We have investigated the wetting behavior and electrochemical response of self-assembled monolayers made from ω-(3-thienyl)-alkyltrichlorosilanes and alkyltrichlorosilanes on indium-tin oxide coated glass slides. Contact angle measurements and cyclic voltammetry show that the properties of the monolayers change as the relative loading of the two components is changed by altering the concentration of the deposition solution. The water contact angles indicate that a fully loaded hydrophobic layer is formed, and both water and hexadecane contact angle measurements show that the tethered thiophenes are at the periphery of the monolayer. However, the hexadecane contact angles also imply that the surface roughness exposes the methylene groups of the monolayer chains. Cyclic voltammograms obtained from mixtures of 11-(3-thienyl)undecyltrichlorosilane and decyltrichlorosilane and mixtures of 16-(3-thienyl)hexadecyltrichlorosilane and hexadecyltrichlorosilane show that as the relative amount of thienyl-bearing chains in the monolayer decreases, the oxidation peak position shifts to higher potential and the peak area, which is related to the amount of charge transferred, decreases. Integration of the oxidation peak area from the cyclic voltammetry experiments yields a coverage that is consistent with a monolayer. Preliminary evidence for formation of a surface-tethered redox-active polymer is observed by cycling the self-assembled monolayers at low potentials.
We have used X-ray photoelectron spectroscopy and external reflectance Fourier transform infrared spectroscopy (ER-FTIR) to investigate the structure and composition of mixed monolayers made from n-alkyl chains and ω-(3-thienyl)alkyltrichlorosilanes. Monolayers made from 11-(3-thienyl)undecyltrichlorosilane (3TUTS) and undecyltrichlorosilane (UTS) and also from 16-(3-thienyl)hexadecyltrichlorosilane (3THTS) and hexadecyltrichlorosilane (HTS) were examined. ER-FTIR indicates that mixed monolayers made from 3TUTS/UTS and 3THTS/HTS form nearly liquidlike monolayers on indium tin oxide (ITO) substrates. The pendant thiophene group is located at the periphery of the self-assembled monolayer (SAM), and it appears to be oriented along the SAM. Both spectroscopic techniques suggest that during assembly of mixed monolayers, the ω-(3-thienyl)alkyltrichlorosilane species preferentially adsorbs, leading to a monolayer enriched in the thiophene-capped component. The information resulting from these studies will be generally useful for tailoring the structure and properties of electroactive monolayers on ITO substrates.
We have investigated the growth of polythiophene from self-assembled monolayers (SAMs) that contain pendant thiophene groups using electrochemistry and atomic force microscopy. The SAMs are formed on indium tin-oxide (ITO) by coadsorption of 11-(3-thienyl)undecyltrichlorosilane (3TUTS) and undecyltrichlorosilane (UTS). By altering the composition of the underlying monolayers we can manipulate the onset of electrochemical polymerization and affect the surface topography of the resultant polythiophene layer. Films made on SAMs that have high loadings of 3TUTS have small, distinct grains, but as the monolayers become enriched in UTS, the grain size increases; however, these films are neither as rough nor as diffuse as films formed on ITO without an underlying SAM. These experiments suggest that the electrochemical growth and structure of the polythiophene layer can be manipulated by tuning the underlying SAM.
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