Effect of leaving group on the structures of alkylsilane SAMs

Multiple transmission and reflection (MTR) infrared spectroscopy has been used to study the kinetics of the formation of self-assembled monolayers (SAM) of octadecylsilanes with different leaving groups, viz. trichloro, trimethoxy, and triethoxy. It was observed that the chlorosilanes form much denser and crystalline-like SAMs and ethoxysilanes form thin SAMs, while methoxysilanes form extremely thin SAMs. The high sensitivity of the MTR IR technique allows the molecular conformations of the alkyl chains and appearance/disappearance of the silanol groups to be scrutinized in detail. This enables the formulation of models for the structures of the SAMs that are in many ways different than the classical picture of silanes on oxide surfaces. We observe that the structure of SAMs depends on the rate of hydrolysis of the leaving groups and thus their chemical nature. SAMs of chlorosilanes resemble a structure of snow moguls or densely packed umbrellas. SAMs of ethoxysilanes, on the other hand, look like stacks of fallen trees, while the molecules of the ultrathin methoxysilane SAMs are lying nearly parallel to the surface, resembling creepers.