Tham khảo tài liệu 'nanomaterials for nanoscience and nanotechnology part 9', 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 97 By using post-specimen lenses we can conveniently switch from low-angle ADF to HAADF imaging mode to study the contrast variations in ADF images. With the increase of the inner collection angle the phase and diffraction contrast slowly decreases the atomic-number contrast becomes dominant but the signal-to-noise ratio of the ADF images decreases. The inner collection angle at for low-angle ADF images is usually greater than 10 mrad and the outer collection angle a2 is generally set at about 50 mrad Fig. 4-12 . For HAADF imaging however the appropriate inner collection angle of the ADF detector depends on the nature of the sample and the orientation of crystalline materials. For imaging polycrystalline materials or small particles the inner collection angle 1 is usually greater than 100 mrad and the outer collection angle 2 can be as high as a few hundred mrad. For imaging zone-axis crystals however 2 is usually set at an angle smaller than the first-order Laue pattern to reduce contributions of high order Bragg scattering and is about 50-60 mrad. The resolution attainable in HAADF images is better than the resolution obtainable in BF STEM or TEM images using the same lenses 4 15 25-27 . Observation of high-resolution details of crystalline specimens on a scale nm can be achieved by using 100 keV electrons 28-32 and a resolution of less than nm has been achieved by using 300 keV electrons 33 34 . Because of the high atomic-number sensitivity and high spatial resolution the HAADF technique is undoubtedly one of the most useful imaging techniques for studying nanoparticles supported catalysts and interfaces in semiconductors ceramics and superconducting materials. The strength of the high-angle scattering which gives the HAADF imaging signal depends on several parameters including 1 large-angle elastically scattered electrons 2 phonon scattered electrons and 3 inelastically scattered electrons. .
