Combinatorial Strategies for Thin Organic Films - Polythiophene via Surface Polymerization by Ion Assisted Deposition
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Combinatorial Strategies for Thin Organic Films Polythiophene via Surface Polymerization by Ion Assisted Deposition Luke Hanley, Sanja Tepavcevic, and Yongsoo Choi Department of Chemistry, University of Illinois at Chicago 845 W. Taylor St., 4500 SES (M/C 111) Chicago, IL 60607-7061, U.S.A. ABSTRACT Combinatorial strategies are applied to surface polymerization by ion assisted deposition (SPIAD) for the production of polythiophene films. Thiophene ion energy in the range of 25 – 200 eV and ion to α-terthiophene neutral ratio are varied to tune the extent of surface polymerization and film photoluminescence. SPIAD polythiophene films are grown by both mass-selected and non-mass-selected ions, while x-ray photoelectron spectroscopy and surface mass spectrometry are applied to determine film chemistry and the extent of polymerization. This work demonstrates that ion kinetic energy and ion to neutral ratio can be varied over a wide range in SPIAD to select films with useful optical or electronic properties. The essentially combinatorial approach of SPIAD can be dramatically extended by use of new ions and/or neutral species. INTRODUCTION Polythiophene and oligothiophenes have been extensively studied for technological application as conducting polymers [1]. Previous work in ion assisted deposition of conducting polymers predominantly utilized atomic ions at kinetic energies up to 100 keV [2]. However, polyatomic ion deposition at hyperthermal energies (1 – 500 eV) can also be used to produce organic films with controlled surface chemistry and nanostructure [3-5]. Combinatorial methods have been widely applied to the production of new materials generally [6] and polymers specifically [7]. Polyatomic ion deposition can also produce surface chemical and contact angle gradients on polymer and other surfaces in a method readily applicable to combinatorial materials growth [8]. Surface polymerization by ion assisted deposition (SPIAD) has recently been described for the production of conducting polymer films, through the co-incident deposition of hyperthermal polyatomic ions and thermal neutrals, as shown schematically in Figure 1 [9]. This paper applies combinatorial strategies to SPIAD for the production of polythiophene films. Ion kinetic energy and ion/neutral ratio are varied to tune the extent of surface polymerization and film photoluminescence. EXPERIMENTAL DETAILS Films are prepared by both mass-selected and non-mass-selected ion deposition using separate instruments described previously [5,10]. Mass-selected deposition of 25 – 200 eV C4H4S+ is performed over several hours at fluences of ~1015 ions/cm2 while α-terthiophene (3T) is simultaneously evaporated from a thermally heated doser at fluences of ~1018/cm2 [9],
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calibrated by a quartz crystal microbalance. Non-mass-selected deposition of 55 – 200 eV ions is also performed using a Kaufman ion source which produces ~65% C4H4S+, with the remaining ions predominantly C3H3+, CHS+, and C2H2S+. Non-mass-selected films are deposited for 10 minutes, cor
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