Nanomechanical study of synthesis of metallic core-shell clusters via coalescence
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Nanomechanical study of synthesis of metallic core-shell clusters via coalescence S. Mizuno and K. Shintani Department of Mechanical Engineering and Intelligent Systems, University of ElectroCommunications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan ABSTRACT Morphological evolution of two metallic clusters of different elements at coalescence is investigated using molecular-dynamics (MD) simulation. All the pair combinations of the elements Ni, Cu, Au, Ag, Pt, and Pd are considered. The final structures of united bimetallic clusters are classified into three categories: epitaxial, core-shell, and alloyed. Which type of structure appears via coalescence depends on the size and temperature of clusters, which can be summarized in an observed structure map. INTRODUCTION Nanoscale particulate metallic materials have attracted much attention of researchers because of their nanotechnological applications to catalysts, magnetic devices, photonic materials, etc. Bimetallic clusters have unique characteristics which cannot be possessed by single-element clusters [1]. Because coalescence of clusters frequently occurs in the gas-phase growth processes [2] or for clusters supported on substrates [3], synthesize of bimetallic clusters via coalescence of clusters of different elements is relevant. In our preceding study [4], we studied coalescence of metallic clusters by means of MD simulations in microcanonical ensemble assuming that coalescence occurs in an isolated system. However, cluster growth processes in experiments are ordinarily temperature-controlled. Hence, in this paper, we perform MD simulations of coalescence of clusters in canonical ensemble. In addition, two elements are added to the elements treated in the preceding study. METHOD OF SIMULATION The generic embedded atom method potential [5] is used for calculating interactions between metallic atoms. The velocity scaling is employed to control the temperature. The time step is set to be 0.1fs in order that thermal movements of the atoms in clusters can be tracked carefully. The six elements of transition and noble metals Ni, Cu, Pd, Ag, Pt, Au are treated. First, clusters which are of icosahedral structure and consist of 13, 55, 147, 309, and 561 atoms of each of the elements are constructed, and are equilibrated at a specified temperature beforehand. Next, two of these clusters of the same number of atoms are put close to each other, where the nearest two atoms that belong to the two clusters, respectively, are within the range of interaction of the potential. The MD calculation under the constant temperature condition is started, and the two clusters begin to coalesce. Temperature is changed from 300K to 900K. RESULTS AND DISCUSSION The simulations are performed for all the pair combinations of the six elements at seven temperatures. The resulting structures of the united clusters are categorized into the three kinds,
epitaxial, core-shell, and alloyed. As a typical example, the results for coalescence of Cu309 cluster and Au309 cluster at 600
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