carboxylated-multiwall carbon nanotube composites produced by in-situ and interfacial polymerization

Materials Chemistry and Physics 135 (2012) 80e87

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Polyindole/ carboxylated-multiwall carbon nanotube composites produced by in-situ and interfacial polymerization Leela Joshi, Arun Kumar Singh, Rajiv Prakash* School of Materials Science and Technology, Institute of Technology, Banaras Hindu University, Varanasi-221005, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2011 Received in revised form 13 March 2012 Accepted 14 April 2012

Composites of polyindole (PIn), a conducting polymer, with carboxylated-multiwalled carbon nanotubes (c-MWCNT/PIn) were synthesized; the synthesis was done using (i) two miscible solvents (in-situ method) and (ii) two immiscible solvents (interfacial method). A tubular composite, with a uniform coating of the polymer over c-MWCNTs, was observed in the case of interfacial synthesis. However, the in-situ synthesis of c-MWCNT/PIn composites exhibited a densely packed spherical morphology, with cMWCNT incorporated within the polymer spheres. The spherical morphology was probably obtained due to fast polymerization kinetics and the formation of micelles in case of in-situ polymerization, whereas tubular morphology was obtained in case of interfacial polymerization due to the sufficient time provided for the growth of polymer chains over the c-MWCNT surfaces. Nanoscale electrical properties of composites, in a metal/(c-MWCNT/PIn) configuration, were studied using current sensing atomic force microscopy. Interfacial c-MWCNT/PIn composite, on Al metal substrate, exhibited a typical rectifying diode behavior. This composite had manifested enormous potential for electronic applications and fabrication of nanoscale organic devices. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Conducting polymer Polyindole Composite materials Polymers Chemical synthesis Electrical characterisation

1. Introduction Rapid progress has been made in the development of conducting polymer and carbon nanotube (CNT) hybrid materials. These composites are a promising route to attain synergistic effect [1e3], and find applications in the fabrication of various electronic devices ranging from super capacitors to biochips [4e6]. Carbon nanotubes possess inherent extraordinary structural, mechanical, electrical, electronic and thermal behavior [7,8]. In the past few years, CNTconducting polymer composites had been developed and used for many promising technological applications such as corrosion resistance, battery, solar cells, sensors, energy storage devices, microelectronics and optoelectronic devices [9e16]. However, inhomogeneous distribution of CNTs in the polymer matrix and their poor interaction with polymer chains had resulted in their inferior properties and restricted their use in technological applications [17]. CNTs should be well dispersed and well interacting with polymer chains in order to provide an effective synergetic effect to the composite. A simple route for functionalization of CNTs had been achieved by an oxidation process, which involves their

* Corresponding author. Tel./fax: þ91 542 2368707. E-mail address: [email protected] (R. Prakash). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.04.026

treatment in a mixture of concentrated HNO3 and H2SO4; after this treatment, the end and side walls of CNTs are decorated mainly with carboxylic groups. CNTs functionalized in this manner attained better dispersion in aqueous phase [18]. Among various conducting polymers, nitrogen heteroatom containing monomers such as pyrrole, carbazole and indole, have been the subject of extensive research due to their better stability, electroactivity and ease of synthesis. Compared to polypyrrole and polycarbazole [19e24], less work had been conducted on polyindole based systems because their synthesis was limited to the electrochemical method (not chemical synthesis) only. Polyindole is an electroactive polymer which exhibits many interesting properties such as good photo luminescence, thermal stability, corrosion inhibition property, high redox activity and, above all, less toxicity [25e32]. This polymer is explored recently for the formation of composites with carbon nanotubes; however, the synthesis is limited to electrochemical method only [33]. In addition, their electrical properties are still to be studied as they are explored for other conducting polymers and their composites [34,35]. In this work, we have described a chemical method for formation of polyindole and c-MWCNT hybrid materials through in-situ and interfacial polymerization routes, and studied the junction properties of composites using nano-contact, in configuration of Al/Composite/Pt nano probe.

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2. Experimental setup 2.1. Chemicals Ammonium peroxodisulphate, nitric acid and sulphuric acid were obtained from Merck, India. Ethanol and Dichloromethane were purchased from Spectrochem Pvt. Ltd, India. Indole and multiwall carbon nanotube (MWCNT) were purchased from Aldrich, MA, USA. All other chemicals used were of analytical grade. Double distilled deionized water was used in all the experiments. Pristine MWCNT sample was acid washed to remove catalysts and was refluxed for carboxylation in a mixture of nitric acid and sulphuric acid [3]. After refluxing, the sample was collected through centrifugation, followed by washing with distilled water and drying in a vacuum oven at 50  C.

2.2. Preparation of polymer and composites For the synthesis of composites, protocol of our recent work was followed [36,37] as shown in Fig. 1. In-situ composite was prepared by dispersing 5wt% (of the monomer) carboxylated-multiwalled carbon nanotube (cMWCNT) in H2SO4 (0.2 M, 10 mL), followed by vigorous sonication and stirring. For complete dispersion of c-MWCNT, indole (100 mg) was dissolved in few ml of ethanol and this solution was added to the dispersed c-MWCNT solution; the resultant solution was set at stirring for 2 h. To the above solution, a solution of 0.2 M ammonium peroxodisulphate (used as oxidizing agent) in H2SO4 (0.2 M, 10 mL) was added drop wise, with constant stirring. For interfacial polymerization, 5wt% (of the monomer by weight) c-MWCNT was dispersed in H2SO4 (0.2 M, 10 ml) solution with vigorous sonication and stirring, followed by addition of oxidizing agent (0.2 M). Monomer solution (100 mg), in dichloromethane (10 ml), was added to the above solution which led to the formation of two distinct layers of aqueous (top) and non-aqueous (bottom) phases; the mixture was then kept for polymerization under slow stirring. After 24 h, the organic phase was filled with dark-green polyindole composite. The aqueous phase was removed from the top and its solid part was collected by centrifugation followed by washing with 0.2 M H2SO4 and further followed by washing with water. Composite was dried and stored in a vacuum desiccator for further use.

Fig. 2. Device configuration of Al/(c-MWCNT/PIn) nano-contact assembly.

2.3. Characterization Electrochemical characterizations were carried out using electrochemical workstation (model CHl7041C), CH-Instrument Inc., Tx, USA. Cyclic voltammetry (CV) was performed in a singlecompartment cell using three-electrode assembly with a Pt disk as a working electrode, Pt plate as an auxiliary electrode and Ag/ AgCl as reference electrode. X-ray diffraction (XRD) patterns were recorded using 18 kW rotating anode (Cu) based (Rigaku, Japan) powder diffractometer operating in the Bragg-Brentano geometry and fitted with a graphite monochromator in the diffracted beam. Data were recorded from 2q ¼ 50 to 600, at a scanning rate of 40/ min, at 6 kW energy. Thermogravimetric analyses (TGA) was done, with Mettler TGA, at a heating rate of 20  C/min (in nitrogen/inert environment). The UVevis absorption spectra of different samples were recorded, in 200e800 nm range, using a Perkin Elmer, Lambda 25 spectrophotometer. Fourier Transform Infrared (FTIR) spectroscopic analysis of the samples were conducted using Thermo (model 5700), Germany from 400 to 4000 cm1, with a resolution of 4 cm1. Scanning Electron Microscopy (SEM) studies were carried out using ZEISS Supra 40 SEM at operating voltage of 5.0 kV. High resolution Transmission Electron Microscope(TEM) image was obtained by HRTEM model Tecnai 30 G2 S-Twin electron microscope, operated at 300 kV accelerating voltage.

Fig. 1. Schematic of in-situ and interfacial growth of polyindole in presence of c-MWCNT.

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5 wt% of c-MWCNT/PIn composites were dissolved in THF (tetra hydrofuran) and coated on flat Al metal substrate by spin coating (at 1000 rpm) technique (Spin Coater; Model PRS 4000, India). The AFM measurement confirmed that the thickness of film was about 50 nm. Spin coated samples, Al/c-MWCNT/PIn (in-situ and interfacial), were dried in vacuum desiccator before characterization. Current sensing Atomic force microscope (Cs-AFM, model PRO 47, NT-MDT, Russia) was used to investigate the electrical properties of the devices. IeV characteristics of nano-layered devices in the configuration Al/c-MWCNT/PIn (in-situ and interfacial), with a nanometer sized Pt tip positioned at nanometric distance from the c-MWCNT/PIn composites film, were studied using Scanning Tunneling Microscope (STM) mode of the AFM at room temperature, as reported by others [38]. The device configuration, with nano-contact assembly, is shown in Fig. 2. A constant normal force of 20 nN was applied between probe tip and thin film.

not shown). After several hours, a dark-green film was formed at the interface. Hydrophobic/hydrophilic nature of oligomers and polymer was deciding factor for migration towards nonaqueous/ aqueous phase in interfacial synthesis. In polyindole case short chain oligomers migrate towards aqueous phase but polymer is incompatable for both the phase and collected at interface. UVevis spectra of in-situ synthesized polymer also exhibited a similar pattern (not shown here) with a difference that after 2 h of in-situ polymerization, a broad absorption started to appear between 635 and 780 nm, which was absent in interfacial polymerization. Probably this absorption was due to the presence of dopant ions which was absent or present in a lower amount in interfacial polymerization. With increase in polymerization time (after 1 h) a significant decrease in the intensities was observed in both the cases due to the formation of solid polymers and its insolubility in the solvent. The UVevis spectra of chemically synthesized polyindole composites were very much similar to the electrochemically derived polyindole or those of its derivatives [41,42].

3. Results and discussion

3.2. FTIR analysis

3.1. Study of polymerization through UVevis. spectroscopy

FTIR spectra of in-situ and interfacial composites of polyindole (c-MWCNT/PIn) are shown in Fig. 4. Strong peak at 740 cm1 corresponds to out of plane deformation of CeH bond of benzene ring in indole moiety. Peaks at 1460 and 1493 cm1, assigned to stretching mode of benzene rings, indicated that benzene ring was not the polymerization site. The strong peak at 3433 cm1 (stretching vibrations), together with the peak at 1570 cm1 (deformation vibrations), present in both the composites, implied that there were still NeH bonds on the polyindole backbone which proved that N site was not the polymerization site, and it occured via 2 and 3 positions. The absence of band at 720 cm1 confirmed that benzene ring (this is a part of indole molecule) was not affected in the polymerization process and 2, 3position of the pyrrole ring (this is a part of indole molecule) was responsible for the polymerization of indole. Both composite’s spectra an exhibited low intensity band at 1709 cm1 due to elongation of carbonyl groups, which was reported for the doped polymer [43,44]. A considerable shift in the peak positions was observed, in the frequency range of 1100e1600 cm1 (due to

2.4. Fabrication and measurement of devices

Growth of the polymer and characteristic peak positions were monitored for both the synthesis routes following UVevis spectra. Indole monomer showed one peak at position 280 nm (inset of Fig. 3). Interfacial polymerization proceeded with change in colour of the upper aqueous phase (light green) followed by film formation at the interface. Three peaks appeared and manifested an increase in intensity with polymerization time (from 0 h to 6 h at different interval) at 280, 380 and 532 nm, when spectra was recorded by collecting the samples from interface of the two solutions, as shown in Fig. 3. It also showed a shift in the base line due to larger particle size and scattering. First peak was due to p-p* transitions, second one was due to np* transitions and third one was due to polaronic excitations as reported earlier [39,40]. This increase in the peak intensities were not observed at longer time and a significant decrease in the intensities were observed after 1e2 h, due to formation of solid polymers at the interface and its insolubility in the solvents (figure

Fig. 3. UVevis spectra of interfacial c-MWCNT/PIn composite.

Fig. 4. FTIR spectra of (A) In-situ c-MWCNT/PIn composite and (B) Interfacial cMWCNT/PIn composite in KBr matrix.

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c-MWCNT/Pin composite gets rid of orientation heterogeneity and directional growth of polymer is observed along the c-MWNT planes, because c-MWNT acts as a hard template for polymer growth. This was further proved by TEM and SEM Techniques. The XRD pattern of both composites were similar, except the broad peak w2q ¼ 10 in case of in-situ polymerization, because large number of dopant ions are present in in-situ method compare of interfacial method; which showed an arrangement of dopant ions in some regular order in the polyindole [47]. 3.4. SEM and TEM analysis

Fig. 5. XRD of (A) c-MWCNT, (B) in-situ c-MWCNT/PIn (C) interfacial c-MWCNT/PIn composites.

different stretching vibrations of benzene rings), in the case of composites (more shifted in case on interfacial composite), as compared to pure polymer [36], probably due interaction of two components, i.e. polymer chains and c-MWCNT. 3.3. XRD analysis XRD pattern of c-MWCNT, in Fig. 5(A), shows diffraction peaks at 2q ¼ 25.69 and 2q ¼ 50.7 corresponding to (002) and (004) reflection planes (interlayer spacing between adjacent graphite layers), respectively, along with the reflection peaks of (100), (101) and (102) corresponding to in plane ordering at position 2q ¼ 42.7, 43.4 , and 48.6 , respectively. Peaks at 2q ¼ 37.43 and 44.41 appeared due to residual catalyst [45]. XRD of polyindole and polyindole composite by both the routes (in-situ and interfacial method) was shown in Fig. 5(BeE). From XRD it is evident that compare to in-situ polyindole, interfacial polyindole has several sharp peaks over large shallow background. So it can be inferred that polymer crystallites are oriented in different directions and showing orientation heterogeneity [46]., whereas interfacial

The surface morphology of c-MWCNT, c-MWCNT/PIn in-situ and c-MWCNT/PIn interfacial composites were analyzed under SEM, as shown in Fig. 6. Two different morphologies were observed for polyindole prepared by two different polymerization methods. Insitu polymerization reaction kinetics was fast as compared to that of interfacial polymerization; this was an important factor for the two different morphologies of the composites. c-MWCNT was totally embedded within the polymer matrix in in-situ composite shown in Fig. 6(B), whereas interfacial composite showed a tubular morphology shown in Fig. 6(C). This difference in morphology was also caused because of formation of metastable micelles in miscible solvents system (organic aqueous emulsion) of in-situ polymerization, in comparison to unstable micelles or no micelles formation in interfacial polymerization. The spherical micelles formed in the reaction mixture (in-situ) became a nucleation center for the further and fast growth of the polymer; the growth was fast enough to restrict the formation of nucleation centre on c-MWNCT and these centers were created within the solution which led to spherical morphology of the in-situ composite with c-MWNCT embedded in it. In case of interfacial polymerization, nucleation sites were created over the c-MWCNT, which worked as a template, and polymerization started at the interface due to the interaction of monomer and oxidizing agent, as shown in Fig. 1. Slow kinetics provided sufficient time to monomers and oligomers to interact with the c-MWCNT surface, and polymer growth took place at the surface of nanotubes and formed tubular composite. The formation of tubular composite and a uniform coating of the polymer were further supported by high resolution TEM analysis as shown in Fig. 7. 3.5. Electrochemical study Redox property and electroactivity of composites were studied using cyclic voltammetry, in 0.5 M H2SO4, at various scan rates vs. Ag/AgCl, as shown in Fig. 8. Before experiment, Pt disk working

Fig. 6. SEM images of (A) c-MWCNT, (B) in-situ c-MWCNT/PIn and (C) interfacial c-MWCNT/PIn composites.

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Fig. 7. TEM image of c-MWCNT/PIn interfacial composite and formation of tubular composite.

electrode was polished with alumina slurry, rinsed and sonicated for 1e2 min for complete cleaning. Five different solutions were prepared by separately dispersing 0.5 mg of the (c-MWNT, polymer (in-situ and interfacial) and composite (in-situ and interfacial) materials) in five different vials containing 200 ml of THF each. Ultrasonic agitation was used for

Scheme 1. Molecular structure of polyindole in redox process (A) First redox step (B) Second redox step.

proper dispersion of the black suspension of c-MWNT. Five set of experiments were performed by modifying the Pt disc by 5 ml of the above prepared solutions one by one; the modification was done using the drop casting method. The Cyclic Voltammogram (CV) of cMWNT is shown in Fig. 8(A) along with bare Pt. In the potential window of 0e0.4 V, small background current was observed (attributed to the capacitive behavior of carbon materials) [48]. No distinct redox peaks were observed except a broad peak with

Fig. 8. (A) CV of bare Pt (a) and c-MWNT (b) at 20 mV, vs. Ag/AgCl in 0.5 M H2SO4. (B) Comparative CV of (a) in-situ (b) interfacial polymer and (c) in-situ (d) interfacial composite at 20 mV/s for in 0.5 M H2SO4. (C) Comparative CV of (a) bare Pt (b) in-situ (c) interfacial polymer and (d) in-situ (e) interfacial composite at 20 mV/s for 20 cycles in 0.5 M H2SO4. (D) CV of interfacial Pin/c-MWCNT composite at different scan rates 5 mV, 20 mV, 50 mV, 100 mV, 200 mV and 500 mV vs. Ag/AgCl in 0.5 M H2SO4. Inset: anodic and cathodic peaks current variation with square root of scan rate.

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Fig. 9. Thermogravimetric analysis of (A) in-situ c-MWCNT/PIn and (B) interfacial c-MWCNT/PIn composites.

some spikes at 0.8e0.9 V due to leaching of c-MWNT from Pt surface. CV of in-situ and interfacial polymers and composite showed two set of redox peaks shown in Fig. 8(B).First redox peak at lower potential (Epa1), was due to the creation of a radical cation and the formation of a stable polycationic intermediate. Second

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prominent peaks (Epa2), were due to deprotonation of the intermediate state, leading to a fully oxidized form of polymer, as shown in Scheme 1 [42]. We can easily see the differences of Epa values, which is due to different conjugation length of polymers in all the systems. Higher potential shift to Epa values signifies the reduction of conjugation length [39]. Compared to polymer, the redox peaks of composites were not well defined and blur due to the incorporation of c-MWNT (which shows capacitive behaviour); but their redox current was enhanced as compared to the polymer. Redox current was much higher for interfacial composite, in comparison to in-situ composite and pristine polymer, probably due to better interactions of delocalized p electrons of the conducting polymers with c-MWCNT surfaces. CV of these systems (recorded for 20 cycles) showed that the polymer may be repeatedly cycled between the conducting (oxidized) and insulating (neutral) states without significant decomposition, as any decrease in peak current was not observed during continuous scan Fig, 8(C). Furthermore, for interfacial composite, peak current swept linearly as a function of square root of scan rate as shown in Fig. 8(D). This demonstrated that for this system the electrochemical process is not diffusion limited and electrode reactions had rapid electron transfer kinetics and are reversible even at higher scan rates [49].

3.6. TGA analysis Thermal analysis of c-MWNT, polymer and composite materials under N2 atmosphere, was carried out to study their degradation behavior. TGA curves showed a two-step weight loss process for

Fig. 10. SEM images of c-MWCNT/PIn interfacial composite thin film coated over Al substrate (A) showing surface region (B) showing crack region.

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polymer and composite shown in Fig. 9(BeD). The first weight loss at w230  C is attributed to the loss of solvent and smaller polymer chains. Second and a prominent weight loss which started at w550  C, inferred due to degradation of skeletal polyindole backbone. TGA curve of c-MWNT shown in Fig. 9(A), shows 25% weight loss of its original weight at 800  C. For pure polymer (in-situ and interfacial) it reached to w70%. In case of in-situ composite, total weight loss reached up to 50% of its original weight, whereas, in case of interfacial composite, it reached to only 37% at 800  C. A significant shift of this degradation temperature towards higher temperature as well as less decomposition of material in case of interfacial composite was due to strong interaction of polymer chains with c-MWCNT. 3.7. Electrical and junction properties of composites using nanocontact Electrical characterization of thin film of c-MWCNT/PIn composites was done using current sensing atomic force microscope. The cMWCNT/PIn composite films, coated over Al substrates (spin coating using THF solution of composites), were also examined under SEM before IeV characterization. In-situ c-MWCNT/PIn composite showed spherical polymer with c-MWCNT in them (figure not shown here); hardly any c-MWCNT was observed over the surface of the thin film, however, fractured surfaces showed the presence of MWCNT in the bulk. Large numbers of c-MWCNT were observed over the surface of the interfacial c-MWCNT/PIn composite film; c-MWCNT, modified with polyindole, were uniformly distributed over the surface as well as in the bulk, as supported by SEM of the top surface and fractured

surface of film shown in Fig. 10 by circle A and circle B respectively. Fig. 11(A) shows the 3d AFM image of c-MWCNT/PIn (interfacial) composite. The tip was probed at different spots of the film and a bias voltage was applied between the conductive tip and the sample. Therefore, when the sample was approached to few angstroms from the tip, tunneling current occurred. The c-MWCNT/PIn (interfacial) composite exhibited a highly nonlinear and asymmetric behaviour with a rectification ratio about 500 at 0.9 V, which resembled a rectifying nanodiode with a turn-on voltage at w0.7 V. The current rectification was reproducible as shown in Fig. 11(B) and Fig. 11(C). The asymmetrical nature of curve was attributed to the difference in the work function of electrodes, which implied different barriers at each electrodeecomposite interface. IeV characteristics of the c-MWCNT/PIn composite, prepared by in-situ method, were nearly linear with a very low value of current (in fA). The electrical parameters of Al/(c-MWCNT/PIn) composite (interfacial) junction such as ideality factor, reverse saturation current and barrier height were analyzed assuming the standard emission-diffusion theory [50,51]. The ideality factor was 2.0, reverse saturation current was 2.24  1014(A/cm2) and the barrier height, evaluated from reverse saturation current at room temperature, was found to be 1.29 eV. The smaller reverse saturation current was attributed to a larger barrier height. The value of ideality factor for Schottky diode was higher than the ideal value (unity). The deviation of ideality factor from unity was probably due to a barrier inhomogeneity. However, the ideality factor of Al/c-MWCNT/PIn (interfacial) Schottky diode measured using nano-contact was much better than previously reported ideally factor of polyindole alone [52,53]. The better ideality factor of

Fig. 11. (A) 3d image of interfacial c-MWCNT/PIn composite and (BeC) IeV characteristics of interfacial Al/(c-MWCNT/PIn) nano-layered device.

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composite device may be due to the presence of MWCNT in polymer matrix, enhancing the charge transport across the interface. 4. Conclusions We have reported, for the first time, synthesis and characterization of polyindole composites with carboxyl functionalized multiwall carbon nanotubes. Composites were synthesized using in-situ and interfacial polymerization techniques. Polymerization was confirmed by investigating UVevis and FTIR spectra. A significant improvement in electrical, electrochemical and thermal properties was observed in both the composites; however, it was much significant in interfacial composites because of the presence of a strong interaction of polymer chains with the surface of cMWCNT, in case of interfacial composites. The morphological and structural evaluations of composites were studied using SEM, TEM and powder X-ray diffractometer. Two different types of morphologies were observed based on the different polymerization techniques. Further, the electrical characteristic of the composites in Al/(c-MWCNT/PIn) configuration were studied using nanocontacts (using conducting platinum metal tip AFM). Interfacial composites manifested a rectification behavior. A cheaper metal, like Al metal, substrate made a good contact available for electrical measurements and device performance study. Nanoscale electronic devices, based on polyindole- c-MWCNT composite, can emerge as a promising candidate in organic electronics. Acknowledgements Authors are thankful to Prof. D. Pandey, SMST, IT, BHU and Prof. K. Kaneto, KIT, Japan for fruitful discussion and suggestions. Authors are thankful to DST-JSPS for funding. References [1] M. Baibarac, P. Gomez-Romero, J. Nanosci. Nanotechnol. 6 (2006) 289. [2] D.W. Hatchett, M. Josowicz, Chem. Rev. 108 (2008) 746. [3] R.K. Srivastava, A. Srivastava, R. Prakash, V.N. Singh, B.R. Mehata, J. Nanosci. Nanotechnol. 9 (2009) 1. [4] M. Hughes, G.Z. Chen, M.S.P. Shaffer, D.J. Fray, A.H. Windle, Chem. Mater. 14 (2002) 1610. [5] J. Wang, J. Dai, T. Yarlagadda, Langmuir 2 (2005) 9. [6] R. Sainz, A.M. Benito, M.T. Martinez, J.F. Galindo, J. Sotres, A.M. Baro, B. Corraze, B.O. Chauvet, W.K. Maser, Adv. Mater. 17 (2005) 278. [7] P.M. Ajayan, Chem. Rev. 99 (1999) 1787. [8] D. Tasis, N. Tagmatarchis, A. Bianco, M. Prato, Chem. Rev. 106 (2006) 1105. [9] I.Y. Jeon, S.W. Kang, L.S. Tan, J.B. Baek, J. Polym. Sci. Pol. Chem. 48 (2010) 3103. [10] E. Kymakis, G.A.J. Amaratunga, Appl. Phys. Lett. 80 (2002) 112.

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