Nucleation center

The secondary and ternary islands will keep growing in approximately concentric fashion, thereby producing a conical structure above the original nucleation centers. This process of kinetic roughening supported by the Schwoebel effect makes a rather bumpy surface structure. Looking finally at a vicinal surface, this will grow rather smoothly when the width of the terraces is smaller than the typical distance between nucleation centers 4i (see below), and becomes bumpy in the opposite case [12,93]. 

The nucleation behavior of transition metal particles is determined by the ratio between the thermal energy of the diffusing atoms and the interaction of the metal atoms at the various nucleation sites. To create very small particles or even single atoms, low temperatures and metal exposures have to be used. The metal was deposited as metal atoms impinging on the surface. The metal exposure is given as the thickness (in monolayer ML) of a hypothetical, uniform, close-packed metal layer. The interaction strength of the metals discussed here was found to rise in the series from Pd < Rh < Co ( Ir) < V [17,32]. Whereas Pd and Rh nucleate preferentially at line defects at 300 K and decorate the point defects at 90 K, point defects are the predominant nucleation center for Co and V at 300 K. At 60 K, Rh nucleates at surface sites between point defects [16,33]. 

These results suggest that the surfaces of the a-Fc203 ellipsoid support play the role of nucleation centers of the precursor particles. The large precursor particles of 20—50nm were formed by aging for 72 h in the absence of any support. The size distribution was relatively narrow and each particle consisted of much smaller particles of 2—3 nm. As a consequence, the support plays an important role in the formation of the well-dispersed precursor nanoclusters. 

Chronoamperometric transients for CO stripping on polycrystalline platinum were measured by McCallum and Fletcher [1977], Love and Lipkowski [1988] were the hrst to present chronoamperometric data for CO stripping on single-crystalline platinum. However, they interpreted their data on the basis of a different model than the one discussed above. Love and Lipkowski considered that the oxidation of the CO adlayer starts at holes or defects in the CO adlayer, where OH adsorbs. These holes act as nucleation centers for the oxidation reaction, and the holes grow as the CO at the perimeter of these holes is oxidized away by OHads- This nucleation and growth (N G) mechanism is fundamentally different from the mean held model presented above, because it does not presume any kind of mixing of CO and OH [Koper et ah, 1998]. Basically, it assumes complete surface immobility of the chemisorbed CO. 

Both the bubble departure frequency / and the number of nucleation centers n are difficult to evaluate. These quantities are known to be dependent on the magnitude of the heat flux, material of construction of the tube, roughness of the inside wall, liquid velocity, and degree of superheat in the liquid elements closest to the tube wall. Koumoutsos et al. (K2) have studied bubble departure in forced-convection boiling, and have formulated an equation for calculating bubble departure size as a function of liquid velocity. 

The important role of surface defects as nucleation centers in metal deposition processes is well-established. At low overpotentials, i.e., low supersaturation, metal de-... 

I believe, it is fair to state that scanning tunneling microscopy and related techniques such as atomic force microscopy have a tremendeous potential in metal deposition studies. The inherent nature of the deposition process which is strongly influenced by the defect structure of the substrate, providing nucleation centers, requires imaging in real space for a detailed picture of the initial stages. This is possible with an STM, the atomic resolution being an extra bonus which helps to understand these processes on... 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

The process described above is expected to produce a random distribution of active and passive spots on the electrode interface. But the electrode surface may also be artificially patterned prior to anodization in order to form nucleation centers for pore growth. This may be a lithographically formed pattern in said passive film or a predetermined pattern of depressions in the electrode material itself, which become pore tips upon subsequent anodization. The latter case applies to silicon electrodes and is discussed in detail in Chapter 9, which is devoted to macropore formation in silicon electrodes. 

This enzyme from E. coli is a tetramer of four identical subunits, each of molecular weight 116,500. Amber and ochre (premature termination) mutants of the enzyme provide a number of enzymically inactive, incomplete peptide chains, identical in sequence with the N-term nal part of the wild-type chains. A subset of these N-terminal peptides, called acceptor peptides, can combine with so-called wild-type chain, to restore enzymic activity (Ullmann et al., 1965, 1967 Ullmann and Perrin, 1970 see also the review by Zabin and Villarejo, 1975). Goldberg (1969) suggested that the acceptor peptides and the independent nucleation centers as evidenced by the following facts ... 

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