Defects steps

Yates JT. 1995. Surface-chemistry at metallic step defect sites. J Vac Sci Technol A 13 1359-1367. 

Stranski-Krastanov growth has been documented for copper on Au(lll) [101, 102], Pt(100) and Pt(lll) [103], for silver on Au(lll) [104, 105], for cadmium on Cu(lll) [106] and for lead on Ag(100) and Ag(lll) [107-109]. In all of these examples, an active metal is deposited onto a low-index plane of a more noble metal. Since the substrate does not undergo electrochemical transformations at the deposition potential, a reproducible surface can be presented to the solution. At the same time, the substrate metal must be carefully prepared and characterized so that the nucleation and growth mechanisms can be clearly identified, and information can be obtained by variation of the density of surface features, including steps, defects and dislocations. 

Fig. 7.27 Photoelectron emission microscopy (PEEM) images of two Fe3C>4 surfaces that were used as model catalyst in the dehydrogenation of ethylbenzene to styrene at 870 K, showing carbonaceous deposits (bright). These graphitic deposits grow in dots and streaks on a surface of low defect density, but form dendritic structures on surfaces rich in point and step defects. (From [78]).

RE etch back or CMP may be used to remove the polysilicon overburden. When RE etch back is used, however, a center seam is etched into the trench. In addition, the RE etch is not self-arresting, and therefore leads to a step at the oxide-polysilicon edge. These defects reduce planarity and make it difficult to reliably cover the trench with the strap film. Center seam and edge step defects in deep trench formation are analogous to the same defects that form when RE etch back is used to remove the... 

Abild-Pedersen F, Lytken O, Engbaek J, Nielsen G, Chorkendorff 1, Norskov JK (2005) Methane activation on Ni(lll) effects of poisons and step defects. Surf Sci 590 127... 

The electro-catalytic oxidation of hydrogen, and reduction of oxygen, at carbon supported platinum based catalysts remain essential surface processes on which the hydrogen PEM fuel cell relies. The particle size (surface structure) and promoting component (as adsorbate or alloy phases) influence the activity and tolerance of the catalyst. The surface chemical behavior of platinum for hydrogen, oxygen, and CO adsorption is considered, in particular with respect to the influence of metal adsorbate and alloy components on close packed and stepped (defect) platinum surfaces. Dynamical measurements (employing supersonic molecular beams) of the... 

The activity, stability, and tolerance of supported platinum-based anode and cathode electrocatalysts in PEM fuel cells clearly depend on a large number of parameters including particle-size distribution, morphology, composition, operating potential, and temperature. Combining what is known of the surface chemical reactivity of reactants, products, and intermediates at well-characterized surfaces with studies correlating electrochemical behavior of simple and modified platinum and platinum alloy surfaces can lead to a better understanding of the electrocatalysis. Steps, defects, and alloyed components clearly influence reactivity at both gas-solid and gas-liquid interfaces and will understandably influence the electrocatalytic activity. 

It is interesting to compare an order of magnitude estimate of the experimental value of the equilibrium constant for this reaction (11) to those obtained from the work of Bengaard et al.(26) at nickel step defect sites and at nickel planes. Experimentally, the onset of carbon formation was found (11) to be at partial pressures of H2, H2S, and... 

A relative high coverage of sulphur (0s > ca. 0.7) is necessary for sulphur to compete effectively with carbon for the step defect sites and to avoid whisker carbon formation. 

Wire-shaped growth of nanostructured PPy with diameter <10 nm, has been obtained by electropolymerization at naturally occurring step defects and artificially formed pit defects of HOPG, in a template assisted electropolymerization where the size of the nanostmctures could be controlled by limiting the pyrrole polymerization time at anodic potentials [242], Electrochemical polymerization of pyrrole within the confines of anodized alumina templates and subsequent metal nanoparticles immobilization on the surface of polymer pillars has been used to make surfaces that show roughness on two independently controllable levels sub-microscopic roughness from polymer pillar dimensions and nanoscopic roughness from the appropriate size selection of metal NPs [243],... 

A single crystal surface exhibits terraces (T) with low Miller index crystal faces and high surface density. They are separated by ledges (L) which may have kinks (K). A terrace between two ledges is a step. Defects on terraces consist of vacancies or adsorbed atoms. Thus the smooth surface may indeed be rough on an atomic scale. This model is called as TLK model (Fig. 2.6). The systematic... 

Under industrial conditions the nitrogen coverage is low (hence the theoretical description applies), but in the bulk phase the catalyst is nitrified, which causes a distorted crystal structure. Through suitable pretreatments, this structure has also been included in model experiment, however, it was not further characterized. After the required activation — a process now well understood — the industrial catalyst shows a distorted iron structure that has been demonstrated to be essential for its catalytic function. This distortion manifests itself as metastable plates in the (111) orientation, which are formed by the topotactic reduction of the magnetite precursor at extremely mild conditions, but also by stress states in the regularly orientated (100) regions of the ammonia iron. These stress states participate in the structme-sensitive activation of nitrogen, which is particularly efficient on the (111) faces, but also on different faces and on strain-induced step defects. These recent theoreticaf ° and experimental resuits require that the discussion about the role of the promoters (especially the potassimn), which is normally considered as closed, is reopened. 

The major problem of top-layer membranes is the apparition of defects induced during the successive deposition, drying and firing steps. Defects are detrimental to selectivity and they render the membranes non-operant, specially in gas phase separation processes. 

Figure 4.19 Illustration of a possible mechanism for top-metal penetration upon evaporation of the top metal contact onto the SAM. The SAM exhibits pinhole defects that can occur at grain boundaries where the natural tilt of the molecules (-20° off the surface normal) are dislocated at differing domains and at step edges where there are single atomic step defects in the underlying metal substrate. These pinhole defects could be the source of the metal shorts upon evaporation or continued usage of the devices. The larger the area, the more defects that will be present, thereby exacerbating the top metal punch through problem.

This is followed by GaN coalescence over the groove regions and subsequent spreading over the Si02 region in the third step. Defects are effectively prevented from propagating to the layer surface by the Si02 mask. 

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