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Fig. 4. In-situ STM images of copper crystallites electrodeposited on gold. Electrolyte: 0.001M CuS04 and 0.01 M H2S04 in Millipore water, (a) Electrodeposition terminated at E = -100 mV, Et = -58 mV, d = 216 nm, 7t = 2.1 nA. (b) Electrodissolution initiated at E = 0 mV, Et = 42 mV, d = 186mn,/t = 2.1 nA.

and island formation (Stranski-Krastanov) [20], and pure islanding (Volmer-Weber)

[21]. Metal ions in solution typically grow on the surface at cathodic overpotentials by the latter two mechanisms [8, 19, 22-24], Growth by the layer-by-layer mechanism proceeds only for a single-monolayer UPD. Excellent atomic resolution of the growth of metal monolayers on metal working electrodes for a number of systems has been obtained by in-situ STM [6, 7, 13, 14, 22-27]. The gold surface reconstructs when small amounts of halides are added [6, 28], lifting the potential-induced (1 x 23) surface reconstruction. These results agree well with results obtained under UHV conditions [6, 28] even though water adsorption is different in the two cases.

Nucleation in bulk deposition occurs near surface imperfections. Steps and kinks are observed for copper on copper [16], gold [22], platinum [23], and graphite [29], Nucleation continues at cathodic overpotentials until the surface is covered. New results indicate that several layers of equally sized nuclei are formed initially whereas crystallites are formed only when the surface concentration of nuclei exceeds a certain limit [11], possibly determined by the surface atom density. Growth proceeds to become three-dimensional, nucleation is then terminated, and crystallites of micron dimensions cover the surface (Fig. 4(a)). If the overpotential is fixed at values small enough to induce bulk nucleation but prevent bulk deposition, the system may enter a state of equilibrium between electrodeposition and -dissolution [11], The amount of metal electrodeposited may thus be controlled with great precision so as to maintain a fraction of the number of metal ions in solution with a corresponding number resting on the surface at small overpotentials. This situation is established at potentials at the leading edge of the cathodic branch of a cyclic voltammogram [11], By comparing cyclic voltammetry with in-situ STM recorded simultaneously it seems that the cathodic current peak of bulk metal electrodeposition reflects the termination of metal deposition rather than a potential of maximum metal growth [11].

The morphology of the copper deposit has been studied also in the presence of organic additives. Organic additives do not appear in the images but their effect is striking. They adsorb onto defect sites, steps, and kinks, and prevent copper from nucleation at these sites. Copper nucleates in a more random manner and when adequate coverage is reached the copper crystallites merge and form a compact layer

[22]. The overall effect of additives is thus to induce lateral growth (2D), as opposed to island formation by 3D growth which dominates in the absence of additives.

As observed by in-situ STM the metal is also dissolved before the maximum current is reached in the voltammogram when the potentials are swept from cathodic to anodic values [11]. Figure 4(b) shows the first stages of copper dissolution (vs. Cu2+/Cu) where copper gradually enters the ionic state from the top layers. When copper dissolves, a small fraction remains and alloys with the gold polycrystalline surface to from nanometer-scale crystallites (Fig. 5(a, b)). In Figure 5(a) the alloy nuclei form

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