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Nanomaterials for Nanoscience and Nanotechnology part 8

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Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 8', 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 85 Figure 4-4 shows a set of images acquired simultaneously of a supported metal catalyst. Figure 4-4a is an SE image of the entrance surface of the catalyst showing small particles and a detailed surface morphology of the catalyst support. Figure 4-4b is the corresponding bright-field BF STEM image revealing small particles with a dark contrast. Metal particles of various shapes and sizes dispersed in or on the catalyst support are clearly revealed in the HAADF image Fig. 4-4c . The exit-surface SE image Fig. 4-4d reveals an unusual surface topography of the catalyst support. Detailed analyses of such sets of images can provide important information about the catalyst distribution of sizes and relative locations of various types of metal particles present in the supported catalyst. The SE images clearly indicate that the small spherical particles are located on the entrance side of the support. The larger metal particles with irregular shapes are located in the interior of the catalyst support since these particles are clearly revealed in the HAADF image but are not shown in either the entrance- or the exit-surface SE images. XEDS analysis of these particles can give information about the composition of the individual nanoparticles. The correlation of HAADF images with SE images and XEDS spectra is very effective in identifying which type of the metal particles is exposed to the reacting molecules during a catalytic reaction. 4.2 STEM instrumentation 4.2.1 Dedicated STEM and STEM attachment in TEM There are two types of STEM instruments commonly used by electron microscopists dedicated STEM DSTEM and STEM attachment in TEM. The DSTEM instruments which were exclusively made by VG Microscopes UK have unique designs and capabilities. A DSTEM microscope uses a cold field-emission gun to generate high-brightness electron probes with sub-nanometer sizes. The sizes of electron probes used in TEM STEM instruments