Molecular dynamics simulations of nanometer cluster behavior

Chun-Cho Chen, Purdue University

Abstract

Molecular dynamics (MD) simulation techniques with Lennard-Jones (LJ) and embedded-atom-method (EAM) potentials have been performed to study the behavior of nanometer diameter clusters. The melting behavior and the solidified structures of single-component LJ and EAM Au clusters as well as two-component LJ and EAM Pd/Pt clusters were investigated. All the clusters show depressed melting temperatures compared with the bulk, which increase with increasing cluster size. The melting temperatures for Au clusters obtained using the EAM potential are higher than those measured experimentally. As clusters become bigger, their lowest energy structure transforms from icosahedral (IC) to face-centered-cubic (FCC). From solidification simulations, the critical size range for this transformation is between 13 and 55 atoms for EAM Au clusters, while for LJ clusters it is larger than 110 atoms. For LJ clusters consisting of two components with different interaction strengths or atomic radii, superheating of the core atoms can be achieved if the core atoms are surrounded by stronger-interacting atoms. Upon solidification clusters tend to have a multi-layer icosahedral (MIC) structure, with the stronger-interacting atoms in the core. If the two components have different atomic radii, the solidified structures do not show significant order. In the EAM Pd/Pt case in which there were 13 Pt atoms and 42 Pd atoms, the solidified structure is MIC with a Pt core surrounded by Pd. Head-on collisions between two LJ clusters and between two EAM Au clusters were investigated. For each system two types of simulations were performed: collisions in which the initial temperature of the clusters was 0K and collisions in which the clusters had a finite initial temperature. All the collisions result in sticking, i.e., a single collision product. Quenched to 0K, the products of LJ collisions display an IC symmetry, whereas the products of the EAM Au collisions do not show significant structural order. The effects of initial impact parameter and initial relative velocity on the collision dynamics of two 55-atom LJ clusters were also studied. Using a mean relative velocity, the critical impact parameter calculated assuming a spherically symmetric cluster-cluster potential is in good agreement with the simulation results. However the sticking probabilities at different impact parameters are smaller than the simulation values obtained for an initial cluster temperature, T* = 0.1, due to the neglect of vibrational motion in the theory.

Degree

Ph.D.

Advisors

Talbot, Purdue University.

Subject Area

Chemical engineering

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