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 structures, bonding, and ring-opening reactions of strained cyclic carbon-based molecules form a key component of standard textbooks. In contrast, the study of strained organometallic molecules containing transition metals is a much more recent development. A wealth of recent research has revealed fascinating nuances in terms of structure, bonding, and reactivity. Building on initial work on strained ferrocenophanes, a broad range of strained organometallic rings composed of a variety of different metals, pi-hydrocarbon ligands, and bridging elements has now been developed. Such strained species can potentially undergo ring-opening reactions to functionalize surfaces and ring-opening polymerization to form easily processed metallopolymers with properties determined by the presence of the metal and spacer. This Review summarizes the current state of knowledge on the preparation, structural characterization, electronic structure, and reactivity of strained organometallic rings with pi-hydrocarbon ligands and d-block metals.
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.
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-...
Ultrathin SiO 2 films with thicknesses between 0.3 and 8 nm were grown on native silicon (Si/SiO 2 ), muscovite mica and polycrystalline gold substrates via repeated application of a binary reaction sequence, which involved the formation of a self-assembled alkylsiloxane monolayer (step A) and UV-ozone oxidation of the hydrocarbon groups (step B). Using octadecyltrichlorosilane as a precursor, SiO 2 films could be grown in a strictly linear, layer-by-layer mode on each of the three substrates with a growth rate of 3.0 ( 0.3 Å per deposition cycle, which corresponds to a monolayer of SiO 2 . The properties of these oxide films (composition, structure, packing density) were found to be essentially identical and independent of the substrate, as evidenced by ellipsometry, infrared reflection, and X-ray photoelectron spectroscopy. Furthermore, the quality of the oxide layers was investigated as a function of the hydrocarbon chain length of the alkylsiloxane monolayer formed in step A, using four different alkyltrichlorosilanes, R-SiCl 3 (R ) C 18 H 37 , C 11 H 23 , C 4 H 9 , CH 3 ), as precursors. For each compound, a linear increase of the SiO 2 film thickness with the number of applied deposition cycles was again observed, but the growth rate increased noticeably from 2.8 Å/cycle for the C 18 and the C 11 compound to 3.2 Å/cycle and 6.5 Å/cycle for the C 4 and the C 1 compound, respectively, concomitant with an increase of the surface roughness in atomic force microscopy images of the oxide films. The packing density of the Si atoms in these films remains essentially constant for longer-chain precursors (R g C 4 ), although the structure of the hydrocarbon layer changes drastically from a highly-ordered, perpendicular alignment on the surface (R ) C 18 ) to a random, isotropic arrangement (R e C 11 ). For films prepared from shorter precursors (R < C 4 ), however, multilayer formation sets in and results in film growth rates clearly beyond the monolayer level. A minimum chain length of about four C atoms is therefore required to restrict the alkylsiloxane film formation (step A) to the monolayer level and to provide reproducible and precise control of the oxide film thickness in this deposition process.
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