Well-ordered facets

The investigation of anodic oxide on various metals shows that at first usually amorphous structures are formed with a dense coverage of the terraces with grains, which change to nano-crystallites with time. The extent and the rate of this change depend on the system under study. This crystallization occurs for Cr within hours [127], whereas Cu keeps the amorphous grain structure for a very few minutes only and develops a well-ordered, faceted, crystalline layer covering the whole electrode surface [128, 129], In the next section, the details of the structure of layers formed on Cu are discussed, followed by a summary of some other more reactive metals like Ni and Cr. 

Fig. 4.15 (a) A 15 K He field ion image of a low temperature field evaporated silicon surface. (b) A computer simulation image of Si with (lxl) surfaces, (c) a 60 K Ne field ion image of a 720°C annealed silicon surface where well ordered atomic structures are developed at a few facets of the Si emitter surface, (d) When the Si tip is annealed at 800°C, almost all the facets are well developed. The atomic structures of all these facets are completely reconstructed. 

Mercuric iodide crystals grown by physical vapor transport on Spacelab 3 exhibited sharp, well-formed facets indicating good internal order (19). This was confirmed by y-ray rocking curves which were approximately one-third the width of the ground control sample. Both electron and hole mobflity were significantly enhanced in the flight crystal. The experiment was repeated on IML-1 with similar results (20). 

Only a handful of the identified types of mesoporous materials have been extensively explored and used. A noteworthy occurrence has been that some of the preparations afforded well developed faceted particles, a la ordered crystals. The examples are MCM-48, SBA-1 and SBA-16 [36]. 

The role of the grain boundary plane needs to be established, especially in view of anisotropic superconducting order parameters. In thin films, considerably smaller grain boundary junctions than those used in our work would be feasible and it should thus be possible to prepare samples with reasonably well defined facets. Of course, at inclinations such as for the 45°/[001] grain boundaries, where asymmetric facets dominate supposedly because of their much lower energy than other inclinations (see Fig. 10.4), symmetric facets may not form, at least not exclusively. Whenever there is coexistence of... 

This is visualized in Fig. 6, which shows the bulk deposition of copper on a flame-annealed polycrystalline gold electrode. The experiment was performed on an approximately 100 nm-wide, rather well-ordered microfticet, which is bordered by relatively rough regions of the sample surface. The orientation of the facet is presumably (100), as indicated by the rectangular shapes of islands and supported by... 

In the field of catalysis, it is essential to achieve a better understanding of the physical and chemical properties of various well-defined crystalline orientations of catalyst particles over a wide range of temperature and pressure. Such insights permit us to interpret the catalytic behavior of small particles at the atomic and molecular level. Surprisingly, SERS is also a suitable in-situ spectroscopy for well-defined surfaces. This was recently demonstrated for oxygen on faceted Ag surfaces, which are not smooth but locally well ordered along the facets (over 10-1000 nm) [44]. 

As pointed out above, the surface free energy f p, q) contains terms which are of cubic or higher order in the step density. A term cubic in p with a positive coefficient could well stabilize the free energy at not too small values of p. In both scenarios the (110) facet has sharp edges running roughly perpendicular to the missing-row direction. 

The arguments advanced here apply equally well to 110 facets. This conclusion is illustrated schematically in Fig. 4. Figure 4a shows the y-plot obtained by Heyraud and Metois for pure Pb . Figure 4b is a modification of Fig. 4a, in which the cusps at 111 and 100 have been deepened, and new cusps have been created at 227) and 110) orientations. The depth of the cusps has been adjusted to produce an equilibrium form by means of the Wulff constmction (Fig. 4c) that matches the experimental equilibrium form of Fig. 2d. The deepening of the cusps needed to produce the desired shape is of the order of a few percent, and falls within the general range of anisotropy measured previously in the case of pure Pb. 

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