Nanostructuring by Scanning Electrochemical Microscopy

A typical configuration of a SECM system is shown in Fig. 36.6. In this case the solution contains oxidized (Ox) species (denoted mediators) that are reduced on the active part of the microelectrode yielding the reduced (Red) species. The figure also shows a possible reaction of the Red species with the electrode, with the reaction rate If is very large, the approach of the tip to the surface will result in an increase in the reduction reaction (current) on the tip because the regeneration of Ox on the tip will be more efficient in a smaller gap. On tfie otfier fiand, if k is close to zero, the only effect of the tip approach to the surface wifi be the depletion of the Ox species in the gap upon reduction, whose diffusion from the bulk of the solution is now hindered by the probe. These two mechanisms, which result in the positive and negative feedback operation modes, can be used to map the reaction rate k, on the surface. 

A different operation mode can be empfoyed in SECM if some species are only generated via a reaction on the substrate and they are electrochemicaUy active for 

FIGURE 36.6 Typical configuration of a scanning electrochemical microscope. In this case the solution contains Ox species (mediators) that are reduced on the active part of the micro-electrode, yielding the reduced Red species. A possible reaction of the Red species with the substrate, with the reaction rate. illustrated. 

SECM has also been used to deposit a gold microstructure on n-Si(lll) and indinm oxide. In these experiments, Au is dissolved anodically from the ultramicro electrode under constant potential according to the reaction 

To deposit Au structures, a Au probe is approached to the surface until a positive feedback is observed. This is due to the regeneration of Cl species on the substrate while Au is deposited from AUCI4 according to the reverse reaction, leading to an increase in the local concentration of Cl. The microelectrode is then left at this position above the substrate for a certain time, after which it is withdrawn from the surface. The potential of the substrate, the electrolyte, and the pH were found to be the most significant parameters determining in determining the rate of Au electrodeposition and its structure (Amman and Mandler, 2001). 

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