Substrate Effects on the Formation of Alkylsiloxane Monolayers

Abstract

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-...

Discontinuous Molecular Films Can Control Metal/Semiconductor Junctions

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Atomic force microscopy of thin organic films on silicon in ultrahigh vacuum and under ambient conditions

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Surface modification of cobalt–chromium–tungsten–nickel alloy using octadecyltrichlorosilanes

“…The exact state of mechanism behind the chemical bonding of OTS on a surface is still debatable. The formation of OTS monolayer on a material surface is highly sensitive to several factors, which include (a) the density of surface hydroxyl groups [32]; (b) reaction temperature [29,33]; (c) reaction environment [34,35]; (d) reaction time [34]; (e) solvent used to deposit OTS [35,36]; (f) water content of the solvent [33]; (g) concentration of OTS [35,37]; (h) solution age [38]; (i) roughness of the underlying substrate [39]; and (j) cleaning procedures after SAM deposition [40].…”