once formed, we did not observe dissociation. In this case, the newly formed clusters have elongated shapes with narrow middle sections (1 to 2 atoms thick) and, as a result, the transition toward compactness is achieved over considerably longer times. Moreover, these clusters exhibit a variety of reconfiguration and migration events. Edge-diffusion, reptation and dislocation propagation are the competing diffusion mechanisms observed for clusters in this range.