In HAADF Z (atomic-number)-contrast imaging mode, the intensity of electron scattering by the atoms depends on the atomic number Z. For the superlattice structure consisting of LaTiO3 and SrTiO3 layers, the brightest features are columns of La ions, the second brightest features are columns of Sr ions, and the Ti ions are weakly visible in between the LaTiO3 and SrTiO3 layers as shown in Figure 3513a. Even though the Z contrasts of La, Sr, and Ti are clear, the oxygen ions are still not visible due to their much lower atomic number. Note that the number of the LaTiO3 and SrTiO3 layers is determined by the deposition recipe.
Figure 3513a. HAADF image of superlattice structures consisting of LaTiO3 and SrTiO3 layers. The imaging optical axis is the  zone axis of the crystals. Adapted from .
Figure 3513b shows high-resolution HAADF Z-contrast STEM images presenting the interface between Si3N4 matrix (on the top of each image) and the mixtures (at the bottom of each image) of Yb2O3 and Al2O3 (a) and Yb2O3 and SiO2 (b). The Yb atoms, visible here as bright spots due to enhanced electron scattering (giving higher Z-contrast), are located periodically along the interface. The images are oriented along the low-index zone axis  of Si3N4.
Figure 3513b. High-resolution STEM images showing the atomic interface between a Si3N4 matrix
grain and two different mixtures. Adapted from 
 A. Ohtomo, D. A. Muller, J. L. Grazul and H. Y. Hwang, Artificial charge-modulation in atomic-scale perovskite titanate superlattices, Nature 419, 378-380.
 A. Ziegler, M. K. Cinibulk, C. Kisielowski, and R. O. Ritchie, Atomic-scale observation of the grain-boundary structure of Yb-doped and heat-treated silicon nitride ceramics, Appl. Phys. Lett. 91, 141906 (2007).