Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon

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Abstract

Silicon-supported alkylsiloxane layers were prepared by reaction of alkylmethyldichlorosilanes and alkyltrichlorosilanes with silicon wafers under two conditions: (1) in the vapor phase and (2) in toluene in the presence of ethyldiisopropylamine. Covalent attachment of di- and trichlorosilanes to the surface of silicon/silicon oxide through SiS−O−Si bonds occurs for the amine-catalyzed reactions. This sets apart this reaction from the self-assembly process that occurs in the reaction between certain trichlorosilanes and hydrated silica with no amine present. The thickness of the layers formed from dichloro- and trichlorosilanes (as assessed by ellipsometry) is on the order of the single molecule sizes and increases gradually with alkyl chain length. The thickness values are considerably smaller (by a factor of ∼0.75) than the length of the fully stretched alkyl chain, which argues for disordered structures of the monolayers. Dynamic advancing and receding contact angles for water, methylene iodide, and hexadecane argue for interaction between the probe fluids and accessible silanol groups (Si−OH) on the surface. Water contact angles increase with alkyl chain length and level at θA/θR = ∼103°/∼90° for relatively long alkyl chains (∼C6 and longer), indicating that these surfaces project disordered methyl and methylene groups toward the probe fluid. n-Hexadecane and methylene iodide contact angles show more complex behavior, which is discussed in the paper. The vapor-phase reaction of di- and trichlorosilanes with silicon wafers yields surfaces that depend dramatically on the alkyl chain of the silane. Alkylsilanes with short and medium chains form polymeric grafted layers with thicknesses ranging from a few nanometers for dichlorosilanes up to tens of nanometers for trichlorosilanes. We suggest a mechanism that involves polycondensation of chlorosilanes into 3-D alkylsiloxanes in the presence of adsorbed water. Dynamic advancing and receding contact angles of water, methylene iodide, and hexadecane on these surfaces are consistently higher than for surfaces prepared in the liquid phase. Alkylsilanes with long alkyl moieties yield approximately monomolecular layers that exhibit wettabilities similar to those for surfaces prepared in the liquid phase.

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