TAILIEUCHUNG - Nanomaterials for Nanoscience and Nanotechnology part 11

Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 11', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Scanning Transmission Electron Microscopy of Nanoparticles 121 Surface compositional analysis of individual nanoparticles is essential for understanding the activity and selectivity of industrial bimetallic or multi-component catalysts used in a variety of chemical reactions. The overall composition of these individual nanoparticles can usually be obtained by the use of high spatial resolution XEDS see section . It is however extremely difficult to extract information about preferential surface segregation or aggregation of individual components in nanoparticles of different sizes. Because of the high-surface sensitivity of Auger electrons it is possible to determine qualitatively and in some cases quantitatively the surface composition of nanoparticles consisting of multiple components. Figure 4-31b shows an Auger electron spectrum obtained from a sample containing palladium silver or palladiumsilver nanoparticles highly dispersed on a high-surface area support. Both the silver and the palladium MNN Auger peaks are clearly revealed. Quantitative analyses of this type of spectrum can provide information about the surface enrichment of specific elements and information about how this enrichment varies with the size of the particles. High spatial resolution scanning Auger microscopy Considerable efforts have been devoted to the development of scanning Auger microscopy SAM and the improvement of its imaging resolution since the pioneering work of MacDonald and Waldrop 122 . The driving force for developing high spatial resolution SAM is related to the characterization of nanoparticles and other nanostructured materials. The best resolution of SAM images obtained in conventional SEM geometry is approximately 30 nm under favorable conditions 123-124 . A spatial resolution of about 10 nm can be achieved using the high current density and the small probes of STEM instruments 125 . A similar or better image resolution may also be achievable in the new generation

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