Monolayer organosilane

Organosilanes, such as trichlorosilanes or trimethylsilanes, can establish SA monolayers on hydroxylated surfaces. Apart from their (covalent) binding to the surface these molecules can also establish a covalent intennolecular network, resulting in an enlranced mechanical stability of the films (figure C2.4.11). In 1980, work was published on the fonnation of SAMs of octadecyltrichlorosilane (OTS) 11171. Subsequently, the use of this material was extended to the fonnation of multilayers 11341. 

In order to prevent the irrevisible adhesion of MEMS microstructures, several studies have been performed to alter the surface of MEMS, either chemically or physically. Chemical alterations have focused on the use of organosilane self-assembled monolayers (SAMs), which prevent the adsorption of ambient moisture and also reduce the inherent attractive forces between the microstructures. Although SAMs are very effective at reducing irreversible adhesion in MEMs, drawbacks include irreproducibility, excess solvent use, and thermal stability. More recent efforts have shifted towards physical alterations in order to increase the surface roughness of MEMS devices. 

Fig. 2.4 Self-assembled monolayer (SAM) structures from organosilane compounds.

In order to investigate the phase transition in the monolayer state, the temperature dependence of the Jt-A isotherm was measured at pH 2. The molecular area at 20 mN rn 1, which is the pressure for the LB transfer of the polymerized monolayer, is plotted as a function of temperature (Figure 2.6). Thermal expansion obviously changes at around 45 °C, indicating that the polymerized monolayer forms a disordered phase above this temperature. The observed temperature (45 °C) can be regarded as the phase transition point from the crystalline phase to the liquid crystalline phase of the polymerized organosilane monolayer. 

Fig. 2.7 Set-up of permeation experiment through organosilane monolayer immobilized on a porous glass plate. Reprinted with permission from [46], K. Ariga and Y. Okahata,J. Am. Chem. Soc., 1989, 77 7, 5618. 1989, American Chemical Society.

Organosilane monolayers are interesting objects for scientific research and can be used in sensitive detectors. However, bulk structures are sometimes required for a... 

Figure 3.10—Formation of bonded organosilanes at the interface of silica gel. Representation of organic monomers and polymers at the surface of silica gel. The arrangement Si-O-Si C is more stable than Si O C. This reaction leads to a carbon content of 4 or 5%. Other reactions can also be used (hydrosilylation in particular). When a monolayer of hydrocarbons is bonded to the surface of silica, they orient in a particular manner at the interface due to their lipophilic and hydrophilic character.

Various other surface chemistries have been used to attach chemical species to electrode surfaces. For example, Watkins et al. activated the carboxylic acid functionalities on carbon electrodes with thionyl chloride and then reacted this surface with amines [17]. Sagiv and his coworkers have recently invented a clever approach for monolayer-by-monolayer deposition of multilayer films based on organosilane chemistry [18]. Finally, Mallouk et al. have also developed a monolayer-by-monolayer approach for synthesizing well-ordered multilayer films [19]. Because of these interesting new synthetic strategies, covalent attachment of functional groups remains an attractive approach for modifying electrode surfaces. 

The higher order alkylchlorosilanes (C8 and C18) have historically been treated in the same way as organosilanes. The reaction inevitably occurs in the liquid phase and is usually followed by a curing step. The extremely low surface of the silicon wafers and the deposited Si02 layers, used for self-assembled-monolayers does not allow a spectroscopic quantification of the surface species. A completely different type of analysis techniques is used here mainly to determine the quality (roughness and uniformity), the adherence (parallel or at random) and the hydrofobicity of the coated layer. 

In the liquid phase modification of Chemical Surface Coating, the treatment of gas 1 and gas 2 are replaced by a modification with e.g. an organosilane. In this case, the thickness of the ceramic precursor can be controlled by varying the amount of water in the reaction phase. When the reaction occurs in an aqueous phase, thick multilayers are created. Reaction circumstances, totally free of water, will yield a monolayer. 

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

Figure 2.10 (a) Langmuir monolayer with an organosilane network, (b) Permeation coefficient of... 

Self-assembled monolayers (SAMs) on gold and other metal surfaces have been extensively studied in biosensors applications.28-30 The thiol-gold chemistry is well known and is much easier controlled than organosilane chemistry (Fig. 14.4).31 34 The... 

Control of pore sizes of known catalysts like zeolites has been known for some time although the use of chemical vapor deposition (CVD) of organosilanes to control pore sizes has been the focus of recent research.7 Other catalysts like silica have been treated with methods like CVD and sol-gel in order to deposit thin films. Monolayer coatings of titanium oxide prepared by sol-gel methods have been recently used to coat silica and such films are active in alcohol dehydrogenation reactions.8... 

---