Localized Electrochemical Nucleation and Growth

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. 

Experiments and simulations show that the characteristics of the nanostructures generated by this procedure are basically given by live parameters the distance between the STM and the substrate, the quantity of material loaded on the tip, the maximum ion current density for the dissolution of the material on the tip, the potential of the substrate, and the diameter of the STM apex. The controlled variation of these five parameters allows tailoring of the diameter and height of the clusters. 

Since the separation between the tip and the surface is such that their respective double layers do not overlap, the nanostmcturing process can be described simply through the diffusion of the ions toward the surface. Thus, the concentration profiles of the diffusing ions dehne effective Nemst potential prohles that can be employed to predict the regions where the oversaturation conditions will contribute to metal nucleation and growth. 

Model calculations are shown in Fig. 36.5, where the tip has been modeled as a hemisphere with a radius a = 50 nm. During the few milliseconds after the 

FIGURE 36.5 Model calculation employed to study the dependence of the cluster diameter on the distance between the STM and the substrate surface for localized electrochemical nucle-ation and growth. The lower part of the figure shows the profile of the Nemst potential for the Co /Co potential as a function of the distance from the tip center. The fines indicate a constant Co concentration. (From Schindler et al., 2000, with permission from the American Institute of Physics.) 

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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. 

W. Schindler, P. Hugelmann, M. Hugelmann, EX. Kartner, Localized electrochemical nucleation and growth of low-dimensional metal structures, J. Electroanal. Chem. 522 (2002) 49-57. 

Atomic force microscopy (AFM) and electrochemical atomic force microscopy (ECAFM) have proven usefiil for the study of nucleation and growth of electrodeposited CP films on A1 alloy [59]. AFM was used to study adhesion between polypyrrole and mild steel [60], whereas electric force microscopy (EFM) has been used to study local variations in the surface potential (work function) of CP films [61]. AFM with a conductive tip permits a nanoscale AC impedance measurement of polymer and electrolyte interfaces, permitting differentiation between highly conductive amorphous regions and less-conductive crystalline regions of the CP film [62]. 

The incorporation of discreet nucleation events into models for the current density has been reviewed by Scharifker et al. [111]. The current density is found by integrating the current over a large number of nucleation sites whose distribution and growth rates depend on the electrochemical potential field and the substrate properties. The process is non-local because the presence of one nucleus affects the controlling field and influences production or growth of other nuclei. It is deterministic because microscopic variables such as the density of nuclei and their rate of formation are incorporated as parameters rather than stochastic variables. Various approaches have been taken to determine the macroscopic current density to overlapping diffusion fields of distributed nuclei under potentiostatic control. 

An analysis of potentiostatic current density transients indicates progressive nucleation and a cluster growth controlled by hemispherical diffusion (cf. Section 6.2), as shown in Fig. 6.37. From the initial part of the transients, the nucleation rate, /, as a function of rj was determined. The number of atoms forming the critical nuclei, Afcrit -2, was determined from the slope of the log/vs. t] plot in the overpotential range - 210 mV < T <- 100 mV. These results show that localized metal deposition under electrochemical conditions using in situ local probe techniques and appropriate poiarization routines seems to be realistic. 

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