Simulation of Morphology and Surface Vibration in Copper and Gold Nanoparticles
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Simulation of Morphology and Surface Vibration in Copper and Gold Nanoparticles Y. Kogure, Y. Kato, T. Nozaki and M. Doyama Teikyo University of Science & Technology Uenohara, Yamanashi 409-0193, Japan ABSTRACT Formation and vibrational states in nanoparticles have been investigated by means of molecular dynamics simulation. The embedded atom method potentials for Cu and Au were adopted to express the interaction between atoms in the crystals. The nanoparticles were formed by cooling the atomic systems of molten states. Surface morphology of the nanoparticles were represented by highlighting the surface atoms, which were distinguished by the potential energy. Simulated surface morphology is not so symmetric as natural nanoparticles. The radial distribution function and the cross sectional view of the particles were also derived to characterize the internal structure. Thermal vibration of sample atoms at elevated temperatures was analyzed and the power spectra were calculated. Excitation of phonon mode is seen in the spectra. INTRODUCTION
Morphology of metallic nanoparticles produced by gas-evaporation technique has extensively been investigated by means of electron microscope observations [1]. Those particles are 10-1000 nm in diameter and have highly symmetric external shape. The static morphology of these particles is strongly affected from surface energy and compared with the Wulff polyhedron. A theoretical calculation of Wulff polyhedron based on the Morse potentials has also been reported [2]. Recently, the embedded atom method (EAM) potentials have been developed [3,4], which can realize the many body nature of the atomic interaction in metals and has successfully been applied to the problems of surface and defects. Molecular dynamics simulations using the EAM potential are performed in the present study, and the process of formation and dynamical nature of the nanoparticles are investigated. The nanoparticle is a suitable system for the molecular dynamics simulation because it is consisted of manageable number of atoms by a computer. The formation of nanoparticles are related with the fundamental nature of crystal growth, and final purpose of the present study is to find the fundamental mechanisms of self-organization in the nanostructures.
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METHOD OF SIMULATION In the molecular dynamics simulation of glassy state, dynamics of 5300 atom systems are treated under free boundary condition. The motion of each atom is traced by integrating the Newton’s equation of motion. The time interval ∆t for the molecular dynamics simulation is chosen to be 5 ×10−15 s, which is about 1/100 of the period of the maximum atomic vibration frequency. The potential function used in the present study has been developed by the present authors [5,6], and has been applied to the simulation of nanoparticles [7]. The potential energy for i-th atom is expressed as 1 E i = F ( ρ i ) + ∑ φ (rij ) . (1) 2 j ≠i Where F ( ρ i ) is the embedding energy for i-th atom.
ρ i is the electron density function and it
is a sum o
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