Surface Structure and Electrochemical Polymerization of Mixed, Thiophene-Capped Monolayers
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Surface Structure and Electrochemical Polymerization of Mixed, Thiophene-Capped Monolayers
Jung F. Kang, Katherine Harrison, and S. Michael Kilbey II Department of Chemical Engineering Clemson University Clemson, SC 29634-0909, U.S.A. ABSTRACT 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-(3thienyl)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. INTRODUCTION Conductive polymers have been proposed for a wide variety of applications, including organic electronic devices, electrochromic displays, and sensors. As the interest in these technologies has advanced, so has the need for materials with enhanced or tailored properties. This is, of course, a multi-faceted problem that can be approached from many angles, including manipulating the physicochemical properties of the materials through synthetic chemistry or post-polymerization modification, altering processing events, or influencing the assembly and organization of the conductive polymers in order to mediate the structure-property relationships of the system. It is this latter aspect that we have been interested in. We have been studying a model system comprised of multicomponent, self-assembled monolayers (SAMs) bearing electroactive thiophene groups at the periphery of the SAM [1,2]. Although similar systems containing pyrrole [3-6], thiophene [7-10], and aniline [11,12] have been investigated, there has been little work on using these layers to create ultra-thin, surfacetethered layers of conductive polymers and studying the structure-property relationships of those films. Our previous work has shown that the wetting and electrochemical behavior of the thiophene-capped, multicomponent monolayers changes as the SAM composition is altered [1,2]. Here we investigate whether the composition of the underlying SAM affects the interfacial structure and mechanical integrity of the film. This work is important because these interfacial layers of conductive polymers may be used as platforms for the applications listed above.
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EXPERIMENT The particular systems used to create self-assembled monolayers on indium tin-oxide surfaces were mixtures of 11-(3-thienyl)undecyltrichlorosilane (3TUTS) and undecyltrichlorosilane (UTS). The procedures used to synthesize these
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