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Growth and Morphology of Polythiophene on Thiophene-Capped Monolayers: 1. Single Component Monolayers Jung F. Kang, John D. Perry, Peng Tian, and S. Michael Kilbey, II* Department of Chemical Engineering, Clemson University, Clemson, South Carolina 29634-0909 Received April 29, 2002. In Final Form: August 22, 2002 Thin polythiophene films were grown at a variety of oxidizing potentials by electrochemical polymerization on indium tin-oxide surfaces that were modified with a thiophene-capped, self-assembled monolayer (SAM) composed of 11-(3-thienyl)-undecyltrichlorosilane (3TUTS). The evolution of those growing films was tracked via cyclic voltammetry and infrared spectroscopy, and the surface morphology was probed with atomic force microscopy. To allow comparison, similar data were obtained for polythiophene films grown on bare indium tin-oxide (ITO) (no SAM). Our results show that 3TUTS layers can nucleate growth of polythiophene films at potentials below the oxidation potential of monomeric thiophene in solution, and the cyclic voltammetry signature is characteristic of a surface-confined, redox-reversible species; however, at these low potentials no polythiophene growth on the bare ITO electrodes is seen. At higher potentials where polythiophene growth is observed on both types of surfaces, an additional amount of charge corresponding to the amount of thiophene groups along the SAM is transferred across the SAM-modified electrodes. Although spectroscopic data reveal no differences in the films grown on bare and 3TUTS-modified ITO, atomic force microscopy results show that the latter class of films are more homogeneous, have lower roughness, and are denser than the corresponding film grown on bare ITO. We attribute this difference in the morphology of the films to the high nucleation density arising from the pendent thiophene rings of the monolayer.
Introduction Interest in thin films of conducting polymers has been largely driven by their proposed use in a wide variety of applications, including rechargeable polymer batteries,1 electrochromic displays,2-4 antistatic materials,5 organic electronic devices,6-10 and sensors.11-13 As the interest in these technologies has advanced, so has the need for materials with enhanced or tailored properties. This is, of course, a multifaceted problem that can be approached from many angles, including manipulating the physicochemical properties of the materials through synthetic chemistry or postpolymerization modification, altering processing events, or influencing the assembly and * Corresponding author. E-mail:
[email protected]. (1) Croce, F.; Persi, L.; Ronci, F.; Scrosati, B. Solid State Ionics 2000, 135, 47-52. (2) Lee, D. S.; Lee, D. D.; Hwang, H. R.; Paik, J. H.; Huh, J. S.; Lim, J. O.; Lee, J. J. J. Mater. Sci.: Mater. Elec. 2001, 12, 41-44. (3) Salaneck, W. R. Philos. Trans. R. Soc. London A 1997, 355, 789799. (4) Somani, P.; Mandale, A. B.; Radhakrishnan, S. Acta Mater. 2000, 48, 2859-2871. (5) Pomposo, J. A.; Rodrı´guez, J.; Grande, H. Synth. Met. 1999, 104, 107-111. (6) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280, 1741-1744. (7) Levi, O.; Yakimov, A. V.; Nassar, H.; Davidov, D.; Pfeiffer, S.; Ho¨rhold, H. H. J. Appl. Phys. 2000, 88, 2548-2552. (8) Pei, J.; Yu, W.-L.; Huang, W.; Heeger, A. J. Macromolecules 2000, 33, 2462-2471. (9) Brown, A. R.; Jarrett, C. P.; de Leeuw, D. M.; Matters, M. Synth. Met. 1997, 88, 37-55. (10) Li, X.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S. C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc. 1998, 120, 2206-2207. (11) Talie, A.; Lee, J. Y.; Lee, Y. K.; Jang, J.; Romagnoli, J. A.; Taguchi, T.; Maeder, E. Thin Solid Films 2000, 363, 163-166. (12) Freund, M.; Lewis, N. S. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 2652-2656. (13) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson, R.; Wudl, F.; Whitten, D. G. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287-12292.
organization of the conductive polymers to mediate the structure-property relationships of the system. Along this latter vein, there has been a considerable effort to examine systems made from self-assembled monolayers (SAMs) that are end-capped with a monomer of the corresponding conductive polymer. It has been demonstrated with a variety of systems that surface-confined monomers of conductive polymers can serve as nucleation sites for the chemical or electrochemical growth of conductive polymer films. Particular systems investigated include SAMs capped with pyrrole,14-22 aniline,23-25 and thiophene26-30 monomers, with most of these studies having been done on silicon or gold substrates. The involvement of the surface-confined monomer units in the polymerization results in a strongly adhered, conductive polymer layer compared to what is achieved by electropolymerization from bulk solution onto an unmodified surface. The functionalized SAMs can also template the growth of conductive polymers. Because the surface-confined monomers that decorate the SAM change (14) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296-301. (15) Guiseppi-Elie, A.; Wilson, A. M.; Tour, J. M.; Brockmann, T. W.; Zhang, P.; Allara, D. L. Langmuir 1995, 11, 1768-1776. (16) Simon, R. A.; Ricco, A. J.; Wrighton, M. S. J. Am. Chem. Soc. 1982, 104, 2031-2034. (17) Wurm, D. B.; Kim, Y.-T. Langmuir 2000, 16, 4533-4538. (18) Smela, E.; Kariss, H.; Yang, Z.; Mecklenberg, M.; Liedberg, B. Langmuir 1998, 14, 2984-2995. (19) Kim, Y.-H.; Kim, Y.-T. Langmuir 1999, 15, 1876-1878. (20) Collard, D. M., Sayre, C. N. J. Electroanal. Chem. 1994, 375, 367-370. (21) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302-306. (22) Wurm, D. B.; Brittain, S. T.; Kim, Y.-T. Langmuir 1996, 12, 3756-3758. (23) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 19831992. (24) Schomburg, K. C.; McCarley, R. L. Langmuir 2001, 17, 19931998. (25) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511-2518.
10.1021/la020397f CCC: $22.00 © 2002 American Chemical Society Published on Web 11/23/2002
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the interfacial environment, the resulting films exhibit morphologies and properties different than the corresponding films polymerized onto the electrode surface without the underlying SAM. For example, Fikus and co-workers showed that when thiophene was chemically polymerized onto SAMs consisting of thiophene-capped alkyltrichlorosilanes on silicon, the root-mean-square roughness measured via atomic force microscopy was less than 0.2 nm.26 Similarly, Collard and Inaoka showed that when methyl thiophene was electropolymerized from SAMs capped with bithiophene or trithiophene, the resultant films were smooth and strongly adhered to the underlying gold substrates. Furthermore, layers of poly(3-octadecylthiophene) exhibited reversible solvatochromism.27 Along similar lines, several groups reported that films of polypyrrole that were electrochemically grown from pyrrole-terminated SAMs were highly resistant to removal by a tape peel test14-16,21 and optically dense14 compared with films grown on gold electrodes modified by alkanethiol monolayers. Such films formed on pyrrole-terminated SAMs also exhibited higher conductivities than films formed on bare gold electrodes.21 Scanning electron microscopy images also revealed that the polypyrrole films grown on the SAM-modified surfaces were extremely smooth and comprised nuclei approximately 80 Å in diameter, while films grown on bare gold electrodes under the same conditions were much rougher and comprised micron-sized grains.14,21,22 Wurm and co-workers arrived at similar findings using pyrrole-capped SAMs and n-hexadecylpyrole as the monomer in solution. In this system, the oxidation potentials of the surface-confined pyrrole and monomer in solution were the same.22 Peel tests showed that the films were strongly bound to the surface and atomic force microscopy images showed that the poly(n-hexadecylpyrrole) films epitaxially grown from the pyrrole-capped SAMs were continuous and consisted of small particles of approximately 100 Å in height, while poly(n-hexadecylpyrrole) films on bare gold surfaces were nonuniform and discontinuous in nature.22 This body of research clearly demonstrates that the presence of the underlying SAM affects surface morphology and properties of thin, conductive polymer films; however, these studies have all involved single-component SAMs, and the impact of the electropolymerization conditions has not been rigorously probed. Our previous work has shown that by using binary SAMs composed of the thiophene-capped and n-alkyltrichlorosilanes, the surface energy, as measured by the water contact angle, can be changed in a continuous fashion as the composition of the mixed SAM is altered.28 This also changes the areal density of the electroactive thiophene groups, which can serve as nucleation sites,14,21,25 along the interface. We have segmented this investigation of the effect of electrosynthetic conditions and monolayer composition on the morphology of the electropolymerized polymer films into two parts. In this first part, we present a thorough and systematic study of the effect of electrosynthetic conditions on the morphology of polythiophene films grown from single-component SAMs made from 11-(3-thienyl)undecyltrichlorosilane on indium tin-oxide (ITO) surfaces. To the best of our knowledge, such detailed morphological studies have not been undertaken, despite the sizable research effort focused on SAMs bearing monomeric units (26) Fikus, A.; Plieth, W.; Appelhans, D.; Ferse, D.; Adler, H.-J.; Adolphi, B.; Schmitt, F.-J. J. Electrochem. Soc. 1999, 146, 4522-4525. (27) Inaoka, S.; Collard, D. M. Langmuir 1999, 15, 3752-3758. (28) Sullivan, J. T.; Harrison, K. E.; Mizzell III, J. P.; Kilbey II, S. M. Langmuir 2000, 16, 9797-9803.
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of conductive polymers. This line of research is pertinent in a system such as ours where the surface-tethered thiophene group and monomer in solution have different oxidation potentials. A subsequent paper31 (which expands on preliminary results appearing elsewhere32) will explore the effect of SAM composition and electrosynthetic conditions on the ultimate surface structure of the electropolymerized films. This work is important because the electrical properties of conductive polymer films created by electrochemical polymerization are strongly correlated with the surface morphology and the film thickness.33-35 The results emerging from this body of work may find application in situations where thin films of conductive polymers are interfaced with other materials. Experimental Section We have previously detailed the procedures we use for substrate cleaning and SAM assembly,28,29 so these elements are described only briefly. The synthesis of the thiophene-capped SAM material has been reported previously, so no mention is needed here.28 The setup of the electrochemical system used for the electropolymerization also remains as beforesa singlecompartment cell and three-electrode arrangement where an Ag/AgNO3 (0.01 M) nonaqueous reference electrode was used and the ITO surface serves as the working electrode. Freshly prepared electrolyte solutions consisting of recrystallized and vacuum-dried supporting electrolyte [0.1 M tetrabutylammonium hexafluorophosphate (Aldrich)] in anhydrous acetonitrile (99.8%, Aldrich) were used.28 Preparation of SAM-Modified Substrates. Monolayers were self-assembled onto ITO-coated glass slides (Delta Technologies) from rigorously dried methylene chloride solutions. The concentration of 3TUTS was 10 mM and deposition was allowed to proceed for a 6-h period. After the deposition, the substrates were removed, rinsed with and sonicated in methylene chloride, and blown dry by a stream of nitrogen. Electrochemical Polymerization. Cyclic voltammetry (CV) experiments were conducted at room temperature (24 ( 1 °C) on a CH instruments (model 600A) electrochemical analyzer. A housing enclosing the electrochemical cell was continuously purged with nitrogen to maintain the relative humidity at approximately 16% ((2%) throughout the experiments. The electrochemical polymerization was carried out from the electrolyte solution containing 0.02 M thiophene (99%, Aldrich), which had been redistilled before use. A scan rate of 100 mV/s was used in all CV experiments. The electrosynthetic condition was manipulated by changing the maximum oxidation potential applied to the 3TUTS-modified electrode. For the various samples, cycles to and from 1.70, 1.66, 1.62, 1.56, 1.50, 1.47, 1.45, and 1.40 V (vs Ag/Ag+) were performed. After CV experiments, films grown on the 3TUTS-modified electrodes were rinsed with ethanol, sonicated in ethanol for 15 min, and blown dry with nitrogen; the films grown on bare ITO were not sonicated. Unless explicitly stated otherwise, all potentials stated in the text should be interpreted as relative to the Ag/Ag+ nonaqueous reference electrode used in these experiments. (At a scan rate of 100 mV/s, the ferrocene/ferrocenium redox couple shows an E1/2 ) 0.095 V versus our nonaqueous reference electrode.) External Reflection Fourier Transform Infrared Spectroscopy (ER-FTIR). The IR experiments were performed with Nicolet Nexus 870 FTIR spectrometer equipped with MCT-A (29) Harrison, K. E.; Kang, J. F.; Haasch, R.; Kilbey II, S. M. Langmuir 2001, 17, 6560-6568. (30) Appelhans, D.; Ferse, D.; Adler, H. J.; Schmitt, F. J. J. Electrochem. Soc. 1999, 146, 4522-4525. (31) Kang, J.-F.; Tian, P.; Kilbey II, S. M., in preparation. (32) Kang, J. F.; Harrison, K. E.; Kilbey II, S. M. In Organic Optoelectronic Materials, Processing and Devices, S. Moss, Ed. Materials Research Society Symposium Series, Vol. 708, 2002. (33) Roncali, J.; Garreau, R.; Yassar, A.; Marque, P. Garnier, F.; and Lemaire, M. J. Phys. Chem. 1987, 91, 6706-6714. (34) Yassar, A.; Roncali, J.; and Garnier, F. Macromolecules 1989, 22, 804-809. (35) Roncali, J.; Yassar, A.; and Garnier, F. J. Chem. Soc., Chem. Commun. 1988, 9, 581-582.
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Figure 1. Ten consecutive cyclic voltammetry sweeps showing growth of polythiophene on SAMs of 3TUTS on ITO. The maximum applied potential was limited to (a) 1.7 V, (b) 1.5 V, and (c) 1.4 V. A scan rate of 0.1 V/s was used and the initial concentration of thiophene in solution was 0.02 M. In each of these figures, ∆ ) 4 × 10-4 A/cm2. detector that was cooled with liquid nitrogen. A Whatman laboratory gas generator (model 75-45) was used to purge the sample compartment with dry, CO2-free air. The Spectra-Tech FT-80 attachment (angle of incidence 80°) was used to collect the spectra. A total of 2000 scans were collected for each spectrum with a resolution of 8 cm-1. The reflection experiments utilized p-polarized light to enhance the sensitivity to dipole moments parallel to the surface normal. The deviation of the band position is (0.3 cm-1. Atomic Force Microscopy (AFM). The AFM topography images of the films were recorded in tapping mode at ambient conditions using Nanoscope 3100 AFM (Digital Instruments). Both height and phase images were recorded simultaneously using the retrace signal. The silicon nitride tips with a resonance frequency of approximately 300 kHz and a spring constant of about 50 N m-1 were used, and the scan rate was 1 Hz. In all AFM images, differences in height are indicated by color: in our results, dark is low, and white is high.
Results and Discussion Since we were interested in the impact that the monolayer has on the early stages of growth of the polythiophene film, we chose to compare the electrochemical behavior and surface morphology of the polythiophene films that were formed on 3TUTS-modified ITO and bare ITO after 10 cyclic voltammetry sweeps had been completed. The electropolymerization experiments were carried out with the potential limited to 1.70, 1.66, 1.62, 1.56, 1.50, 1.47, 1.45, and 1.40 V. (Again, unless otherwise stated, all potentials are versus the Ag/AgNO3 nonaqueous reference electrode used.) We chose this range of potentials for several reasons: 1.7 V was selected as an upper limit because previous experiments showed that at 1.8 V the thiophene-capped SAMs overoxidize and degrade;28,38 and 1.4 V was selected as a lower limit because it is near the oxidation peak potential of the 3-substituted thienyl groups of the SAM;28 but also below the oxidation potential of monomeric thiophene in acetonitrile, which at these experimental conditions shows the beginning of an oxidation peak at approximately 1.5 V.32,36 The growth of polythiophene was evidenced by the broad redox waves in the cyclic voltammograms, which expand vertically on each successive cycle, and, at the higher potentials, also by the appearance of a reddish/brownish film on the working electrodes. The peak corresponding (36) Waltman, R. J.; Bargon, J.; Diaz, A. F. J. Phys. Chem. 1983, 87, 1459-1463. (37) Roncali, J. in Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998; Chapter 12. (38) Downard, A. J.; Pletcher, D. J. Electroanal. Chem. 1986, 206, 147-152.
to oxidation of the thiophene also shifts to lower potentials with each successive cycle, due to the facility with which polymerization of thiophene occurs on polythiophene. In the case of the SAM-modified anodes, as shown by Figure 1, there are two clear changes in the cyclic voltammograms that occur as the maximum potential is limited: First, there is the expected decrease in the amount of polymer formed, as indicated by decreasing areas beneath the redox waves; second, the difference between the oxidation and reduction peak potentials decreases as the maximum applied voltage is reduced. These two patterns of behavior were preserved over the entire range of potentialcontrolled CV experiments performed on 3TUTS-modifed ITO. In addition, careful inspection of the voltammograms produced from the 3TUTS-modified anodes that were cycled to 1.40 (Figure 1c) and 1.45 V reveals that the oxidation and reduction peak potentials are essentially identical over the 10 cycles, indicating the presence of a surface confined, reversible redox species. No evidence of polythiophene growth was seen at or below 1.35 V. In some respects, the electrochemical polymerization of thiophene onto bare ITO electrodes produced similar results; the area under the redox waves decreased as the maximum applied potential was decreased, and on successive cycles, the oxidation of thiophene was facilitated by the presence of the polymer. As expected, no polythiophene growth was evident unless the maximum applied potential was approximately 1.5 V. Also, for all of the various maximum potentials that were investigated, the difference between the redox peak potentials did not diminish to zero. As noted previously, these samples were not sonicated because the films were easily disengaged from the ITO substrate. We also found that polythiophene films could be easily removed from the unmodified ITO surface by a peel test with Scotch tape, but the films on the 3TUTS-modified ITO could not be removed in this manner, most likely because of chemical bonding between the thiophene groups of the SAM and polymer film.14-16,21 To appreciate the impact that the 3TUTS monolayers and maximum potential have on the growth of thin polythiophene films, we calculated the charge transferred during the reductive sweep, normalized by the apparent area of the ITO electrode, as a function of cycle number for the various samples. (The area beneath the reduction wave was calculated by numeric integration between 0 and 1.3 V.) The result of recasting the data in this manner is shown in Figure 2. As seen in Figure 2, parts a and b, the relationship between the reduction charge density and cycle number is linear when the sweeps are carried out to low and moderate potentials. At higher potentials after
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Figure 3. Average charge transferred per cycle, which was calculated using the slope of the line relating charge density to cycle number, for the 3TUTS-modified and bare ITO slides. The 3TUTS SAMs nucleate the growth of the polythiophene films at potentials below the oxidation potential for monomeric thiophene in solution (∼1.5 V). At higher potentials, the difference in the amount of charge transferred per cycle corresponds to the areal density of thiophene groups of the 3TUTS SAMs. The error bars that result from multiple experiments are approximately the size of the datum markers.
Figure 2. Charge density, obtained from reductive waves of CV experiments, versus number of cyclic voltammetry sweeps. In (a) the ITO was modified by a SAM of 3TUTS but in (b) it was not. The lines are presented as guides to the eye and the error bars resulting from multiple experiments are within the size of the datum markers. In these experiments the concentration of thiophene in solution was 0.02 M and scan rate was 0.1 V/s.
a few cycles, however, the relationship becomes sublinear, and the deviation from the linear trend is more drastic when the surface is not modified with the 3TUTS. Using electrochemical quartz crystal microgravimetry, Wurm and Kim showed that the amount of poly(N-hexadecylpyrrole) electropolymerized from a pyrrole-capped SAM increased linearly as a few CV scans were performed; however, linear growth of the film was not seen on bare gold electrodes, but deterioration and detachment of the film were noted in this case.17 To gain some additional measure of how the SAM and the incremental changes in maximum applied potential were affecting the growth of the polythiophene films and to compare directly the two sets of data shown in Figure 2, we calculated the average charge transferred per cycle for the films grown by cycling to the various potentials. For all cases, we took the average charge transferred per cycle to be equal to the slope of the line that relates the reductive charge density with cycle number (Figure 2), and the result of recasting the data in this manner is shown in Figure 3. As can be seen in this figure, at potentials less than 1.5 V, growth occurs on the 3TUTSmodified surface, but not on the bare electrode. This
suggests that at low potentials, the dense interfacial layer of 3-substituted thiophenes on the electrode nucleates the electropolymerization, whereas without the thiophenebearing SAM, higher applied potentials are required to commence growth of the film. While other researchers have argued for surface nucleated growth of conductive polymer films from SAMs bearing the pendent monomer units based on secondary characteristics such as the smooth morphology and strong adhesion exhibited by the resulting conductive polymer overlayer,14,21,22,39 our evidence is based on the CV behavior, which directly reflects the electrochemical growth of the film. As the maximum applied potential increases above 1.45 V, the difference in the average amount of charge transferred per cycle between the 3TUTS-modified and bare ITO surfaces remains fairly constant at ∼0.10 mC cm-2 cycle-1 until the maximum applied potential reaches 1.7 V. Because the charge density can be converted to an areal density, this difference in the average charge transferred per cycle can also be thought of as a difference in the amount of thiophene (per unit area per cycle) undergoing reduction. If, as is typically done, a fractional excess charge attributed to the oxidation or reduction of polythiophene of 0.25 equiv/mol is assumed,36,37 then the “extra” 0.10 mC/cm2 transferred per cycle translates into ∼2.5 × 1015 molecules/cm2. This areal density is similar to what we have previously reported as the surface density of thiophene groups in a SAM of 3TUTS on ITO based on the apparent area of the ITO surface.28 This simple calculation also indicates that at the lower potentials where growth was seen on the 3TUTS-modified surfaces only, the amount of thiophene added to the growing film per cycle is on the order of one monolayer worth of material, or less. At this stage it is unclear as to why no difference in charge density between the two types of surfaces is seen at 1.7 V. We suspect that this is due to the roughness of films on the bare ITO: as will be discussed below, the films formed on the bare ITO are much rougher (39) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759-762.
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Figure 4. Two-dimensional AFM topography images of the polythiophene films produced by ten CV sweeps on 3TUTS-modified ITO (a-d) and bare ITO (e and f). In these experiments, the potential was swept from 0 to (a) 1.7 V, (b) 1.62 V, (c) 1.5 V, (d) 1.4 V, (e) 1.5 V, and (f) 1.7 V. In all of the experiments, the concentration of thiophene in solution was 0.02 M and the scan rate was 0.1 V/s.
than those formed on the 3TUTS-modified surfaces, so the open structure of the growing polythiophene film on the bare electrode results in a higher surface area, which is not accounted for in the calculations. The surface topography of the films produced on 3TUTSmodified ITO at the various maximum applied potentials was investigated via atomic force microscopy and the results are displayed in Figure 4a-d. To aid comparison between these four images, the scan size and height scale were held constant. The two-dimensional images show that the polythiophene films are quite homogeneous and growing in sharply defined regions. The root-mean-square (RMS) roughness decreased as the maximum applied potential was decreased: the RMS roughness was measured to be 13, 8, 6, and 5 nm for the sample that was limited to 1.7, 1.62, 1.5, and 1.4 V, respectively. (It should be noted that bare ITO has an RMS roughness of ∼5 nm.) For comparison purposes, topographical images produced from films deposited on bare ITO are shown as Figure 4, parts e and f. These two images show the films produced from 10 CV sweeps to oxidizing potentials of 1.5 and 1.7 V, respectively. These films are considerably rougher than the films grown from the 3TUTS-modified ITO; the RMS roughness values are 25 nm for bare ITO cycled to 1.5 V (Figure 4e) and 174 nm for bare ITO cycled to 1.7 V (Figure 4f). We attribute the difference in roughness to differences in how the films are formed.39,40 Similar to what Willicut and McCarley explained for pyrroles,39 with the thiophenecapped SAM, the polythiophene is formed on a surface of similar character that is rich in nucleation sites, which gives rise to a smoother surface; on the other hand, the bare ITO has a significantly different surface energy than polythiophene or 3TUTS28 and presents no nucleating moieties, so coarser films are formed on these nonwetting (40) Lukkari, J.; Alanko, M.; Pitkanen, V.; Kleemola, K.; Kankare, J. J. Phys. Chem. 1994, 98, 8525-8535.
surfaces. There is one other aspect that is interesting to note when films grown on both types of surfaces by cycling to the same maximum potential are compared: Based on the difference between the reductive charge densities, the amount of polythiophene grown on the two types of surfaces (under the same electrosynthetic conditions) differs by approximately one monolayer worth of material. This point, coupled with the more drastic variations seen in the surface morphology of the polythiophene grown on the bare ITO surfaces, suggests that not only is the surface morphology of the films drastically impacted by the underlying SAM, but the density of the films is also affected; the films grown on the 3TUTS-modified ITO must be more compact. Thus, not only does the 3TUTS nucleate growth, but it also dictates the morphological characters smooth versus rough, dense versus poroussof the film even after several cycles to high oxidizing potentials, where several monolayers worth of thiophene are being added to the film on each sweep. This persistent influence of the underlying SAM, even after it is buried by several layers of deposited polythiophene, is quite remarkable given that the electrochemical oxidative polymerization of thiophene is a very uncontrolled process. We also used ER-FTIR spectroscopy to track the development of structure during the growth of the polythiophene films on the 3TUTS-modified surfaces. Three distinct modes signifying R-R′ coupling between thiophene groups were seen (Figure 5): The CHβ out-ofplane deformation mode was seen at 791-792 cm-1, and although significantly weaker, signals from the νs(CHβ) mode at 3066 cm-1 and νoop(CH) mode at 702 cm-1 were also seen.41,42 As shown in Figure 5, the intensity of the signals from these modes increases as the number of (41) Delabouglise, D.; Garreau, R.; Lemaire, M.; Roncali, J. New J. Chem. 1988, 12, 155-161. (42) Tourillon, G.; Garnier, F. J. Electroanal. Chem. 1982, 135, 173178.
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interfacial topography of the polythiophene film, and consequently tuning the surface energy of the interfacial SAM allows the morphology of the film to be manipulated.31,32 Conclusions
Figure 5. ER-FTIR spectra of polythiophene films on SAMmodified ITO electrodes. The scans, from top to bottom, were taken from films subjected to 10 (X), five (V), and one (I) cyclic voltammetry sweeps from (a) 0 to 1.4 V, and from (b) 0 to 1.62 V.
sweeps was increased. In a fashion analogous to the data presented in Figure 2a, the intensity of the out-of-plane deformation mode at 791 cm-1 increased linearly with sweep number, except at the highest potential, where a sublinear increase was seen after a few scans. As the data suggest, there were no appreciable differences in these signals between the films polymerized at the different maximum potentials; however, as expected, the signals were easier to distinguish at higher maximum potentials because more polythiophene was present on the surface. Also, there is little difference in the ER-FTIR signatures resulting from films produced on the 3TUTS-modified compared with those deposited on bare ITO substrates. This suggests that while the SAMs do not alter the structure of the polythiophene overlayer, the compatibilization and nucleation density afforded by the pendent thienyl groups of the SAM drastically impacts the
By using cyclic voltammetry and FTIR spectroscopy we have tracked the growth of thin polythiophene films on bare ITO substrates and surfaces decorated with a monolayer bearing thiophene groups at various applied maximum potentials. At low oxidizing potentials there is a window where the 3TUTS monolayers nucleate the growth of polythiophene films, but bare ITO surfaces show no growth. This “SAM-nucleated” growth is attributed to the lower oxidation potential of the 3-substituted thiophene rings that decorate the periphery of the SAM. Once a high enough potential is reached so that polythiophene films are growing on both the 3TUTS-modified and bare ITO surfaces, the difference in the average charge transferred per cycle seems to be constant and roughly equivalent to the excess charge required for reduction (or oxidation) of the thienyl groups of the 3TUTS monolayer. In addition to nucleating the growth of the polythiophene film, the 3TUTS monolayer becomes incorporated into the growing film, as evidenced by the robust adhesion of the film to the substrate. Films produced on the 3TUTS-modifed surfaces also exhibit significantly lower roughness and higher density than films deposited on bare ITO surfaces. In total, these results indicate that the molecular-level structure of thin films can be manipulated by the electrosynthetic conditions and the surface-modifying monolayer. Whether such morphological effects translate into improved properties remains to be tested. Acknowledgment. This work was supported in part by the ERC Program of the National Science Foundation under Award Number EEC-9731680 and by an Untenured Faculty Award from 3M. Thanks are also due to Prof. Igor Luzinov for assistance with the AFM work and Prof. Stephen Creager for helpful discussions. LA020397F
