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Langmuir 1990,6, 1164-1172
Langmuir-Blodgett Manipulation of Poly(3-Alkylthiophenes) Itsuo Watanabe,+ Keith Hong, and Michael F. Rubner* Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received August 11, 1989. I n Final Form: January 9, 1990 Langmuir-Blodgett (LB) multilayer thin films were fabricated from mixed monolayers containing stearic acid and various poly(3-alkylthiophenes). It has been found that mixed LB films containing as much as 80 mol % of poly(3-alkylthiophenes) form stable monolayers at the air-water interface that can be deposited onto solid substrates as Y-type films by the vertical lifting method. The LB films exhibit well-definedlayered structures as determined by optical, capacitance, and X-ray diffraction measurements. Multilayer thin films containing highly ordered domains of cadmium stearate can be formed with polythiophenes substituted with alkyl chain lengths ranging from 4 to 18 carbon atoms. The molecular organization and the electrical and optical properties of the films depend on the length of the alkyl chain of the polythiophene backbone. All of these LB films can be rendered electrically conductive by doping with strong oxidizing agents. In some cases, conductivities as high as 2 S/cm were obtained via this doping process.
Introduction The Langmuir-Blodgett (LB) technique continues to receive considerable attention because it offers one of the few means for the preparation of highly ordered organic systems with molecular architectures and thicknesses that are controllable at the molecular level. Although a number of novel electronic based on insulating LB films have been fabricated and evaluated, the true promise of this approach will only be realized when LB materials with a wide range of electrical and optical properties become available. The availability of a diverse inventory of electrically and optically responsive materials will make it possible to fabricate novel thin-film structures with electrical and optical properties that can be tailored a t the molecular level. Indeed, recent theoretical calculationsss6 have indicated that organic LB superlattices fabricated with alternating layers of semiconductive polymers with different work functions can be engineered to create a variety of novel optical and electrical devices. Toward this goal, a number of researchers have recently developed electrically conductive materials suitable for LB film formation. These new materials can be conveniently divided into three general classes: molecular charge transfer salts,'+ surface active ring compounds,lOJ1and t Visiting scientist from Hitachi Chemical, Tsukuba, Japan.
(1) Batey, J.; Roberts, G. G.; Petty, M. C. Thin Solid Films 1983,99, 283. (2) Thomas, N. J.: Petty, M. C.: Roberts, G. G.: Hall, H. Y. Electron. Lett. 1984,20, 838. (3) Lloyd, J. P.; Petty, M. C.; Roberts, G. G.; Lecomber, P. G.; Spear, W. E. Thin Solid Films 1983.99. 297. (4) Vincett, P. S.; Roberta,' G. 'G. Thin Solid Films 1980, 68, 135. (5) Saxena, A.; Gunton, J. D. Synth. Met. 1986, 15,23. (6) Saxena, A,; Gunton, J. D. Synth. Met. 1987,20,185. (7) (a) Ruaudel-Teixier, A.; Vandevyver, M.; Barraud, A. Mol. Cryst. Li9. Cryst. 1985, 120, 319. (b) Barraud, A,; Lesieur, P.; Ruaudel-Teixier, A.; Vandevyver, M. Thin Solid Films 1985,134,195. (c) Vandevyer,
M.; Richard, J.; Barraud, A.; Ruaudel-Teixier, A.; Lequan, M.; Lequan, R. M. J. Chem. Phys. 1987,87,6754. (8) (a) Nakamura, T.; Matsumoto, M.; Takei, F.; Sekiguchi, T.; Manda, E.; Kawabata, Y . Chem. Lett. 1986, 1986, 709. (b) Kawabata, Y.; Matsumoto, M.; Tanaka, M.; Sekiguchi, T. Koizumi, H.; Manda, E.; Nakamura, T.; Saito, G. Synth. Met. 1987,19,663. (c) Ikegami, K.; Kuroda, S.; Sugi, M.; Saito, M.; Iizima, S.; Nakamura, T.; Matsumoto, M.; Kawabata, Y.; Saito, G. Synth. Met. 1987, 19, 669.
conjugated polymers.l2-'6 Polymer-based materials are particularly attractive due to the high level of mechanical and thermal stability exhibited by their multilayers. In addition, the recent d i s c ~ v e r y l of ~ -soluble ~~ conducting polymers allows for the first time an opportunity to fabricate LB multilayer films from a wide variety of electrically conductive polymers. Thus, it is in principle now possible to control the molecular and supermolecular organization of electroactive polymers. The problem one encounters, however, is that most of the soluble conjugated polymers of interest do not form true monolayers at the air-water interface. For example, the soluble alkyl-substituted polythiophenes can only be manipulated into LB films by using a horizontal film lifting technique.'6 The quality and uniformity of the resultant films are also difficult to control. In this paper, we show that it is possible to fabricate high-quality LB films from a variety of soluble alkyl-substituted polythiophenes by simply dispersing the polymer with suitable portions of stearic acid. This technique is generally applicable to any soluble conjugated polymer system, (9) Dhindsa, A. S.; Bryce, M. R.; Lloyd, J. P.; Petty, M. C. Synth. Met. 1987,22, 185. (10) Fuiiki. M.: Tabei. H. Lonrrmuir 1988. 4. 320. (11) Baker; S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid -Films .. .. 1983. . - - -, 99. - - , 5.1. - -. (12) Shimidzu, T.; Iyoda, T.; Ando, M.; Ohtani, A.; Kaneko, T,; Honda, K. Thin Solid Films 1988,160,67. (13) (a) Hong, K.; Rubner, M. F. Thin Solid Films 1988, 160, 187. (b)Hong, K.; Rubner, M. F. Thin Solid Films 1989,179, 215. (c) Hong, K.; Rosner, R. B.; Rubner, M. F. Chem. Materials 1990,2,82. (14) (a) Watanabe, I.; Hong, K.; Rubner, M. F.; Loh, I. H. Synth. Met. 1989, 28, C473. (b) Watanabe, I.; Hong, K.; Rubner, J. Chem. Soc., Chem. Commun. 1989, 123. (c) Watanabe, I.; Hong, K.; Rubner, M. F. Thin Solid Films 1989, 179, 199. (15) Nishikata, Y.; Kakimoto, M.; Imai, Y. J. Chem. Soc., Chem. Commun. 1988,1040. (16) Logsdon, P. B.; Jiripfleger; Prasad, P. N. Synth. Met. 1988,26, 369
(17) Sugimoto, R.; Takeda, S.; Gu, H. B.; Yoshino, K. Chem. Express 1986, 1, 635. (18) Elsenbaumer, R. L.; Jen, K. Y.; Oboodi, R. Synth. Met. 1986, 15, 169. (19) Sato, M.; Tanaka, S.; Kaeriyama, K. J. Chem. Soc., Chem. Commun. 1986,87. (20) Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F. Macromolecles 1987,20, 212.
0743-7463/90/2406-ll64$02.50/0 0 1990 American Chemical Society
LB Manipulation of Poly(3-alkylthiophenes)
Langmuir, Vol. 6, No. 6,1990 1165
Figure 1. Chemical structure of the poly(3-alkylthiophenes). Table I. Molecular Weights and Polydispersities for PIAT polymer polymerization procedure Mn Mw Mw/Mn ferric chloride P-BT 11400 73700 6.47 P-BT diiodo coupling 11400 24900 2.18 P-HT ferric chloride 18000 75700 4.21 P-OT ferric chloride 10000 37800 3.78 P-ODT ferric chloride 10100 29700 2.93 P-ODT diiodo coupling 6600 9500 1.44
thereby opening the door to the thin-film processing of electroactive polymers.
Experimental Section The 3-alkylthiophene monomers 3-butyl-, 3-hexyl-, 3-octyl-, and 3-octadecylthiophene were synthesized via the coupling of appropriate alkylmagnesium bromides with 3-bromothiophene in the presence of Ni(dPPP)C12 (dPPP = PhzP(CH2)3PPh2) following the procedure of Tamao et a1.21 Soluble poly(3-alkylthiophenes) (P3AT) (see Figure 1) such as poly(3-butylthiophene) (P-BT), poly(3-hexylthiophene) (P-HT), poly(3-octylthiophene) (P-OT), and poly(3-octadecylthiophene) (P-ODT) were synthesized according to the methods of Sugimoto et al.17 or Elsenbaumer et al.18 In the former procedure, P-BT, P-HT, P-OT, and P-ODT were chemically synthesized in chloroform solutions containing FeCl3. In the latter procedure, 3-butyland 3-octadecylthiophene were iodinated to create substituted 2,5-diiodothiophenes, which were subsequently converted into polymer by Grignard coupling in the presence of Ni(dPPP)ClZ. All polymers were purified by Soxhlet extractions with methanol and acetone for 1 week, respectively. The P3AT polymers prepared by use of the diiodo coupling method were consistently lower in molecular weight than their counterparts prepared by use of FeCl3. Table I displays the molecular weights of all of the polymers evaluated in this study. These values were determined by gel permeation chromatography (GPC) using tetrahydrofuran as a carrier solvent and are therefore presented as polystyrene equivalents. It should also be mentioned that the polymer chains prepared via the diiodo coupling method are terminated with iodine atoms (both ends) as a natural consequence of the coupling chemistry used to create these materials. Despite these differences, the general trends observed for these materials were essentially independent of preparation technique. Thus, the majority of this paper will focus on the formation of LB films from P3AT prepared via the FeCl3 method. Comments about differences in the behavior of these materials attributable to the preparation technique will noted in the text. Stearic acid (Fluka Chemical) was purified by recrystallization from acetone. The water used as subphase was purified with a Milli-Q purification system (Millipore Corp.). Cadmium chloride (Fluka Chemical) was used without further purification. HPLC grade chloroform (Aldrich) was used for the preparation of spreading solutions. Monolayers were spread from chloroform solutions (typical concentration ca. 1 mg of total solute/mL) onto a purified aqueous subphase containing 2 x 10-4 M CdC12. The surface pressure-area isotherms were measured on a Lauda film balance a t 20 "C with a compression speed of 5 A2 molecule-' min-1. The multilayers were built up by the vertical dipping method at 15-28 mN/m and 20 "C. All of the mixed monolayers exhibited excellent stability against (21) Tamao, K.; Kodama, S.; Nakajima, I.; Kumada, M.; Minato, M.; Suzuki, K. Tetrahedron 1982,38, 3347.
Area Per Molecule (A2)
Figure 2. Pressure-area isotherms of mixed monolayers containing P3AT and SA in different molar ratios. collapse as indicated by the ability of the film to maintain ita area for days at a given constant pressure (decreases in area due to expected annealing effects were observed). A dipping speed of 5 mm/min was used for the first dip and was increased to 10 mm/min for subsequent dips. Drying times of at least 2 h were used between the first and second dips. This time was reduced to 15 min for all subsequent dips. Transfer ratios close to unity were observed for all monolayers. The mixed LB films were built onto hydrophobic glass slides (created by treatment with 1,1,1,3,3,3-hexamethyldisilazane) for optical absorption and X-ray diffraction measurements. Visible and near-infrared absorption spectra of the multilayers were measured with either a Oriel Instaspec System 250 multichannel spectrophotometer or a Caryl7D (Varian Associates) spectrophotometer. Low-angle X-ray diffraction measurements of the multilayers recorded with a 28 ranging from 3O to 16" were performed on a Rigaku Rotaflex RU300 unit with a copper K a target. Capacitance measurements were made on Ag/LB f i / A l sandwich structures created by first depositing a variablethickness (stepped LB film) film onto a Ag-coated glass slide and then vacuum-evaporating A1 top electrodes having active areas of 0.1 and 0.2 cm2 onto the LB film. Comparison of equalthickness capacitors created with two different top electrode sizes made it possible to better evaluate the quality of the devices. In all cases, the capacitance of the multilayer thin films was found to scale directly with the size of the top electrode. The electrode configuration utilized was similar to that described by Geddes.22 Capacitance was measured with an HP Model 4275A impedance analyzer operating at a frequency of 100 kHz at room temperature. The P3AT mixed LB films were chemically doped with NOPFe in acetonitrile solutions (0.5-2 mg/mL) for 10 min at 20 OC. In-plane conductivities were measured by using the standard four-probe technique23 at room temperature utilizing the van der Pauw configuration. Film thicknesses were estimated with a surface profilometer (Dektak) on multilayer films comprised of at least 50 layers, which allowed determination of the average thickness per monolayer to an accuracy of about i 5 A. In some cases, the films were overcoated with a uniform layer of aluminium and subsequently measured to prevent penetration of the probe into the soft LB films.
Results The pressure-area isotherms of monolayers formed from mixtures of various P3ATs and stearic acid in different molar ratios (based on the molecular weight of the polythiophene repeat unit) are presented in Figure 2. These isotherms were recorded at 20 "C and are of polymer prepared by the ferric chloride procedure. In order to determine the effect that the added polymer has on the iso(22) Geddes, N. G.; Parker, W. J.; Sambles, J. R.; Jarvis, D. J.; Couch, N . R. Thin Solid Films 1989, 168, 151. (23) Van der Pauw,L. J . Philips Res. Rep. 1958, 13 (l), 1.
Watanabe et al.
1166 Langmuir, Vol. 6, No. 6, 1990
therm of cadmium stearate, the area per molecule axis is based solely on the number of stearic acid molecules spread onto the subphase. For comparison, the pressurearea isotherm of a pure cadmium stearate monolayer recorded under similar conditions is shown with the curves of the poly(3-butylthiophene) system. Figure 2 clearly demonstrates that condensed monolayers can be formed from a variety of P3AT/SA mixtures. Monolayers containing as much as 80 mol % of the P3AT component form stable (to film collapse a t constant pressure), condensed phases that can be readily transferred into Y-type multilayer thin films by using the vertical lifting method. With the exception of the P-ODT/SA 0 system, all monolayers display a distinct phase transi0 20 40 60 80 100 Mole '4 o f P 3 A T tion a t elevated pressures (between 20 and 40 mN/m), indicating that a molecular reorganization is taking place Figure 3. Limiting area per molecule as a function of the mole percent of P3AT for the mixed monolayers of P3AT and SA. within the monolayer. For these latter materials, the surOpen circles represent P-HT, open squares represent P-BT,clwd face pressure initially increases with decreasing area until squares represent P-OT, and closed circles represent P-ODT. a critical transition pressure is reached, a t which point the surface pressure remains essentially constant (or rises tigation. For P-BT, P-HT, and P-OT, the area per molmuch less steeply) with decreasing area. The pressure ecule directly estimated from a monolayer of the pure only begins to rise again (very steeply) when the area polymer (around 5 Az/molecule) is very close to the value per molecule approaches that of a pure cadmium stearobtained by linear extrapolation of the data generated ate monolayer. These results indicate that, at a critical from the mixed monolayers and a monolayer of pure cadsurface pressure, the P3AT molecules are rejected from mium stearate. The limiting area per molecule of these a mixed monolayer phase, creating a multilayer organimixed monolayers therefore follows a simple additivity zation comprised of randomly oriented P3AT molecules rule with P-HT displaying a slight positive deviation from stacked on top of (or less likely underneath) a well-orideal additivity and P-OT and P-BT a slight negative dered condensed phase of cadmium stearate. In gendeviation. Thus, the average area per molecule occueral, the pressure a t which the polymer molecules are pied by the repeat units of the polymer chain is essenexcluded from the cadmium stearate monolayer increases tially independent of the monolayer composition. The as the length of the alkyl chain of the polythiophene slight variations from ideal additivity most likely simply decreases. It has also been found that the lower molereflect the difficulty of obtaining accurate values for the cule weight polymers prepared by using the diiodo coulimiting area of a monolayer of the pure polymer. The pling technique are rejected from the mixed monolayer additive nature of the limiting areas indicates that the a t higher surface pressures than their higher molecular P3AT and stearic acid molecules are not miscible a t the weight counterparts. I t is not clear, however, whether molecular level but rather form separate domains within this is a true molecular weight effect or whether it is related the monolayer^.^^ In addition, the small area per moleto the fact that the lower molecular weight polymer chains cule contributed by the P3AT repeat units (around 5 are terminated with bulky iodine atoms which interact A2/molecule) clearly demonstrates t h a t the alkylwith the subphase more strongly due to their more polar substituted polythiophenes do not form true condensed nature. The latter effect is most likely dominant. monolayers at the air-water interface as only a small portion of the actual chain is in contact with the water surIn contrast to the mixed monolayers created from subface. In the case of the P-ODT/SA mixed system, the stituted polythiophene chains containing alkyl groups with limiting area per molecule occupied by the polymer repeat eight or fewer carbon atoms, the P-ODT/SA mixed monounits as determined by the interce t of the linear region layers show no indication of a phase transition at eleof the curve in Figure 3 (about 7 Hf)z/molecule)is signifvated surface pressures. Apparently, there is a strong icantly lower than that actually measured on the pure enough interaction between the long alkyl chains penpolymer (about 10 A2/molecule). The limiting area per dent to the conjugated polythiophene backbones and the molecule of the mixed monolayers of P-ODT therefore hydrocarbon tails of the cadmium stearate molecules to shows a large negative deviation from ideal behavior in prevent rejection of the polymer molecules at higher presthe high P-ODT content regime. This suggests a higher sures. Alternatively, the higher degree of molecular cohedegree of molecular mixing within this system as comsion present between the hydrocarbon chains of the polypared to the mixed monolayers created from the other thiophene molecules could make domains of these molpolymers. Partial miscibility of the two components within ecules too rigid to be squeezed from the cadmium stearate the mixed monolayer would be expected to modify the phase. The strong interchain interactions of these longer molecular organization of either the polymer or cadhydrocarbon tails could also drive side-chain crystallization, again producing more rigid polythiophene domains. mium stearate phase. As mentioned earlier, all of the mixed P3AT systems Figure 3 displays the limiting area per molecule as a can be easily transferred by using the vertical dipping function of the mole percent of P3AT for the mixed monomethod onto a variety of solid substrates as Y-type mullayers of P3AT and stearic acid. The average area occutilayer films. The exception to this is the 5/1 mole ratio pied per molecule was estimated from the isotherm curves P-ODT/SA mixed monolayer which could not be transby extrapolating the steepest region before the transiferred via the vertical lifting method due to its excestion to zero surface pressure. In this case, the area per sively rigid nature. It is interesting to note, however, that molecule axis was based on the average molecular weight of the P3AT repeat unit and stearic acid, (MP~AT + MsA)/~. (24) (a) Murakata, T.; Miyashita, T.; Matsuda, M. Macromolecules As can be seen, a linear relationship is observed for all 1988,21, 2730. (b) Murakata, T.; Miyashita, T.; Matsuda, M. Macromixed systems over the composition range under invesmolecules 1989, 22, 2706. c
Langmuir, Vol. 6, No. 6, 1990 1167
LB Manipulation of Poly(3-alkylthiophenes) 0.4
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a 5/1 mole ratio P-ODT/SA mixed monolayer created from the lower molecular weight polymer synthesized by the diiodo coupling method could be transferred reproducibly by the vertical lifting technique. Apparently, the mixed monolayer formed from the lower molecular weight polymer is still flexible enough to allow reproducible transfer. There are a number of established methodologies that can be utilized to verify the reproducibility of the transfer process. The majority of these methods simply measure a specific property of the monolayer as a function of the number of layers transferred onto a substrate. A linear relationship between the magnitude of the property of interest and the number of layers present in the film usually indicates that each monolayer contributes, on the average, an equal amount to the overall property being measured. Optical absorbance and capacitance measurements were used in this study to assess the overall quality of the transfer process. Figure 4 shows the visible absorption spectra obtained from multilayer thin films comprised of 68 layers of the various mixed monolayers containing a 1/1 molar ratio of P3AT/SA. These spectra are nearly identical with those obtained from solvent-cast films prepared from the * pure P3ATs. The onset wavelength of the ~ - r transition of the polythiophene conjugated backbone determined from these spectra is about 650 nm (2eV), which is typical for the polythiophene~.~~ The wavelength of maximum optical absorbance for this interband transition, however, depends on the particular substituted polythiophene present in the film with values ranging from about 440 to 550 nm. A plot of the optical absorbance of multilayer films with different polythiophene compositions (at the wavelength of maximum absorbance) as a function of the number of layers transferred is presented in Figure 5 for the P-HT/SA system. Similar linear plots were generated for all of the mixed monolayers transferred by the vertical dipping method. Thus, optical absorbance measurements indicate that all of the mixed P3ATISA monolayers are reproducibly and uniformly transferred into multilayer thin films. The same conclusion could also be made from capacitance measurements. Plots of the reciprocal capacitance (l/C) versus the number of layers deposited for a variety of mixed P3AT/SA LB films all exhibited linear relationships. For example, Figure 6 shows the recipro(25) Chung, T. C.; Kaufman, J. H.; Heeger, A. J.; Wudl, F. Phys. Reu. B 1984, 30,702.
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Figure 5. Relationship between absorbance (at wavelength of maximum absorbance) and the number of deposited layers in LB films containing P-HT and SA in different molar ratios. Open squares represent a 1/1 mole ratio, closed circles represent a 2/1 mole ratio, and open circles represent a 5/1 mole ratio. Monolayer films were transferred at 20 mN/m.
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Figure 6. Plots of the reciprocal capacitance (l/C) versus the number of layers for multilayers fabricated from P-BT/SA mixed monolayers containing 1 / 2 (open circles) and 2 / 1 (closed circles) molar ratios. Monolayer fiis were transferred at 20 mN/m.
cal capacitance plots for multilayers fabricated from P-BT/SA mixed monolayers containing either 33 or 66 mol % P-BT. The linear relationship confirms that the transfer process is completely reproducible up to at least 50 layers. By use of the thickness values generated via profilometer measurements (25 A/monolayer for the 33 mol % film and 32 A/monolayer for the 66 mol ?6 film), it is possible to estimate the dielectric constant of the mixed LB films from the slopes of these plots.22 Thus, the dielectric constants of the P-BT mixed LB films with molar ratios of 1 / 2 and 2 / 1 (P-BT/SA) were estimated to be 3.2 and 3.5, respectively. These values are in the range expected for two-component films containing a low dielectric constant material (the dielectric constant of a pure cadmium stearate LB film is about 2.7)26 mixed with a higher dielectric constant material (the dielectric constant of a free standing film of P-HT, for example, was found to be about 8.8), although the higher polythiophene content films display dielectric constants that are lower than that expected for a simple two-phase heterogeneous system.27 Low-angle X-ray diffraction was used to probe the bilayer structures of the multilayer thin films fabricated (26) Sugi, M.; Fukui, T.; Iizima, S. Mol. Cryst. Liq. Cryst. 1979, 50,
183.
(27) Electrical Transport and Optical Properties of Inhomogeneou Media; Garland, J. C., Tanner, D. B., Eds; American Institute of Physics Conference Proceedings, No. 40., New York, 1978.
1168 Langmuir, Vol. 6, No. 6, 1990
Watanabe et al.
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Figure 7. Low-angle X-ray scattering curves of LB films (34 layers) containg P3AT and SA in different molar ratios. Monolayer films were transferred at 20 mN/m. from the mixed P3AT/SA monolayers. Figure 7 shows the low-angle X-ray scattering curves obtained from multilayer films (34 layers) of mixtures of various P3ATs and SA. For the multilayer films fabricated from P-BT, P-HT, and P-OT, a set of at least seven (001) Bragg reflections is readily observed. The diffraction patterns of the P-ODT/SA multilayers also display a number of clearly defined (001)reflections, although they are weaker in intensity and significantly broader. With the exception of the P-HT/SA system, all multilayer structures show evidence of increased disorder and/or decreased domain size (decreased peak intensities and increased peak widths) when the mole ratio of P3AT to stearic acid is increased to 5/1. The diffraction patterns generated from multilayers of the P-HT/SA system, on the other hand, remain remarkably similar over the entire composition range. The X-ray data indicate that the LB films fabricated from PSAT/ SA mixed monolayers exhibit well-defined multilayer structures. The bilayer d-spacings calculated from these data are all about 50 8, irrespective of the composition of the mixed LB film and the transfer pressure. This value is the same bilayer repeat distance calculated for a LB multilayer film of pure cadmium stearate prepared under the same conditions as the mixed films. The overall diffraction patterns of the mixed LB films were also identical with that obtained from a pure cadmium stearate LB film. These results strongly suggest that the origin of the X-ray patterns in the mixed LB films is diffraction from cadmium planes located within highly ordered molecular stacks of cadmium stearate molecules. Thus, a two-phase microstructure consisting of well-ordered domains of cadmium stearate molecules and relatively disordered polythiophene molecules is clearly indicated. The P3AT/SA mixed LB films are normally insulating; however, they can be converted into electrically conductive films by doping with chemical oxidants such as NOPFs. Oxidation of the polythiophene molecules within the multilayer films dramatically changes the optical prop-
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Figure 9. In-plane conductivity of LB films (34 layers) doped with NOPF6 as a function of the mole percent of P3AT in the films. Open circles represent P-HT,open squares repreaent P-BT, closed squares represent P-OT, and closed circles represent P-ODT. Monolayer films were transferred at 20 mN/m. erties of the films. For example, Figure 8 shows visiblenear-infrared absorption spectra of PBAT/SA mixed LB films (2/1 mole ratio) recorded after exposure to acetonitrile solutions containing NOPFa. In all films, the strong interband transition of the neutral conjugated backbone decreases dramatically after doping and two new absorption bands appear at about 800 and 1800 nm. These latter two bands correspond to the formation of bipolaron defect states along the polymer backbone, as is well documented in the literature.28 These same features also appear in the spectra of solvent-cast films of the pure P3ATs after doping. Figure 9 shows the in-plane conductivity of LB films (containing 34 layers) doped with NOPFe as a function of the mole percent of P3AT in the films. As expected, the conductivity increases as the amount of P3AT in the film increases with a maximum conductivity of about 2 S/cm being obtained in the mixed LB film of P-HT/SA with a molar ratio of 5/1. In each case, the in-plane conductivity approaches the value obtained from a pure freestanding film of the respective NOPFG-doped P3AT as the polymer content in the film approaches 100%. The electrical conductivities of the doped LB films were found to decay by about 1 order of magnitude every 10-20 days (28) Handbook of Conducting Polymers; Skotheim, T. J.; Ed.; Marcel Dekker: New York, 1986; Vols. l and 2.
Langmuir, Vol. 6, NO.6,1990 1169
LB Manipulation of Poly(3-alkylthiophenes) after exposure to atmospheric conditions, with the exact rate of decay being highly dependent on the water content of the film before doping and the level of atmospheric humidity. Doping with NOPFs from solution apparently disrupts the layered stacking of the cadmium stearate molecules, as no diffraction peaks were observed after doping. Similar results were obtained when pure LB films of cadmium stearate were exposed to the doping solution, indicating that the oxidizing agent penetrates and disrupts the hydrophilic planes of the bilayer structure. It has been found, however, that dopants introduced into the mixed LB films via the gas phase modify, but do not destroy, the layered structure of the LB film. The details of this work as well as a complete description of the electrical and optical properties of mixed LB films oxidized with different doping agents will be presented in a separate publication.
Discussion The manipulation of surface-active polymer molecules via the Langmuir-Blodgett technique has recently emerged as a viable means to control the thickness and molecular architecture of ultrathin polymer films. One of the main obstacles encountered with this approach, however, is the high degree of film rigidity usually associated with monolayers of high molecular weight polymers, which tends to make the transfer of these materials into LB films very difficult. Indeed, often times only the horizontal lifting technique can be used to prepare films with many molecular layers. This, in turn, limits the flexibility of the transfer process and makes it difficult to obtain specific molecular organizations that would normally be readily fabricated with the vertical lifting method. The extension of this approach to non-surfaceactive polymers is a particularly interesting challenge. In this case, the polymer molecules tend to aggregate at the air-water interface forming unstable clusters or islands of randomly stacked polymer molecules which are extremely difficult to transfer into multilayer structures. The alkyl-substituted polythiophenes fall into this latter category, as they are structurally devoid of any groups sufficiently hydrophilic to render them surface active. When spread onto the air-water interface of a LB trough, they simply coalesce into poorly ordered multilayer islands that are sometimes large enough to be observed with the unaided eye. This is true even for polythiophenes fitted with hydrocarbon tails that contain up to 18 carbon atoms. Such surface films tend to be unstable at high surface pressures and can only be transferred into multilayer thin films via the horizontal lifting method. The quality of these films, as judged by their optical uniformity, is also very poor. In short, by themselves, these materials are not well suited for manipulation by the LB technique. By mixing these materials with suitable portions of stearic acid, however, we can create surface films that are flexible and stable enough to be transferred into multilayer LB films by using the vertical lifting technique. The stearic acid in this case simply disperses the polymer into much smaller domains that become uniformly distributed throughout the cadmium stearate monolayer. In addition, the cadmium stearate molecules effectively isolate the polymer molecules from each other, thereby preventing them from aggregatinginto larger clusters during film manipulation. For dilute solutions in which the polymer concentrations in the spreading solutions are below the critical concentration for overlap, i.e., the polymer molecules in solution essentially exist as iso-
lated polymer coils, it is highly probable that the mixed monolayers formed at the air-water interface from low P3AT/SA mole ratios are comprised of single, isolated polymer molecules dispersed throughout a cadmium stearate matrix. As the concentration of the polymer molecules in the mixed monolayer increases, however, individual polymer chains in contact with each other will be capable of coalescing into larger aggregates within the film. In any event, it is clear that the cadmium stearate molecules greatly enhance the transferability of polymeric materials that would otherwise be difficult to manipulate with the LB technique. Considering first the mixed monolayers formed from P-BT, P-HT, and P-OT, it can be seen from Figure 2 that the two-phase mixed monolayer state is not stable a t high surface pressures. The polymer domains are rejected from the monolayer, thereby allowing the cadmium stearate molecules to maximize their own intermolecular interactions. The pressure at which the polymer molecules are squeezed out appears to depend on the flexibility of the polymer chain. As the alkyl chain length of the polythiophene side group increases from butyl to hexyl to octyl, the interchain interactions of the backbone become more shielded and the polymer chain becomes more flexible. The more rigid domains of the poly(3butylthiophene) molecules therefore require more energy to reject than the more flexible domains of the poly(3octylthiophene) molecules. In all cases, the alkyl groups of the polythiophene molecules are too short to promote any significant intermolecular interactions with the cadmium stearate molecules. For the P-ODT/SA system, the situation is somewhat different. The hydrocarbons chains pendent to the polythiophene backbone are now long enough to develop relatively strong intermolecular forces between both the cadmium stearate hydrocarbon tails and the hydrocarbon tails of neighboring polymer molecules or segments. In fact, it is possible that at high P-ODT/SA mole ratios the cadmium stearate molecules may be molecularly dispersed with the polymer molecules. NEXAFS experim e n t have ~ ~ ~recently shown that there is a higher level of molecular orientation of the hydrocarbon chains in LB multilayer films fabricated from the P-ODT/SA system as compared to the other mixed multilayer films. This implies that some preferred orientation of the hydrocarbon groups of the polythiophene chains exists within the films in addition to the expected orientation of the cadmium stearate molecules. The net result of all of these effects is that the P-ODT/SA system retains a mixed monolayer organization throughout the full range of surface pressures. The squeezing out phenomenon observed for the P-BT, P-HT, and P-OT-based systems implies that multilayer thin films fabricated from surface films transferred at high pressures should exhibit different structures than those transferred at lower pressures (below the transition). Specifically, if the two-layer structure created at high surface pressures at the air-water interface were retained in the multilayer, then the final multilayer structure should consist of layers of polymer molecules sandwiched between bilayers of cadmium stearate molecules. In addition, the distance between the molecular planes ~~
~~~~
(29) (a) Skotheim, T. A.; Yang, X. Q.;Chen, J.; Hale, P. D.; Inagaki, T.; Samuelson, L.; Tripathy, S.;Hong, K.; Rubner, M. F.; denBoer, M. L.; Okamoto, Y. Synth. Met. 1989,28, C229. (b) Yang, X. Q.;Chen, J.; Hale, P. D.; Inagaki, T.; Skotheim, T. A,; Okamoto, Y.; Samuelson, L.; Tripathy, S.;Hong, K.; Watanabe, I.; Rubner, M. F.; denBoer, M. L. Langmuir 1989, 5, 1288. (c) Skotheim, T. A,; Yang, X. Q.; Chen, J.; Inagaki, T.; den Boer, M.; Tripathy, S.;Samuelson, L.; Rubner, M. F.; Hong, K.; Watanabe, I.; Okamoto, Y. Thin Solid Films, in press.
1170 Langmuir, Vol. 6, No. 6, 1990 of the cadmium stearate molecules should increase as the amount of polymer in the mixed monolayers increases. As mentioned in the Results section, low-angle X-ray diffraction analysis of the multilayer thin films revealed that the bilayer repeat distance for all film compositions was the same as that of a LB film of pure cadmium stearate. Identical diffraction patterns and similar bilayer repeat distances were also observed when surface films with different compositions were transferred a t pressures above the transition. These results indicate that the bilayer structure is only maintained under high surface pressure at the air-water interface and relaxes back to a twophase mixed monolayer during fiim transfer. Thus, regardless of the transfer pressure, the mixed monolayers reorganize during film transfer to form a multilayer structure comprised of randomly oriented polythiophene molecules dispersed throughout a matrix of well-ordered molecular stacks of cadmium stearate molecules. Although in all cases the cadmium stearate molecules manage to organize within the multilayer films in their usual bilayer stacking modification, the pressure at which the mixed monolayers are transferred does influence the structure and properties of the film. Specifically,the optical density per monolayer and average thickness per monclayer were found to be highly dependent on transfer pressure. For example, mixed monolayers that contained a mole ratio of P-HT/SA of 2 / 1 that were transferred at pressures above the transition observed in the isotherms (28 mN/m) consistently displayed an absorbance that was about 1.4 times as large as the absorbance of films prepared a t pressures below the transition (20 mN/m). This difference in optical density was found to be less dramatic in films fabricated from mixed monolayers with lower polythiophene contents and even more dramatic when mixed monolayers with higher polythiophene contents were transferred into multilayer films. In addition, the average thickness per layer of the 2/1 multilayer film prepared a t higher pressures (about 44 A/layer) is about 1.4 times as large as the film prepared a t lower pressures (about 32 &layer). Clearly, multilayer films fabricated a t high pressures from the two-layer surface films exhibit a larger number of polythiophene molecules per unit area than those fabricated a t lower pressures and hence a different molecular organization. It therefore appears that the two-layer structure formed a t high surface pressures is disrupted during the transfer process, thereby allowing the cadmium stearate molecules to reorganize into well-defined molecular stacks. This reorganization, however, does not allow the molecules to re-form the same type of molecular morphology that existed at low pressures, particularly when high polythiophene content monolayers are transferred. The polymer molecules that are rejected from the mixed monolayer and forced closer together at higher surface pressures are reinserted into the multilayer thin film with a domain density (number of polymer domains per unit volume) that is higher than that obtained from mixed films transferred at lower pressures. It is also possible that a number of the polymer molecules aggregate into clusters at high surface pressures, resulting in larger size polymer domains that are not completely reinserted into the multilayer film. This latter phenomenon would be most prevalent in the higher polythiophene content LB films. The result of all of these effects would be a thicker film with a higher optical density as compared to one fabricated a t lower transfer pressures. The important point, nevertheless, is that the cadmium stearate molecules manage to establish well-ordered molecular stacks within the multilayer regardless of the transfer pressure.
Watanabe et al.
Figure 10. Schematic model for a multilayer film containing P3AT and SA with a low P3AT content. The LB film in this case is shown to be deposited on a hydrophilic substrate.
As indicated from the above discussion, the LB films formed from the P-BT-, P-HT-, and P-OT-based mixed monolayers are comprised of domains of polythiophene molecules uniformly dispersed throughout a matrix of well-ordered molecular stacks of cadmium stearate molecules. For these multilayer thin films, the average thickness per monolayer is essentially controlled by the cadmium stearate molecules up to mole ratios of P3AT/SA of about 1/1. In this composition range, the average thickness per monolayer as determined by surface profilometer measurements is very close to the value estimated from bilayer d-spacings extracted from X-ray measurements (about 25 A/layer). An exploded view of a schematic representation of the mixed LB films in this composition range is shown in Figure 10. Near-edge X-ray absorption fine structure analysis29 (NEXAFS) has confirmed that the hydrocarbon tails of the cadmium stearate molecules are highly oriented within the multilayer films whereas the poly(3-alkyl thiophene) chains are randomly distributed. In addition, preliminary reflection/ absorption FTIR measurement~~o using films fabricated from deuterated stearic acid molecules clearly show that the cadmium stearate molecules are oriented nearly normal to the substrate direction and the hydrocarbon tails of the polythiophene chains are randomly arranged in the films. This is in contrast to films formed from the P-ODT/SA system, in which the hydrocarbon tails of the polymer are preferentially aligned within the multilayer film. As the polythiophene content in the LB films increases above a mole ratio of 1/1, the average thickness per layer increases, reaching a upper value of about 50 A/layer for films containing a 5/1 mole ratio of P3AT/SA. The thickness per layer of select LB films as determined by Dektak measurements is displayed in Table 11. This increase in layer thickness represents a transition from a cadmium stearate dominated molecular organization to a polythiophene-dominated matrix. The films containing a high mole ratio of P3AT/SA are best viewed as a continuous or near-continuous matrix of polythiophene molecules throughout which are dispersed cadmium stearate domains. The ability of the cadmium stearate molecules to form well-ordered domains of sufficient size to effectively diffract X-rays in this composition range appears to be most pronounced in the P-HT-based system, which displays (30) Watanabe, I.; Rubner, M. F., results to be published.
Langmuir, Vol. 6, No. 6,1990 1171
LB Manipulation of Poly(3-alkylthiophenes) Table 11. Thickness. per Layer of the P3AT/SA Mixed LB Films P-BT/SA P-HT/SA P-ODT/SA mole mole mole ratio thickness, 8, ratio thickness, 8, ratio thickness, 8, 1/2b l/lb 2flb
25 25 32
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27 25 32 44 44
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strong X-ray diffraction patterns over its entire composition range. The change from a cadmium stearate dominated matrix to a polythiophene-dominatedmatrix with increasing polythiophene content is also indicated by optical absorption measurements. Figure 11shows the maximum absorbance of the polythiophene interband transition as a function of the mole percent of polythiophene in the mixed film (films were transferred at low surface pressures). Note that in some cases the optical absorbance appears to go t o a zero value a t compositions above 0 % polythiophene. This is due to a contribution to the absorbance resulting from scattering from the cadmium stearate domains. A pure LB film of cadmium stearate exhibits a translucent appearance due to scattering from large crystalline domains within the film. The optical absorbance due to scattering in the wavelength region of the polythiophene interband transition (400-500 nm) is about 0.2. When mixed films are fabricated with the P3ATs, the cadmium stearate domains formed during film compression are smaller in size than they would be in a pure LB film of cadmium stearate and therefore scatter less light. In addition, as the polythiophene content in the film increases, the cadmium stearate domains become smaller and the films appear essentially transparent (albeit highly colored). Thus, the measured optical absorbance is due to a contribution from scattering, which is decreasing with increasing polymer content, and a contribution due to the polythiophene interband transition, which is increasing with polymer content. The net result in some cases is a curve that suggests zero absorbance at nonzero polythiophene contents. If it is assumed that the changing contribution due to scattering is insignificant for the compositions evalu-
ated in this study, then it is possible to ascertain how the contribution due to polythiophene absorption varies with polymer content. For compositions below about 50 mol 96, the absorption appears to increase linearly with increasing P3AT content; a result that would have been anticipated from the a-A isotherm data. Above this level, however, the visible absorbance of the film increases in a nonlinear fashion with increasing P3AT content, suggesting that the increase in thickness per monolayer that occurs at higher polythiophene content is accompanied by a fundamental change in the molecular organization of the film. Thus, although a linear relationship was found between the average area occupied per molecule and the mixed monolayer composition over the entire composition range, it appears that a significant reorganization of the structure in the high polythiophene content mixed monolayers occurs during film transfer. The driving force for this reorganization is clearly the formation of molecular stacks of cadmium stearate molecules. As indicated by X-ray results, the creation of separate domains of well-ordered cadmium stearate molecules from high polymer content mixed monolayers is most favorable in the P-HT/SA system and least favorable in the P-ODT/SA system. In this latter case, there is a strong enough interaction between the hydrocarbon chains of the polymer and the cadmium stearate molecules to inhibit the formation of a well-defined phase of cadmium stearate. Given that the mixed LB films are two-phase systems consisting of separate domains of polythiophene dispersed throughout a matrix of ordered cadmium stearate molecules, it is possible to explain the dependence of the in-plane conductivity on the mole percent of P3AT present in the LB film via a simple percolation model. Doping of the low polythiophene content LB films creates isolated islands of conducting polymer in an insulating matrix of cadmium stearate molecules. Conduction in this case occurs by hopping or tunneling between the conducting islands and, as a result, is very low. As the concentration of the conducting phase increases, a percolation threshold is reached at which point a continuous pathway across the film via the conducting domains is established. The electrical properties of the film then become dominated by the intrinsic electrical properties of the conducting phase of the P3AT. Figure 9 indicates that the percolation threshold occurs between 20 and 40 mol ?6 P3AT. Above this range, the conductivity of the film slowly approaches the value of a pure film of conducting P3AT. Similar arguments were used by Tasaka et al.31 to explain the concentration dependence of the conductivity of LB films comprised of domains of iodinedoped quinquethiophene (a five-ring thiophene oligomer) molecules dispersed throughout a cadmium stearate matrix.
Conclusion The LB manipulation of mixed monolayers containing various mole ratios of poly(3-alkylthiophene)and cadmium stearate produces two-phase multilayer thin films comprised of well-ordered domains of cadmium stearate and relatively disordered domains of the polymer. Monolayers prepared from poly(3-alkylthiophenes)with short alkyl groups (less than eight carbons) undergo a phase transition at elevated surface pressures in which the polymer molecules are rejected from the mixed two-phase monolayer, thereby allowing the cadmium stearate molecules to achieve their optimum packing geometry. The (31) Tasaka, S.; Katz, H. E.; Hutton, R. S.; Orenstein, J.; Frederickson, G . H.; Wang, T. T. Synth. Met. 1986, 16, 17.
Langmuir 1990, 6, 1172-1179
1172
long alkyl chain of poly(3-octadecylthiophene),on the other hand, encourages stronger interactions between the two phases and promotes a higher level of molecular mixing in the monolayer. The net result is a mixed monolayer that retains its two-phase morphology at elevated surface pressures and forms multilayer structures with smaller or more poorly ordered cadmium stearate domains. Oxidation of the multilayers creates electrically conductive LB films with the level of conductivity being determined by the poly(3-alkylthiophene) content of the film. Conductivities as high as 2 S/cm can be achieved via this doping process. The conversion from an insulating thin film to a conductive thin film is accompanied by a dramatic change in optical properties reflecting the creation of new localized defect states within the band gap of the polythiophene component. This technique therefore allows one the ability to prepare uniform thin films of electrically conductive polymers in which the thickness of the film can be controlled at the molecular level. This approach can also be utilized to prepare organic super-
lattices comprised of functionally different molecular layers of electroactive polymers and/or insulating polymers. The fabrication and properties of such novel molecular architectures will be discussed in a future publication.
Acknowledgment. We acknowledge Dr. Enid Sichel, Yading Wang, Haskell Beckham, and Shuwei Sun of M.I.T. for their contributions to this research. We are also grateful to Dr. Ron Elsenbaumer of Allied Signal for providing some of the substituted polythiophenes used in this work. Partial support of this research was provided by the National Science Foundation and the U.S. Army Electronics Technology and Devices Laboratory (via the Scientific Services Program administered by Batelle). In addition, we thank Hitachi Chemical of Japan for support of I.W. Registry No. BT, 34722-01-5; HT, 1693-86-3; UT, 6501662-8; UDT, 104934-54-5; P-BT, 98837-51-5; P-HT, 10493450-1; P-OT, 104934-51-2; P-ODT, 104934-55-6; SA, 57-11-4; NOPF6, 16921-91-8;cadmium stearate, 2223-93-0.
Electrical Characterization of Dipalmitoylphosphatidylethanolamine and Cadmium Stearate Films on Platinum Surfaces in Aqueous Solutions Thomas L. Fare Naval Research Laboratory, Center for BiolMolecular Science and Engineering, Code 6090, Washington, DC 20375 Received April 20, 1989. I n Pinal Form: January 5, 1990 The electrical properties of multilayers of both cadmium stearate and dipalmitoylphosphatidylethanolamine/dipalmitoylphosphatidic acid films transferred onto platinum wires by using the Lang-
muir-Blodgett process have been investigated. The electrical measurements of the films were performed in situ. The impedance of the lipid films on the platinum is determined as a function of the lipid surface pressure of the film in the trough and was found to increase with increasing surface pressure. Although the films could be transferrred at relatively low pressures, it was found that a relatively high pressure is necessary to obtain an impedance comparable to black lipid membrane impedance. The impedance is used to determine the coverage of the platinum as a function of surface pressure and area per molecule. The coverage of the electrode is investigated for a variety of dipping speeds at fixed ionic strengths and pHs of the trough. The effects of water on the impedance of the multilayers of the cadmium stearate films are discussed. The electrical impedance of the films is compared to the impedance of lipid films prepared under other conditions. These results serve as the basis for analyzing lipid and fatty acid films in aqueous environments for sensor applications. Introduction
The electrical characterization of lipid and fatty acid films on metal substrates has been studied by several re~earchers.l-~ These films are deposited one layer a t a (1) Mann, B.; Kuhn, H. J. Appl. Phys. 1971, 42, 4398. (2) Procarione, W. L.; Kauffman, W. J. Chem. Phys. Lipids 1974,12,
time by the Langmuir-Blodgett technique5so that a careful analysis of the film properties may be performed as a function of film deposition parameters and number of layers. The electrical characterization of these deposited films has usually been carried out under vacuum or under dry ambient conditions after although some early work on the electrical characterization was done with an electrolyte as the counter electrode to the
251.
(3) Taylor, D. M.; Mahboubian-Jones, M. G. B. Thin Solid Films
1982, 87, 167. (4) Roberts, G. G.; Vincett, P. S.; Barlow, W. A. J . Phys. C. 1978,11,
2077.
( 5 ) Gaines, G. L. Insoluble Monolayers at Liquid-Gus Interfaces, Wiley Interscience: New York, 1966; Chapter 8.
This article not subject to U.S. Copyright. Published 1990 by the American Chemical Society
