Fabrication of Conducting Polymer Films Containing Gold

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Fabrication of Conducting Polymer Films Containing Gold Nanoparticles with Photo-Induced Patterning Sung Huh and Seung Bin Kim* Department of Chemistry, Pohang UniVersity of Science and Technology, San 31, Hyojadong, Namgu, Pohang 790-784, Republic of Korea ReceiVed: September 10, 2009; ReVised Manuscript ReceiVed: December 22, 2009

A novel multifunctional nanocomposite (P3OT-AuNPs-CI) consisting of gold nanoparticles coated with the conducting polymer poly(3-octylthiophene) and a molecule containing the cinnamate moiety has been developed. The cinnamate moiety is well-known for its photoreactive properties; that is, it can be rendered reactive through exposure to UV irradiation. Photoreactive cross-linking of this moiety results in aggregation of the AuNPs. In this study, we introduce a new approach to patterning the conducting polymer by making use of photoreactive gold nanoparticles. We can obtain patterned P3OT-AuNPs-CI thin films by using this fast and facile UV irradiation system. This new nanocomposite not only provides a convenient patterning technique that offers an alternative to conventional lithography for the patterning of conducting polymers, but is also expected to enable the control of the conductivity in the fabrication of submicro- and microscale electronic devices. We suggest that this new multifunctional nanomaterial can be used in the manufacture of microelectronic devices and circuits, solar cells, diodes, and chemical sensors. 1. Introduction Nanomaterials and nanocomposites have recently attracted significant interest because of their unique chemical and physical properties.1-3 One interesting aspect of nanocomposites is the emergence of synergistic properties due to the combination of the two different components. In particular, nanocomposites formed by decorating a conducting polymer (e.g., polyaniline, polypyrrole, and polythiophene) with metal nanoparticles such as Ag, Au, Pt, or Pd have received much attention in the past few years.4-7 In these studies, it was found that the nanocomposites formed by combination of conducting polymers with metal nanoparticles have potential applications in catalysis, chemical and biological sensors, transistors, electronic devices, and elsewhere. The major challenge for the development of applications of conducting polymers is the design of appropriate patterning processes. A variety of techniques for the patterning of conducting polymers have been suggested, for example, soft lithography,8,9 microcontact printing,10 inkjet printing,11 photolithography,12 dip pen nanolithography, and imprinting nanolithography.13,14 Most patterning processes use a top-down approach in which nanostructures are fabricated with nanolithography (e.g., electron, ion beam, or nanoimprint patterning) and etching techniques (e.g., wet- or dry-chemical etching). In most cases, a mold or stamp for the desired pattern or a prepatterned substrate is required for the lithography or printing process, so delicate steps are needed that involve the separation of the mold and the spatial removal of the residual layer after printing with various etching techniques. In this paper, a new convenient method for patterning a conducting polymer containing photoreactive gold nanoparticles is presented. This hybrid material can be patterned rapidly and facilely by using UV irradiation with a shadow mask and does not require difficult and expensive lithography processes. * To whom all correspondence should be addressed. Tel: 82-54-2792106. Fax: 82-54-279-3399. E-mail: [email protected].

Conventional photolithography is usually carried out by modifying the molecular structure by adding an organic chromophore or by using a photoresist, whereas this new process does not require deformation of the polymer structure and uses a photoresist-free imprinting method. The main principle of this process is that the gold nanoparticles are shielded from the conducting polymer and photoreactive group, and act as a framework for patterning the conducting polymer upon UV irradiation. Thus, it can be regarded as a bottom-up approach, which includes making a building block caused by the selfassembled AuNPs, as well as a top-down approach including selective patterning of the feature by shadow mask. Our method for preparation of poly(3-octylthiophene)stabilized gold nanoparticles (P3OT-AuNPs) makes use of the specific interaction between the sulfur atoms of polythiophene and the gold surfaces, as described in a previous study.15 In the next step, we add a newly synthesized molecule (12,13-dithiatetracosane-1,24-dicinnamate) (CI), which contains a gold nanoparticle-capping group and the cinnamate moiety, onto the surfaces of the P3OT-AuNPs.16 Finally, the multifunctional nanocomposite containing poly(thiophene)-coated gold nanoparticles and the cinnamate moiety (P3OT-AuNPs-CI) are fabricated. According to our previous research, cinnamate moieties on the surfaces of gold nanoparticles can act as bridges that covalently link the CI-AuNPs in organic solvent.17 We also found that the particles are covalently cross-linked in the film state during the UV irradiation process. After the reaction, the film particles do not redisperse in toluene because of the change in solubility due to aggregation. In this approach, we used a commercial TEM grid to carry out the simple patterning process on the particle film. The grid enables the selective cross-linking of the nanoparticles to produce a patterned surface. In addition, the texture can be varied by altering the contact mask design. This simple process can be applied to the patterning of the newly developed nanocomposite, P3OT-AuNPs-CI. Scheme 1 illustrates the whole process, from the fabrication of P3OT-

10.1021/jp908743y  2010 American Chemical Society Published on Web 01/28/2010

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SCHEME 1: Schematic Diagram of the P3OT-AuNPs-CI Patterning Process

AuNPs-CI to its patterning. The nanocomposite can be directly patterned on the substrate by using UV irradiation, after quick washing for the selective removal of the nonexposed portion. In addition to its convenient patterning, this new nanocomposite has sufficient electrical conductivity for the fabrication of electronic devices. It is also expected to have conducting properties that are tunable to the required application through control of the doping level, the species of conducting polymer, the proportion of Au to the conducting polymer, etc. 2. Experimental Section 2.1. Materials. Cinnamoyl chloride (98%), bis(11-hydroxyundecyl) disulfide (99%), triethylamine (99%), tetraoctylammonium bromide (99%), gold(III) chloride hydrate (99.999%), and sodium borohydride (98%) were purchased from Aldrich. Poly(3-octylthiophene-2,5-diyl) was purchased from Rieke Metals, Inc. All other reagents were used without any further purification. 2.2. Synthesis of 12,13-Dithia-tetracosane-1,24-dicinnamate (CI). The cinnamate-containing disulfide stabilizer (i.e., 12,13-dithia-tetracosane-1,24-dicinnamate) was obtained as described in a previous paper.17 Cinnamoyl chloride (73.3 mg, 0.440 mmol) and bis(11-hydroxyundecyl) disulfide (85.4 mg, 0.210 mmol) were added to anhydrous THF (20 mL) and triethylamine (TEA) (0.142 mL, 104 mg, 1.02 mmol). The solution was then refluxed for 1.5 h and stirred (overnight) at room temperature. After removing the solvent by means of rotary evaporation, the residue was taken up in chloroform. The organic phase was washed with water and the aqueous layer was back-extracted with chloroform. The combined organic layers were dried over anhydrous MgSO4, leaving a yellow crude solid on the bottom of the flask. This material was purified by means of recrystallization (three times) from G.R. grade ethanol and drying (in vacuum) at room temperature to give a white solid in a yield of 61%. 1 H NMR (CDCl3): δ (ppm) 1.30 (s, -CH2, 28H), 1.68 (m, SCCH2, OCCH2, 8H), 2.68 (t, J ) 6.6 Hz, CH2S, 4H), 4.20 (s, OCH2, 4H) 6.45 (d, J ) 15.9 Hz, CHCO, 2H), 7.38 (d, J ) 9.0 Hz, aromatic C(3)H, C(4)H, C(5)H), 7.52 (d, J ) 8.7 Hz, aromatic C(2)H, C(6)H), 7.69 (d, J ) 15.9 Hz, CHCHCO, 2H).

HRMS-FAB: calcd. for C40H58O4S2 ) 666.38. Found: 667.72 [M + H]+. Melting point: 70-71 °C. 2.3. Preparation of Poly(3-octylthiophene)-Modified Gold Nanoparticles (P3OT-AuNPs). To prepare gold nanoparticles modified with P3OT, a two-phase system18,19 was used with tetraoctylammonium bromide (0.219 g, 50 mM) in toluene (8 mL) as the phase-transfer reagent. After addition of an aqueous solution of HAuCl4 (0.039 g, 30 mM) into the toluene solution, P3OT (0.031 g, 0.09 mmol; Mn ) 34580) dissolved in 3 mL of toluene was added to this mixture. After vigorous stirring, 2.5 mL of an aqueous solution of NaBH4 (0.037 g, 400 mM) was added dropwise to the mixture, which was then allowed to react for 3 h. The gold nanoparticles were extracted from the organic phase; their presence was confirmed with UV/vis spectroscopy. The immobilization of P3OT on the surfaces of the nanoparticles was confirmed by using Fourier transform infrared (FTIR) and Raman spectroscopy. 2.4. Preparation of P3OT-AuNPs-CI. To prepare the bifunctionalized gold nanoparticles, the place exchange reaction was used.20,21 The P3OT-AuNPs (0.017 g, 0.43 µmol for P3OT) in toluene (3 mL) were dissolved in CI solution (8.0 µmol) in toluene (4 mL). The solution contained around 20:1 molar ratio of CI/P3OT by considering one molecule of CI has 2 sulfurs and one molecule of P3OT has 118 sulfurs (units) in our polymer. Below 20:1 molar ratio of CI/P3OT, there was no appearance of the bands due to the CdO stretching vibration at 1709 cm-1 and the CdC stretching vibration at 1634 cm-1 related to CI molecule on the FTIR spectrum. Moreover, patterning effect was not observed. We confirmed the reproducibility of experiment using 20:1 molar ratio of CI/P3OT for patterning effect and conductivity measurement. The solution was stirred for 4 h at 30 °C. It is assumed that the replacement of P3OT molecules with CI molecules occurs because the affinity of the sulfur atoms in CI with the gold nanoparticle surface is higher than that of P3OT. The replacing molecule must have the appropriate chain length: a patterned film cannot be obtained from the P3OT-AuNPs when a short-chained CI (e.g., 3,4-dithia-hexane-1,6-dicinnamate) is used. After sufficient place exchange reaction had occurred, the solution was evaporated and washed with methanol.

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2.5. Measurements. The 1H NMR spectra were recorded on a Bruker Aspect 300 MHz by using tetramethylsilane as the reference. The FTIR spectroscopic measurements were carried out at a spectral resolution of 4 cm-1 by using a Bomem DA8 FTIR spectrometer equipped with a liquid-nitrogen-cooled mercury-cadmium-telluride (MCT) detector. The samples were coated onto NaCl windows [25 mm (diameter) × 2 mm (thickness)] for all of the FTIR spectra. The UV/vis spectra were obtained with a Shimadzu UV 1601 spectrometer. The UV absorptions of the samples were measured for dilute solutions in chloroform. The formation of the particles was confirmed by carrying out transmission electron microscopy (TEM) measurements. Samples suitable for examination were prepared by evaporating a drop of the P3OT-AuNPs solution (0.1 wt % in toluene) onto an amorphous carbon film supported on a copper grid (400 mesh). The samples were examined with a JEOL2100EX electron microscope operating at 200 kV. Typical phase-contrast images were obtained. Optical microscopy images were collected by using an Olympus Model BX51TRF microscope (Olympus Co., Tokyo, Japan) equipped with a digital camera. The patterned P3OT-AuNPs-CI was also characterized by using a field emission scanning electron microscope (JSM-7500F; JEOL Ltd., Tokyo, Japan) operating at an accelerating voltage of 5 kV. The energy-dispersive X-ray spectroscopy (EDX) mode of SEM was used for the elemental analyses of the patterned films. The morphologies of the films were determined by using atomic force microscopy (AFM) in the noncontact mode (Veeco Digital Instruments Dimension 3100). The resistivities of the films were measured with four-point probe measurements.22 2.6. Photoreaction Patterning Process. To pattern P3OTAuNPs-CI on a substrate, a 5 wt % toluene solution of the composite was spin-coated onto a Si wafer (1 cm × 1 cm) for 1 min at 2000 rpm. After the film had been held in vacuum for 24 h, a TEM grid was attached as the photomasking material (normal type copper grid: 400 and 2,000 mesh) onto the film surface before irradiation with UV light. A high-pressure 1.0 kW Hg-lamp system (Altech, model ALHg-1000) was employed as the UV-light source, together with an optical filter (Milles Griot, Model 03-FCG-179), which transmits a band beam of 260-380 nm. The optically filtered UV-light intensity was 50 mW/cm2. The exposure dose was measured by using an International Light photometer (model IL 1350) with a sensor (model SED 240). After removing the TEM grid from the film surface, the film was briefly washed with toluene and dried. The patterns were examined with optical microscopy, SEM, and AFM. 3. Results and Discussion 3.1. Characterization of the P3OT-AuNPs. The TEM observations showed that most of the gold nanoparticles are isolated and confirmed that their average size was 6.16 ( 0.73 nm (Figure 1a). It was calculated by collected TEM images including the 83 isolated particles (data not shown). Histogram of particle diameters was inserted in Figure 1a. The resulting particles have good solubility in many organic solvents, including chloroform, benzene, and toluene. Figure 1b shows the UV/ vis spectra of P3OT, the P3OT-AuNPs, and the CI-AuNPs. An absorption maximum corresponding to the π-π* transition of the conjugated polymer chains is present around 450 nm.23,24 The absorption of the P3OT-AuNPs from 500 to 600 nm is due to two factors, the self-orientation of the polythiophene on the surface of the nanoparticles,25 and the surface plasmon resonance on the gold nanoparticles, similar to the spectrum of CI-AuNPs in Figure 1b. The broadband provides clear evidence

Huh and Kim

Figure 1. (a) TEM image of P3OT-AuNPs and (b) UV/vis spectra of P3OT, P3OT-AuNPs, and CI-AuNPs.

Figure 2. FTIR spectra of P3OT, P3OT-AuNPs, P3OT-AuNPs, and CI-AuNPs.

of the formation of the nanoparticles and the immobilization of the polymer on the particles. 3.2. Characterization of the P3OT-AuNPs-CI. Figure 2 shows the FTIR spectra of the four samples: P3OT, P3OTAuNPs, P3OT-AuNPs-CI, and CI-AuNPs.26 The P3OT spectrum contains vibrational bands due to the CH3 stretching mode at 2955 cm-1, the CH2 antisymmetric stretching mode at 2924 cm-1, the CH2 symmetric stretching mode at 2854 cm-1, the CdC ring stretching mode at 1458 cm-1, the CH2 bending mode at 1377 cm-1, the CH deformation at 824 cm-1, and the ) C-S stretching mode at 715 cm-1.27,28 As shown in Figure 2, the bands in the spectrum of P3OT-AuNPs resemble those of P3OT, providing clear evidence that P3OT is immobilized on the particles. Moreover, the spectrum of P3OT-AuNPs-CI includes bands due to the CdO stretching vibration at 1709 cm-1 and the CdC stretching vibration at 1634 cm-1, and is thus similar

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Figure 3. Optical microscopy images of the P3OT-AuNPs-CI: (a) 37 µm and (b) 7.5 µm P3OT-AuNPs-CI patterns on the Si wafer.

to the spectrum of CI-AuNPs. These results confirm that the CI molecules were incorporated to the particles surface. We expected to observe the band shift in vibration modes of P3OT due to exchanged CI molecules, representing clear evidence that indicate the replacement of P3OT molecules or interaction between CI and P3OT. However, the spectrum change was not sufficiently distinctive in comparison to the FTIR spectrum of P3OT-AuNPs. We attempted to analyze the spectrum change quantitatively with repeated rinsing process in order to remove

exchanged P3OT, but the changes in the band were not clearly observed. The absence of the S-S stretching mode of P3OTAuNPs-CI in the Raman spectrum provides spectroscopic evidence of the binding on the gold surface (data not shown).29 As a result, it was confirmed that CI molecules were attached on the gold nanoparticles surface and the bifunctionalized gold nanoparticles were fabricated. 3.3. Patterned P3OT-AuNPs-CI. Before preparing the bifunctional gold nanoparticles, we also obtained a patterned

Figure 4. SEM images and energy dispersive X-ray analysis (EDX) spectra of the P3OT-AuNPs-CI: (a) 37 µm and (b) 7.5 µm P3OT-AuNPs-CI patterns on the Si wafer. Panels c and d show the EDX spectra of the patterned and nonpatterned areas, respectively.

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Figure 5. AFM images of the 7.5 µm P3OT-AuNPs-CI pattern: (a) two-dimensional (2D) and (b) three-dimensional (3D) surface morphologies. The height is about 100 nm.

film of CI-AuNPs by using UV irradiation and confirmed pattern formation by optical microscopy (data not shown). We found that the particles were covalently cross-linked in the film state by the exposure to UV irradiation. This cross-linking results from the dimerization of the cinnamate groups on the particles during photoreaction. After the reaction, the film of nanoparticles does not redisperse in toluene because of the change in their solubility resulting from aggregation. We observed that many small sheets, which are assumed to be macro cross-linked nanoparticle arrays, precipitated on the bottom of the container after ultrasonification. It was confirmed that the CI-AuNPs could be patterned directly onto the substrate by applying UV irradiation through the TEM copper grid as a contact mask. The patterning of the P3OT-AuNPs-CI thin film was carried out with the same procedure as used for the CI-AuNPs. The thickness of the film was controlled by varying the nanocomposite concentration and the spinning rate. Figure 3 shows optical microscopy images of the patterned P3OT-AuNPs-CI thin film obtained after UV irradiation. The patterned area of the film persists, whereas the masked area is removed by toluene. The masked area of the film was not exposed to the light due to the blocking effect from the contact mask, so the TEM grid provides selective cross-linking of the nanoparticles and thus a patterned surface. The square widths were 35 µm (400 mesh) and 7.5 µm (2000 mesh). After performing experiments for various UV exposure times, we found that the patterned features were clearest for irradiation of around 5 J/cm2. We also investigated this behavior by using SEM (Figure 4). The energy dispersive X-ray (EDX) spectra in Figure 4, panels c and d, show the different components of the patterned and nonpatterned areas respectively, and confirm that the film remains on the patterned area after the lift-off process and even after ultrasonification. Figure 5 shows the twodimensional (2D) and three-dimensional (3D) surface morphologies of the 7.5 µm P3OT-AuNPs-CI pattern, which were obtained with AFM. The height is about 100 nm. The resolution expected to be increased by fabricating higher precision contact mask. The resistivities of the P3OT-AuNPs and the P3OT-AuNPsCI were determined by using four-probe measurements. On the P3OT-AuNPs nanocomposite, the conductivity of the pure (undoped) film was found to be 1.69 × 10-3 S cm-1. The iodinedoped film was determined to have higher electrical conductivity, approximately 0.479 S cm-1. The results obtained for the sample conductivities are in agreement with those reported in previous paper.14 We also determined the conductivities of the

P3OT-AuNPs-CI film, and found values similar to those of the P3OT-AuNPs: 1.54 × 10-3 (undoped film) and 0.610 S cm-1 (doped film). We also found that the intrinsic conducting properties are not changed by the UV patterning treatment. We confirmed the stability of the conducting polymer with respect to UV irradiation by using UV/vis spectroscopy, which showed that there is no change in the structure of the polymer during the patterning process. We suggest that it is possible to control the conducting properties of the nanocomposite by varying the molar ratio of gold to polythiophene and the reaction conditions (e.g., the temperature, the reaction time, etc.) for place exchange on the gold surface as well as by adjusting the doping level of the conducting polymer. 4. Conclusions We have demonstrated a novel process for the patterning of conducting polymer/photoreactive gold nanoparticles. The P3OTAuNPs-CI film is microscale patterned by covalently bonded gold nanoparticles that are cross-linked by the cinnamate moieties on their surfaces upon UV irradiation. The P3OTAuNPs-CI film has stable conducting properties before and after the patterning process. This simple patterning system can be applied to various substrates such as glass, steel, wood, and any other nonorganic-soluble substrate. This improved patterning process also enables large-scale manufacturing in industrial applications, as well as use in micro- and nanoscale devices. Moreover, this new nanocomposite is expected to have conducting properties that are tunable to the required application and sufficient for the fabrication of electronic devices. Our results demonstrate the potential of controllable fabrications of conducting polymer-based nanomaterials. Our research into the development of materials suitable for a wide range of applications is continuing. Acknowledgment. This study was supported by the Second Stage of the Brain Korea 21 Project. References and Notes (1) Mirkin, C. A.; Letsinger, R. L. Nature 1996, 382, 607–609. (2) Boal, K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albercht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746–748. (3) Maheshwari, V.; Saraf, R. F. Science 2006, 312, 150–1504. (4) Tseng, R. J.; Huang, J.; Ouyang, J.; Richard, B.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080. (5) Majumdar, G.; Goswami, M.; Sarma, T. K.; Paul, A.; Chattopadhyay, A. Langmuir 2005, 21, 1663–1667.

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