Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 10', 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 109 Figure 4-21. HAADF image of bimetallic particles supported on zeolite crystallites connected by alumina binders and electron nanodiffraction patterns obtained from the six nanoparticles labeled in the HAADF image. Some of the diffraction spots are clearly split along certain crystallographic orientations spot splitting in the diffraction patterns 3 and 4 . buttons of their CEND patterns. Before this technique can be effectively and reliably utilized to extract the sample information coded in CEND patterns of small particles many experimental difficulties such as particle stability contamination and accurate control of beam defocus have to be overcome. When metal atoms aggregate from the vapor phase or in a liquid they usually form a crystal having shapes of regular pentagonal bi-prisms or icosahedra see Chapter 3 for details . Their internal structure is a complex arrangement of five or twenty twinned components. Large metal particles with shapes of cuboctahedron decahe- 110 Liu Figure 4-22. Experimental a and simulated b coherent electron nanodiffraction patterns of a silver nanoparticle with an icosahedral shape. The incident beam was along the five-fold symmetry axis of the icosahedral particle. dron icosahedron and other multiple-twinned structures have been observed 74-76 . For particles with sizes smaller than 2 nm in diameter however it is difficult to unambiguously determine their shape by imaging techniques. CEND technique can provide information about the shape of clean metallic nanoparticles. For example a large portion of clean silver nanoparticles with a size smaller than 3 nm in diameter was observed to give unique CEND patterns exhibiting almost five-fold symmetry. These clean silver nanoparticles were formed by in situ deposition in a UHV STEM instrument MIDAS . Figure 4-22a shows such a CEND pattern and Fig. 4-22b shows a simulated CEND pattern of a small icosahedron with the incident .
