High-angle annular dark-field (HAADF) images are formed by high-angle scattered electron with angles of about 100mrad
and are taken by STEM technique.
Scattering intensities of such electrons strongly depends on the atomic number Z at the scattering center. Thus, it is also called "Z contrast image".
The figure shows HAADF images of Al72Ni20Co8 decagonal quasicrystal, which is the first HAADF image of the quasicrsytal. Since the atomic number of the Al atom is as small as half of those of Ni and Co atoms, scatterings from Al atoms give small contributes for the images. Thus, only Ni and Co atoms are seen in the image. The bright spots correspond to the Ni and Co atoms.
Based on the HAADF image, a new structural model of Al72Ni20Co8 decagonal quasicrystal is constructed. The model is composed of two types of cluster column, Pentagonal and Star-shaped clusters as indicated P and S in the figure.
The arrangement of P and S clutsers
has good agreement with vertices of the rhombic Penrose tiling.
