Abstract
The successful correction of aberrations in scanning transmission electron microscopy (STEM) has improved lateral resolution by more than a factor of two [1], brought greatly enhanced sensitivity for imaging individual atoms, and depth resolution of a few nanometers [2-4]. In zone axis crystals, columnar channeling tends to reduce the depth resolution, and image simulations are necessary to establish the depth sensitivity [5]. To illustrate this capability, images from Si nanowires grown by a Au-catalyzed vapour-liquid-solid method will be presented [6]. A high density of Au atoms are seen not only on the surface of the nanowires, but, by changing focus, are seen also in the bulk of the nanowire, as shown in Fig. 1. The gold atoms move under the influence of the beam, and may not be present in the as-grown material. However, substitutional and three interstitial configurations are found, which are confirmed by density functional calculations to be stable (substitutional) or metastable (interstitial) sites. Furthermore, the number density of the different configurations correlates with their calculated defect formation energies. As a second example of the identification of a point defect complex, the origin of ferromagnetism in Co-doped anatase will be shown by a combination of imaging, EELS and theory to be a Co-Ti+3-VO complex [7].
