On Molecular Dynamics Simulation of Metal Clusters and Their XRD and ELNES Spectra
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L10.3.1
On Molecular Dynamics Simulation of Metal Clusters and Their XRD and ELNES Spectra G. D'Agostino1 and M. Gusso2 Unità “Materiali”, ENEA - Ente per le Nuove Tecnologie l'Energia e l'Ambiente 1 ENEA CR “Casaccia”, sp 59 CP 2400 Roma, ITALY 2 ENEA CR “Brindisi”, SS 7 Appia km 713 Brindisi, ITALY ABSTRACT We report on molecular dynamics calculation of few selected metal clusters by means of classical many-body potential. Despite simplicity of the approach, structural and thermodynamic properties have been achieved, such as an estimation of melting temperature lowering with size. As expected, coexistence ranges are observed instead of a single temperature transition. Phonon spectra have been also evaluated. Thermal atomic displacements at low temperature have been, in turn, estimated by means of Bose statistics. This work is intended as a step toward computer aided interpretation of cluster characterization by X-ray's or electron beams. Therefore, X-rays diffraction patterns and near edge electron energy loss have been simulated for different model structures in order to provide insights on those characterization technique capabilities. INTRODUCTION Metal clusters exhibit interesting features both at academic and applied level. Liquid-solid coexistence represent a typical finite size phenomenon that sheds light on several aspects of solid-state physics including theory of nucleation and phase transitions. On the other hand, metal clusters have been extensively employed in catalysis and as doping particles for silicon or polymer based materials, due to their optic activity and magnetic properties. The present work will be focused on metal clusters of nano-metric size that correspond to aggregates of typically a thousand atoms. Functional properties of clusters mostly relate to their electronic properties. A proper computational approach to such problem would involve “ab initio” calculations that can be performed by means of several assessed quantum chemical codes or by “Car Parrinello” ones. Unfortunately, all such approaches require a huge amount of computational time and memory that is beyond our means and the purposes of the present work. Nevertheless qualitative results may be obtained by means of classical molecular dynamics simulation. Furthermore, even the most sophisticated “ab Initio” simulation can hardly include the effects of matrix-cluster interaction. Actually, this should involve tens of thousands of atoms that is far away from nowadays computational power. There are many different experimental characterization techniques to probe structural and functional properties of clusters. This paper will be limited to electro-magnetic (e.m) radiation scattering only. More precisely X-Rays Diffraction (XRD) and Electron Energy Loss Near Edge (ELNES) spectra will be explicitly simulated. It is worth stressing that any other e.m. spectroscopy such as XANES, EXAFS, XPS can be approached within the same scenario. The former techniques were preferred as they are easily achieved by means of a transmission electron
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