Morphology Maps of Small Particles
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time imaging of the structural transformations in small particles by lijima and Ichihashi[3] followed by a series of observations of these transformations in transition metals on oxides and carbon[4-8] indicated analogous behavior in small particles. Dundurs et al[9] theoretically evaluated the energies of the decahedral MTP for various positions of the disclination and showed that the energy differences between the Sc and the Dc were indeed small. The extension of the idea of a configurational space of morphologies, where the particles transformed rapidly, led to the construction of a "phase map" of the morphologies as functions of temperature and size[10]. The map while providing a possible interpretation for the formation of exceptionally large Ics in the electron diffraction studies by inert gas aggregation methods on unsupported Ag particles[l 1,12] has not been rigorously verified by experiments. In this paper we report the results of a detailed real time high resolution electron microscopy analysis of the structural transformations of Au particles on a SiO substrate and show for the first time, that structurally fluctuating particles have preferred morphology regimes. A probability model based on the potential energies of the small particles is then developed to construct morphology maps of fluctuating small particles. EXPERIMENTAL METHODS Samples were prepared by depositing small particles of Au onto crushed SiO at a vacuum of 2x10.6 Torr and then transferred to a Hitachi H-9000, 300kV transmission electron microscope 135 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
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Figure 1: Examples from the basis set (a) Single crystal (b) Single twin (c) Icosahedral MTP (d) Decahedral MTP
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maintained at a typical vacuum of 10-7 Torr. The sample was then annealed in the microscope at a beam flux of 85 Amps/cm 2 to obtain well separated particles. The anneal also elevated the particles onto cup-like mounds of the substrate. The particles were excited into undergoing morphological transformations by varying the beam flux and their behavior recorded in real time using a SONY EV09800 Hi-8 video recorder connected to a fiber optically coupled GATAN TV camera. The images were captured at the standard NTSC rate of 30 frames per second and then transferred to a workstation through an image grabber for further analysis using SEMPER, a image processing program. To ensure a good statistical count each observation lasted for more than three minutes. During this time several hundred image frames were recorded. These images were then analyzed by using a data reduction scheme. A basis set of good images was selected from each recording and completely characterized using simulated images and experimental images obtained by earlier workers[ 13-16]. Each basis set consisted of (a) the single crystal (b) the decahedral MTP (c) the icosahedral MTP (d) the single twin in various orientations, featureless structures were identified as indistinguishable and structures with features that could not be classifi
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