Preparation of organic monolayers on uniform and patterned silicon substrates
The growth of self-assembled alkylsiloxane monolayers on uniform and patterned Si substrates was studied at room temp. using at. force microscopy (AFM), contact angle measurements and quartz crystal microbalance (QCM) gravimetry. Immersion of oxidized Si substrates in a millimolar soln. of octadecyltrichlorosilane (OTS) gave ordered octadecylsiloxane islands with a height close to 2.5 nm. In the area between these islands an addnl. - presumably disordered - adsorbate layer with a height of .apprx.0.6 nm can be identified. The overall uptake-curves show subtle but significant deviations from the generally assumed 1st-order Langmuir adsorption kinetics. A nearly perfect fit, however, can be achieved from a simple model considering the adsorption of initially disordered species which subsequently transform into ordered islands. In this model, the disordered species are believed to occupy a larger surface area per entity and hence prevent adsorption of further mols. before rearrangement takes place. In contrast to oxidized Si substrates, H-terminated areas on Si substrates appear to remain uncoated after immersion into an OTS soln. Considering these results, a laser direct writing technique was used to create arbitrarily patterned Si substrates which expose H-terminated as well as oxidized areas. Starting with a uniformly H-terminated Si surface this technique allows for writing oxide lines with a lateral resoln. around 500 nm suitable for the selective coating with an alkylsiloxane monolayer.
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