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Poly(o-anisidine) Langmuir-Schaefer Films: Fabrication and Characterization Manoj K. Ram,† Sandro Carrara,‡ Sergio Paddeu,‡ Elisabetta Maccioni,† and Claudio Nicolini*,†,‡ Institute of Biophysics, University of Genoa, Via Giotto 2, 16153 Genoa, Italy, and EL.B.A Foundation, Via A. Moro 15, 57033 Marciana Marina, (LI) Italy Received June 14, 1996. In Final Form: December 2, 1996X The effect of pH on the Langmuir monolayer behavior was investigated for the poly(o-anisidine) (POAS) conducting polymer. The Langmuir-Schaefer (LS) films were prepared at pH 1 of the subphase, where the doping during the monolayer formation was essential for the high quality of the conducting polymer film. The LS films of POAS were characterized by using Fourier transform infrared spectroscopy, UV-vis absorption, X-ray diffraction, and ellipsometry techniques. The thickness of one monolayer of POAS film was estimated to 24 Å by using X-ray reflectivity and ellipsometric measurements. The uniformity of such POAS LS films was investigated for the redoping in 1 M HCl (protonic acid) for achieving the higher conductivity. The conductivity of the POAS LS film was shown to vary between 0.1 and 10-9 S/cm. Direct current through conducting LS films of POAS was observed as a function of monolayers. Further, the effects of different protonic acids on the electrical behavior of the POAS LS films were also investigated at length.
Introduction The polyanilines have become prominent among other conducting polymers due to their ease of doping by protonic acids and chemical stability in the doped and undoped states.1,2 Excellent stability of the conducting polyaniline films in both air and water, including the ease of transition between the insulating and the conducting states with voltage, has prompted researchers to use them in electrochromic display, gas sensor, photovoltaic, battery, and other microelectronic devices.3-7 Recently, it has been demonstrated that the polyaniline can be processed from the solution in the form of films and fibers.8 This ease of processibility is essential for its use in various applications.5 Further, efforts have been made in increasing the solubility of the polyaniline by using substituents groups (alkyl or alkoxy) in monomer or polymeric chains without significant loss of its main characteristics.9,10 Such a procedure increases the range of techniques that can be employed to process this class of conducting polymers. Of particular interest is the Langmuir-Blodgett (LB) technique, which enables conducting polymers to have a degree of order at the molecular level.11-13 In the recent past, * Corresponding author. Institute of Biophysics, University of Genoa: tel., +39.10.6516052; fax, +39.10.6507721; e-mail, director@i bf.unige.it. † EL.B.A Foundation. ‡ University of Genoa. X Abstract published in Advance ACS Abstracts, April 1, 1997. (1) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc. Faraday Trans. 1, 1986, 86, 2385. (2) Tzou, K.; Gregory, R. V. Synth. Met. 1993, 53, 365. Yang, C. Y.; Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1993, 53, 293. (3) Genies, E. M.; Lapkowski, M.; Tsintavis, C. New J. Chem. 1988, 12, 181. (4) Misra, S. C. K.; Ram, M. K.; Pandey, S. S.; Malhotra, B. D.; Chandra, S. Appl. Phys. Lett. 1992, 61, 1219. (5) Miyasaka, T.; Koyama, K.; Watanabe, T. Chem. Lett. 1990, 627. (6) Dogan, S.; Akbulut, U.; Yalcin, T.; Suzer, S. Synth. Met. 1993, 60, 30. (7) Gardner, J. W.; Bartlett, P. N. Synth. Met. 1993, 55-57, 3665. (8) Cao, Y.; Smith, P.; Heeger, A. J. Synth. Met. 1989, 32, 263. (9) Mello, S. V.; Mattoso, L. H. C.; Santos, J. R., Jr.; Goncalves, D.; Faria, R. M.; Oliveira, O. N., Jr. Electrochim. Acta 1995, 40, 12. (10) Leclerc, M.; Guay, J.; Dao, L. H. Macromolecules 1989, 22, 649. (11) Rubner, M. F.; Skotheim, T. A. Congugated Polymers; Bredas, J. L., Silbey, R., Eds.; Kluwer: Amsterdam, 1991, pp 363-403. (12) Roberts, G., Ed. Langmuir Blodgett Films; Plenum Press: New York, 1990.
S0743-7463(96)00588-4 CCC: $14.00
the deposition of quasi-ordered and ordered LangmuirBlodgett films of polyaniline were reported, where emeraldine base dissolved in N-methylpyrrolidinone (NMP)/ chloroform (CHCl3) could be used to cast LB films using an aqueous subphase containing neutral or acidic water.14-16 The deposited LB films of polyaniline used to give rise to an irregular surface after a certain number of layers due to the presence of NMP.14 Recently, the horizontal lifting technique [Langmuir-Schaefer (LS)] was employed for the fabrication of ultrathin films of polyanilines, where each successive deposited film was dried by blowing nitrogen flux. In fact, the poly(oanisidine) (POAS) and poly(ethoxyaniline ) are soluble in chloroform.17 Though attempt was made for the fabrication of the LB and the LS films of poly(ethoxyaniline), these films gave rise to the lesser conductivity.18 In fact, the poly(o-anisidine) is found to be a suitable material for the fabrication of the LB/LS films due to its solubility in chloroform and intermediate conductivity (0.1 S/cm) compared to that of the polyaniline.19 Thus, it is necessary to have a deeper understanding for the Langmuir film formation of the POAS conducting polymer at the airwater interface and the detailed optical and electrical properties of its LS films. Very high quality of film morphology and stability in electrical properties are necessary for the use of such films in the molecular electronics devices and the sensors applications.19-20 Keeping all this in mind, this paper discusses the various parameters ruling the quality of the Langmuir film as well as optical, structural, and electrical characterizations of the HCl-doped poly(o-anisidine) LS films. The electrical properties of the POAS LS films on an interdigited (13) Cheung, J. H.; Punkka, E.; Rikukawa, M.; Rosner, R. B.; Royappa, A. J.; Rubner, M. F. Thin Solid Films 1992, 210-211, 246. (14) Ram, M. K.; Sundaresan, N. S.; Malhotra, B. D. J. Phys. Chem. 1993, 97, 11580. Ramanathan, K.; Ram, M. K.; Murthy, A. S. N.; Malhotra, B. D. J. Mater. Sci. Eng. 1995, C3, 159. (15) Goncalves, D.; Bulhoes, L. O. S.; Mello, S. V.; Mattoso, L. H. C.; Faria, R. M.; Oliveira, O. N., Jr. Thin Solid Films 1994, 243, 544. (16) Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244, 990. (17) Macinnes, D.; Funt, B. L. Synth. Met. 1988, 25, 235. (18) Mello, S. V.; Mattoso, L. C. H.; Santos, J. R., Jr.; Goncalves, D.; Faria, R. M.; Oliveira, O. N., Jr. Electrochim. Acta 1995, 40 (12), 18511855. (19) Vince, P. S.; Robert, G. G. Thin Solid Films 1980, 68, 135. Ram, M. K.; Carrara, S.; Paddeu, S.; Nicolini, C. Thin Solid Films, in press. (20) Nicolini, C. Biosens. Bioelectron. 1995, 10, 105.
© 1997 American Chemical Society
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Figure 1. The structure of the poly(o-anisidine) (POAS) conducting polymer: (a) emeraldine base form (y ) 1/2), (b) emeraldine salt form.
electrode were experimentally carried out by varying the number of monolayers. Further, the time-dependent electrical properties of various protonic acids doping on the POAS LS film were investigated at length. Experimental Section Synthesis of the POAS Conducting Polymer. The monomer (o-anisidine), oxidizing agents, and various reagents were procured from Sigma for the synthesis of the poly(o-anisidine) conducting polymer. The POAS was chemically synthesized by oxidative polymerization of o-anisidine using ammonium peroxydisulfate [(NH4)2S2O8)] under controlled conditions. Distilled o-anisidine (27 mL, 0.219 M) was added in 1 M HCl solution, and the solution was cooled to 0-4 °C in an ice bath. Ammonium peroxydisulfate (11.5 g, 0.05 M) was also dissolved in 200 mL of 1 M HCl solution, and the solution was precooled to 4 °C. Later, ammonium peroxydisulfate in 1 M HCl solution was added slowly in o-anisidine solution, and the reaction was continued for 12 h.21 The dark green precipitate of the POAS recovered from the reaction vessel was filtered and washed by using 1 M HCl for the removal of oxidant and oligomers. This precipitate was further washed using deionized water, methanol, and diethyl ether for the elimination of the low molecular weight polymer as well as oligomers (all violet color). Further, this precipitate was heated at 100 °C in a temperature-controlled oven. The green powder thus obtained was the emeraldine salt (ES) of the POAS conducting polymer. Such an ES form of POAS was subsequently treated by using aqueous ammonia for 24 h. Then, it was washed by using distilled water and acetone for several times and dried for 6 h at 100 °C in a temperature-controlled oven. The dark blue powder thus obtained was the emeraldine base form of the POAS conducting polymer. The emeraldine base was used for the fabrication of Langmuir-Schaefer films. Figure 1a shows the general structure of the POAS conducting polymer, where y ) 1/2, 1, and 0 are related to emeraldine, leucoemeraldine, and pernigraniline structures, respectively. Figure 1b shows the emeraldine salt form of the POAS conducting polymer. Formation of POAS LS Films. A stock solution of the POAS was prepared by using 1 mg:5 mL of CHCl3 for immediate use. The resulting solution was also filtered with a solvent-resistant filter (0.5 µm). The Langmuir trough of MDT Corp. (240 mm × 100 mm in size and 300 mL in volume) was used for the deposition of the LS film. An appropriate 70 µL of solution was spread onto the air-water interface, containing the aqueous subphase of different pH (6.4-1). The HCl acid was used for maintaining the different aqueous subphases. After the various isotherms were recorded, it appeared that the film forming at pH 1 showed higher collapse pressure. So, pH 1 of subphase (using 1 M H2SO4 or 1 M HClO4) was also used for the deposition of LS films. The deposition pressure was carefully chosen to be 25 mN/m with a barrier compression speed of 100 cm2/min for the fabrication of the POAS LS films. Different numbers of monolayers were transferred onto quartz, silicon, and interdigited electrode substrates (containing chromium electrodes previously cleaned with ethanol and chloroform) by the horizontal lifting (LangmuirSchaefer) method. Many literatures sources are available which identify such fabrications as LB films,3,18,19 but we are presenting the deposited films to be Langmuir-Schaefer (LS) films, referring to deposition principle. Optical Characterizations of LS Films. The vibrational bands of POAS LS films deposited on the [111] single crystal were measured by Fourier transform infrared (FTIR) spectro(21) Annapoorni, S.; Sundaresan, N. S.; Pandey, S. S.; Malhotra, B. D. J. Appl. Phys. 1993, 74, 2109.
Langmuir, Vol. 13, No. 10, 1997 2761 photometer (Perkin-Elmer 1760). The sample chamber of the spectrophotometer was continuously purged with nitrogen gas for 15-20 min before the data collection as well as during the measurements for the elimination of the water vapor absorption. For each sample, 30 interferograms were recorded, averaged, and Fourier-transformed to produce a spectrum with a nominal resolution of 4 cm-1.22 FTIR spectra of POAS LS films were obtained after proper subtraction to a substrate silicon base line. The FTIR spectra of the powdered POAS conducting polymer were also measured in KBr pellets in the similar conditions as discussed. The UV-absorption of the POAS LS films on quartz substrates were measured using the UV-spectrophotometer (Jasco model 7800). X-ray Diffraction. The X-ray diffraction measurement of POAS LS films deposited on silicon substrate was carried out on a diffractometer (AMUR-K) with a linear position-sensitive detector. A Philips high-voltage generator (PW 1830) with Cuanode X-ray long fine tube (35 kV, 35 mA) was applied as a source of X-ray radiation, where the linear resolution of the detector was 0.3 mm. The data was collected by using a wavelength of 1.54 Å from Cu K radiation. The detector was then moved in a vertical direction at a constant speed to measure the small angular regions of the film.23,24 Ellipsometric Measurements. The ellipsometric measurements were performed using a PCSA null ellipsometer LEPh-2 (special Design and Production Bureau for Scientific Devices of the Siberian branch of the Russian Academy of Sciences, Novosibirsk) using a He-Ne laser (wavelength 632.8 nm). The accuracy of the device was 0.02° with respect to Ψ and ∆. The angle of incidence was kept at 70°. The data were processed according to the two layer model, in which the lower layer accounted for the imperfections of the silicon substrate and the upper one represented the deposited POAS films. The applicability of such a model for the interpretation of measurements of thin layers deposited onto silicon substrate was shown elsewhere.25 Electrical Measurements. The electrical characterization was performed using a Keithley electrometer (model 6517) as well as an operational amplifier in inverting configuration. Current-voltage (I-V) characteristics were obtained by potential step of 0.05 V. The two electrodes, which were interdigited, were spaced to 50 µm between any two pairs. Each track in the interdigited electrode was 50 µm in width and 40 nm in height.
Results and Discussion Pressure-Area Isotherms for POAS Langmuir Films at pH 6.4 to pH 1. The stability of the Langmuir monolayer is usually associated with a high collapse pressure, a steep increase in the pressure curve in the condensed phase, and a small hysteresis in the compression-expansion cycle. The choice of the appropriate subphase has been proven to be extremely important for the deposition of LB films of the polyanilines.9 Figure 2 (curve 1) shows the pressure-area isotherm for the POAS Langmuir film at pH 6.4. It does not show a steep increase in pressure in the condensed phase, where the yielding (breaking) point for the Langmuir monolayer has a surface pressure of 42 mN/m. It reveals that the higher pH does not produce a good Langmuir monolayer.9 In addition, the wrinkles on the aqueous subphase at pH 6.4 can also be seen from naked eye. So, a careful investigation was performed by using different pH values of the subphases. When the pH is maintained at either 5 (curve 2) or 4 (curve 3), we still do not get the steep increase in the pressurearea isotherm behavior. But, when the pH of the aqueous (22) Bramanti, E.; Benedetti, E.; Nicolini, C.; Berzina, T.; Erokhin, V.; Benedetti, E. Biochimica et Biophysica Acta (communicated in 1996). (23) Sukhorukov, G.; Lobyshev, V.; Erokhin, V. Mol. Mater. 1992, 1, 91. (24) Facci, P.; Erokhin, V.; Tronin, A.; Nicolini, C. J. Phys. Chem. 1994, 98, 13323. (25) Kim, Y. M.; Foster, C.; Chiang, J.; Heeger, A. J. Synth. Met. 1989, 29, E285.
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Figure 3. The low-angle X-ray diffraction pattern of 10 monolayers of the POAS LS films deposited at pH 1.
Figure 2. (a) Variation of the pressure-area isotherm as a function of pH using HCl protonic acid viz.: pH 6.4 (curve 1), pH 5 (curve 2), pH 4 (curve 3), pH 3 (curve 4), pH 2 (curve 5), and pH 1 (curve 6). (b) Variation of area per molecule vs pH using HCl protonic acid as subphase of the POAS Langmuir monolayer.
subphase is lowered between 3 and 1 as shown in Figure 2a(curve 4-6), the POAS molecules on the water surface suffer and get doped. In this process, more steep rises in pressure-area can be seen. The Langmuir monolayer at lower pH (1-3) gives rise to higher collapse pressure (∼50 mN/m) than that at higher pH (4-6.4) (∼42 mN/m). The importance of doping has been confirmed for the stable monolayers, which is probably associated to the ordering of the molecules on water surface. It can be speculated that the HCl doping organizes the polymer chain of the POAS Langmuir film. The solid phase transition starts appearing at the molecular area of 26-28 Å2 between pH 1 and 3. The isotherm at pH 1 (curve 6) also attributes to the fact that there is no change in molecular area at the air-water interface between a pressure of 22-28 mN/ m. So an attempt was made for the estimation of molecular area of POAS at the air-water interface by extrapolating the pressure-area isotherm curve at different pH values of the subphases.11,18 In fact, the formation of the Langmuir monolayer for polyaniline was shown in the literature using pH 1 as subphase.13 Figure 2b shows such molecular area of POAS in the solid phase at the air-water interface as a function of different pH values of the subphases. For the calculation of molecular area at condensed phase, one repeating unit of POAS was taken into account as shown in Figure 1a. The molecular area was estimated to be 55 Å2 for pH 6.4. The average molecular area is shown to be decreasing as a function of pH of the subphase. When the POAS monolayer is formed at lower pH, the polymer chain orients and is simultaneously doped due to the presence of protonic acid. So the area obtained at 1 pH is found to be about 20 Å2, which should be the expected area of the aniline repeat unit.9,18 The cross-sectional area of the aniline repeat unit at the air-water interface was already estimated to be 20 Å2.9 So, 25 mN/m pressure was carefully selected for the
Figure 4. Ellipsometric measurements on the POAS LS films for thickness as a function of monolayers.
fabrication of LS films of the POAS conducting polymer using the subphase of pH 1 at a compression speed of 100 cm2/min. X-ray Diffraction Measurements. The structure and thickness of 10 POAS monolayers of LS films made from pH 1 subphase were investigated using low-angle X-ray diffraction. The POAS LS films are amorphous as shown in Figure 3. The presence of Keissing fringes can be seen in the range of 1-2° in 2θ. Although the Langmuir monolayer molecules are fixed with high precision in the LS films, these still do not form the crystallites. Moreover, the X-ray pattern shows amorphous films in the larger planes as shown in Figure 3. It attributes that the structure does not depend upon the technique of deposition. The thickness of the film was estimated to be 24 ( 1 Å for each layer. This value arises from the difference in 2θ of the Keissing fringes at lowest angles (1-2°). In fact, the X-ray-diffracted pattern found for the POAS LS film is similar to that of an electrochemical film measured till an angle (2θ) of 8°.17 It is needed to perform the higher angle measurements in X-ray diffraction studies for observing some crystallinity of POAS LS films. The thickness of the film was also measured using ellipsometry. Ellipsometric Measurements. To study the film structure and thickness further, we performed ellipsometric measurements of POAS LS films deposited on silicon substrate as shown in Figure 4. We measured the thickness of the films till 40 monolayers. Since POAS LS film is highly anisotropic, its shape can be approximated by rotation of the ellipsoid with the axis in the process of measurements. So, we kept the angle of incidence at 70° by properly optimizing our measurement system. The thicknesses of the POAS LS films as a function of the monolayers are shown in Figure 4. The thickness and density of the monolayers are inevitably linked to the orientation of the molecules. This means that at the deposited surface pressure at 25 mN/m, the POAS molecules may be oriented parallel to the substrate surface and give rise to a uniform deposition of the monolayers. The value of the thickness obtained for each monolayer is 24 ( 2 Å, which corresponds to the thickness measured
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Figure 6. FTIR spectra of POAS for the powder form in the KBr pellet (curve 1), and for 50 monolayers of LS films on silicon substrate (curve 2) deposited at pH 1.
Figure 5. (a) UV-vis spectra of the POAS LS films of 20 monolayers made in pH 1 (curve 1), the film treated in 1 M HCl for 5 min (curve 2), and the undoped LS film in aqueous ammonia (curve 3), and (b) UV-vis absorption spectra of POAS LS film doped in 1 M HCl as a function of the number of monolayers: curve 1 (2 layers), curve 2 (5 layers), curve 3 (8 layers), curve 4 (15 layers), curve 5 (20 layers), curve 6 (25 layers), curve 7 (30 layers).
by X-ray diffraction. It shows the linear fitting of the experimental points. A little change in the experimental data points with respect to the best linear fit after 20 monolayers can be noticed. The little decrease in film thickness after 20 layers may be linked to less transfer of the Langmuir monolayer molecules. Interestingly, the average thickness of one monolayer calculated from the ellipsometric technique quite resembles with the X-ray diffraction measurement. It reveals the uniform deposition of POAS LS films. The uniformity in redoping using 1 M HCl acid of POAS LS films was investigated by using the UV-vis absorption technique. UV-Absorption of POAS LS Film. The optical spectra of the POAS LS film of 20 monolayers are shown in Figure 5a. Figure 5a (curve 1) shows the two sharp absorption bands at 340 and 750 nm for the film made at subphase pH 1. The observed peak seen at 340 nm can be attributed to a Π-Π* transition centered on the benzoid ring (interband transition), and the peak at 750 nm is perhaps due to the defect (HCl molecule) incorporated when the emeraldine form of the POAS monolayer was formed at pH 1. When the films are doped in 1 M HCl, curve 2 (Figure 5a) shows the absorption bands at 352 and 840 nm, besides the appearance of the new peak at 450 nm. The absorption peak seen at 450 nm is due to phase segregation of fully protonated and unprotonated regions or the presence of polarons in the POAS conducting polymer films. The observed Π-Π* transition is sharper and varies between 340 and 352 nm for lightly to the degenerately doped POAS LS films. Further, the LS film treated in aqueous ammonia shows the characteristic bands of the emeraldine form at 330 and 600 nm in Figure 5a (curve 3). The band seen at 330 nm is due to interband transition (Π-Π*), whereas the observed peak at 600 nm
is because of a n-Π* transition from the nonbonding nitrogen lone pair to the conduction band (Π*).26 Figure 5b shows the UV-vis s[ectra of doped POAS LS films in 1 M HCl as the number of monolayers deposited on the quartz substrate. The intensity of the broad bands from 280 to 1080 nm show a distinct increase in the increment of monolayers. The magnitude in UV-vis absorption bands at 352, 450, and 850 nm are found to increase as a function of monolayers. It can be attributed that the LS films can be readily produced in the doped state and the doping level can be enhanced by the simultaneous doping of the film in 1 M HCl medium. The uniform increase of the UV-visible spectra with the number of monolayers is also related to the conductivity of the film.14 The highest conductivity of the polyaniline can be achieved in the treatment of the films to the 1 M HCl solution.4 Besides this, the UV-vis studies also reveal the maintenance for the uniformity of the LS films of POAS in treatment to 1 M HCl solution. The doping process is somewhat similar to that for the polyaniline LB film.27,28 The effect of doping during the film formation has also been tested using FTIR study. FTIR Spectroscopy on Bulk and LS Films. Further, the FTIR spectroscopic measurement was performed to identify the functionality of each group of POAS LS films as shown in Figure 6. The various vibrational bands of the POAS LS films are compared to the spectra obtained in powder form. Figure 6 (curve 1) shows the FTIR spectrum of the POAS powder in KBr pellet. The observed peaks at 3400, 2830, 2923, 1715, 1681, 1561, 1487,1375, 1300, 1237, 1179, 1000, 879, 800, 711, 702, 600, and 418 cm-1 are the characteristic bands of the POAS conducting polymer. The bands seen at 1000, 879, 800, 711, 702, 600, and 418 cm-1 are linked to the o-substituted aromatic rings. The observed bands at 1237, 1250, and 1615 can be due to the C-N and CdN linkage,4 whereas the bands at 1375, 1300, 1487, 2830, and 2923 cm-1 are because of the -CH3 substituted group. The strong band at 1600 cm-1 reveals the electrical conductivity of the sample. The band near 1000-1300 cm-1 can also be attributed to the typical electron-phonon interaction of the conducting polymeric system.25 Curve 2 in Figure 6 shows the FTIR spectra of 50 monolayers of the HCl-doped POAS LS films on a [111] single crystal. It shows the characteristic bands of the POAS conducting polymer. The bands, which are emanating due to the presence of the HCl molecule at 1325 and 1375 cm-1, can also be observed in Figure 6 (curve 2). It underlines the fact that HCl ions are incorporated in POAS LS films during the Langmuir film (26) Kim, Y. H.; Phillips, S. O.; Nowak, M. J.; Spiegel, D.; Foster, C. M.; Yu, W.; Chiang, J. C.; Heeger, A. J. Synth. Met. 1989, 29, E296. (27) Lacroix, J. C.; Garcia, P.; Audiere, J. P.; Clement, R.; Kahn, O. Synth. Met. 1991, 44, 117. (28) Ram, M. K.; Malhotra, B. D. Polymer 1996, in press.
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Figure 8. Variation of current as a function of the number of monolayers. The LS films are made at pH 1 and doped in 1 M HCl.
formation. In fact, these peaks are absent in the emeraldine base form of polyanilines.14 It should be noticed that curves 1 and 2 were presented in the normalized mode for comparison studies between the bands of the LS films and the powder form of the POAS conducting polymer. The intensity magnitude of the peaks pertaining to 800-1700 cm-1 in LS films (curve 2) are more resolved than those of the powder form (curve 1) of conducting polymer. The detailed FTIR investigations are under progress to understand the presence of different splitting vibrational levels of POAS LS films. Further sections deal with the characteristics of the electrical findings on POAS LS films. Electrical Conductivity on Doped LS Films. We performed the electrical measurements of POAS LS films on interdigited electrodes with the perspectives as (1) the interdigited chromium electrode metals do not diffuse in polyaniline film, (2) there is no pinning effect of conducting polymer films, which generally occurs in the sandwiched (metal-LB film-metal) type of configuration, and (3) the interdigited electrodes have a fixed size and equal separation from one electrode to another. We carried out the electrical measurements on such a type of interdigited electrodes depositing various numbers of Langmuir monolayers. Figure 7a shows the current vs voltage (I-V) measurements of one monolayer LS film doped in 1 M HCl. A potential step of 0.05 V/min was applied for the I-V measurement. The I-V curve (Figure 7a) of one monolayer does not show the ohmic behavior. It can be explained by taking into account that the thickness of a single monolayer is nearly 24 ( 1 Å, while the height of the electrode is 40 nm. So, it is possible to have some breaking of the film at the electrode.30 When the potential is reversed, the hysteresis in the current-voltage can be observed. Such hysteresis of I-V characteristics for one monolayer has also been observed for charge transfer complexes in the metal-LB films-metal structure.29
Hysteresis may be arising due to the instability of the charge defect in one monolayer of the POAS LS film. Figure 7b shows the I-V characteristics of 40 monolayers of doped films of the POAS conducting polymer. The conductivity of the film measured at 0.6 V was estimated to be 0.1 S/cm. Curve 2 in Figure 7b shows the I-V characteristics for the same LS films measured after one month time period. It shows the stable electrical behavior of HCldoped POAS LS films. In fact, 1 M HCl-doped electrochemical films or powder state POAS conducting polymers reveal the stable behavior of the conductivity measurements. Further, we have performed the I-V study as a number of monolayers on such POAS LS films. Figure 8 shows the current vs number of monolayers plot for a measured potential of 0.6 V. The increment in the magnitude of the current values for the number of monolayers is observed for a potential value of 0.6 V. It reveals that the current flowing through the POAS LS film increases with the increment in the number of monolayers, i.e. the electrical conductivity increases in successive deposition of the film. The conductivity of the film was found to be 0.1 S/cm for the 40 monolayers containing the LS films of the POAS conducting polymer. On the other hand, Figure 8 shows the relationship between the current increase and the material transferred onto the substrate; strictly speaking it is not linear. This figure shows the best fit following the linear relationship of the experimental data, which shows that the data points are not exactly positioned on the best fit curve. The interpretation can be rivulet to the fact that there may be little change of material transferred during lifting after 20 layers of deposition. This fact could also be related to some other experimental results from electron microscopy or from atomic and tunneling scanning microscopy, which show us that POAS films are amorphous structures. The gradual increment in the current magnitude is linked to an increase in the charge defects (polarons/bipolarons) as a function of number of monolayers. Effect of Dopants on the Electrical Conductivity. Moreover, to study the film in the undoped state, 40 monolayers of the POAS LS films were undoped in aqueous ammonia to obtain the emeraldine base form of the POAS conducting polymer. It shows a current value of picoampere in Figure 9. The conductivity of the film was calculated to be 10-9 S/cm. The similar value of conductivity has also been obtained for polyaniline LB films.31 It is an interesting aspect that the aqueous ammonia treatment completely releases the dopant HCl ions from such an ordered structure of LS films and shows the conductivity similar to that of the bulk POAS conducting
(29) Geddes, N. J.; Sambles, J. R.; Jarvis, D. J.; Parker, W. G.; Sandman, S. D. J. Appl. Phys. 1992, 71, 756.
(30) Carrara, S.; Gussoni, A.; Erokhin, V.; Nicolini, C. J. Mater. Sci.: Mater. Electron. 1995, 6, 79.
Figure 7. (a) Current-voltage characteristics of one monolayer of POAS LS film on the interdigited electrode made at pH 1 and consequently doped at 1 M HCl. (b) Variation of current-voltage characteristics of 40 monolayers of POAS LS film on the interdigited electrode doped in 1 M HCl, immediately measured (curve 1) and after one month (curve 2).
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Langmuir, Vol. 13, No. 10, 1997 2765
Figure 9. Variation of current-voltage characteristics for the undoped form of 40 monolayers of POAS LS films.
polymer.31 We checked the conductivity of undoped POAS LS films as a function of monolayers, but regardless of the increment of the thickness, the conductivity was found to be a value of ∼10-9 S/cm. Further, the effect of dopants on the electrical conductivity of the POAS LS film were studied. However, to show the dopants’ effect on the electrical conductivity, the LS films of POAS were simultaneously made in pH 1 of H2SO4 and HClO4, separately. These films were subsequently doped in 1 M H2SO4 and 1 M HClO4 acids, very similar to the HCl doping of the POAS LS films. POAS LS films of 40 monolayers were used for the electrical characterization. Figure 10a,b shows the I-V characteristics for 1 M H2SO4- and 1 M HClO4-doped POAS LS films, where the films consist of 40 monolayers. The magnitude of current for 1 M H2SO4-doped LS films are found to be more than that for 1 M HCl-doped films for the same thickness of the films. When the films are measured after 3 h in each case, the change in the current value can be observed, as shown in curve 2 of Figure 10a,b; whereas the film made in HCl does not show any variation of the current after the one month time period, as shown in Figure 7b. There is also a 30% decrease in the magnitude of current for the 1 M H2SO4-doped LS films as shown in Figure 10a (curve 2). Figure 10b (curve 2) also reveals the similar decrease in the magnitude of current for 1 M HClO4-doped LS films. The HClO4-doped LS films also attribute the lesser conductivity in comparison to HCl and H2SO4 dopants, besides the unstability in I-V characteristics. In general, the LS films formed by the H2SO4 and HClO4 protonic acids were found to be more unstable in the electrical behavior. Conclusions The doping has been proven to be essential for obtaining a highly stable Langmuir monolayer for the POAS conducting polymer. The pressure-area isotherm behavior shows high collapse pressure for the subphase containing between pH 1 and 3. The Langmuir film formation and the monolayer stability can be significantly improved by using an acidic subphase between pH 1 and 3. The uniformity of the deposited POAS LS films can be maintained after redoping with 1 M HCl. The thickness (31) Pearson, C.; Dhindsa, A. S.; Petty, M. C.; Bryce, M. R. J. Mater. Sci.: Mater. Electron. 1992, 210/211, 257.
Figure 10. (a) I-V characteristics of 40 monolayers of POAS LS films and of those doped in 1 M H2SO4 protonic acid: (curve 1) just made film, (curve 2) measured after 3 h. (b) I-V characteristics of 40 monolayers of POAS LS films and of those doped in 1 M HClO4 protonic acid: (curve 1) just made film, (curve 2) measured after 3 h.
of one monolayer estimated from the X-ray diffraction as well as ellipsometry measurement was found to be 24 ( 2 Å. It has been shown that monolayers are transferred onto solid substrates in doped states and the doping level can be increased using 1 M HCl, protonic acid, as evidenced from UV-vis spectra. The value of the conductivity was estimated to be 0.1 S/cm, which was same for the chemically synthesized doped form of POAS, while the emeraldine base form of the POAS LS films showed the electrical conductivity in the range of 10-9 S/cm, which was the same as that for the chemically synthesized form of the POAS conducting polymer. HCl-doped LS films did not show any time degradation on the electrical characteristics, whereas H2SO4 and HClO4 protonic acids attributed to a large (30%) decrease in current magnitude within short interval of time. The detailed investigation reported here and the emerging properties point to the possible application in molecular electronics, gas sensor, and battery fields for the HCl-doped POAS LS film, whereas the instabilities seen for some kinds of dopants suggest to look forward for a better characterization of the electrical behavior of these films before any practical applications. Acknowledgment. The authors are thankful to Drs. Erokhin and Facci for their helpful discussions during the preparation of the manuscript. Thanks are also due to Mr. A. Rossi and Miss C. Rando for their help in carrying out the experiments. EL.B.A. Foundation and the University of Genoa are gratefully acknowledged for the financial support. LA960588G
