Using high-resolution electron microscopy, anisotropic elasticity calculations, and image simulations, typical contrast was identified for the pure edge threading dislocations in GaN layers grown by molecular-beam epitaxy on sapphire. Their atomic structure was found to exhibit 5/7, or 8 atom cycles. The two configurations were observed at a similar frequency for isolated dislocations and low-angle boundaries. Coincidence grain boundaries have been studied and they are all made of pure-edge dislocations with the above identified atomic structures. A topological analysis of high-angle boundaries has been carried out in order to determine the defect content at the interfaces. The defects introduced on deviation from coincidence are associated with steps and their Burgers vectors correspond to the smallest vectors of the displacement shift complete set. At the interfaces, defect free steps, which belong to the sides or diagonal of the coincident site lattice unit cell, have been found to occur. The reconstruction of some boundaries was only possible by taking into account the occurrence of structural units which exhibit 4-atom ring cycles for the dislocation cores. In nonsymmetric interfaces, a new structural unit made of 5/4/7 atom rings has been found to constitute the core of one grain boundary dislocation.

The concept of single atom catalysis offers maximum noble metal efficiency for the development of low-cost catalytic materials. Among possible applications are catalytic materials for proton exchange membrane fuel cells. In the present review, recent efforts towards the fabrication of single atom catalysts on nanostructured ceria and their reactivity are discussed in the prospect of their employment as anode catalysts. The remarkable performance and the durability of the ceria-based anode catalysts with ultra-low Pt loading result from the interplay between two states associated with supported atomically dispersed Pt and sub-nanometer Pt particles. The occurrence of these two states is a consequence of strong interactions between Pt and nanostructured ceria that yield atomically dispersed species under oxidizing conditions and sub-nanometer Pt particles under reducing conditions. The square-planar arrangement of four O atoms on {100} nanoterraces has been identified as the key structural element on the surface of the nanostructured ceria where Pt is anchored in the form of Pt²⁺ species. The conversion of Pt²⁺ species to sub-nanometer Pt particles is triggered by a redox process involving Ce³⁺ centers. The latter emerge due to either oxygen vacancies or adsorption of reducing agents. The unique properties of the sub-nanometer Pt particles arise from metal–support interactions involving charge transfer, structural flexibility, and spillover phenomena. The abundance of specific adsorption sites similar to those on {100} nanoterraces determines the ideal (maximum) Pt loading in Pt–CeO_x films that still allows reversible switching between the atomically dispersed Pt and sub-nanometer particles yielding high activity and durability during fuel cell operation.