Preparation of Ultrafine Supported Clusters for Electron Microscopy Studies
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irradiation damage problems. They are inevitable, but they are interesting. Poppa et al. [5] have studied electron irradiation effects on vapor deposited Pd particles on oxide supports. These authors have studied the changes in the particle size and morphology induced by electron beam damage of the support. Others [6] have performed similar research on carbon-supported molecular clusters of various metals namely Pt, Ni, etc. These studies throw significant light on the crystal structure and on stability of these clusters. " There have been some recent developments where a very similar technique (small probe microscopy) is used to obtain atomic images from bulk materials; to date it has not been applied to small particles but it probably will be in the near future. " [7]. We have performed this work in a quantitative manner for the first time, refining the work of previous workers [8, 9]. McComb et. al [10] have examined osmium carbonyl clusters using HAADF imaging in STEM and compared the images with simulated images. Using such techniques, whether one wants to study simple diffusion of metal atoms on a support surface, or stability of metal particles or interaction between metal/support, the issue of sample preparation is usually highly understated. For quantitative electron microscopy of small supported particles, the requirements are even more demanding. In order to attain reliable statistical analysis, there is a need for many separated particles; thus, two issues must be addressed: (1) Preparation of thin, uniform supports of a weakly scattering material, such as carbon, so that background contribution to the image intensity is insignificant. (2) Deposition of controlled sizes of metal clusters, with random distribution. EXPERIMENTAL Our experiments were twofold: sample preparation, and high-angle dark field image acquisition in STEM followed by image analysis. Sample preparation involves two steps: one, preparing thin, low-noise supports, and two, controlled deposition of isolated particles on these supports. I. Supports Various supports are of interest in catalysis, because metal/support interactions dictate the effectiveness of the catalyst system for certain chemical reactions. Various model supports can be prepared for reliable microscopic analysis. Preparing low background supports to deposit small metal particles is crucial. These supports should possess the following properties:
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1. They should be weakly scattering; therefore, they should be made of a low atomic number (Z) material such as carbon, boron, beryllium, silica etc. A weakly scattering support produces lower background intensity. 2. They should be thin. 3. They should not etch or contaminate under the electron beam. 4. The thickness of these supports should be uniform over the areas being analyzed. Thickness fluctuations, appearing as speckle in low-angle dark field images, can be suppressed under high-angle scattering conditions, and are prevalent for amorphous substrates. Crystalline substrates such as graphite, are less likely to d
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