Electroactive polymer patterns with metal incorporation on a polymeric substrate.

INTRODUCTION

The potential for using electroactive polymers as active materials in optoelectronics (1, 2), microelectronics (3), sensors (4), and related areas (5) has recently stimulated significant research interest. The doped, conductive forms of electroactive polymers are being evaluated as possible alternatives to metals as connecting wires and conductive channels, since the conductivity of these materials can be tuned over a wide range by changing the dopant and/or doping level. A number of techniques, such as photolithography (6, 7), e-beam writing (8), laser writing (9) and surface-templated deposition (10), have been successfully demonstrated for the formation of patterned microfeatures based on conductive polymers such as polyaniline, polypyrrole, and polythiophene.

In the present work, we report on two methods for patterning electroactive polymer on polymeric substrates. In the first method, the pattern formation was developed from photo-induced reaction between polyaniline (PANI) and viologen through a mask. Conventionally, photolithographic patterning of conducting polymer films requires the coating of a resist layer, exposure and development of the photoresist (11). In the present work, through proper surface functionalization of the polymeric substrate, a photoresist was not required, which allows for simplicity and cost-saving. The photo-induced reaction between PANI and viologen immobilized on the substrate results in the doping of the PANI (12). The difference in solubility between the doped (i.e., irradiated) and undoped regions allows the use of a solvent to dissolve the soluble, undoped and nonconducting parts, thereby generating patterns on the substrate.

The second approach involved selective surface deposition via plasma polymerization through a mask. Recently, plasma polymerization has been recognized as another important method to obtain thin films of conductive polymer (13-19). As reported earlier, the chemical structures of plasma-polymerized conductive polymers are rather different from conventional polymers and are dependent on the plasma polymerization conditions (13, 17, 20). Plasma-polymerized conductive polymer films have been characterized as high quality, adherent and pinhole-free with a high degree of cross-linking and branching (18, 21, 22). In the present work, we demonstrate that patterns on the microscale and nanoscale can be conveniently fabricated on the surfaces of polymeric substrates through a mask. In both techniques, metal incorporation on the electroactive patterns can be further realized, which gives rise to potential applications in sensor technology, nanoelectronics, and catalysis. Characterization of the electroactive polymer films and patterns was carried out using UV-vis absorption spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and sheet resistance measurements.

EXPERIMENTAL

Preparation of PANI-Viologen Film

LDPE films (0.125 mm in thickness) obtained from Goodfellow Inc. were washed in acetone for 0.5 hr using an ultrasonic bath to remove surface impurities. The films were then dried under reduced pressure and cut into strips of 2 cm X 4 cm. The washed LDPE films were plasma-treated with argon plasma for 30 s using an Anatech SP100 plasma system. The plasma power applied was 35 W at a radio frequency of 40 kHz. The pretreated films were exposed to air for approximately 5 minutes to facilitate the formation of surface oxide and peroxide groups (23).

The graft copolymerization of vinyl benzyl chloride (VBC) with LDPE films was first carried out as follows. A small amount of VBC monomer was placed on both surfaces of the argon plasma-pretreated LDPE film, and the films were then sandwiched between two pieces of quartz plate. This assembly was inserted into a Pyrex tube and then exposed to UV-irradiation in a Riko rotary photochemical reactor (RH400-10W) at 24[degrees]C - 28[degrees]C for 40 minutes. The graft-copolymerized films were extracted from the quartz plates and then washed thoroughly with N-dimethylformamide (DMF) to remove the VBC homopolymer. The viologen moieties were introduced via the reaction of the VBC-graft copolymerized films with an equimolar mixture of dichloro-para-xylene and 4.4'-bipyridine (0.06 M of each) in DMF at 60[degrees]C for 20 h. The reacted films were washed with DMF followed by doubly distilled water to remove unreacted reactants and homopolymers. The viologen-grafted films were finally dried under reduced pressure. Schematic representations of the VBC graftcopolymerized and viologen-functionalized LDPE film can be found in a previous paper of Zhao, Neoh, and Kang (12).

Polyaniline was coated onto the viologen-grafted films as follows. 0.23 g (NH4)[.sub.2][S.sub.2][O.sub.8] and 0.4 ml aniline monomer were each dissolved in 20 ml of 1-M HCl[O.sub.4] in a 50-ml beaker. The two beakers were then kept in an ice bath for 30 min to cool the solutions to 0[degrees]C. The oxidant ((N[H.sub.4])[.sub.2][S.sub.2][O.sub.8]) solution was dripped gradually into the beaker containing the aniline-HCl[O.sub.4] solution under stirring over a period of 10 min. Then the viologen-grafted films were immersed in the polymerizing mixture, hanging freely in the reaction mixture from stainless steel hooks. The beaker was then covered with Al foil and polymerization was carried out for 2 hr at 0[degrees]C. The polyaniline coated films were washed with 1-M HCl[O.sub.4] before being washed with 0.5-M NaOH for 1 hr or with doubly distilled water for 4 hr to obtain the 50% oxidized emeraldine-base (EB) thin films. These films were finally dried under reduced pressure.

Pattern Formation of PANI-Viologen Film by Photo-Irradiation and Plasma Treatment

Photopatterning on the PANI-viologen film on the LDPE substrate was carried out by exposing the films through a mask to UV irradiation between 24[degrees]C to 28[degrees]C for 1 hr, using a 1-kW Hg lamp in the Riko rotary reactor. The masks tested were commercial photomask (Photronics Singapore Pte Ltd.) and a 0.2-[micro]m [Al.sub.2][O.sub.3] disc. After irradiation, the sample was rinsed with copious amounts of N-methylpyrrolidinone (NMP), followed by washing with doubly distilled water and dried under reduced pressure. NMP was chosen since it can readily dissolve EB but not PANI in the doped salt form.

Plasma treatment was carried out using a Samco Basic Plasma Kit (Model BP-1). In a typical experiment, the as-synthesized PANI-viologen films were placed on the lower electrode and covered with a porous [Al.sub.2][O.sub.3] mask. The mask was taped down around the edges. After the system was evacuated to lower than 5 Pa, argon gas was introduced into the reactor. During the treatment, the flow rate of argon gas was fixed at 25 ml/min, the pressure at 100 Pa, and the RF power at 35 W. Various irradiation times were employed to induce the reaction between PANI and viologen.

Pattern Formation via Plasma Polymerization of Aniline

The precleaned LDPE thin film was placed on a slide and covered by an [Al.sub.2][O.sub.3] porous mask. The periphery of the mask was sealed with transparent tape to prevent the diffusion of aniline under the mask. Plasma polymerization was carried out using the same equipment described above. In a typical experiment, the substrate films were pretreated for 10 s by argon plasma. All parameters, including carrier gas, pressure and RF power, were the same as mentioned in the previous section. After the pretreatment, argon was bubbled through a reservoir of aniline at room temperature, and the aniline monomer was carried by the argon gas into the reactor. In this step, the argon gas flow rate and the pressure of reactor were kept the same as the conditions of the pretreatment step. The polymerization time was fixed at 30 min.

Incorporation of Metals/Metal Ions

The chemically synthesized polyaniline coating or plasma-polymerized polyaniline film was treated with hydrazine (30 wt% in water) for 10 min to reduce the polyaniline followed by thorough washing with water. The reduced films were then immediately pumped dry and immersed in Au[Cl.sub.3] or Pd(N[O.sub.3])[.sub.2] acid solutions for 10 min, followed by thorough washing with water. Standard Au[Cl.sub.3] and Pd(N[O.sub.3])[.sub.2] acid solutions (100 ppm of metal cations in 0.5-M HCl or HN[O.sub.3], respectively) were obtained by diluting the concentrated metal acid solutions (1000 ppm of metal cations in 0.5-M HCl or HN[O.sub.3] from Merck) with the respective acid.

Sample Characterization

UV-visible absorption spectra measurements were carried out on a Shimadzu UV-3101 PC scanning spectrometer, with pristine LDPE films as reference. XPS analysis of the films was made on an AXIS HSi spectrometer (Kratos Analytical Ltd.) using the monochromatized Al K[alpha] X-ray source (1486.6 eV photons) at a constant dwell time of 100 ms and a pass energy of 40 eV. The anode voltage was 15 kV and the anode current was 10 mA. The pressure in the analysis chamber was maintained at 5.0 X [10.sup.-8] Torr or lower during each measurement. The films were mounted on the standard sample studs by means of double-sided adhesive tape. The core-level signals were obtained at a photoelectron take-off angle of 90[degrees] (with respect to the sample surface). All binding energies (BEs) were referenced to the C 1s hydrocarbon peak at 284.6 eV. In the peak synthesis, the linewidth (full width at half maximum or FWHM) of the Gaussian peaks was maintained constant for all components in a particular spectrum. Surface elemental stoichiometries were determined from peak-area ratios, after correcting with the experimentally determined sensitivity factors, and were reliable to [+ or -]5%.

SEM images and energy dispersive X-ray (EDX) spectrum of the micropatterns on the films were obtained using a scanning electron microscope (Jeol JSM 5600LV). The surface morphology of the patterned films was also analyzed using an atomic force microscope (Nanoscope IIIa). The AFM image was obtained in the air using the tapping mode at a scan rate of 1.0 Hz. Sheet resistances (Rs) of the films were monitored using the standard two-probe method (Conductivity = 1/(Rs X thickness of the film)).

RESULTS AND DISCUSSION

Photo-Irradiated PANI-Viologen System

The surface graft copolymerization of VBC and viologen on the LDPE substrate and the photo-induced doping of PANI deposited on the LDPE substrate with surface graft copolymerized viologen moieties were described in detail in our previous publication (12). The sheet resistance of the EB-viologen film decreased sharply from > [10.sup.10] [ohm]/sq before UV-irradiation to 5 X [10.sup.4] [ohm]/sq after 30 minutes of irradiation (12). In the present work we take advantage of the photo-induced reaction between PANI and viologen to fabricate the micropatterns on the surface of polymeric substrate.

After UV irradiation of the PANI-viologen film through a mask for 1 h, the parts of the PANI-viologen film that were exposed to UV irradiation changed from blue to green, indicating the conversion of the EB coating to the doped state, whereas the unexposed parts remained blue. After the film was immersed in NMP, the unexposed parts dissolved readily, leaving only the irradiated region since NMP is a good solvent for EB (24) but not the doped salt. An image of a circuit pattern on the surface of the LDPE substrate obtained using the above-described process is shown in Fig. 1. The developed circuit represents the parts exposed to the UV irradiation, while the background is the viologen-grafted LDPE film without the EB coating, which had dissolved. The pattern matches that of the commercial mask used during the selective UV irradiation of the PANI-viologen film and exhibits well-defined edges. The width of the fine lines is about 50 [micro]m.

In the next part of the investigation, an [Al.sub.2][O.sub.3] porous mask with pore size of 0.2 [micro]m was employed to achieve finer resolution. The SEM images of the pristine [Al.sub.2][O.sub.3] mask and the PANI-viologen film after UV irradiation for 1 hr without the mask are shown in Figs. 2a and 2b, respectively. The surface of the PANI-viologen film after UV irradiation for 1 hr without the mask is relatively smooth. The surface morphology of the PANI-viologen film after UV irradiation for 1 hr through the [Al.sub.2][O.sub.3] mask can be delineated by AFM. Figures 2c and 2d show the 3-D and 2-D AFM images of the PANI-viologen film after UV irradiation with the [Al.sub.2][O.sub.3] mask and subsequent treatment with NMP. Relief images were formed on the surface of the PANI-viologen film, which corresponded to the pattern of the [Al.sub.2][O.sub.3] mask. However, the average size of the "dots" is about 0.5 [micro]m, in contrast to the 0.2-[micro]m pore of the mask. This difference is likely due to the diffraction of the UV irradiation around the holes of the [Al.sub.2][O.sub.3] mask, which resulted in a region of doped PANI that was larger than the pore size.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

After the formation of the patterns on the PANI-viologen film, metals can be selectively incorporated on the patterns. To facilitate the metal deposition, the PANI-viologen film was first treated with hydrazine to reduce PANI to its fully reduced form, leucoemeraldine (LM). The immersion of the PANI-viologen film (with the PANI in the LM state) into acidic gold chloride solution resulted in the reduction of Au[Cl.sub.3] to [Au.sup.0] and the deposition of the elemental gold on the patterns. The reaction of PANI film and gold chloride in acid solution has been described in an earlier publication (25). Figure 3 shows the SEM image and energy dispersive X-ray (EDX) spectrum of the patterned PANI-viologen film after reduction with hydrazine for 10 min and reaction with a 100-ppm Au[Cl.sub.3] solution for 10 min. From Fig. 3a it can be seen that the distribution of those spots on the film is again consistent with the pristine [Al.sub.2][O.sub.3] mask. However, a comparison of Fig. 3a and Fig. 2d shows that the average size of the spots has decreased to about 0.2 [micro]m after treatment with hydrazine and gold chloride solution. As mentioned above, the diffraction of the UV irradiation around the edges of the holes resulted in the doping of a wider area than the actual hole. However, it is possible that the reactions in the peripheral region did not proceed to the same extent as that in the central region. Hence, the interactions between PANI and viologen chains may be weaker in the former, resulting in the loss of the PANI during treatment with hydrazine and gold chloride solution. Figure 3b shows the EDX spectrum of a point on one of the microdots on the PANI-viologen film after treatment with gold chloride. A strong gold signal at 2.2 KeV (26) was obtained. On the other hand, there is no discernible gold signal for the parts outside the microdots. These results confirm that the microdots are polyaniline incorporated with gold.

[FIGURE 3 OMITTED]

Palladium was incorporated on the microdot pattern of the PANI-viologen film using a method similar to that for the incorporation of gold, as mentioned above. The XPS Au 4f and Pd 3d core-level spectra of the microdot patterned PANI-viologen film after treatment with hydrazine and Au[Cl.sub.3] and Pd(N[O.sub.3])[.sub.2] solutions are shown in Figs. 4a and 4b, respectively. In Fig. 4a, the spectrum can be resolved into two component peaks (Au 4[f.sub.7/2] and Au 4[f.sub.5/2]) at 84 eV and 87.7 eV (27). This confirms the presence of elemental gold. In Fig. 4b the Pd 3d core-level spectrum can be curve-fitted with three spin-orbit-split doublets. The most prominent doublet having BEs at about 335 eV (Pd 3[d.sub.5/2]) and 340 eV (Pd 3[d.sub.3/2]) is attributed to the [Pd.sup.0] species. The doublets having BEs at about 338 eV (Pd 3[d.sub.5/2]) and 343 eV (Pd 3[d.sub.3/2]) are assigned to the [Pd.sup.2+] species, whereas the intermediate doublet is assigned to the Pd-N complex (represented as Pd*) (28, 29, 30). The dominant [Pd.sup.0] peaks confirm that palladium ions in solution were reduced to the elemental state and deposited on the surface of microdot pattern on the PANI-viologen film.

[FIGURE 4 OMITTED]

Plasma Treatment of PANI-Viologen Film

The UV-visible absorption spectra of PANI-viologen films before and after plasma treatment are compared in Fig. 5a. The absorption spectra of EB films before irradiation show two absorption peaks at around 325 nm and 620 nm, which are due to [pi]-[pi]* transition of the benzenoid rings and the exciton absorption of the quinoid rings, respectively, as expected of PANI in the EB state (31). After plasma treatment, the PANI-viologen films showed a slight color change from blue to dark green. A new band appeared at 430 nm, which indicates the formation of the polaron/bipolaron structures (32, 33) characteristic of doped and conductive PANI. However, sheet-resistance measurement shows that the films are not conductive. The UV-visible absorption spectrum also showed a blue shift in the exciton peak (Fig. 5a). A shift in the exciton absorbance band to shorter wavelengths indicates that the effective conjugated length of the molecular chains in the conjugated chromophores has been reduced (15). Thus, the plasma treatment may have resulted in the breaking of the long-chain PANI molecules. The PANI-viologen films remained nonconductive with a sheet resistance of greater than [10.sup.10] [ohm]/sq after 30 min of plasma irradiation.

[FIGURE 5 OMITTED]

On the other hand, as mentioned earlier, the UV-irradiation of the PANI-viologen film resulted in a distinct color change, and the changes in the absorption spectra are shown in Fig. 5b. With an increase in the time of UV irradiation, there is a decrease in the exciton absorption peak intensity and the appearance of new bands at 430 nm and beyond 800 nm. These two new bands are due to the formation of the polaron/bipolaron structure and are characteristic of doped and conductive PANI (31).

The XPS N 1s and Cl 2p core-level spectra of the PANI-viologen films before and after plasma irradiation are shown in Fig. 6. The N 1s spectrum of the PANI-viologen film before irradiation (Fig. 6a) shows a typical structure of EB with two major components of almost equal intensity at about 398.2 eV and 399.4 eV, which are assigned to the imine (-N=) and amine (-NH-) species respectively (34). The minor peaks at BEs greater than 400 eV can be attributed to the positively charged nitrogen on the PANI backbone ([N.sup.+]/N = 0.12). From Fig. 6b, the chlorine signal (from the viologen) was found to be insignificant before irradiation, which implies that the PANI coating covering the viologen was thicker than 7.5 nm, which is regarded as the probing depth of the XPS technique in an organic matrix (35).

The N 1s core level spectra after plasma treatment of the PANI-viologen films are shown in Figs. 6c and 6e. As the irradiation time increases, the intensity of the imine group decreases while that of the amine group increases. As mentioned above, the long-chain PANI structures may be destroyed to form short-chains, which may have similar structures as plasma-polymerized aniline (36). From Figs. 6a, c and e, it is obvious that the intensity of the positively charged component ([N.sup.+]) does not increase substantially with plasma treatment. Thus, the XPS results are consistent with the UV absorption spectra (Fig. 5a), and since the conductivity of PANI is correlated with the [N.sup.+] component, it is clear that viologen and PANI do not react under plasma treatment to result in the doping of PANI. As a comparison, the N 1 s core level spectrum of the PANI-viologen film after UV irradiation is shown in Fig. 6g. The spectrum clearly shows that the proportion of positively charged nitrogen ([N.sup.+]/N = 0.30) is much higher than that in the plasma-treated sample.

After plasma treatment, there is an increase in the intensity of the Cl 2p signals, as shown in Figs. 6d and 6f. The Cl 2p core-level spectra can be deconvoluted into two-spin-orbit split doublets (Cl 2[p.sub.3/2] and Cl 2[p.sub.1/2]), with the BE for the Cl 2[p.sub.3/2] peaks at 198.6 and 200.2 eV (27, 36). The higher binding energy component is assigned to covalent chlorine (-Cl), while the lower binding energy component is assigned to the intermediate chloride species (Cl*), which has been widely observed (37-39). The increase in the Cl signal intensity upon plasma treatment may be due to some electrostatic interactions with the PANI, but this increase in signal intensity is small compared with that observed when the PANI-viologen film was irradiated with UV (Fig. 6h). In this case, the predominant species in Cl 2 p core level spectrum is the [Cl.sup.-] anions (with the Cl 2[p.sub.3/2] and Cl 2[p.sub.1/2] peak components at 197 and 198.6 eV). The [Cl.sup.-] anions served as counterions to the [N.sup.+] species to maintain charge neutrality (([Cl.sup.-] + Cl*)/[N.sup.+] = 1.16). The greater interactions between the [Cl.sup.-] anions and the [N.sup.+] species of the PANI may result in the PANI coating "sinking in" and/or the migration of the [Cl.sup.-] from the viologen layer towards the upper PANI layer.

Plasma-Polymerized Aniline System

The plasma-polymerization of aniline directly onto an LDPE substrate as a thin film was described in a previous work (40). The aniline polymer synthesized in this manner has a different structure from polyaniline derived from conventional methods. The C/N ratio of the former is 5.1 instead of about 6 for the latter. The lower C/N ratio may be due to the breaking-up of the benzene rings under electron bombardment from the glow discharge. Figure 7 shows the SEM and AFM images of aniline plasma polymerized through the [Al.sub.2][O.sub.3] mask on the LDPE film. Nanosized clusters (100-150 nm) of plasma-polymerized aniline can be clearly seen. The average cluster size is less than the pore size of [Al.sub.2][O.sub.3] thin film, in contrast to the microdots obtained from the UV irradiation of PANI-viologen films through the [Al.sub.2][O.sub.3] mask. In the plasma-polymerization process, there may be growth of the aniline polymer on the inner surface of the pores, similar to the process observed in oxidative polymerization in solution medium as reported earlier (41). This partial blockage of the pores would result in smaller-than-expected polymer clusters on the substrate.

[FIGURE 6 OMITTED]

The XPS Au 4f and Pd 3d core-level spectra of the plasma-polymerized polyaniline (using RF power of 35 W) on LDPE after reaction with hydrazine and subsequently with Au[Cl.sub.3] (100 ppm in 0.5 M HCl) and Pd(N[O.sub.3])[.sub.2] (100 ppm in 0.5 M HN[O.sub.3]) for 10 min are shown in Fig. 8. In Fig. 8a, the Au 4[f.sub.7/2] component peak at 84 eV confirms the presence of [Au.sup.0] (27). Thus, the plasma-polymerized aniline has an ability similar to that of the polyaniline synthesized via the conventional chemical method to reduce [Au.sup.3+] to [Au.sup.0] (29). The Pd 3d core-level spectrum in Fig. 8b shows a dominant doublet at 338 eV and 339.5 eV, which is attributed to the [Pd.sup.2+] species. The amount of [Pd.sup.0] (doublet at 335 eV and 336.5 eV (27)) present on the film is insignificant. Hence, it is quite clear that the [Pd.sup.2+] is the main state of the palladium deposited on the plasma-polymerized aniline surface. This result is different from that obtained with the PANI-viologen film discussed in an earlier section and also those obtained with conventionally synthesized PANI in the fully reduced form either as free-standing films (29) or coatings (40). In the latter three systems, [Pd.sup.0] is the predominant species incorporated. This difference may be due to the fact that the plasma-polymerized aniline comprises shorter chains than the conventionally synthesized polyaniline, and the ability of the former to reduce the metal salt is decreased. In addition, hydrolysis reaction (40) may reduce the number of reaction sites in the thin plasma-polymerized layer significantly, resulting in the complexation of the Pd species with the plasma-polymerized aniline rather than a complete reduction to the metallic state. This difference between the plasma-polymerized aniline and the conventionally synthesized PANI was not observed in the reduction of Au[Cl.sub.3], since the higher reduction potential of Au[Cl.sub.3] ([Au.sup.3+] + 3 e [right arrow] Au and [Pd.sup.2-] + 2 [e.sup.-] [right arrow] Pd are 1.42 V and 0.951 V, respectively) (25) allows the reaction to proceed more readily.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

CONCLUSION

Two methods for forming micropatterns and nanosized structures of conducting polymer with metal incorporation on an LDPE substrate were demonstrated. In the first method, uniform micropatterns using photosensitive PANI-viologen film can be successfully generated with UV irradiation through a mask. The use of a mask with submicron-sized holes in this method results in an increase in size of the micropatterns compared with the original mask because of light diffraction around the edges of the holes. The use of Ar plasma instead of UV irradiation was unsuccessful for inducing the doping reaction between PANI and viologen. However, plasma polymerization of aniline through a mask results in nanosized clusters. The size of such clusters is slightly less than that of the holes in the mask owing to some deposition of the aniline polymer on the inner surfaces of the holes. The micropatterns fabricated by either method can be treated with metal salt solutions for the incorporation of metal or metal ions.

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LUPING ZHAO, JINGGONG WANG, K. G. NEOH*, and E. T. KANG

Department of Chemical and Environmental Engineering

National University of Singapore

Kent Ridge, Singapore 119260

*To whom correspondence should be addressed. E-mail: [e-mail]