The formation and growth of self-assembled octadecylsiloxane monolayers on native silicon and mica substrates have been studied using atomic force microscopy, ellipsometry, and infrared spectroscopy. Submonolayer ODS films of varying surface coverages were prepared by immersing the substrates into dilute solutions of octadecyltrichlorosilane in toluene for different periods of time, and the submonolayer film structures were compared between mica and silicon substrates for different water contents of the adsorbate solutions and for different time delays between solution preparation and substrate immersion (solution age). It was found that, in general, both a continuous growth (formation of disordered, liquidlike submonolayers) and an island-type growth (formation of organized assemblies with vertically aligned hydrocarbon chains) are involved in the formation of ODS monolayers, whereby the relative contributions depend strongly on the solution properties. With increasing water content or increasing age of the adsorbate solution, island-type growth is strongly favored on both silicon and mica surfaces, which indicates the kinetically controlled formation of larger, preordered aggregates of silanol molecules as the primary hydrolysis products in solution. For identical conditions of film preparation, both the degree of structural order in the submonolayer films and the overall adsorption rate was found to be higher on mica in comparison to silicon. The higher structural order was interpreted as a consequence of the lower hydroxyl group concentration and a correspondingly enhanced surface diffusion rate of weakly bound film molecules on a mica substrate. The enhanced adsorption rate, on the other hand, points to some additional activation of a mica surface with respect to silanol adsorption, which might be related to its ionic composition containing mobile surface charges in contrast to the covalent, neutral character of a native silicon surface.

The formation of alkylsiloxane monolayers O x Si−(CH2) n −Y with different hydrocarbon chain lengths (n = 10, 16, 17) and different terminal substituents (Y = CH3, COOCH3, CN, Br) on native silicon (Si/SiO2) was studied by means of in situ internal reflection IR spectroscopy (ATR) at the interface between a Si ATR crystal and the precursor solution. The growth of the ν(CH2) stretching absorptions of the monolayer films, monitored with s-polarized and p-polarized radiation, provided information on the monolayer formation rates and on structural changes in the course of the growth process. The film molecules adsorb initially in a random, disordered configuration. With increasing coverage, the hydrocarbon chains gradually align and stand up on the surface. Their final orientation in the complete monolayer films depends both on the chain length and on the type of terminal substitution, whereby chain tilt angles between 7° for O x Si−(CH2)17−CH3 and 21° for O x Si−(CH2)16−CN and O x Si−(CH2)16−Br were found. The film growth follows essentially a Langmuir model of irreversible adsorption, from which the adsorption rate constants were derived. Whereas the chain length and the terminal substituent have relatively small influences on the adsorption rates, a higher water content of the precursor solutions strongly accelerates the film formation and, in addition, causes significant deviations from a Langmuir growth model. These findings were interpreted as a consequence of polycondensation of the precursor molecules in solution.

Monolayers of octadecylsiloxane were formed on native silicon (Si/SiO2) and glass surfaces by adsorption from dilute solutions of octadecyltrichlor osilane and were investigated by polarization- and angle-dependent external reflection infrared spectroscopy. In contrast to metal substrates, both the parallel and perpendicular vibrational components of the adsorbate can be detected on these dielectric surfaces. The monolayer reflection spectra show significant changes as a function of the light incidence angle and the polarization of the infrared radiation, which contain detailed information on the surface orientation of the film molecules. Spectral simulations based on classical electromagnetic theory yield an average 10° tilt angle of the hydrocarbon chains with respect to the surface normal on both silicon and glass surfaces. Despite this apparent structural identity of the monolayer films on silicon and glass, significant differences are observed in the monolayer reflection spectra resulting from purely optical effects of the substrate.

Submonolayers of octadecylsiloxane (ODS) were prepared by adsorption from dilute solutions of octadecyltrichlorosilane (OTS) onto a series of different substrates: mica, native silicon (Si/SiO2), and mica coated with a defined number nSiO of SiO2 monolayers (nSiO ) 1, 2, 4, 6). Atomic force microscopy (AFM) was used to investigate the adsorption rate and the submonolayer island morphology as a function of the substrate composition. Two types of substrate effects were observedsfirst, an abrupt change of the shape, size, and height distribution of the submonolayer islands between mica and SiO2-coated mica or silicon substrates, and second, an exponential decrease of the adsorption rate with nSiO up to a thickness of about 6 SiO2 monolayers. The first effect is independent of the SiO2 film thickness and the nature of the underlying substrate (mica or Si) and is therefore believed to arise from the different surface concentrations of OH groups on mica and SiO2 surfaces. The adsorption rate decrease with nSiO, in contrast, appears to be a long-range, bulk effect of mica and might reflect an electrostatic interaction between the negatively charged mica surface and the polar head groups of the film molecules, which accelerates the adsorption in comparison to that of an uncharged substrate such as silicon.Self-assembled monolayers (SAMs) formed on solid substrates by spontaneous assembly of amphiphilic molecules from dilute solutions are often considered as solidstate analogs to Langmuir-Blodgett (LB) films prepared on a liquid subphase and transferred to a solid support. 1 Despite the striking similarities between these two classes of highly organized, supramolecular systems, regarding the type of film molecules and their uniform, densely packed assembly on the substrate surface, SAM films are generally strongly chemisorbed and often show pronounced, substrate-dependent properties unknown for LB films but rather typical for epitaxial overlayers. Organothiol molecules, for example, adsorb on coinage metal surfaces (Au, Ag, Cu) via specific sulfur-metal bonds onto a predefined coordination site lattice (e.g. 3-fold hollow sites on a Au(111) surface), whereby the lattice spacing and the lattice geometry of the particular metal determine the packing density and the surface orientation of the film molecules. 2 Other classes of SAM films, on the other hand, such as alkylsiloxane monolayers formed from alkylsilanol precursors on a variety of OH-terminated surfaces 3-5 (Si/SiO 2 , Al 2 O 3 , glass, mica, etc.), appear to lack any substrate influences 3,6,7 and are considered as the closest relatives to LB films known to date. 8 One major difference still existssthe covalent linkage between the silanol film molecules and the surface hydroxyl groups under Si-O-Si bond formationswhose role in the film growth process is still under debate. Whereas in some reports a substrate-decoupled growth mechanism was proposed, 7,8 in which the monolayer forms on a thin layer of adsorbed water and is sparsely anchored to the substrate via Si-O-...

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A novel procedure for a controlled monolayer growth of silicon oxide films on silicon substrates is presented. It is based on a binary A−B reaction sequence involving the formation of an alkylsiloxane monolayer through self-assembling from solution (step A) followed by UV/ozone oxidation of the hydrocarbon groups (step B). Repeated application of this A−B cycle results in a layer-by-layer growth of the oxide film with a strictly linear thickness increase of 2.7 Å per cycle, as evidenced by ellipsometry and infrared reflection spectroscopy.

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