Nucleation multilayer formation

Nucleation and growth processes of the metal lattice. Understanding of the nucleation and growth of surface nuclei, formation of monolayers and multilayers, and growth of coherent bulk deposit is based on knowledge of condensed-matter physics and physical chemistry of surfaces. 

At higher overpotentials the nucleation rate increases faster than the step (Chapter 3) propagation rate, and the deposition of each layer proceeds with the formation of a large number of nuclei. This is the multinuclear multilayer growth. Armstrong and Harrison (13) have shown that initially, the theoretical current-time transient for the two-dimensional nucleation (Fig. 7.7) has a rising section, then passes through several damped oscillations, and finally, levels out to a steady state. 

Figure 7.7 also shows the theoretical i-t transients for the formation of successive layers under conditions of progressive nucleation. The theoretical current-time transient for three-dimensional nucleation is shown in Figure 7.8. The difference between 2D and 3D nucleation (Fig. 7.7 and 7.8) is in the absence of damped oscillations in the latter case. A comparison between the theoretical and experimental transients for the 2D polynuclear multilayer growth is shown in Figure 7.9. 

Moller etal. [462] have performed in situ STM observations of Ni electrodeposition on reconstructed Au(lll) electrodes. Ni nucleation proceeded in three distinct potential-dependent steps. The same group of researchers [463] has studied electrodeposition and electrodissolution of Ni on Au(lOO) electrodes. Pronounced differences were observed for the nucleation and submonolayer growth on the reconstructed and unreconstructed surfaces. On perfectly reconstructed Au(lOO), the formation of Ni islands started at overpotentials significantly higher (rj > 100 mV) than on unreconstructed surface (rj > 40 mV), where Ni monolayer islands were formed. Dissolution of the Ni film exhibited better monolayer stability in comparison to the multilayer deposit. 

The results obtained in the system C xQikt)/C x clearly correspond to 3D Me phase formation on the native substrate involving spiral growth and/or 2D nucleation and multilayer growth. It is evident that experiments in such systems are solely carried out to demonstrate local metal deposition, but not for surface heterostructuring and modification. 

At low overpotentials nucleation of the Ni deposit starts at the elbows of the reconstruction (Fig. 4(b)), followed by anisotropic growth of monolayer islands perpendictilar to the double rows of the reconstruction (Fig. 4(c)). For multilayer coverages a layer-by-layer growth of the Ni thin film was observed up to thicknesses of six layers. Under UHV conditions, this system exhibits a similar nucleation behavior but the subsequent growth proceeds isotropically and in a more three-dimensional fashion [27]. Both in UHV and in file electrochemical environment the nucleation of islands is preceded by the formation of dqiressions at the elbows. This indicates that Au surface atoms at these sites are replaced by Ni atoms, which subsequently act as centers for adlayer island nucleation [28]. Ihis demonstrates r-reaching mechanistic similarities for deposition at the metal-vacuum and the metal-electrolyte interface, even in complex cases. 

Fig. 5.1 a For compact microcubic structure formation, the PB-CD nanoparticles undergo nucleation process within the LbL flask conducting to the formation of microcrystals that support a mesoscale self-assembly process and a final supramolecular conversion to compact microcubic structures, b Cyclic voltammograms (CVs) for self-assembly PAH/PB-CD multilayers onto ITO electrode containing three bilayers at various scan rate 10-200 mV s Electrolyte KCl— 0.2 mol T = 25 °C. Adapted with permission from [30]... 

There is a sharp separation between adsorbed water and liquid water, in which an adsorbent may be suspended and dissolved. In adsorbed water the structure is imposed by the adsorption field, while in liquid water the (dis)order is the one characteristic of the bulk phase. This state of affairs suggests that the adsorbed-to-liquid transition is a phase transition. On another side, water on many surfaces continuously undergoes adsorbed-to-liquid transition and vice versa. Familiar examples of adsorbents where the state of water frequently cycles between the adsorbed and liquid phases are soils and the skin of terrestrial mammalians. It is also noted that the formation of liquid water in clouds may occur via heterogeneous nucleation (i.e., via multilayer adsorption), either spontaneously on dust particles or artificially on Agl crystals formed by condensation of sublimated Agl,... 

The investigation of the initial stages of reaction-diffusion in multilayers, carried out during recent years, by differential scanning calorimetry (DSC), proved that the stage of intermediate phase nucleation at solid-state reaction does take place. DSC experiments [6-8] have shown that the formation of a new phase in multilayers can involve two stages. For example, the curve illustrating the dependence of heat flux on time at formation of NbAls in multilayers Nb/Al (obtained by deposition) has two maxima. X-ray analysis and electron microscopy confirmed that both peaks correspond to the formation of the phase NbAls. Similar curves with two peaks are obtained for such systems as Co/Al, Ni/Al, Ti/Al, Ni/amorphous Si, and V/amorphous Si. 

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