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Nanomaterials for Nanoscience and Nanotechnology part 5

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Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Transmission Electron Microscopy and Spectroscopy of Nanoparticles 49 Figure 3-11. A pair of a bright-field and b dark-field images from Pt nanocrystals showing a ring shape contrast in the dark field image see the particles indicated by arrowheads which is likely due to the diffuse scattering from the surface reconstructed relaxed layers. The dark field image was recorded using a small objective aperture as shown in the inserted electron diffraction pattern positioned on the 111 and 200 reflection rings. 50 Wang 10 nm smaller than the extinction distances of Au 111 15.9 nm and Au 200 17.9 nm . This type of contrast is due to the diffuse scattering generated from the top few atomic layers of the nanocrystal owing to surface reconstruction relaxation. A non periodic perturbation to the crystal lattice would produce diffuse scattering that is distributed between the Bragg reflections in the entire diffraction plane. In the angular selection range of the aperture the particles whose Bragg reflections are not falling into the aperture should be completely dark but the diffuse scattering from the particles would be partially collected resulting in a bright ring at the edge of the particle. This is another way to image the reconstruction relaxation of the surface atoms in nanocrystals. 3.3.3 Twining structure and stacking faults Twining is one of the most popular planar defects in nanocrystals and it is frequently observed for fee structured metallic nanocrystals. Twining is the result of two subgrains by sharing a common crystallographic plane thus the structure of one subgrain is the mirror reflection of the other by the twin plane. Figure 3-12a gives a HRTEM image of a Pt particle oriented along 110 which is composed of two grains connected at a twin relation. The twin plan is indicated by a dotted line. The fee structured metallic nanocrystals usually have 111 twins which is the fundamental defect mechanism for the growth of spherical-like particles see Section 3.2.4