Investigation of the formation and structure of self-assembled alkylsiloxane monolayers on silicon using in situ attenuated total reflection infrared spectroscopy

The formation of alkylsiloxane monolayers OxSi-(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 nu(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 degrees for OxSi-(CH2)(17)-CH3 and 21 degrees for OxSi-(CH2)(16)-CN and OxSi-(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.