SAMs quality

Phosphonic acid based SAMs [7c], which bind preferentially to alumina surfaces, can be prepared similarly, but it was also found experimentally that spin-coating a solution of 0.1% (w/w) SAM from a solvent such as toluene, followed by baking for 10-30 min on a hot plate at 100-120 °C, then a good rinse in dean toluene, and blow dry produced superior results in terms of phosphonic acid-SAM quality compared with overnight soaking in the same solution. This may be because of... 

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. 

For advanced electrochemical applications of SAMs in this area, their design is, therefore, a key issue. While SAMs are often perceived to form easily well-defined structures, a closer look into the literature reveals that thiol SAMs, in fact, very often lack the structural quality anticipated. Contrasting their ease of preparation, orga-nosulfur SAMs represent systems whose structure is determined by a complex interplay of interactions and if those are not properly taken into account, a SAM of limited structural quality and performance will result. To optimize SAMs for electrochemical applications and to exploit their properties for electrochemical nanotechnology it is, therefore, crucial to identify the factors controlling their structure. For this reason we start with an account of the structural aspects of SAMs. 

SAMs, in general, and thiol SAMs, in particular, are very often perceived as systems that easily form layers of high structural quality and this view is reflected in oversimplifying cartoons where a SAM is represented by a two-dimensional crystalline arrangement of molecules on a surface, similar to the one depicted in Figure 5.1b. For some systems one can get quite close to this ideal picture, as seen from Figure 5.2a, however, the more common case exemplified by Figure 5.2b is quite different. While... 

Figure 5.2 STM images ofthiols SAM on Au/mica illustrating the range of structural qualities, (a) high-quality SAM of dodecane thiol prepared from a solution of dodecane thiocyanate [111] and a low-quality aromatic SAM of methylbiphenyl butane thiol (MBP4) (b) [90].

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]. 

While some features, such as the formation of UPD islands, were commonly reported for various systems (different thiols and metals, that is, Ag and Cu) differing interpretations were given with respect to the details such as formation, extension or height, possibly due to the sometimes difficult interpretation of data that, furthermore, can vary with the details of the system and the experimental conditions applied. Some of the issues could be resolved in a recent study on high-quality aromatic SAMs where the UPD process could be extremely slowed down to allow time-resolved in-situ studies [43]. 

Thus, the quality of the SAM does play a role in the overall growth kinetics and resulting film thickness. The role of chain transfer has not been quantified but it clearly does not prevent photo-SlP. 

High quality SAMs of alkyhrichlorosilane derivatives are not simple to produce, mainly because of the need to carefully control the amount of water in solution (126,143,144). Whereas incomplete monolayers are formed in the absence of water (127,128), excess water results in facile polymerization in solution and polysiloxane deposition of the surface (133). Extraction of surface moisture, followed by OTS hydrolysis and subsequent surface adsorption, may be the mechanism of SAM formation (145). A moisture quantity of 0.15 mg/100 mL solvent has been suggested as the optimum condition for the formation of closely packed monolayers. X-ray photoelectron spectroscopy (xps) studies confirm the complete surface reaction of the —SiCl3 groups, upon the formation of a complete SAM (146). Infrared spectroscopy has been used to provide direct evidence for the full hydrolysis of methylchlorosilanes to methylsilanoles at the solid/gas interface, by surface water on a hydrated silica (147). 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobility of individual molecules is restricted. This lack of mobility is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibility problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quality. Alkyl silanes remain, however, ideal materials for surface modification and functionalization applications, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

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