Crystallization Growth and Micropatterning on Self-Assembled

School of AdVanced Materials Engineering, Kookmin UniVersity, Sungbuk-gu, ... Department of Semiconductor Display Engineering, Hoseo UniVersity, Asan ...
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11252

J. Phys. Chem. C 2007, 111, 11252-11258

Crystallization Growth and Micropatterning on Self-Assembled Conductive Polymer Nanofilms Jin-Yeol Kim,*,† Min-Hee Kwon,† Jae-Taek Kim,† Sijoong Kwon,† Dae-Woo Ihm,‡ and Young-Kun Min| School of AdVanced Materials Engineering, Kookmin UniVersity, Sungbuk-gu, Seoul 136-702, Korea, Department of Semiconductor Display Engineering, Hoseo UniVersity, Asan 336-795, Korea, and Department of Chemistry, Graduate School of Science, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ReceiVed: December 5, 2006; In Final Form: April 3, 2007

A process for producing conducting polymer nanofilms is demonstrated. In this process, organic polymers, which have a π-conjugated system, become self-assembling by depositing polymerization in the gas phase. The polypyrrole nanoscale films, having several crystalline morphologies, were successfully fabricated by a self-organizing technique of monomer, and their fine-pitched patterns can also be easily formed by microcontact printing, using selective self-assembly. Compared with traditional polymerization and film-coating procedures, this process reduces the percolation threshold for electrical conductivity, increases the conductivity by several orders of magnitude, and simultaneously improves the thermal stability and light transmittance. These conductive thin films, possessing polycrystalline structures, have a very high conductivity and electron mobility and are capable of being applied to organic optoelectronic films for electrical devices such as the organic EL and semiconductor.

Introduction π-Electron-conjugated polymers have attracted considerable attention in the past few decades because of their electronics/ physical properties and potential application in electronic devices.1-5 Particularly, their nanofilms and/or nanostructured materials are known to play an important role in optoelectronic devices, ranging from organic transistors1 and electronic flat panel displays2 to chemical sensors3 and artificial actuators,4 both as interconnecting and as active components. Optoelectronic devices often require polymeric transparent electrodes to improve device performance and for the manufacture of devices with sole polymer components.6-9 For applications requiring both conductivity and good optical properties, these polymers are blended with tougher insulating polymer matrixes, resulting in mechanically resistant and easy-to-process conducting polymer blends.10-11 In these blends, conductivity is achieved by percolation of the dispersed conducting polymer phase hosted in an insulating polymer matrix and spin-coated or/and cast for film formation. However, only thin films can be produced with these procedures, and the conductive films are unstable when processed at temperatures above the Tg of the matrix polymer. To overcome these drawbacks, we propose a different and new concept to produce the doped π-conjugated polymer films that are stable at high temperatures, highly conductive, and highly transparent at nanoscale in a continuous process without using any matrix or binder polymers. Using this new procedure, we can also easily realize well-defined crystalline structured thin films with a fine-pitched pattern. The * To whom correspondence should be addressed. Phone: 82-2-9104663. Fax: 82-2-919-1369. E-mail: [email protected]. † Kookmin University. ‡ Hoseo University. | The University of Tokyo.

electronic devices, especially photoelectronic displays and organic semiconductors etc., in which a high light transmittance, electronic conductivity, and electron mobility are required, need ultrathin films and single crystalline structures for more improved performance of the devices. In polymeric materials, most research into microstructure formation during solidification has focused on single-crystal growth, ranging from faced crystals to symmetric dendrites. These growth forms can be perturbed by heterogeneities, yielding a rich variety of polycrystalline growth patterns.12-16 We have developed a vapor-phase-deposited polymerization technique17 using a self-assembly method to create conducting polymer ultrathin films and patterning at the nano- or microscale (Schemes 1 and 2). Here, the vapor-phase polymerization technique is one of the nanofabrication techniques based on a bottom-up processing method which can utilize well the organic arrangement of macromolecules to produce ordered aggregates at the scale of a nanolayer. This new process is the self-assembly polymerization technique progressing in the gas phase. The thin films are prepared in a continuous process without any additional doping process and without using any polymeric binders for film formation. Conventionally, the conducting polymers having a π-conjugated bond have been synthesized by either an oxidatative chemical or electrochemical polymerization of a monomer in the liquid phase, and these polymers are blended for film formation with tougher insulating polymer matrixes. Self-assembly of polymeric supramolecules is a powerful tool for producing functional materials that combine several properties and may respond to external conditions. In this paper, we present studies on the structure and properties of the conductive ultrathin films prepared by selfassembling polypyrrole, PPy, which is grown on the substrate

10.1021/jp0683622 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/11/2007

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SCHEME 1: Schematic Outline of the Procedure To Fabricate Self-Assembled PPy Nanofilms and Patterned PPy Thin Films by Using the VDP Technique

SCHEME 2: Schematic Outline of the Procedure To Produce Self-Assembled PPy Patterned Films Using Microcontact Printing and Selective PPy Layer Deposition by the VDP Method

film in the gas phase and chemically deposited. Distinctive features of these conductive ultrathin films of p-doped conducting polymer include high electrical conductivity and their having various polycrystalline microstructures. Furthermore, the selective growth of polymer nanofilms can also be easily prepared by deposition of self-assembled PPy layers on patterned oxidant layers formed using nanocontact printing or imprinting methods as shown in Scheme 2. Experimental Section Preparation of PPy Nanofilms and Growth of Crystalline Structures. PPy nanofilms were prepared by gas-phasedeposited polymerization using a self-assembly method. The electrically conductive PPy thin films were directly grown on the polymeric film substrates by polymerizing pyrrole monomer in the vapor phase (as shown in Scheme 1). Solutions of ferric chloride hexahydrate (FeCl3‚6H2O; 5-10 wt %) in methanol were prepared as an oxidant. First, the FeCl3‚6H2O solution was pretreated on clean bare polymeric substrate films (poly(ethylene terephthalate), PET; polyimide, PI; polycarbonate, PC; etc.) by dip or spin coating. In this study pyrrole monomer (Aldrich,

GR), FeCl3‚6H2O (Aldrich, 98%), and MeOH (Aldrich, 98%) were used without any further purification. After being dried at 60 °C, in the second stage, polymeric substrate films pretreated with FeCl3‚6H2O were exposed to a pyrrole vapor for 10 s to 60 min in a vapor deposition chamber under ambient conditions. In the third stage, after the completion of polymerization, the grown PPy nanofilms were directly produced through the washing process, using methanol to remove the unreacted oxidant, pyrrole monomer, and byproducts, and dried for 3 min at 80 °C. The concentration of oxidant, deposition temperature, and polymerization time with pyrrole were moderately controlled to obtain a desirable conductive PPy film thickness and crystalline structures. To improve the adhesive properties of the base film, we adapted the corona or plasma treatment using O2 gas at 13.56 MHz rf power before the oxidant coating. Micropatterning of PPy Nanofilms. Micropatterning of PPy nanofilms is possible in accordance with the selective vaporphase deposition of PPy thin films on a patterned oxidant formed by microcontact printing (as shown in Scheme 2) in the first stage of the above-mentioned process. In the case of this process, the plasma is also used for the activated surface of the base film instead of a polymer matrix so that adhesiveness between the base materials and the conductive PPy layer is improved. Measurements for PPy Nanofilms. The thickness and the conductivity of conductive PPy thin films grown on the plastic substrates were measured using scanning electron microscopy (SEM) (JSM-633F, Jeol) and the standard four-probe technique (Loresta-GP, Mitsubishi Chemical), respectively. The polymer morphologies were observed with an atomic force microscope (Nanoscope III a DI) and a polarized optical microscope. A nearinfrared Raman measurement system with an optical fiber probe was used for the identification of microcrystalline structures on PPy nanofilms. A Q-switched Nd:YAG laser (Spectra Physics X30-106QA, wavelength 1064 nm, repetition rate 10 kHz, pulse duration 100 ns) was used as the near-infrared Raman excitation source. Raman signals were transported through a 300 µm collection fiber to the entrance slit of a single polychromator (Horiba Jobin-Yvon, TRIAX320). An InP/InGaAsP nearinfrared multichannel detector (Hamamatsu Photonics K. K., intensified CCD, quantum efficiency ca. 1% for 1-1.4 µm) was coupled with a relay lens to the exit slit of the polychromator. Results and Discussion The self-assembly of molecules or small clusters, that is, the spontaneous association of atomic or molecular building blocks under conditions of equilibrium, is emerging as a successful

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Kim et al.

Figure 1. (I, II) Microscopic and AFM surface images of self-assembled PPy nanofilms, respectively. (III) Thickness of the nanofilm measured by SEM. (IV) AFM image and profile of the micropattern formed by microcontact printing using selective self-assembly.

Figure 2. Polarized optical microscopy images of the PPy polycrystalline morphologies: (I) disordered dendritic growth forms seen in PPy deposited at a temperature of 15 °C, (II) dendrites are fully grown at a temperature of 40 °C; (III) single-crystal needles are grown at a temperature of 80 °C (a, b, and c show crystalline forms grown when the pyrrole is exposed in a VDP chamber for 30 s, 10 min, and 60 min, respectively). The scales are 100 × 100 µm.

chemical strategy to create well-defined structures of nanometer dimensions, with potential applications in many areas of nanotechnology. This vapor-phase-deposited polymerization (VDP) using a self-assembly process was developed to have the advantages of both producing ultrathin films with high purity and processing in a dry condition. In this work, to produce PPy nanofilms, oxidants such as FeCl3, Fe(ClO4), and iron(III) salts of organic acids/inorganic acids containing organic radicals were needed. Conventionally, iron(III) salts can be conveniently used not only as “dopant” ions for π-conjugated polymers but also as an “oxidant” for polymerization. Pyrrole monomer has a relatively high vapor pressure, and the polymerization of PPy in the in situ vapor phase can be readily initiated by exposing an oxidant to pyrrole vapor. By the above-mentioned method, the conductive PPy film was assembled on a substrate film. Their thickness was freely controlled at between 20 and 100 nm according to the deposition tine, and their fine-pitched patterns can also be easily formed by microcontact printing,

using selective self-assembly as shown in Figure 1. Compared with traditional polymerization and film-coating procedures, this process reduces the percolation threshold for electrical conductivity, increases the conductivity by several orders of magnitude, and simultaneously improves the thermal stability and light transmittance. These conductive thin films, possessing polycrystalline structures, have a very high level of conductivity and electron mobility. In particular, the growth of PPy crystalline entities was strongly dependent on the deposition temperature and time. Figure 2 shows a two-dimensional view of (scale 100 µm × 100 µm) polarized optical microscopy images of the PPy layer surfaces deposited at several stages. For comparison, the crystalline structures of poly(3,4-(ethylenedioxy)thiophene) and polyaniline films prepared by the same procedure are shown in Figure 3. Figure 2, images I-a, II-a, and III-a, shows the initial stage of growth when the pyrrole was exposed in the VDP chamber for 30 s at 15, 40, and 80 °C, respectively. At the low

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Figure 3. (I, II) Polarized optical microscopy images of poly(3,4-(ethylenedioxy)thiophene), polycrystalline morphologies that have grown during 10 and 60 min at a temperature of 40 °C, respectively. (III) That of polyaniline grown for 60 min at a temperature of 40 °C. The scales are 100 × 100 µm.

Figure 4. Two- and three-dimensional AFM images of the typical topography, the sample from image II-b in Figure 2. The scan area was 50 × 50 µm.

temperature of 15 °C, the nucleation of fine dendrites was generated as shown in Figure 2, image I-a. At 40 and 80 °C, the initial stages of dendritic (image II-a) and needle-shaped (image III-a) crystal morphology were generated with a lower growth rate, respectively. The growth pattern observed at 40 °C can be explained by assuming that the dendrite probably advanced through the nucleated liquid-phase pyrrole monomers as it grew out from the solid-liquid interface into the liquid at high supercooling. The supercooling seems to provide the driving force for the dendritic growth. The boundary of the stable and unstable interfaces for pyrrole crystallization appears to be formed at about 80 °C since the single-crystal needle grows from a stable interface into the liquid by the formation of an oval projection and the dendrite grows from an unstable interface. Initial stages of dendritic growth, fully grown dendrites, and early stages of crystal growth occurred on the dried oxidant which was exposed to the three different temperatures for 10 and 60 min, as shown in parts b and c of Figure 2, respectively. When deposition was performed for 10 min at 15 °C, dendritic growth ceased and pyrrole polymerization commenced between the dendrite grains (image I-b) and grains formed by the dendritic growth were pushed back and agglomerated around each other according to the increase in deposition time (image I-c). However, symmetric dendrites had fully grown at the intermediate temperature, as shown in Figure 2, image II-b. The dendrites had both primary arms and secondary arms but not tertiary arms since they are grown in the thin film. Figure 4 shows typical two- and three-dimensional atomic force microscopy (AFM) images of the sample from image II-b. The primary arm grows with a high velocity through the loosely packed planes until it touches other dendrites grown from the nearby nucleation sites. The crystallographic planes of the growing dendrites have a 90° angle from each other and form 4-fold symmetric dendrites, but the growing directions of the arm were often deflected by the other dendrites or foreign

particles. The curling primary arms and extra arms that appeared in the upper and lower parts of Figure 2b resulted from a sequential deflection of the dendrite tips and formed a polycrystalline structure. The deflection was caused by the interaction between the primary dendrite arms having secondary arms. As the deposition time increased, the tips of the primary arms were cut, pushed, or split by the tips of other growing dendrites as dendritic growth of pyrrole proceeds by a regular arrangement of pyrrole monomers which have a week binding energy. The primary arms having a low strength in the grains were pushed back in all directions by the surrounding dendrite grains and by the polymerized pyrroles, as shown in image II-c. Similar crystalline structures have been observed by V. Ferreiro et al.14 They described how the crystalline morphology of poly(ethylene oxide) (PEO) mixed with poly(methyl methacrylate) (PMMA) in a thin-film geometry can be “tuned” through spherulitic, seaweed, symmetric dendritic, and fractal dendritic patterns through the adjustment of the PMMA composition. However, when deposition was performed for 10 min at 80 °C, a mixture of dendrites and single-crystal needles was obtained, which represents a boundary condition changing from dendritic growth to single-crystal growth. The single-crystal growth is feasible at the lower growth rate and at the lower supercooling. However, the supercooling at this temperature was not low enough to form a fully grown single crystal and led to dendritic growth from the side of the single crystal. In the case of a 30 min deposition time, rod-shaped crystals (III-c image) were grown from the single-crystal needles and thick dendrite arms consumed their small arms through the polymerization that occurred at the interdendritic regions. Finally, in the case of a longer deposition time, these dendrites and single-crystal needles were frozen at the higher polymerization rate. New dendrites and needles formed, owing to a low catalytic effect on the covered polypyrrole surface. However, in the case of poly(3,4-(ethylenedioxy)thiophene) and polyaniline, their crystallization patterns were very different from those of Ppy, as shown in Figure 3. In particular, the crystallization pattern of polyaniline has a circular shaped domain. However, the crystallized PPy films show a very smooth surface that results in a good reflection of light and the surface becoming mirrorlike. Furthermore, these structures, having polycrystalline structures of several types, grow to an 80 µm size as the deposition temperature increases. The size and shape of the crystalline superstructures in each image illustrate the qualitative differences in the morphologies and rates that can be covered by the reaction conditions. According to Billia and Trivedi18 and Wunderlich,19 the crystallization of polymeric materials under the processing conditions normally occurs far from equilibrium and the properties of these materials depend strongly on the growth conditions. A wide range of crystal growth patterns can be observed, depending on the extent of undercooling or supersaturation. The supercooling seems to provide the driving force

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Figure 5. Conductivity change of PPy films according to the deposition time or crystallization. Curves 1, 2, and 3 are the conductivity change of PPy films deposited for 1-30 min at temperatures of 15, 55, and 80 °C, respectively.

for the dentritic growth as mentioned earlier. It is also speculated that the oxidants as impurities present in the polymerization process act as catalysts for dendritic growth patterns, as found in the ceystallization of certain small-molecule fluids with impurities.20,21 The 1064 nm excited Raman spectrum and refractive index were used for the identification of microcrystalline structures grown on PPy nanofilms. Raman absorption bands arising from C-C and C-N stretching are clearly different between the amorphous and crystalline areas. Thus, in the crystalline domains, the relative Raman intensities of the 1582 and 1476 cm-1 peaks of symmetrical CdC and C-C stretching, the 932 cm-1 peak of ring deformation due to a bipolaron, and the 1380 cm-1 peak of N-C stretching distinctly increase. However, the relative intensities of the C-H in-plane bending peaks are not changed in spite of crystalline growth. From these results, we can speculate that these crystalline-induced Raman intensities may be caused by the closely packed structure or π-π streak, originating between molecules due to crystallization. For the films having crystalline morphologies, we also observed the anisotropic property of the refractive index. However, further research is needed to understand the origin of this ubiquitous polymer crystallization morphology. Electrical properties are very important because many applications are based on electrical behavior. Conducting polymers are straightforwardly prepared by several methods, and their electronic states can be reversibly changed between insulating and conducting states by redox reactions. Conductivity is the product of two important factors: the number of carrier electrons or holes and carrier mobility, which in a loose sense is the case in which a carrier moves through a material. The electrical conductivities of most conductive polymers are in the same range as those of inorganic semiconductors, but there is some difference according to the degree of crystallinity, purity, and a lack of defects in these materials. In this work, we obtained PPy nanofilms that have a high crystallinity and purity, and their electrical properties are observed. In Figure 5, we show that the conductivity changes of PPy films according to the deposition time or crystallization plotted between 1 and 30 min at temperatures of 15, 55, and 80 °C. At the initial stage of polymerization of pyrrole, its conductivity increases very quickly with time due to the deposition of pyrrole. The conductivity no longer increases after approximately 10 min of deposition. The conductivity is 102 S/cm at the initial stage of deposition and up to 1000 S/cm at over 10 min of deposition. However, in the case of low-temperature conditions, the behavior of the conductivity increase is slower and lower than that at high-

Kim et al. temperature deposition as shown in Figure 5. The crystalline structures and crystallinity are different according to the deposition temperature and higher conductivities are shown at high crystallinity and for needle-shaped crystals than dendritic morphologies. Here, the crystalline morphologies depend on the temperature of film growth as shown in Figure 2. The conductivity no longer increases at over 30 min of deposition, and at this time, the film thickness is almost bulky at 150200 nm. However, the conductivity of the PPy film strongly depends on crystalline growth. In general, in the case of metal films (Au, Pb, Sn, Te, etc.), the degree of film thickness dependence on the conductivity is related to its mobility and correlates with the crystallinity of the film26 and the surface scattering, discontinuities in the films, quantum size effect, island formation, and other factors.27 The surface scattering was attributed to the thickness being less than or comparable to the mean free path of the charge carriers. In this case, the PPy films contained partially charged domains dispersed in neutral polymer chains. The metallic properties of conductive polymers have been known to be due to partially charged domains (polarons or bipolaron lattice structure), stemming from a defect in the conjugated chains of the neutral polymer.28 This may lead to conductivity depending on the film thickness. However, the conductivity as a function of the deposition time or crystallinity is related to the difference in pyrrole diffusion that affects the surface morphology of PPy. For the demonstration of conductivity as a function of crystallinity, we used a method employing a scanning probe microscope. Several methods have been proposed for measuring the conductance of single molecules.22-25 One of these is a method employing a scanning probe microscope tip as an electrode. It was demonstrated that the conductive scanning probe microscope could be used for the electrical characterization of molecules, self-assembled monolayers, or thin films. In this measurement, a scanning probe microscope equipped with a diamond tip cantilever was used to measure the electrical conductivity of a PPy crystal, and the electrical contact on the other side of the template was achieved with a conducting gold paste. Figure 6 shows the change of the I-V characteristics at different contact points. All the I-V curves display exponential increases until they saturate at the highest full-scale current of 100 nA. These curves represent the typical I-V traces obtained reproducibily depending on the doping states of the films. In particular, in local I-V measurements, shown in traces 1 and 2, crystalline regions of the sample showed ohmic behavior. This ohmic behavior has more vertical form at the fully grown dendritic or needle-type crystal than that at the amorphous or disordered dendritic type. Thus, the crystalline regions show a higher current response than the amorphous regions as the bias voltage increases. It was shown that a linear relationship between the voltage and the current was observed when the bias voltage was in the range from -0.5 to +0.5 V, indicating that the conductivity is constant. However, when a relatively high voltage (>0.5 V) is biased, the conductivity of the PPy film increases with increasing bias voltage. That is, the PPy nanofilms that have crystalline structures exhibit nonlinear I-V characteristics at high bias voltage. As mensioned above, the conductive PPy thin films made by the vapor-phase polymerization method show good electroconductivity because they can be grown with the microstructure having uniform and compact morphologies. In general, the spin-cast film prepared from a PPy solution in organics has the surface morphologies of fine protrusion.29 The conductivity of the resulting film was lower than 10 S/cm, and the light transmittance in the visible range was lower than 70%.

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Figure 6. I-V characteristics of PPy nanofilms obtained from marking spots in the film shown in an AFM image of the sample (upper inset). The lower inset is a plot in the range of -0.3 to +0.3 V.

The transmittance of PPy films was controlled by the thickness of the film. Light transmission of a conductive PPy film itself is measured by subtracting the spectrum of the substrate film (PET) from that of PPy deposited on the PET film. Its optical characteristics showed a high transmittance value above 90% when the thickness of the film was between 20 and 40 nm. Electronic absorption bands due to the polymer moiety are also observed clearly in the UV-vis spectra of the PPy films. In the case of these PPy films, a broad absorption band is apparent in the 460 nm region. This absorption band is attributed to transitions from the valence band to the uppermost bipolaron band,30-31 thus dominated by π-π* transitions. Recently, thin polymer films having high transparency and conductivity have been used as optical films or antistatic coating technologies for displays. As mentioned above, the conductive PPy thin films made by the vapor-phase polymerization method show good electroconductive and optical properties which might be attributed not only to the growth of a thin film with a nanoscale thickness and to a surface structure of high integrity but also to film formation without using any binder or matrix polymer. Conventionally, for a concuctive PPy film requiring both conductivity and good optical properties, these polymers are blended with tougher insulating polymer matrixes, resulting in mechanically resistant and easy-to-process conducting polymer blends.10,11 To overcome these drawbacks, we propose a different and new concept to produce doped π-conjugated PPy films that are stable at high temperatures, highly conductive, and highly transparent at nanoscale in a continuous process without using any matrix or binder polymers. Accordingly, this process can also largely reduce the percolation threshold for electrical conductivity because the films are composed of only pure PPy component and, thus, are not used as insulating host polymer matrixes for film formation, even if their thickness is very thin, i.e., at the nanolevel. In conclusion, we investigated anew the gas-phase-deposited polymerization technique using self-assembly of a monomer to

produce conducting polymer ultrathin films or patterning at the nanoscale. The surface of conductive PPy nanofilms shows much uniformity, compactness, or tight morphology. We have shown that the crystallizations in polypyrrole films are grown in several patterns. Polymerization of the vapor-phase pyrrole has two distinct regimes at low temperatures. Crystallization of pyrrole occurring in the first regime consists of two stages: a nucleation stage and a growth stage. Nucleation and growth mechanisms prevail in the early stage of the polymerization. The crystallization ceases and is followed by the catalytic polymerization of pyrrole in the second regime. Growth conditions for a single-crystal pyrrole of the needle type occur at over 80 °C and at the nucleation and growth stages. Their crystalline morphologies also show different forms according to the depositing monomer species. In addition, the thin polymer films, possessing high transparency and conductivity, have been used as optical films. These organic conducting polymers are used as the antistatic coating in flat panel displays. These properties are sufficient for the thin film to be applicable to the organic optoelectronic devices to achieve all-polymer devices. Acknowledgment. This work was supported in part by the ERC program of MOST/KOSEF (Grant R11-2005-048-00000). References and Notes (1) Yao, Z.; Poatma, H. W. Ch.; Balents, L.; Dekker, C. Nature 1999, 402, 273. (2) Normile, D. Science 1995, 286, 2056. (3) Kong, J.; Franklin, N. R.; Zhou, C.; Chapline, M. G.; Peng, S.; Cho, K.; Dai, H. Science 2000, 287, 622. (4) Baughman, R. H.; Cui, C.; Zakhidov, A. A.; Iqbal, Z.; Barisci, J. N.; Spinks, G. M.; Wallace, G. G.; Mazzoldi, A.; De rossi, D.; Rinzler, A. G.; Jaschinski, O.; Roth, S.; Kertesz, M. Science 1999, 284, 1340. (5) Sirringhaus, H.; et. al. Science 2000, 290, 2123. (6) Gustafsson, G.; Cao, Y.; Tracy, G. M.; Klavetter, F.; Colaneri, N.; Heeger, A. J. Nature 1992, 357, 477.

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