SAMs defects

However, SAMs are rarely structurally perfect and typically contain defects where crystalline domains meet, at step-edges, and where the electrode is not coated with the SAM. Defects of this kind all facilitate mass transport to the electrode surface where efficient electron transfer can take place. A key objective in characterizing SAMs is to map out the nature, size and distribution of the pinholes and other defects. Undoubtedly, scanning probe microscopy, such as the AFM and STM techniques discussed earlier in Chapter 3, play important roles in this area. However, voltammetry is an extremely powerful approach for detecting defects in SAMs when in contact with solution. This extraordinary sensitivity arises from the ability to routinely detect currents at the nanoamp and picoamp levels which... 

Other excellent example of metal ion trapping by SAMs has been proposed by Kolb et al. In this case, SAMs of N-containing thiols were used to trap Pd(ll) ions from solution. The Pd(II) ions are then reduced electrochemically to produce a monolayer of metallic Pd onto the organic monolayer. This approach was used as an alternative to metallize thiol SAMs from vapor phase, where the diffusion of metal atoms through SAM defects destroys the metal-thiol-metal device. 

Hyperbranched poly(acrylic acid) films have been grown on SAMs of alkanoic acid [137]. These films, rich in carboxylic acid groups could be utilized to selectively bind metal ions or serve as sites for further derivatization. Wurm et al. have studied electrochemically induced epitaxial polymerization [135] of iV-alkylpyrrole in solution onto a SAM composed of ((A -pyrrolyl)-n-undecyl)disulfide. These optically smooth films of long chain poly(iV-alkylpyrrole) have excellent stability in air. The Collard group [142] has studied the kinetics of electro-oxidatively polymerized polyaniline and polypyrrole on surfaces modified with SAMs formation of conducting films is initially blocked by the monolayer but nucleation at SAM defect sites leads to eventual deposition of rough polymer films. 

SAM) and TEM. An Auger electron spectrometer with high spatial resolution imaging capability was developed especially for the detection of small particles and defects which might be present in the ULSI regime this enabled the inspection of wafers up to 200 mm in diameter [2.150]. 

By examining the dispersion properties of surface acoustic waves, the layer thickness and mechanical properties of layered solids can be obtained using the SAM. It can be used to analyze the wear damage progress [104], and detect the defects of thermally sprayed coatings [105]. 

Complete characterisation of complex samples thus becomes possible, and the SAM is used to identify embedded defects and surface particles in semiconductor devices, as well as the study of metal matrix composites and grain boundary analysis. 

Detailed studies about metal deposition from the gas phase onto SAMs have been published [108-110], The central question for the system substrate/SAM/deposit there (as well as in electrochemistry) is the exact location of the deposited metal On top of the SAM or underneath Three clearly different situations are easily foreseen (Fig. 31). (1) Metal on top of the SAM. Depending on a strong or weak chemical interaction between metal and SAM (e.g., functional end group of the SAM), the deposit will spread out on top of the SAM or it will cluster on the SAM. (2) Metal penetrating the SAM (e.g., at defects in the SAM) and connecting to the metal substrate underneath the SAM. This configuration is often pictured as a mushroom, with a thin connective neck and a large, bulky head. (3) Deposited metal is inserted be-... 

Most metals are covered by an oxide (impervious and insulating, or more often, as with Al, defect-ridden). In contrast, gold has no oxide, and has the advantage of making SAMs with thiols. However, Au atoms migrate somewhat after deposition to minimize total energy, and migrate even more under an electric field... 

LAJs based on liquid metal electrodes have been extensively used in different geometries and modes to incorporate and study a large number of organic compounds [76, 85, 88, 106-108, 132, 160-171]. The wide use of Hg-based electrodes relies on the properties of this metal (1) it is highly conductive, (2) it forms well-ordered SAMs in a few seconds [166], (3) its surface, as a liquid, is free of structural features that cause defects in adsorbed monolayers and (4) it can form a... 

The two metal surfaces covered by SAMs are brought into contact by the use of a micro-manipulator in the presence of a liquid medium, such as hexadecane the presence of hexadecane transforms the defects of the SAMs into insulating sites. The use of a semitransparent solid surface (Au or Ag) allows (1) evaluation of the contact area by collecting the image of the contact area by a mirror and (2) electrical measurements under irradiation of the SAMs through the Au surface. The disadvantage of Hg-based junctions is related to the environmental unfriendly characteristics of Hg, which prevent any application. For this main reason, these junctions are valuable only as versatile test-beds for organic electronics. 

Figure 5.1 Illustration of (a) molecular architecture of a SAM-forming molecule, (b) an idealized SAM and (c) a more realistic description with molecular defects (1), domain boundaries (2) and substrate topography such as steps (3).

The possibilities afforded by SAM-controlled electrochemical metal deposition were already demonstrated some time ago by Sondag-Huethorst et al. [36] who used patterned SAMs as templates to deposit metal structures with line widths below 100 nm. While this initial work illustrated the potential of SAM-controlled deposition on the nanometer scale further activities towards technological exploitation have been surprisingly moderate and mostly concerned with basic studies on metal deposition on uniform, alkane thiol-based SAMs [37-40] that have been extended in more recent years to aromatic thiols [41-43]. A major reason for the slow development of this area is that electrochemical metal deposition with, in principle, the advantage of better control via the electrochemical potential compared to none-lectrochemical methods such as electroless metal deposition or evaporation, is quite critical in conjunction with SAMs. Relying on their ability to act as barriers for charge transfer and particle diffusion, the minimization of defects in and control of the structural quality of SAMs are key to their performance and set the limits for their nanotechnological applications. 

Figure 5.7 The role of stress caused by lattice mismatch between SAM and substrate illustrated in (a) and (b) by a cross-section of a SAM (x-z plane), indicated adsorption sides (x-y plane) and the molecule-substrate interaction potential V where the solid circles indicate the energy of an adsorption site for a particular SAM molecule, (a) For rigid molecules, stress is mainly released by defect formation in SAM, which results in a layer of rather low crystallinity and small domains, (b) Molecules...

Figure 5.13 (a) Potential drop across a SAM and the adjacent double layer (DL) in a region free of defects shown in top part. Solid and dashed lines are two examples of different positions of the Fermi level Ep. Dash-dot line indicates the course of an equipotential line representing the potential at the outer surface of the SAM including defects, (b) Illustration of processes involved in electrochemical metal deposition. For details, see text. 

STM and CVs are applied. This is exacerbated by the fact that the extent to which UPD features are suppressed in the CVs depends sensitively on the quality of the SAM. For such a pronounced quenching a good film quality is required, that is, a low defect density is required. To achieve this reproducibly is quite critical as has been pointed out in the literature [40, 183, 204—206]. Therefore, it is no surprise that substantial variations in the blocking properties refiected in the CVs have been observed [39, 183, 203, 207-209]. 

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).

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