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Vibrational Properties of Silver Nanoparticles and Nanocrystalline Materials Ralf Meyer D´epartement de Physique, Universit´e de Montr´eal and Groupe de Recherche en Physique et Technologie des Couches Minces (GCM) C.P. 6128 succursale centre-ville, Montr´eal (Qu´ebec) H3C 3J7, Canada

ABSTRACT The vibrational density of states of silver in the form of a free cluster, a single crystal and a nanocrystalline material has been calculated with the help of molecular-dynamics simulations. The model for the nanocrystalline material was derived by the simulation of pressureless sintering of nanometer sized silver particles. The results show a broadening of the vibrational density of states in the case of the cluster and the nanocrystalline material.

INTRODUCTION In recent years materials scientists have paid much attention to the study of nanomaterials (see e.g. [1]). One interesting problem within this context is the vibrational density of states (VDOS) of nanocrystalline metals since the small grain sizes in these materials lead to significant changes in this quantity. Several inelastic neutron neutron scattering studies of pure metals and alloys have been published [2–7]. In two of these studies a linear onset of the VDOS at low energies has been observed [6, 7]. In this work, results of the calculation of the VDOS of silver as a free cluster, a single crystal, and a nanocrystalline material from molecular-dynamics simulations are presented. The model for the nanocrystalline material was obtained from a simulation of the pressureless sintering of nanoclusters. By this method a model material with 92.5 % of the density of the perfect solid was obtained which contains a number of nanometer sized voids. The results for the VDOS show a significant broadening in the case of the cluster and the nanocrystalline material. However, no linear onset of the VDOS is observed in the low energy regime.

COMPUTATIONAL METHODS Molecular-dynamics simulations employing an embedded-atom method (EAM) potential [8] for silver have been performed. Details of the potential will be published elsewhere [9]. From these simulations the VDOS of the systems under investigation were calculated from the velocity autocorrelation function hv(t ) v(0)i whose Fourier-transform is proportional to the VDOS [10]. Three different configurations were used in the calculations. An Ag1205 cluster with a diameter of 3.3 nm was constructed by cutting a spherical region out of a regular fcc lattice. Simulations of single crystalline silver were based on a supercell of 20  20  20 cubic fcc elementary cells (32 000 atoms). Much more efforts were required to obtain a model for the nanocrystalline material. For this purpose, 32 randomly rotated copies of the Ag1205 cluster (after equilibration) were arranged B8.8.1

Figure 1. Upper panel: Configuration of nanocrystalline silver after pressureless sintering over 1.5 ns. The edges of the nearly cubic cell have a length of about 9 nm. Lower panel: 0.4 nm thick cross-section of the same configuration. B8.8.2

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