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

The formation of alkylsiloxane monolayers OxSi-(CH2)(n)-Y with different hy drocarbon chain lengths (n = 10, 16, 17) and different terminal substituent s (Y = CH3, COOCH3, CN, Br) on native silicon (Si/SiO2) was studied by mean s of in situ internal reflection IR spectroscopy (ATR) at the interface bet ween 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 fi lm molecules adsorb initially in a random, disordered configuration. With i ncreasing coverage, the hydrocarbon chains gradually align and stand up on the surface. Their final orientation in the complete monolayer films depend s both on the chain length and on the type of terminal substitution, whereb y 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 fo llows essentially a Langmuir model of irreversible adsorption, from which t he adsorption rate constants were derived. Whereas the chain length and the terminal substituent have relatively small influences on the adsorption ra tes, 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.