New Method for Fabricating a Mixed Monolayer Using Self-assembly of Trichlorosilanes and Mapping of Different Molecules in the Mixed Monolayer Using a Frictional Force Microscopy with a Tip Chemically Modified with Fluoroalkyltrichlorosilanes for Chemical Sensing

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Abstract
An alkyltrichlorosilane/fluoroalkyltrichlorosilane mixed monolayer was fabricated by the self-assembly of an octadecyltrichlorosilane (OTS) monolayer having pinholes with 10–200 nm radii and following adsorption of (2-(perfluorooctyl)ethyl)trichlorosilane (FS-17) into the pinholes. The adsorption of FS-17 into the pinholes was confirmed using an atomic force microscope (AFM) by measuring the hole depths in the OTS monolayer, which decreased by a molecular length of FS-17 after the adsorption of the FS-17. The OTS and FS-17 molecules in the mixed monolayer were mapped using a frictional force microscope (FFM) whose tip was chemically modified with FS-17 molecules. In the case of the conventional observation using a FFM whose tip is not chemically modified, the frictional forces of the tip on the FS-17 molecules in the pinholes were smaller than those on the OTS monolayer. In contrast, the frictional forces of the chemically modified tip on FS-17 molecules in the pinholes were larger than those on the OTS monolayer. These results indicate that the FS-17 molecules on the tip have a critical influence on the frictional force between the tip and the molecules, and that the chemically modified tip may be used for discriminating different chemical groups in the monolayer. We propose that our new techniques for fabricating and estimating molecular distributions of the mixed monolayer will be useful in the development of molecular electronic devices.

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