SAM.

Wafers of silicon covered with hydroxyl end-groups (SiOH) were synthesized by this method and immediately probed in order to avoid contamination of the surface by the environment due to the high reactivity of SiOH groups. 

Crafting of Functionalized SAMs onto Silicon Wafer 

Three different techniques are frequently used to obtain SAMs Langmuir-Blod-gett techniques, involving an air-water interface to transfer the assembled film to a solid substrate solution adsorption of film molecules onto the substrate and a vapor-phase molecular self-assembling technique [2], which uses vapor deposition of the film onto the substrate. Our functionalized SAMs were prepared by the last of these techniques, which had been slightly improved in the laboratory 

The silicon wafers were placed above a previously de-aired solution consisting of a mixture of 100 pL organosilane and 3 mb paraffin. The vapor-phase deposition of the molecular film on the substrate was performed in a vacuum chamber (50 min at 5 x 10 Torr) at room temperature. 

PDMS samples were crosslinked under nitrogen in a glovebox using tetrakis(di-methylsiloxy) silane as a crosslinker, and a platinum-based catalyst AU the chemicals were supplied by ABCR (Karlsmhe, Germany). The classification of PDMS substrates refers to the length of the chains before crossHnking (determined by SEC) (Table 3.1). Then, PDMS 1.5k is the hardest substrate, whereas 53k refers to the softest one. Flor/s law of rubber elasticity, which represents 

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From the variation of contact angle with SAM composition in Fig. X-3, what do you think the measuring liquid is What would the plot in Fig. X-3 look like if, say, n-decane were used instead ... 

The acid monolayers adsorb via physical forces [30] however, the interactions between the head group and the surface are very strong [29]. While chemisorption controls the SAMs created from alkylthiols or silanes, it is often preceded by a physical adsorption step [42]. This has been shown quantitatively by FTIR for siloxane polymers chemisorbing to alumina illustrated in Fig. XI-2. The fact that irreversible chemisorption is preceded by physical adsorption explains the utility of equilibrium adsorption models for these processes. 

Alkanethiols and other sulfur-bearing hydrocarbons covalently attach to metal surfaces alkanethiol onto gold is the most widely studied of these systems [27-29,31,32,45]. These SAMs are ordered provided the alkane chain contains nine or more carbons [32]. Binary solutions of two alkanethiols also appear... 

SAMs are generating attention for numerous potential uses ranging from chromatography [SO] to substrates for liquid crystal alignment [SI]. Most attention has been focused on future application as nonlinear optical devices [49] however, their use to control electron transfer at electrochemical surfaces has already been realized [S2], In addition, they provide ideal model surfaces for studies of protein adsorption [S3]. 

Investigate the differences between LB films and self-assembled monolayer SAMs (Chapter XI). Which are finding more practical use, and what are the potential applications of each ... 

Self-assembled monolayers (SAMs) are molecular layers tliat fonn spontaneously upon adsorjDtion by immersing a substrate into a dilute solution of tire surface-active material in an organic solvent [115]. This is probably tire most comprehensive definition and includes compounds tliat adsorb spontaneously but are neither specifically bonded to tire substrate nor have intennolecular interactions which force tire molecules to organize tliemselves in tire sense tliat a defined orientation is adopted. Some polymers, for example, belong to tliis class. They might be attached to tire substrate via weak van der Waals interactions only. 

Not aii moiecuies are suited for estabiishing SAMs. The majority of cases studied have invoived assembiy of aikyi-chain-based entities. The moiecuies of seif-organizing chemicai compounds aii have a simiiar stmcture. The spontaneous nature of fiim fonnation is due to the interaction energies of the monoiayers. These can be considered in tenns of tiiree main components (figure C2.4.i0) [121], which cooperativeiy estabiish stabiiity, order and orientation in the monoiayer. 

In contrast to tire preparation of LB films, tliat of SAMs is fairly simple and no special equipment is required. The inorganic substrate is simply immersed into a dilute solution of tire surface active material in an organic solvent (typically in tire mM range) and removed after an extended period ( 24 h). Subsequently, tire sample is rinsed extensively witli tire solvent to remove any excess material (wet chemical preparation). 

More recently, alternative chemistries have been employed to coat oxide surfaces with SAMs. These have included carboxylic 1129, 1301, hydroxamic 11311, phosphonic 1124, 1321 and phosphoric acids 11331. Potential applications of SAMs on oxide surfaces range from protective coatings and adhesive layers to biosensors. 

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. 

Figure C2.4.11. The fonnation of SAMs from OTS on a silicon oxide substrate.

Otlier lithographical means include micromachining [175], photopatteming [176] or electron beam patterning [1771, which have been demonstrated on alkanetliiolate/Au SAMs, alkanetliiolate and organo-siloxane on Si and Ti and alkanetliiolates on GaAs. 

SAMs of tliiolates on gold are generally resistant to strong acids or bases [175, 178 and 179], are not destroyed by solvents [180] and can witlistand physiological environments [181, 182 and 183]. However, tliey also show some degradation if exposed to tire ambient atmosphere for sufficiently extended periods [184]. 

Thermal stability. The tliennal stability of SAMs is, similarly to LB films, an important parameter for potential applications. It was found tliat SA films containing alkyl chains show some stability before an increase in tire number of gauche confonnations occurs, resulting in melting and irreversible changes in tire film. The disordering of tire... 

SAMs tliat are made out of stmctures capable of fonning strong intennolecular hydrogen bonds have been studied especially in view of tlieir expected high thennal and chemical stability [186, 187],... 

A good survey of tire chemical and physical film characteristics of highly organized SAMs is given in [123],... 

Experiments witli chemically grafted SAMs displayed much larger wear resistance tlian films produced by tire LB technique [188]. Also it was found tliat wear properties of SAMs can be furtlier improved by chemically grafting CgQ molecules onto SAM surfaces [189]. 

Regarding protein adsorjrtion properties, differently tenrrinated SAMs on gold have also been investigated [2251. It was found Arat Are nature of Are adsorbate chain stmcture was Are most importairt parameter for Are observed behaviour towards protein aird cell adsorjrtion. 

Covalent immobilization of proteins on nricrostmctured gold surfaces was studied in [226]. On Arese substrates, which were prepared by pCP aird etching. Are immobilization sites of proteins could be spatially controlled using air amino-reactive SAM. The whole process, i.e. production of Are micropattemed substrate including SAM exchairge aird protein immobilization, took a reasonably small amount of time ( 24 h), providing some flexibility in the experimental work. 

The examples described above are only a small selection out of a tremendous number of investigations of LB films aird SAMs. This number is still increasing aird it is expected tlrat ultrathin orgairic films will play a central role in botlr fundamental aird applied sciences in tire future. 

Gu Y, Akhremitchev B B, Waiker G C and Waideck D H 1999 Structurai characterization and eiectron tunneiing at n-Si/Si02/SAM/iiquid interface J. Phys. Chem. B 103 5220-6... 

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