Self-assembly of octadecyltrichlorosilane monolayers on mica
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I. INTRODUCTION
The goal of synthesizing organic surfaces of controlled structure and functionality, outside high vacuum conditions, has engaged increasing effort in recent years. The phenomenon of self-assembly has emerged as a promising means to this end. Widely used in technology1'2 and of increasing academic interest,3"9 the method allows the rapid formation of single layers of densely packed oriented organic molecules on a solid substrate. While self-assembled films are similar in their high degree of order to those formed by classical Langmuir-Blodgett deposition,1011 they do not require the careful dipping procedure, and in many instances the resulting films are more robust owing to more secure attachment to the substrate. Self-assembly of a monolayer occurs spontaneously when a solid substrate is immersed in a dilute solution of the appropriate surfactant. This transforms the surface chemistry of the substrate to that of the adsorbed surfactant tail. Depending on the chemistry of the surfactant, a formerly lyophilic substrate may be rendered lyophobic, actually emerging dry when it is removed from the solution. Many types of molecules can display self-assembly, but all include a chemical group which interacts with the substrate, making substrate properties important. The substrate-adsorbate interactions may involve physisorption, as is the case for fatty acids,11213 or chemisorption, as is the case for trichlorosilanes on oxide surfaces1"9 and thiols on gold.14"17 Chemisorbed films are of particular interest because of their expected physical and chemical robustness. The chemical group which terminates the monolayer of molecules is limited only by the availability of J. Mater. Res., Vol. 5, No. 8, Aug 1990
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surfactants which contain that group. This makes it possible to design a monolayer film to meet specific needs. Films of tailored chemical functionality have been shown to enhance the formation of very thin layers of noble metals on oxides.18 Surface-modified electrodes are under study as a means to modify electrochemical reactions.1419 Reactive end groups on monolayers have been used to attach another species or to design multilayers.41011 Lubricating films, eventually of fluorocarbons, are of special interest to tribologists.20 Looking to the future, many further applications of this approach to molecular-scale engineering seem assured. To date the materials used as substrates for selfassembled films have been chosen because of their reactivity with the adsorbed end of the surfactant molecule, producing a tightly bound layer. Examples of this include using gold and silver as a substrate for thiol adsorption14"17 and oxides for silane adsorption.1"9 Silicon, germanium, zinc selenide, glass, and alumina are among the oxides which have been used. A different variety of material, which shows promise as a film substrate, is muscovite mica. The utility of mica derives from its translucency and from its molecularly smooth cleavage at the basal plane, to
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