Nucleation metal deposits

Eitch indented into the tube. Tube 48 was a clean copper tube that ad 50 longitudinal flutes pressed into the wall (Gener Electric double-flute profile, Diedrich, U.S. Patent 3,244,601, Apr. 5, 1966). Tubes 47 and 39 had a specially patterned porous sintered-metal deposit on the boihng side to promote nucleate boiling (Minton, U.S. 

FIGURE 36.1 Schematic illustration of some electrochemical techniques employed for surface nanostructuring (a) tip-induced local metal deposition (b) defect nanostructuring (c) localized electrochemical nucleation and growth d) electronic contact nanostructuring. 

As pointed out above, an STM tip can be used to nucleate and grow single clusters. In this type of experiment, cluster deposition on a STM tip is achieved when it is retracted about 10 to 20 run from the substrate surface. Under these conditions, where the feedback loop is disabled, absence of mechanical contact between the tip and the substrate in ensured. Then a positive potential pulse is applied to the tip, the metal deposited on it is dissolved, and it diffuses toward the substrate surface, where a nucleus develops and grows to yield a cluster, typically 20 nm wide. 

Fig. 10). With the completion of the structure transition, the current should drop to zero, which is indeed the case except for peak B, where a slight leak current is seen (ascribed to the side reaction Cu++ I c > Cu+). According to the theory by Bewick, Fleischmann and Thirsk (BFT) the transients can be used to distinguish between instantaneous and progressive nucleation [45], A corresponding analysis revealed that the falling part of the transients agrees well with the model for instantaneous nucleation, while the rising part shows a systematic deviation. This was explained by the existence of surface defects on a real electrode in contrast to the ideal case of a defect-free surface assumed in the theoretical model. By including an adsorption term in the BFT theory to account for Cu deposition at defects, the experimentally obtained transients could indeed be reproduced very well [44], We shall return to the important role of surface defects in metal deposition later (sec. 3.2).

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... 

The phenomenon of nucleation considered is not limited to metal deposition. The same principles apply to the formation of layers of certain organic adsorbates, and the formation of oxide and similar films. We consider the kinetics of the growth of two-dimensional layers in greater detail. While the three-dimensional case is just as important, the mathematical treatment is more complicated, and the analytical results that have been obtained are based on fairly rough approximations details can be found in Ref. 3. 

As far as SAM-controlled electrochemical metal deposition is concerned, substantial interest derives from microelectronics with its need to control the generation of interconnects and, thus, to understand the influence of organic layers on the metal nucleation and growth. However, the scope of this topic reaches well bqfond that, as illustrated by the substantial range of potential applications where small-scaled metal structures are of crucial importance, for example in electrochemical [31] and optical [32] sensing, molecular electronics [33], for plasmonics [34], or as metamaterials [35]. 

A series of nucleation and growth models was developed by, for example, Bewick et al. (11), Armstrong and Harrison (16), and Scharifker and Hills (17). Amblart et al. (18) have shown that nickel epitaxial growth starts with the formation of three-dimensional epitaxial crystallites. An electrochemical model for the process of electroless metal depositions (mixed-potential theory) was suggested by Paunovic (14) and Saito (14b). 

Figure 7.7. Potentiostatic current-time transient for the metal deposition together with theoretical currents for individual layers (1-5). Two-dimensional progressive nucleation taking overlap into account. (From Ref. 13, with permission from the Electrochemical Society.)...

Reactive metals are of interest for two primary reasons (1) reaction with the uppermost part of the SAM which can drive uniform nucleation with no penetration and (2) for electropositive metals, injection of electrons into the SAM to create a favorable dipole at the metal/SAM interface for device operation. With respect to the first, as opposed to the results with non-reactive metal deposition, some reports of reactive metal deposition appear to show prevention of metal penetration with the avoidance of short-circuits across the M junction. In general, serious concerns remain that some of metal atoms react destructively with the SAM backbone to produce inorganic species, e.g., carbides and oxides in the case of aggressive metals such as titanium. 

The discovery of the heterogeneity of surfaces, and in particular of dislocations (see Section 7.12.12), was made in the 1930s (Taylor, 1936), but there had been theoretical work on metal deposition at an earlier time. The model of the surface employed by these earlier workers (Kossel, 1927 Stranski, 1928 Erdey-Gruz, and Volmer, 193 l)was a flat plane without steps and edges to which the adions produced by ion transfer from the double layer could surface diffuse. The only way a metal could grow on a perfect planar surface without growth sites was by nucleation of the deposited atoms, rather than diffusion to the growth sites shown in Fig. 7.134. 

Electrolytic metal deposition ( electroplating ) is an empirical art widely in use to cover corrosion-sensitive surfaces with a thin protecting metal layer, e.g. of tin, nickel, zinc, etc. The complete plating process comprises several partial processes such as mass transport, charge transfer, adsorption of adatoms, surface diffusion of adatoms, and finally nucleation and crystal growth. 

Recently a series of dialkylpyrrolidinium (Pyr+) cations have been studied in our laboratory 7-9). These cations are reduced at relatively positive potentials and could be investigated electrochemically as low concentration reactants in the presence of (C4H9)4N+ electrolytes. Using cyclic voltammetry, polarography and coulometry, it was shown that Pyr+ react by a reversible le transfer. The products are insoluble solids which deposit on the cathode and incorporate Pyr+ and mercury from the cathode. Both the cation and the metal can be regenerated by oxidation. Quantitative analysis of current-time transients, from potential step experiments, showed that the kinetics of the process involve nucleation and growth and resemble metal deposition. 

See also crystallization overpotential (polarization), - nucleation and growth kinetics, - equilibrium forms of crystals and droplets, - half-crystal position, - Kaischew, - metal deposition, - supersaturation, - Stranski, - Zeldovich. 

Electroless plating — An autocatalytic process of metal deposition on a substrate by reduction of metal ions from solution without using an external source of electrons. It is promoted by specific reductants, namely formaldehyde, sodium hypophosphide, sodium boro-hydride, dialkylamine borane, and hydrazine. Electroless deposition has been used to produce different metal (e.g., nickel, cobalt, copper, gold, platinum, palladium, silver) and alloy coatings. It can be applied to any type of substrate including non-conductors. Some substrates are intrinsic catalytic for the electroless deposition other can be catalyzed usually by sensibilization followed by Pd nucleation also, in some non-catalytic metallic substrates the electroless process can be induced by an initial application of an appropriate potential pulse. In practical terms, the evaluation of the catalytic activity of a substrate for the electroless deposition of a given metal is... 

Nucleation and growth kinetics — Nucleation-and-growth is the principal mechanism of phase transformation in electrochemical systems, widely seen in gas evolution, metal deposition, anodic film formation reactions, and polymer film deposition, etc. It is also seen in solid-state phase transformations (e.g., battery materials). It is characterized by the complex coupling of two processes (nucleation and phase growth of the new phase, typically a crystal), and may also involve a third process (diffusion) at high rates of reaction. In the absence of diffusion, the observed electric current due to the nucleation and growth of a large number of independent crystals is [i]... 

Surface modification of a polymer prior to metallization is widely used to improve adhesion. The most common surface modifications employed are electric discharge (corona and plasma) and, more recently, ion-beam treatments QJ- Several mechanisms have been proposed for the improved adhesion after such surface modifications (2). These include mechanical interlocking, the elimination of weak boundary layers, electrostatic attractions, and chemical bonding. All of these can play a role in adhesion depending on the surface modification used, metal/polymer system, type of metal deposition, and the extent of polymer preparation employed. However, for low power, short exposure modifications, the formation of new chemical species which can provide nucleation and chemical bonding sites for subsequent overlayers is considered to be of prime importance (3-51. 

With respect to the Pd/Al203 model catalysts described below, STM was used to examine the structure of the AI2O3 support and the nucleation and growth of metal deposits (e.g.. References (34,63,73,101,215) and references cited therein), providing information about the size, shape, and height of palladium nanoparticles. In some cases, even atomically resolved images of individual palladium nanoparticles were obtained (206). 

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