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Langmuir and Langmuir-Blodgett Films of Poly[2-methoxy-5-(n-hexyloxy)-p-phenylenevinylene] Marystela Ferreira,*,† Carlos J. L. Constantino,‡ Clarissa A. Olivati,† M. L. Vega,† De´bora T. Balogh,† Ricardo F. Aroca,§ Roberto M. Faria,† and Osvaldo N. Oliveira Jr.† Instituto de Fı´sica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, CP 369, 13560-970 Sa˜ o Carlos/SP, Brazil, Depto de Fı´sica, Quı´mica e Biologia, Faculdade de Cieˆ ncias e Tecnologia, Universidade Estadual Paulista, CP 467, 19060-900 Presidente Prudente/SP, Brazil, and Materials and Surface Science Group, School of Physical Sciences, University of Windsor, N9B 3P4 Windsor, Ontario, Canada Received April 17, 2003. In Final Form: July 31, 2003 Langmuir films have been fabricated from poly[(2-methoxy-5-n-hexyloxy)-p-phenylenevinylene] (OC1OC6PPV). The stability and the area per monomer for condensed films indicate the formation of true monolayers with a very small extent of aggregation, which is unusual for polymer films. This is attributed to the linearity of the alkyl side chain. The Y-type Langmuir-Blodgett (LB) films produced from Langmuir films of OC1OC6-PPV have distinctive features compared to those of cast films, probably due to the organization in the LB films whereas the molecules are randomly oriented in cast films. Infrared absorption spectra recorded for both transmission and reflection modes indicate that OC1OC6-PPV molecules are anchored to the substrate by the lateral groups. This is confirmed by the Raman spectrum, in which a distortion of the vinylene group was observed, and by surface enhanced fluorescence (SEF) on an LB monolayer deposited onto Ag nanoparticles. The more homogeneous nature of the LB films in comparison with the case of cast films was demonstrated by optical microscopy and fluorescence measurements where the emission spectra were essentially the same for different regions of an LB film but showed dispersion in cast films. The LB films also displayed reversible photoconductivity.
Introduction The electrical, luminescent, and photoconducting properties of conjugated polymers make them suitable for applications in optoelectronic devices.1-3 Among the various types of conjugated polymers, poly(p-phenylenevinylene) (PPV) and its derivatives have been the most investigated to produce polymer light emitting diodes (PLEDs), where thin films of the polymer are deposited onto a transparent conducting electrode, generally glass coated with indium-tin oxide (ITO). The polymer films may be fabricated using several techniques, including the Langmuir-Blodgett (LB) method whereby nanostructured films may be achieved with controllable thickness, low number of defects, and some degree of organization at the molecular scale. Using LB films is therefore attractive, since the homogeneity and organization of the polymer molecules may affect positively the optical and electrical properties of the device. The LB technique also has the advantage of requiring a very small amount of polymer material to fabricate the films, in contrast to the cases of other techniques such as spin coating or casting. There are few reports on the fabrication of LB films from PPV derivatives.4-8 Sluch et al.4 reported the photo* To whom correspondence should be addresssed. † Universidade de Sa ˜ o Paulo. ‡ Universidade Estadual Paulista. § University of Windsor. (1) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Holmes, A. B. Nature 1990, 347, 539-541. (2) Greenham, N. C.; Moratti, S. C.; Bradley, D. D. C.; Friend, R. H.; Holmes, A. B. Nature 1993, 365, 628-630. (3) Hide, F.; Diaz-Garcia, M. A.; Schwartz, B. J.; Heeger, A. J. Science 1996, 273, 1833-1836. (4) Sluch, M. I.; Pearson, C.; Petty, M. C.; Halim, M.; Samuel, I. D. W. Synth. Met. 1998, 94, 285-289. (5) Wu, Z. K.; Wu, S. X.; Liao, J. H.; Fu, D. G.; Liang, Y. Q. Synth. Met. 2002, 130, 35-38.
and electroluminescence properties of LB films from poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene], MEH-PPV. Jung et al.7,8 used MEH-PPV LB films as the emitting layer to improve the quantum efficiency of LEDs. Wu et al.6 investigated the LB film characteristics of poly[2-methoxy-5-(n-hexadecyloxy)-p-phenylenevinylene] using UV-visible polarized spectroscopy and linear dichroism infrared analysis and showed that the polymer chains were oriented. Wu et al.5 prepared Langmuir and LB films from poly[2-methoxy-5-(n-hexadecyloxy)-p-phenylenevinylene] and suggested on the basis of atomic force microscopy (AFM) images and UV polarized spectra that the film exhibits the main chains aligned in the LB dipping direction. Molecular organization has been shown to improve the optical properties of thin films from PPV derivatives,9,10 which also occurs for LB films from thermally converted PPV.11 In this paper we report on Langmuir and LB films of the PPV derivative poly[(2-methoxy-5-n-hexyloxy)-pphenylenevinylene] (OC1OC6-PPV), whose structure is shown in the inset of Figure 1. Surface pressure-molecular area (π-A) and surface potential-molecular area (∆VA) isotherms were used to characterize the polymer film at the air-water interface, while the deposited LB films and cast films were characterized with UV-visible (6) Wu, Z.; Wu, S.; Liang, Y. Langmuir 2001, 17, 7267-7273. (7) Jung, G. Y.; Pearson, C.; Horsburg, L. E.; Samuel, I. D. W.; Monkman, A. P.; Petty, M. C. J. Phys. D.: Appl. Phys. 2000, 33, 10291035. (8) Jung, G. Y.; Pearson, C.; Kilitziraki, M.; Horsburgh, L. E.; Monkman, A. P.; Samuel, I. D. W.; Petty, M. C. J. Mater. Chem. 2000, 10, 163-167. (9) Kalinowski, J. J. Phys. D: Appl. Phys. 1999, 32, R179-R250. (10) McBranch, D.; Campbell, I. H.; Smith, D. L. Appl. Phys. Lett. 1995, 66, 1175-1177. (11) Marletta, A.; Gonc¸ alves, D.; Oliveira, O. N., Jr.; Faria, R. M.; Guimara˜es, F. E. G. Macromolecules 2000, 33, 5886-5890.
10.1021/la0346595 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/16/2003
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Figure 1. Surface pressure-area and surface potential-area isotherms for pure OC1OC6-PPV on a water subphase.
absorption, Fourier transform infrared (FTIR) spectroscopy using both transmission and reflection-absorption (RAIRS) modes, and Raman scattering, fluorescence, and cyclic voltammetry measurements. AFM was carried out to estimate film thickness while the morphology of LB and cast films was determined using both AFM and optical microscopy images. The conductivity and photoconductivity of the LB film were determined from current versus potential measurements. The importance of the nanostructured nature of the LB films becomes apparent in the comparison with cast films from OC1OC6-PPV. Experimental Details
Ferreira et al. transfer ratio (TR) for the LB films of ∼1.0 in the upstrokes and 0.9 in the downstrokes. The thickness and roughness of the LB films were measured using AFM, with a Nanoscope IIIa from Digital Instruments. AFM images were obtained in the tapping mode, employing a silicon tip and using a resonance frequency of approximately 300 kHz and a scan rate of 0.71 Hz, and the scan areas were 10 × 10 µm2. Prior to the measurements, the films were treated in a vacuum for at least 12 h to remove residual solvents and moisture. The cast and 41-layer LB films were characterized by FTIR spectroscopy either in the transmission mode on silicon wafers using a BOMEM MICHELSON FT (MB 102, 4 cm-1 spectral resolution, and 64 scans) or in RAIRS mode on Ag mirrors using a BOMEM DA3 (MCT detector, 4 cm-1 spectral resolution, 1032 scans, 80° incident angle, and evacuating the sample chamber until 0.5 Torr). The emission and Raman scattering spectra were recorded at room temperature (22 °C) using the laser lines at 514.5 nm (5× lens and 20 µW) and at 633 nm (30 µW), in a Renishaw Research Raman Microscope System RM2000 equipped with a Leica microscope (DMLM series), a 1800 g/mm grating, a spectral resolution of ∼3 cm-1, and a Peltier cooled (-70 °C) CCD array detector. Surface enhanced fluorescence (SEF) was obtained using the RM2000, 50× lens, 488 nm laser line (28 µW), 20-s collection time, and two accumulations for a one-layer LB film deposited onto a glass substrate half coated with a Ag nanoparticle film (6 nm mass thickness). Data acquisition and analysis were carried out using the WIRE software for Windows and Galactic Industries GRAMS/32. UV-vis absorption measurements were carried out in a HITACHI U-2001 spectrophotometer in the range between 350 and 800 nm. Cyclic voltammograms were obtained on 41-layer LB films deposited onto ITO (1.0 cm2), using an Autolab PGSTAT30a. The measurements were performed at room temperature and with a scan rate of 50 mV s-1. The counter electrode was a platinum sheet with an area of 1.5 cm2, and the quasi-reference electrode was a Ag/Ag+ wire. An electrolytic solution of 0.1 M LiClO4 (Aldrich) in acetonitrile (Aldrich) was used in all electrochemical experiments. All the film formation studies and characterizations were carried out in the dark to prevent photo-oxidation. For the electrical measurements an 82-layer LB film of OC1OC6-PPV was deposited on a glass substrate coated with an interdigitated chromium-gold array, whose fabrication is described in detail elsewhere.13 The electrical characterization was carried out using a Keithley 238 (High Source Voltage Unit) in the dark and under illumination at room temperature.
The polymer OC1OC6-PPV was synthesized using a procedure similar to that described in ref 12 with a weight-average molecular weight (Mw) of ∼1.8 × 105 g mol-1 and a degree of polymerization (by weight) of ∼760 as determined by high performance size exclusion chromatography (HPSEC) measurements. For the Langmuir and LB experiments, the material was dissolved in chloroform (0.2 mg mL-1) while cast films were deposited from chloroform solutions (0.5 mg mL-1) onto glass coated with ITO. Langmuir and LB films were produced with a KSV5000 Langmuir trough housed in a class 10,000 clean room. Langmuir films were spread onto ultrapure water obtained from a Millipore Milli-Q system (resistivity 18.2 MΩ cm). All experiments were conducted at room temperature (22 °C). Film compression was carried out at a barrier speed of 10 mm min-1. π-A and ∆V-A isotherms were conducted with a Wilhelmy plate and a Kelvin probe, respectively, both provided by KSV. Stability measurements of the Langmuir films were performed by keeping the surface pressure at 30 mN m-1 while the time evolution of the area per monomer was recorded and also by performing successive cycles of compression-decompression of the films. The molecular weight of the polymer-repeating unit (232 g mol-1) was adopted for all area calculations. LB films were produced by transferring pure OC1OC6-PPV Langmuir films onto one of the following substrates: glass coated with ITO (Asahi Glass Co., Japan), interdigitated electrodes of a chromium-gold array, or silicon wafers (Aldrich). Y-type LB films with either 41 or 82 layers were transferred at a surface pressure of 30 mN m-1 with a typical dipping speed of 10 mm min-1 for the upstrokes and 15 mm min-1 for the downstrokes. These nonconventional high speeds were necessary to reach the
Langmuir Films of OC1OC6-PPV. Figure 1 shows the π-A isotherms of pure OC1OC6-PPV film, featuring a collapse pressure of ∼40 mN m-1 and an extrapolated area of approximately 22-25 Å2 per monomer for the condensed phase, calculated using the molecular weight of the polymer-repeating unit. This value is close to that obtained for OC1OC6-PPV in the CPK model using the Hyperchem program (∼24 Å2), indicating the formation of monomolecular structures. Aggregation, if present, occurs to a very limited extent. The slight change in the π-A isotherm at ∼17 mN m-1 appears not to be associated with collapse, since the ∆V-A isotherm did not show significant changes in this region. The area of 22-25 Å2 is similar to the one obtained by Wu et al. for another PPV derivative, poly[2-methoxy-5-(n-hexadecyloxy)-p-phenylenevinylene] (MH-PPV),6 ∼28 Å2, assuming a molecule with the aromatic and vinyl groups oriented perpendicularly to the water surface. A possible arrangement is shown in Chart 1, which is similar to that suggested for poly[2-methoxy-5-(n-hexadecyloxy)-p-phenylenevinylene].6 Implicit in this chart is the assumption that a true monolayer is formed. Interestingly, the true monolayer appears to be formed for OC1OC6-PPV despite its having only six
(12) Marconi, F. M.; Bianchi, R. F.; Faria, R. M.; Balogh, D. T. Mol. Cryst. Liq. Cryst. 2002, 374, 475-480.
(13) Olivati, C. A.; Bianchi, R. F.; Marconi, F. M.; Balogh, D. T.; Faria, R. M. Mol. Cryst. Liq. Cryst. 2002, 374, 451-456.
Results and Discussion
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Chart 1
Figure 2. Electronic absorption spectra for a OC1OC6-PPV chloroform solution, in an LB film (41 layers) and a cast film. Also shown are the spectra of a silver nanoparticles film and a film coated with a one-layer LB film of OC1OC6-PPV.
carbons. Wu et al.6 suggested that a long alkyl chain would be necessary for the formation of true monolayers on the basis of the evidence that poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) did not form monolayers (extrapolated area of only ∼2.7 Å2).4 It seems that the linearity of the side chain may play a more important role, since both linear side chain polymers (MHPPV and OC1OC6-PPV) formed true monolayers, but the branched one (MEH-PPV) did not. Stability for the Langmuir films from OC1OC6-PPV was attained in hysteresis measurements after two compression-decompression cycles, with the π-A isotherms being the same in subsequent runs. The stability was confirmed in experiments where the surface pressure was kept at 30 mN m-1, with small decreases in area per monomer of 3% and 12% after 1 and 4 h, respectively. The surface potential of the OC1OC6-PPV films is not zero at large areas per molecule, which might denote some degree of aggregation immediately after spreading, as is common for polymer Langmuir films. The large surface potential, that is, approximately 800 mV, for the condensed phase indicates the contribution of large dipole moments in the OC1OC6PPV molecule that are perpendicular to the water surface. We calculated the vertical contribution of a monomer using Hyperchem software, but the value of approximately 0.50.6 D is much smaller than that required to give the measured surface potential, if one assumes that the molecules are embedded in a medium of dielectric constant between 2 or 3 and 6 or 7, for the air and water-film interfaces, respectively.14 Therefore, the dipole moment contribution per monomer appears to be much larger in the polymer than that if the monomers were separated. Characterization of OC1OC6-PPV LB Films. Figure 2 shows the UV-vis absorption spectra for a 41-layer LB film and a cast film, both deposited from a chloroform OC1OC6-PPV solution onto ITO. The solution spectrum for OC1OC6-PPV in chloroform exhibits a strong absorption band maximum at 490 nm, which is attributed to the π-π* electronic transition of the conjugated polymer backbone. The absorption spectra for cast and LB films are similar, with a broader band red-shifted at 515 nm, indicating that J-aggregation occurs in the films. The red shift is consistent with observations by Liu et al.15 for LB films from alkoxyl-substituted poly(cyanoterephthalylidene) (C16-CNPPV). Note that no blue shifts were observed in (14) Dynarowicz, P.; Cavalli, A.; Filho, D. A. S.; Santos, M. C.; Oliveira, O. N., Jr. Chem. Phys. Lett. 2000, 326, 39-44. (15) Liu, Y.; Li, Q.; Xu, Y.; Jiang, X.; Zhu, D. Synth. Met. 1997, 85, 1279-1280.
the films. Blue shifts may indicate H-aggregation6 but may also be caused by photo-oxidation of the vinyl group of the PPV derivatives to carbonyl groups upon exposure to light. The photo-oxidation causes the conjugation length to decrease, leading to a colorless material when the degradation is completed.12,16 Figure 2 also shows the spectra recorded for both a Ag nanoparticle film and this film covered with the LB monolayer. The broad shape of the Ag plasmon absorption band is due to the large distribution of the sizes and shapes of the Ag nanoparticles while the change in its shape by depositing the LB monolayer comes from changes in the dielectric constant of the medium where the Ag is embedded, from air to the organic material.17 Note that the 488 nm laser line is in resonance with the Ag plasmon absorption band, a necessary condition to achieve SEF.18 Figure 3 shows the FTIR and Raman spectra for the 41-layer LB film of OC1OC6-PPV whose main bands are assigned in Table 1. The complementary nature of these techniques is readily apparent in the assignments. The band at 970 cm-1, attributed to the vinylic CsH wagging in the Raman spectrum, is evidence of the slight distortion of the vinylene group in relation to the planar trans form.19 Although the film was fabricated under dark conditions, the bands around 1598 and 1670 cm-1 in the FTIR spectrum indicate some minor photodegradation. They are assigned, respectively, to the asymmetric aromatic ring and carbonyl groups (CdO conjugated aldehyde) stretching. The anisotropy in the LB film was investigated using RAIRS. Figure 4 presents the FTIR spectra recorded in the transmission mode for bulk OC1OC6-PPV and the 41-layer LB film, and in RAIRS mode for the 41-layer LB film. Differences in relative intensities are noted between the spectra for the LB films and also in the comparison between the latter spectra and the spectrum from the bulk sample. The molecular organization of the LB film can be inferred considering the surface selection rules20 and some key molecular bonds.21 For instance, the bands at 970 cm-1 (vinylic CsH wagging) and 854 cm-1 (aromatic CsH wagging), whose induced molecular dipoles are (16) Bianchi, R. F.; Balogh, D. T.; Tinami, M.; Faria, R. M.; Irene, E. A. Submitted. (17) Weitz, D. A.; Garoff, S.; Nitzan, A. J. Chem. Phys. 1983, 78, 5324-5338. (18) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783-826. (19) Sakamoto, A.; Furukawa, Y.; Tasumi, M. J. Phys. Chem. 1992, 96, 1490-1494. (20) Antunes, P. A.; Constantino, C. J. L.; Duff, J.; Aroca, R. Appl. Spectrosc. 2001, 55, 1341-1346 and references therein. (21) Guo, T.-F.; Yang, Y. Appl. Phys. Lett. 2002, 80, 148-150.
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Figure 3. Raman and FTIR spectra of OC1OC6-PPV in an LB film (41 layers). Table 1. FTIR Bands for OC1OC6-PPV for LB Films LB film
assignment
3060 2956 2935 2865 1670 1598 1509 1463 1417 1351 1260 1210 1120 1040 970 854
ν(CHdCH) vinyl νas(CH3) νas(CH2) νs(CH3), νs(CH2) CdO aldehyde ν(CdC) (aromatics) breath (CsC) aromatic δas(CH3) breath (CsC) aromatic δs(CH3) νas(C-O-C) ν(aryl-oxygen), CH2 CH2 arylsOsCH2 vinylic CsH wagging aromatic CsH wagging
perpendicular to the plane that contains both the aromatic and the vinylic groups (in a planar trans form), are stronger in the transmission mode and weaker in the RAIRS mode. This shows that the OC1OC6-PPV molecules are attached to the substrate surface by the lateral groups (R1 or R2) with the aromatic ring practically perpendicular to the substrate. In addition, the bands at 1509 and 1417 cm-1 assigned to the ring deformation are strong in the RAIRS spectrum for the LB film, which is consistent with the arrangement suggested for OC1OC6-PPV molecules in Chart 1. The results above show unequivocally that the side groups R1 and R2 of the OC1OC6-PPV molecule are preferentially oriented in the LB film, with the vinyl group outside the plane. However, unlike the case of thermally converted PPV,11 the preferential orientation did not lead to significant shifts in the absorption spectra of the LB film in comparison to cast films (see Figure 2). The reason for such a difference lies in the fact that in the LB film from OC1OC6-PPV the side groups are preferentially oriented but the conjugated chains are apparently less organized. Therefore, one should not expect the degree of conjugation (and consequently the position of the absorption peak) to depend on whether the film is cast or LB.
Furthermore, for the parent PPV the thermal conversion process probably differs from cast to LB films and may lead to a different degree of conjugation. Fluorescence spectra obtained at different regions of a sample with a 514.5 nm excitation laser line are shown in Figures 5 and 6 for cast and 41-layer LB films, respectively. The insets in these figures show the optical images recorded with a 50× lens of the regions from which the spectra were collected. The spectra are reproducible for the LB film (Figure 6) but not for the cast film (Figure 5). Moreover, the morphology displayed in the optical image is reproduced in other parts of the film for an LB film, whereas for the cast film the size of the aggregates varies from region to region. As for the spectra themselves, two explanations have been proposed in the literature for the observation of broader, structureless emission spectra for cast films: (i) According to Samuel et al.,22 this could be due to interchain excitation involving molecular arrangements such as dimers. For the LB film, the welldefined vibronic structure in the emission spectra would be associated with intrachain excitation. (ii) Alternatively, the broader emission spectra could be due to defects in the film, which produce disorder and shorter conjugation lengths, in comparison to the case of the LB films.23 Figure 7 shows the first SEF curve for PPV derivatives as far as we know. The average enhancement factor, that is, the ratio between the maximum emission of the monolayer on Ag and on glass, is approximately 6, which agrees with the estimated enhancement obtained with the electromagnetic model.17 The fluorescence enhancement offers further evidence that OC1OC6-PPV is anchored to the substrate in the LB film by the R1 or R2 groups. Otherwise, if the emitter group were in contact with Ag, the fluorescence would be vanished rather than be enhanced, with the energy being transferred directly from the emitter group to Ag nanoparticles.24 (22) Samuel, I. D. W.; Rumbles, G.; Collison, C. J.; Friend, R.; Moratti, S. C.; Holmes, A. B. Synth. Met. 1997, 84, 497-500. (23) Marleta, A.; Gonc¸ alves, D.; Oliveira, O. N., Jr.; Faria, R. M.; Guimara˜es, F. E. G. Adv. Mater. 2000, 12, 69-74.
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Figure 4. FTIR transmission spectra of bulk and LB films (41 layers) and RAIRS spectrum of a LB film (41 layers) of OC1OC6-PPV.
Figure 5. Fluorescence spectra of a cast film of OC1OC6-PPV. The inset shows the optical image (50×) of a region of the film.
Distinct morphologies were also observed in AFM images for a cast and a 41-layer LB film onto ITO (Figure 8). The morphology is always globular, but in the LB film there are more globules in a given area. The thickness of cast films is three times that of the 41-layer LB film, as expected, but the roughness is about two times smaller (24) Antunes, P. A.; Constantino, C. J. L.; Aroca, R. F. Langmuir 2001, 17, 2958-2964 and references therein.
(see Table 2). While one should expect less rough LB films, which are more homogeneous in terms of surface distribution than the cast films, it should also be considered that the LB films are anisotropic. Data from the Langmuir monolayers as well as from the LB films presented here indicate that the OC1OC6-PPV molecules are anchored by the side groups (R1 or R2) with the chain perpendicular to the substrate. The aggregates formed in the solid films
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Figure 6. Fluorescence spectra of a LB film (41 layers) of OC1OC6-PPV. The inset shows the optical image (50×) of a region of the film; all other regions presented identical images.
Figure 7. Emission spectra recorded with the 488 nm laser line for the LB monolayer deposited on glass and on a Ag nanoparticle (SEF).
may therefore also be oriented perpendicularly to the substrate, yielding a large roughness. A similar result was observed with lignin LB films in which perpendicularly oriented aggregates led to films that were rougher than cast films of the same lignin.25 The electroactivity of LB films from OC1OC6-PPV was investigated by cyclic voltammetry. Figure 9 shows three
voltammetric cycles for cast and LB films deposited onto ITO, using a sweeping rate of 50 mV s-1 in an electrolytic solution of 0.1 M LiClO4 in acetonitrile. For both figures, the first cycle gives an onset potential for oxidation at (25) Pasquini, D.; Balogh, D. T.; Antunes, P. A.; Constantino, C. J. L.; Curvelo, A. A. S.; Aroca, R. F.; Oliveira, O. N., Jr. Langmuir 2002, 18, 6593-6596.
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Figure 9. Cyclic voltammograms for (a) cast and (b) LB films from OC1OC6-PPV with a sweeping rate of 50 mV s-1. All measurements were carried out with 0.1 M LiClO4 in acetonitrile.
Figure 8. Tapping mode AFM images of OC1OC6-PPV (a) in a cast film onto ITO and (b) in a LB film deposited onto ITO (82 layers). Table 2. Roughness and Thickness of a 41-Layer LB Film and a Cast Film of OC1OC6-PPV roughness (rms) (nm) thickness (nm)
cast
LB
7.3 230.2
14.2 70.5
∼ +0.4 V (cast film) and +0.28 V (LB film). In the subsequent cycles the voltammograms remain unaltered with an oxidation potential of +0.20 V for the cast film and +0.16 V for the LB film. Apparently, the change in potential for the first cycle is caused by the overpotential in the first p-doping.26 For OC1OC6-PPV cast films a broad peak appears, with predominance of capacitive current. This effect could correspond to a dedoping process of OC1OC6-PPV. This difference in the voltammograms for cast and LB films may be caused by the molecular orientation in the LB films, as observed in the FTIR measurements. Figure 10a shows I versus V curves while Figure 10b shows how conductivity (σ) varies with the light intensity for an interdigitated chromium-gold array coated with an 82-layer LB film. The curves are linear, indicating Ohmic behavior. From Figure 10a it is possible to estimate the conductivity (σ) for each light intensity by fitting the curves, using the parameters l (thickness) ) 100 nm and (26) Yang, C.; Zheng, J.; Fan, L.; Li, Y. Supramol. Sci. 1998, 5, 519522.
Figure 10. (a) Current vs voltage at different illumination conditions for OC1OC6-PPV. (b) Conductivity vs intensity for OC1OC6-PPV.
A (area) ) 2 nm2. The conductivity increases from 6.3 × 10-8 S m-1 under dark to 4.4 × 10-7 S m-1 at an illumination of 152 mW cm-2, showing the photoconduction property of this material. This dark conductivity is
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in the range of undoped semiconductor materials.27 The photoconduction effect is reversible and is not affected by possible photo-oxidation reactions, since the material conductivity returns to the initial value under the dark condition and is reproducible in further cycles of illumination. The fatigue of the photoconduction property is now under investigation. Conclusion A conjugated polymer, OC1OC6-PPV, was successfully spread at the air-water interface and formed stable monolayers, with an area per monomer consistent with the aromatic rings of the main chain being perpendicular to the air-water interface. Y-type LB films were successfully transferred onto different solid substrates. FTIR (transmission and RAIRS modes) spectra for LB films showed that the OC1OC6-PPV molecules must be attached (27) Friend, R. H. In Physics and Chemistry of Electrons and Ions in Condensed Matter; D. Reidel Publishing Company: Dordrecht, The Netherlands, 1984; pp 625-651.
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to the substrate surface by the lateral groups (R1 or R2) with the aromatic ring practically perpendicular to the substrate. In addition, a comparison of the Raman spectra from the films and bulk sample points to a slight distortion of the vinylene group in relation to the planar trans form. FTIR, RAIRS, fluorescence spectra, and cyclic voltammetry showed remarkably different behaviors between cast and LB films, probably due to the molecular organization in the LB films, in contrast to the randomly oriented molecules in cast films. Surface enhanced fluorescence was observed for the LB monolayer deposited onto a Ag nanoparticle film in comparison with the LB monolayer on glass. The LB films also showed reversible photoconductive properties. Acknowledgment. The authors are grateful to FAPESP, CNPq, IMMP/MCT (Brazil), and NSERC (Canada) for the financial support and to Professor Ricardo Zanatta from IFSC/USP for the Raman facilities used in the SEF measurements. LA0346595
