oriented either ªface-onº or ªedge-onº on the surface. The generation of micrometer-sized, stable, and highly organized domains of ªedge-onº or ªface-onº oriented zinc porphyrin hexamer assemblies can be controlled by the addition of coordinating axial ligands, which control the supramolecular architecture formed. Such a precise and unique control over surface organization, as well as its visualization with STM, is of great interest for future applications of these porphyrin assemblies. Future research will be directed to obtaining even more control over the organization of the porphyrin hexamers by varying conditions such as the temperature and concentration of the self-assembling components. In addition, the effect of other coordinating, e.g., chiral ligands, which can induce chirality into the arrays, and the replacement of the Zn II ions by Mn III ions, which can provide the molecules with magnetic and catalytic properties, resulting in well-defined functional surfaces, will be studied. ExperimentalSTM measurements were carried out in the constant current mode using a home-built low-current STM. For each experiment the HOPG surface was freshly cleaved and the STM tips were mechanically cut from a Pt:Ir (80:20) wire. A drop of a nearly saturated solution of molecules or complexes in 1-phenyloctane was brought to the surface. Typically, an STM image (1024 lines 1024 points) was recorded over a period of 10 min. All STM experiments were carried out at least in duplicate, and the raw data were processed only by the application of background flattening. Before and after the experiments the piezo was calibrated in-situ by lowering the bias voltage to 100 mV and raising the tunneling current to 50 pA, which allowed imaging of the HOPG surface underneath the molecules.

The atomic structures of two symmetric [001] tilt grain boundaries in yttria-stabilized cubic-zirconia, ⌺5 (310) and near-⌺13 (510), are studied by Z-contrast scanning transmission electron microscopy. Both boundaries are composed of periodic arrays of highly symmetric structural units, with a distinct unit for each boundary. Oxygen K-edge electron energy-loss spectra show that the oxygen coordination is similar between the bulk and grain boundary, indicating that oxygen ions within the grain boundary reside in distorted tetrahedral sites. Atomic models of the grain boundaries are proposed that are consistent with the experimental data. The core structures are different from previously studied metal or oxide grain boundaries and are unique to the fluorite structure. Yttrium segregation to the grain boundaries is also investigated by electron energy-loss spectroscopy. Yttrium is found to segregate preferentially to the ⌺5 grain boundary, and the spatial distribution of the segregation layer is confined to within 1 nm of the boundary plane.