SAM-modified electrodes

SAMs controlling electrochemistry, whereas the reverse holds for the other one, both topics are inseparably intertwined as exemplified by the underpotential deposition of metal on SAM-modified electrodes where patterned SAMs allow localization of metal UPD, which in turn affects the monolayer. 

Given the tremendous development of SAMs over the past two decades it is dear that this chapter is able to cover only a fraction of the spectrum of topics related to the combination of SAMs and electrochemistry. For a comprehensive picture the reader is referred to a number of additional review articles, one of which is the excellent and extensive account of organized monolayers on electrodes by Finklea [23]. Besides this one, which comprehensively covers the literature up to the mid-1990s, other more focused reviews are available that address various developments over the past decade in areas of sensor development and electroanalytical applications [22, 24—28] and electrochemical metal deposition on SAM-modified electrodes [29, 30]. 

Figure 5.12 Possibilities to deposit metal onto a SAM-modified electrode, (a) intercalation of metal at the SAM/substrate interface, (b) deposition originating at the substrate with subsequent mushroom-like growth, (c) deposition on top of a SAM resulting in a metal-SAM-metal sandwich structure.

Figure 5.17 Illustration of different pathways for growth of a U PD layer on a SAM-modified electrode, (a) Uniform penetration across the whole SAM area, (b) deposition starting at a major defect with subsequent penetration at edge ofUPD island (1), penetration through SAM across the whole U PD area due to the distortion of SAM structure by UPD (2) and growth of UPD island through initial defect only (3).

Another common feature of metal UPD on SAM-modified electrodes is the pronounced suppression of nucleation that for bare Au occurs at steps unless the step density is very low [218]. Even though nucleation still occurs mostly at steps for... 

In a SAM-modified electrode, the SAM functions as a dielectric, reducing the permeability of the monolayer by electrolyte and decreasing the charging current [23, 30, 48]. However, a thinner film [30, 49], one with a higher dielectric constant (e.g., containing unsaturated carbons [50]), or one which is more permeable to electrolyte (i.e., a more defective SAM), exhibits increased Cji because the resulting electrical double layer is better organized [23, 30, 48[. The double-layer capacitance is, thus, a convenient check of monolayer quality under the conditions of electrochemical electron-transfer measurements. 

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