Effect of solvents and concentration on the formation of a self-assembled monolayer of octadecylsiloxane on silicon (001)

We have explored the effects of solvent, adsorber concentration, and environment on the formation of high-coverage monolayers of alkylsiloxanes on silicon oxide surfaces from alkylsiloxane solutions. Specifically, we have varied the solvent polarity and the concentration of octadecyltrichlorosilane (OTS) used for the deposition process. We found that for a wide range of concentrations (25 muM to 2.5 mM) and normal laboratory air humidity (RH 45-85%) OTS dissolved in heptane causes the formation of a full-coverage self-assembled monolayer on hydrophilic silicon oxide. The resulting self-assembled layers were studied by atomic force microscopy, ellipsometry, and X-ray reflectometry. Deposition of OTS from dodecane solutions resulted in multilayered films. In contrast, the use of heptane as solvent (in which the solubility of water is at intermediate level between toluene and dodecane) caused the formation of high-quality monolayers. We found consistent and reproducible results for the effect of solvents (dodecane and heptane) and conditions on the formation of OTS layers on Si/SiO2 surfaces. The substrates, which are covered by a full monolayer, function as well-defined ultraflat hydrophobic surfaces for other experiments.