An AFM study of the growth kinetics of the self-assembled octadecylsiloxane monolayer on oxidized silicon

Ex situ atomic force microscopy (AFM) was used to study the growth of the self-assembled octadecylsiloxane monolayer on silicon oxide at room temperature altering the immersion time of freshly oxidized Si(1 0 0) samples in a millimolar solution of octadecyltrichlorosilane (OTS) in toluene with a comparatively low water content. Two types of islands differing in size have been identified, small circular islands with a diameter around 0.1 μm and larger islands with a sometimes branched shape indicative for a growth according to diffusion limited aggregation (DLA). Both islands exhibit a height close to 2.5 nm, i.e. the height of the final monolayer, suggesting a vertical orientation of the octadecylsiloxane monomers within these islands. The large islands grow in size upon increasing the immersion time maintaining a constant height. At the same time their number decreases. A quantitative analysis of the AFM images reveals two regimes: a period with nearly linear growth up to a coverage close to 0.75 ML followed by a comparatively slow saturation at higher coverages. In comparison with first-order Langmuir adsorption kinetics a substantially lower uptake in the first growth regime takes place suggesting that physisorbed species not yet grafted to the surface are removed during the cleaning and rinsing steps prior to ex situ examination.