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In STEM mode, if the detected electrons are from elastical and quasi-elastical scattering at high angles between ~ 50 – 100 mrad, the imaging mode is called high-angle annular dark- field (HAADF) mode. However, in many cases of HAADF imaging, the annular detection angle was practically set to be 60 - 170 mrad due to the limitation of combinations between aperture sizes and camera lengths. In HAADF STEM, the "high-angle" means beyond the angle at which diffraction maxima (spots) can be found. High-angle scattered electrons are few in number of elections and are mostly induced by Rutherford scattering. Due to absence of diffraction intensity, the key advantage of HAADF for imaging and analysis is that they are usually insensitive to crystalline structure and orientation but strongly dependant on atomic number (see Z-contrast), meaning that the collected electrons strongly depends on chemical composition. If the electron probe is less than one atom dimension in diameter (e.g. in many aberration-corrected STEMs), atom column resolution for composition analysis can therefore be obtained. However, we need to be aware that the thickness variation in STEM specimen also modifies the scattering signal.
Figures 3994 (a) and (c) show HAADF images of Si [110] taken in a Cs-corrected STEM system at 30 kV and 60 kV, respectively. Si (silicon) dumbbells, in which two Si columns are separated by 136 pm, are clearly shown in both cases. In Figure 3994 (b), the Fourier transform of the HAADF image (a) contains a clear -224 spot (corresponding to the lattice spacing of 111 pm), while in Figure 3994 (d), the Fourier transform of the HAADF image (c) shows clear -115 and -440 spots (corresponding to the lattice spacings of 105 and 96 pm, respectively).
![HAADF images of Si [110] taken at 30 kV and 60 kV](image1/2235b.jpg)
Figure 3994. (a) and (c) HAADF images of Si [110] taken at 30 kV and 60 kV, respectively, and (c) and (d) The Fourier transform of the HAADF image (a) and (b).
[1]
Table 3994. History of development of HAADF-STEM technique.
DATE |
MILESTONE |
1970 - 1979 |
Dedicated to image various heavy atoms, including U acetate, organic matter with U dimer, and U chloride, on carbon film [2, 3] |
Late 1980s & early 1990s |
Chemical imaging of
inorganic crystals at near-atomic scale, e.g. Pt catalyst clusters [4], ion-implanted
Si [5], superconductor material, YBa2Cu3O7-δ [6-7]. |
[1] Takeo Sasaki, Hidetaka Sawada, Fumio Hosokawa, Yuji Kohno, Takeshi Tomita, Toshikatsu Kaneyama, Yukihito Kondo, Koji Kimoto, Yuta Sato, and Kazu Suenaga, Performance of low-voltage STEM/TEM with delta corrector and cold field emission gun, Journal of Electron Microscopy 59(Supplement): S7–S13 (2010).
[2] Crewe AV, Wall J, Langmore J (1970) Visibility of single atoms. Science 168:1338–1340.
[3] Isaacson M, Kopf D, Ohtsuki M, Utlaut M (1979) Atomic imaging using the dark-field
annular detector. Ultramicroscopy 4:101–104.
[4] Liu J, Cowley JM (1990) High-angle ADF and high-resolution SE imaging of supported
catalyst clusters. Ultramicroscopy 34:119–128.
[5] Pennycook SJ (1989) Z-contrast STEM for materials science. Ultramicroscopy 30:58–69.
[6] Pennycook SJ, Boatner LA (1988) Chemically sensitive structure-imaging with a scanning
transmission electron microscope. Nature 336:565–567.
[7] Chisholm MF, Pennycook SJ (2006) Direct imaging of dislocation core structures by
Z-contrast STEM. Philos Mag 86:4699–4725.
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