Preparation of Transparent Conductive Multilayered Films Using

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Langmuir 2008, 24, 10467-10473

10467

Preparation of Transparent Conductive Multilayered Films Using Active Pentafluorophenyl Ester Modified Multiwalled Carbon Nanotubes Hye Jin Park,†,§ Junkyung Kim,§ Ji Young Chang,*,† and Patrick Theato*,‡ Department of Materials Science and Engineering, Seoul National UniVersity, Seoul, 151-744, Korea, Hybrid Materials Center, Korea Institute of Science and Technology, Seoul, 136-791, Korea, and Institute of Organic Chemistry, UniVersity of Mainz, Duesbergweg 10-14, D-55099, Mainz, Germany ReceiVed April 29, 2008. ReVised Manuscript ReceiVed June 11, 2008 A mutilayered film was prepared by layer-by-layer (LBL) assembly of active ester modified multiwalled carbon nanotubes (MWCNTs) and poly(allylamine hydrochloride) (PAH). For this purpose, carboxylic groups on the surface of the oxidized MWCNTs were converted to the acyl chlorides by their reaction with thionyl chloride. Subsequent reaction of the acyl chlorides with pentafluorophenol formed the active esters. These active ester modified MWCNTs (MWCNTs-COOC6F5) were air-stable and moisture resistant, but showed a high reactivity toward primary or secondary amines resulting in amide bonds. For the preparation of a multilayered film, the surface of a quartz slide was first activated and sacrificial double layers of PAH and poly(sodium 4-styrene sulfonate) (PSS) were deposited. Subsequently, LBL assembly of MWCNTs-COOC6F5 and PAH was then conducted on these double layers [(PAH/PSS)2]. In the process of the assembly, a reaction occurred between the active ester on the surface of MWCNTs and the amine groups of polyallylamine yielding amide bonds, which resulted in a mechanically stable thin film. A free-standing film was obtained after dissolving the sacrificial layer [(PAH/PSS)2] in a concentrated aqueous NaOH solution. The surface resistance of the multilayered film with 20 bilayers decreased to around 10 kΩ while remaining a reasonable transparency (70% at 500 nm).

Introduction Carbon nanotubes (CNTs) have remarkable electronic and mechanical properties. They are considered to be one of the most attractive fillers for producing conductive polymeric nanocomposites,1 which have desirable antistatic and electronic dissipation properties. In recent years, transparent conductive films based on CNTs and an organic polymer have attracted increasing attention because of their versatile use in many applications, including in organic photovoltaic (OPV) devices, flexible displays, and electrochromic devices.2,3 Transparent conductive films comprising CNTs and a polymer matrix have a great advantage over conventional metal coated glasses in terms of their flexibility.4 In order to achieve transparent and highly conductive films, CNTs should be dispersed finely in an ultrathin polymer matrix. Accordingly, the preparation of a stable CNT dispersion in an appropriate solvent and the prevention of reaggregation producing in-plane cluster during solvent drying are important issues.5 In this context, several methods have been reported for * To whom correspondence should be addressed. Dr. Patrick Theato; Tel: (+49)6131-39-26256; Fax: (+49) 6131-39-24778; E-mail: theato@ uni-mainz.de; Prof. Ji Young Chang; Tel: (+82)2-880-7190; Fax: (+82)2885-1748; E-mail: [email protected]. † Seoul National University. § Korea Institute of Science and Technology. ‡ University of Mainz. (1) Dresselhaus, M. S.; Dresselhaus, G.; Avouris, P. Carbon Nanotubes: Synthesis, Structure, Properties, and Applications; Springer-Verlag Berlin Heidelberg: New York, 2001. (2) Zhang, D.; Ryu, K.; Liu, X.; Polikarpov, E.; Ly, J.; Tompson, M. E.; Zhou, C. Nano Lett. 2006, 6, 1880–1886. (3) Small, W. R.; Masdarolomoor, F.; Wallace, G. G.; Panhuis, M. i. h. J. Mater. Chem. 2007, 17, 4359–4361. (4) Cao, Q.; Hur, S.-H.; Zhu, Z.-T.; Sun, Y. G.; Wang, C.-J.; Meitl, M. A.; Shim, M.; Rogers, J. A. AdV. Mater. 2006, 18, 304–309. (5) Bocharova, V.; Kiriy, A.; Oertel, U.; Stamm, M.; Stoffelbach, F.; Jerome, R.; Detrembleur, C. J. Phys. Chem. B 2006, 110, 14640–14644.

the preparation of thin CNT networks such as vacuum filtration,6 spray coating,7,8 and Langmuir-Blodgett deposition.9,10 Another interesting approach to this objective is layer-bylayer (LBL) assembly. LBL assembly has been widely used for the preparation of thin polymeric films since its first introduction by Decher et al.11 Generally, a solid substrate is first electrically charged, followed by the deposition of polyelectrolytes possessing opposite charge in an alternating cycle. Consequently, multilayered films consisting of two types of polymers having opposite charges are built up, due to the electrostatic interaction between these two polyelectrolytes. The incorporation of single-walled carbon nanotubes (SWCNTs) into thin films prepared by a LBL assembly technique has also been reported. For this purpose, water dispersed SWCNTs that provide charges on their surface have been prepared by oxidizing SWCNTs, and thus exhibiting carboxylic acid groups12,13 or by complexation with ionic ligands such as ionic pyrene derivatives.14 Cui et al. reported on transparent carbon nanotube containing thin films that have been prepared by LBL assembly using oxidized SWCNTs, which opened routes toward advanced applications, such as an acoustic actuators and sensors.15 (6) Wu, Z.; Chen, Z.; Du, X.; Logan, J. M.; Sippel, J.; Nikolou, M.; Kamaras, K.; Reynolds, J. R.; Tanner, D. B.; Hebard, A. F.; Rinzler, A. G. Science 2004, 305, 1273–1276. (7) Kaempgen, M.; Duesberg, G. S.; Roth, S. Appl. Surf. Sci. 2005, 252, 425– 429. (8) Andrade, M. J.; Lima, M. D.; Ska´kalova´, V.; Bergmann, C. P.; Roth, S. Phys. Stat. Sol. (RRL) 2007, 1, 178–180. (9) Kim, Y.; Minami, N.; Zhu, W.; Kazaoui, S.; Azumi, R.; Matsumoto, M Jpn. J. Appl. Phys 2003, 42, 7629–7634. (10) Li, J.; Zhang, Y. Carbon 2007, 45, 493–498. (11) Decher, G. Science 1997, 277, 1232–1237. (12) Rouse, J. H.; Lillehei, P. T. Nano Lett. 2003, 3, 59–62. (13) He, P.; Bayachou, M. Langmuir 2005, 21, 6086–6092. (14) Paloniemi, H.; Lukkarinen, M.; Aaritalo, T.; Areva, S.; Leiro, J.; Heinonen, M.; Haapakka, K.; Lukkari, J. Langmuir 2006, 22, 74–83. (15) Yu, X.; Rajamani, R.; Stelson, K. A.; Cui, T. Sens. Actuators, A 2006, 132, 626–631.

10.1021/la801341t CCC: $40.75  2008 American Chemical Society Published on Web 08/21/2008

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Scheme 1. Preparation of Pentafluorophenyl Ester Modified MWCNTs

Recently, Theato et al. reported the successful preparation of multilayers on the basis of covalent bonds between gold nanoparticles and poly(allylamine) utilizing gold nanoparticles exhibiting active pentafluorophenyl esters on their surface, which have the potential to be used as biomolecular sensors.16 Such films might find application as biomolecular sensors or electrochromic devices by taking advantage of the reactivity of remaining pentafluorophenyl esters moieties toward potential analytes. A covalent binding of an analyte may result in a change of conductivity or absorbance of the film. However, multilayered films containing nanoobjects of various kinds (CNTs, AuNP, etc.) that have been prepared by the LBL deposition method suffer from the potential instabilities because the electrostatic interactions between the layers tend to fail under certain conditions, such as strong electrolyte treatment. Accordingly, covalent bonds between the layers could improve the chemical resistance of such multilayered films and several methods for building-up multilayered films by formation of covalent bonds have been reported. Kotov et al. solved this problem by introducing covalent bonds between the layers via a post reaction. A multilayered film was prepared using electrostatic interactions between poly(sodium 4-styrenesulfonate) wrapped around CNTs and a diphenylamine-4-diazoresin. The ionic bonds between the sulfonate anions and diazonium cations were then converted to covalent bonds by UV irradiation.17 In another report,18 they demonstrated a thermal cross-linking reaction of a multilayered film consisting of a polyethylenimine and a MWCNTs layer. After every fifth deposition cycle, a layer of MWCNTs was replaced with a layer of poly(acrylic acid) and the film was heated resulting in amide cross-links between polyethylenimine and poly(acrylic acid). The film was further cross-linked by the reaction of the remaining free amine groups with glutaraldehyde. In this paper, we adopt the idea of the active esters on the surface of a nanoparticle and present a new method to prepare a multilayered film consisting of MWCNTs in which all layers are covalently connected. Pentafluorophenyl active esters are air-stable, moisture resistant and show a high reactivity toward primary or secondary amines resulting in amide bonds.19 Such active ester modified MWCNTs (MWCNTs-COOC6F5) can be better dispersed in various organic solvents than their precursors, oxidized MWCNTs and a multilayered film can be obtained by the LBL assembly through the covalent coupling reaction between the active ester and a polyamine. (16) Roth, P. J.; Theato, P. Chem. Mater. 2008, 20, 1614–1621. (17) Qin, S.; Qin, D.; Ford, W. T.; Zhang, Y.; Kotov, N. A. Chem. Mater. 2005, 17, 2131–2135. (18) Olek, M.; Ostrander, J.; Jurga, S.; Mohwald, H.; Kotov, N.; Kempa, K.; Giersig, M. Nano Lett. 2004, 4, 1889–1895. (19) Eberhardt, M.; Mruk, R.; Zentel, R.; Theato, P. Eur. Polym. J. 2005, 41, 1569–1575.

Experimental Section Materials. The MWCNTs used in this study were supplied by Nanotube and Nanodevice Laboratory at Korea University. The MWCNTs were produced by chemical vapor deposition (CVD) using Fe-Mo/Al2O3 as a catalyst and C2H4 as a carbon source. Their diameters were 10∼30 nm and the ash content was below 8 wt%. Poly(allylamine hydrochloride) (PAH; Mw ) 70 000), poly(sodium 4-styrene sulfonate) (PSS; Mw ) 70 000) and pentafluorophenol (>+99%) were purchased from Sigma-Aldrich Co. Thionyl chloride was obtained from Fluka. Amine terminated poly(ethylene glycol) with molecular weight of 550 (NH2-PEG550) was synthesized following a previously reported procedure.21 4-Nitro-7-piperazinobenzofurazan (NBD-PZ) was synthesized by reaction of 4-nitro7-chlorobenzofurazan with piperazine. All solvents used in this study were spectrophotometry/HPLC grade and used without further purification unless otherwise stated. Oxidation of MWCNTs. A mixture of MWCNT (400 mg) in a solution of H2SO4 (98%, 75 mL) and HNO3 (70%, 25 mL) was stirred for 2 h at room temperature. The mixture was then diluted with 500 mL of deionized water and the solids were isolated by filtration through a polytetrafluoroethylene (PTFE) membrane (ALBET, 0.2 µm pore size) and rinsed with deionized water several times. The black powder collected from the membrane was further treated by stirring with a solution of H2SO4 (98%, 40 mL) and H2O2 (30%, 10 mL) for 1 h at room temperature. After dilution with 300 mL of deionized water, the MWCNTs were isolated by filtration through a PTFE membrane and rinsed again with deionized water until the pH of the filtrate reached around 7. The resulting oxidized MWCNTs could be stored after freeze-drying overnight. Functionalization. The oxidized MWCNTs (150 mg) were stirred in a mixture of thionyl chloride (SOCl2; 120 mL) and N,Ndimethylformamide (DMF; 6 mL) at 70 °C for 24 h. The excess SOCl2 was then removed by distillation. The remaining solids (MWCNTs-COCl) were washed with dry tetrahydrofuran (THF), isolated by filtration through a PTFE membrane and dried in a vacuum oven at room temperature. MWCNTs-COCl (100 mg) were dispersed in 25 mL dry dichloromethane (CH2Cl2) by sonication in an ultrasonic bath (Elma Transsonic TI-H-5) for 20 min at 45 kHz. Pentafluorophenol (1 g) and pyridine (0.44 mL) were added to the suspension, and the mixture was stirred at room temperature for 24 h. The solids were isolated by filtration through a PTFE membrane and washed in ethanol under sonication. After filtration, the solids were washed several times with CH2Cl2 and freeze-dried overnight. Reaction of Active Pentafluorophenyl Ester Modified MWCNTs (MWCNTs-COOC6F5) with Amines. Dodecylamine or NH2-PEG550 (10 mM) in THF (10 mL) was added to a dispersion of MWCNTs-COOC6F5 (5 mg) in THF (20 mL) and the solution was stirred for 3 h at room temperature. The resulting powder was isolated by filtration through a PTFE membrane and then rinsed with THF several times. The powder was freeze-dried overnight. The modified MWCNTs (5 mg) were redispersed in THF (20 mL) to evaluate their solubility. (20) Liang, Z.; Wang, Q. Langmuir 2004, 20, 9600–9606. (21) Mongondry, P.; Bonnans-Plaisance, C.; Jean, M.; Tassin, J. F. Macromol. Rapid Commun. 2003, 24, 681–685.

Transparent ConductiVe Multilayered Films Reaction of MWCNTs-COOC6F5 with a Fluorescence Dye. 4-Nitro-7-piperazinobenzofurazan (NBD-PZ) was used as a fluorophore. Either oxidized MWCNTs (MWCNTs-COOH) or MWCNTsCOOC6F5 (10 mg) were suspended in a solution of NBD-PZ (1 mg) in THF (1 mL) under mild sonication for 1 h. MWCNTs were isolated by filtration and unreacted NBD-PZ was thoroughly removed by repeated suspending in THF under ultrasonication and filtration through a PTFE membrane (0.2-µm pore size). Afterward, MWCNTs were dispersed in acetone under sonication for 10 min and then the suspension was dropped onto a slightly inclined glass slide for fluorescence imaging. Preparation of LBL Films. A quartz slide was used as a substrate. The substrate was cleaned by treatment with a solution of 30% H2O2 and concentrated sulfuric acid (3:7, v/v) at room temperature for 1 h, rinsed several times with water, and then dried under nitrogen. After cleaning, the substrate was negatively charged on its surface by means of a basic RCA treatment:22 the substrate was immersed into a solution containing 5 parts of H2O, 1 part of NH3, and 1 part of H2O2, heated to 70 °C for 30 min and afterward thoroughly rinsed with deionized water. The solutions for LBL assembly were prepared as follows. The active ester modified MWCNTs (MWCNTs-COOC6F5) were dispersed in DMF (1 mg/mL) under mild sonication in an ultrasonic bath (Elma Transsonic TI-H-5) for 2 h. The dispersion was centrifuged at 5000 rpm, and the supernatant was collected and used for LBL assembly. A PAH solution (1 wt%) and a PSS solution (1 wt%) were prepared in water. The negatively charged substrate was immersed in a PAH solution in water (1 wt%) for 20 min, rinsed with water 3×, and then dried with nitrogen. The substrate was then immersed into a solution of oppositely charged PSS in water (1 wt%) for 20 min and subjected to the same rinsing and drying procedures. This cycle was repeated once again to prepare a homogeneous polyelectrolyte multilayer on the substrate. A layer of MWCNTs-COOC6F5 was then deposited onto the amine terminated multilayer by immersing the polyelectrolyte coated substrate into a dispersion of MWCNTs-COOC6F5 in DMF for 20 min and rinsing in DMF 3×. After drying with nitrogen, LBL assembly was repeated with a solution of PAH in water and a dispersion of MWCNTs-COOC6F5 in DMF in sequence. To denote LBL assemblies, [(PAH/PSS)2(PAH/MWCNTs-COOC6F5)n], was used in which n represents the number of repeated dipping cycles in a solution of PAH and a dispersion of MWCNTs-COOC6F5. Characterization. FT-IR measurements were carried out on a Bruker Vector 22 Fourier transform infrared spectrometer with an attenuated total reflectance (ATR) sample stage. X-ray photoelectron spectroscopy (XPS; PHI 5800 ESCA System) was performed using a monochromatic Al KR radiation and operating at 15 kV and 20 mA. The background pressure of 2 × 10-10 torr maintained during the analysis and the main peak at 284.5 eV was assigned to the C1s binding energy for the MWCNTs. Thermogravimetric analysis was performed with a Perkin-Elmer Pyris 1 TGA instrument under nitrogen. An Olympus BX 51 microscope was used to obtain fluorescence images. Electrical conductivity measurements have been performed in a four-point probe setup. A DC precision power source (Keithley; Model 6220), nanovoltmeter (Keithley; Model 2182A) and a four-point cylindrical probe head (JANDEL Engineering Ltd.) were used. UV-visible spectra were measured with a Jasco V-570 spectrophotometer. SEM images were obtained with a Nova NanoSEM 600 (FEI Co.). TEM images were taken on a field emission gun transmission electron microscope (Tecnai G2) at an accelerating voltage of 200 kV.

Results and Discussion Pentafluorophenyl ester modified MWCNTs (MWCNTsCOOC6F5) were prepared as described in Scheme 1. First, the MWCNTs were oxidized with acid solutions, H2SO4/HNO3 and H2SO4/H2O2, resulting in carboxylic groups on the surface of MWCNTs. These carboxylic groups were then converted to the (22) Quinn, J. F.; Caruso, F. Macromolecules 2005, 38, 3414–3419.

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Figure 1. Photographs of modified MWCNTs dispersed in THF taken one day after their preparation: (a) MWCNTs-CONH-PEG550, (b) MWCNTs-CONH-C10H21, (c) MWCNTs-COOC6F5, and (d) MWCNTsCOOH.

acyl chlorides by their reaction with thionyl chloride. Subsequent reaction of the acyl chlorides with pentafluorophenol formed the respective active esters (MWCNTs-COOC6F5). MWCNTsCOOC6F5 were better dispersible than their precursors, MWCNTsCOOH in THF. Dispersing the functionalized MWCNTs in THF with a concentration of 0.25 mg/mL by ultrasonication resulted in a precipitation of most MWCNTs-COOH within 1 h, while the equivalent dispersion of MWCNTs-COOC6F5 was stable over one month. Exemplary pictures of dispersions of different functionalized MWCNTs in THF taken one day after their preparation are shown in Figure 1. MWCNTs-COOC6F5 were characterized using FT-IR and UV-vis spectroscopy, XPS, and TGA. The FT-IR spectra of MWCNTs-COOH and MWCNTs-COOC6F5 are shown in the Supporting Information, Figure S1. A band corresponding to the CdO stretching vibrations of the carboxyl and carbonyl groups of the MWCNTs-COOH appeared at ∼1730 cm-1.23,24 After esterification with pentafluorophenol, a broadband at around 1750 cm-1 showed up, which can be associated with the ester carbonyl stretching frequency typically found for the active ester. The typical signals for the pentafluorophenyl group (C-F bonds) were found at 1520 cm-1 and 1000 cm-1. In the UV-vis spectrum of MWCNTs-COOH, no significant peak was observed. However, after esterification, the UV-vis spectrum of MWCNTs-COOC6F5 showed a peak with the maximum intensity at 265 nm and a shoulder at 290 nm (Figure S2, Supporting Information). These absorption peaks could be assigned to the pentafluorophenyl group and are in agreement with the peaks observed for pentafluorophenol. The amount of pentafluorophenyl ester groups on the surface of MWCNTs-COOC6F5 was estimated by TGA. We presumed that the total mass loss until 300 °C in the TGA thermogram could be attributed to the loss of the ester groups on the MWCNT surface. Therefore, using the mass loss and the molecular weight of the functional group, the mole ratio of pentafluorophenyl ester groups to MWCNT carbon atoms was calculated to be about 0.004 (Figure S3, Supporting Information).25 Additional evidence that the MWCNTs-COOC6F5 exhibits active ester groups on the surface can be found in the XPS spectra. Figure 2, parts a and b show the XPS spectra for pristine MWCNTs and MWCNTs-COOC6F5, respectively, along with the corresponding high-resolution F 1s spectra (insets). The pristine MWCNTs showed a graphite-like C 1s peak at 284.6 eV and a O 1s peak at 531.0 eV, which is commonly observed and (23) Osswald, S.; Havel, M.; Gogotsi, Y. J. Raman Spectrosc. 2007, 38, 728– 736. (24) Liu, L.; Qin, Y.; Guo, Z.-X.; Zhu, D. Carbon 2003, 41, 331–335. (25) Stephenson, J. J.; Hudson, J. L.; Leonard, A. D.; Price, B. K.; Tour, J. M. Chem. Mater. 2007, 19, 3491–3498.

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Figure 2. XPS survey spectra of (a) pristine MWCNTs, (b) MWCNTs-COOC6F5, and (c) [(PAH/PSS)2(PAH/MWCNTs-COOC6F5)20]. The insets of (a) and (b) show high-resolution F 1s spectra. The inset of (c) shows a high resolution N 1s spectrum. Table 1. Atomic % Concentration of MWCNTs based on XPS Analysis atomic % concentration sample

C1s

N1s O1s F1s

pristine MWCNTs 97.90 0.00 2.10 0.00 MWCNTs-COOC6F5 93.84 0.00 5.59 0.57 [(PAH/PSS)2(PAH/MWCNTs-COOC6F5)20] 89.48 2.56 7.95 0.01

due to physically adsorbed oxygen or intrinsic defects (Figure 2a).26 However, the XPS spectrum of MWCNTs-COOC6F5 revealed a new peak at 685.0 eV, which is attributable to F 1s of the pentafluorophenyl ester (Figure 2b).20,27,28 The F 1s peak was more clearly observed in the high-resolution F 1s spectrum given in the inset. Table 1 summarizes the atomic percent concentrations of the pristine MWCNTs and MWCNTsCOOC6F5. In order to demonstrate the reactive character of active ester functionalized MWCNTs-COOC6F5 toward amines, a sample was allowed to react with either dodecylamine or amine terminated poly(ethylene glycol) (NH2-PEG550; MW ) 550). For this purpose, the respective amine was added to the dispersion of MWCNTs-COOC6F5 in THF, and the reaction was allowed to stir for 3 h at rt. The resulting surfacefunctionalized MWCNTs showed a much better dispersibility in THF, as shown in Figure 1. All dispersions were stable over 3 months, suggesting that the dodecylamine or NH2-PEG550 was covalently attached to the surface of MWCNTs-COOC6F5 via an amide linkage. The reactivity of MWCNTs-COOC6F5 toward amines was further studied using a fluorescent dye. We chose 4-nitro-7piperazinobenzofuranzan (NBD-PZ) as a fluorescent dye because it possesses a secondary amine group and shows only a relatively weak hydrophobic or π-π interaction with MWCNTs.29-31 NBD-PZ was allowed to react with MWCNTsCOOC6F5 dispersed in THF at rt. Afterward, the functionalized CNTs were washed several times with THF and acetone under ultrasonication to remove any physically adsorbed dye on the MWCNTs. For comparison, MWCNTs-COOH were treated with NBD-PZ under the same reaction conditions in order to exclude (26) Okpalugo, T. I. T.; Papakonstantinou, P.; Murphy, H.; McLaughlin, J.; Brown, N. M. D. Carbon 2005, 43, 153–161. (27) Francesch, L.; Borros, S.; Knoll, W.; Forch, R. Langmuir 2007, 23, 3927– 3931. (28) Lee, K. M.; Li, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 4122–4123. (29) Fernando, K. A. S.; Lin, Y.; Wang, W.; Kumar, S.; Zhou, B.; Xie, S. Y.; Cureton, L. T.; Sun, Y. P. J. Am. Chem. Soc. 2004, 126, 10234–10235. (30) Ehli, C.; Rahman, G. M. A.; Jux, N.; Balbinot, D.; Guldi, D. M.; Paolucci, F.; Marcaccio, M.; Paolucci, D.; Melle-Franco, M.; Zerbetto, F.; Campidelli, S.; Prato, M. J. Am. Chem. Soc. 2006, 128, 11222–11231. (31) Rohit, P.; Washburn, S.; Richard, S.; Richard, E. C.; Michael, R. F. Appl. Phys. Lett. 2003, 83, 1219–1221.

Figure 3. Optical (a,c) and fluorescent images (b,d) of MWCNTs-COOH (a,b) and MWCNTs-COOC6F5 (c,d) after immobilization of NBD-PZ.

any electrostatically driven adsorption. Figure 3 shows the optical and fluorescent images of MWCNTs after reacting with NBDPZ. Before reacting with NBD-PZ, both MWCNTs did not show any fluorescence. The sample prepared from the MWCNTsCOOH after the reaction with the fluorescent dye did not show any fluorescence either, suggesting that the binding affinity of the dye to the MWCNTs-COOH was not sufficient and it could be removed by the washing procedure (Figure 3b). Alternatively, the sample prepared from MWCNT-COOC6F5 showed a strong fluorescence, indicating that the fluorescent dye was immobilized on the MWCNTs through the reaction of the secondary amine group of NBD-PZ with the pentafluorophenyl ester groups on the surface of the MWCNT-COOC6F5 (Figure 3d). Altogether, these different reactions of amines with the active ester functionalized MWCNTs demonstrated that MWCNTs-COOC6F5 are useful nanomaterials, which combine two advantages over pristine MWCNTs: (i) improved dispersibility and (ii) selective reactivity. Thus, we took advantage of the high reactivity of the MWCNTs-COOC6F5 toward amines to prepare a multilayered thin film. LBL assembly was performed using MWCNTs-COOC6F5 and poly(allylamine hydrochloride) (PAH) on a quartz slide. The surface of the

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Scheme 2. Schematic Representation of the LBL Assembly of MWCNTs-COOC6F5 and PAH

quartz slide was first activated by means of a basic RCA treatment, resulting in a negatively charged surface. In the next, poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS) were deposited in sequence in order to prepare a sacrificial polyelectrolyte double layer ([PAH/ PSS]2) on the substrate. Then, the LBL assembly of MWCNTsCOOC6F5 and PAH was performed on the sacrificial double layer. A layer of MWCNTs-COOC6F5 was immobilized by reacting with the amine groups of the subjacent PAH layer. After successful reaction of the MWCNTs-COOC6F5, the film was rinsed three times in DMF to remove pentafluorophenol, which is formed as a byproduct of the reaction, and then dried with nitrogen. Repeated sequential deposition of PAH and MWCNTs-COOC6F5 resulted in a thin film consisting of multilayers, which are cross-linked via amide bonds, as shown in Scheme 2. The LBL assembly process of [(PAH/PSS)2(PAH/MWCNTsCOOC6F5)n] was monitored with UV-vis spectroscopy. After each deposited layer, the film was washed with DMF, dried in a N2-stream and then a UV-vis spectrum was recorded. The UV-vis absorbance at λ ) 215 nm and λ ) 265 nm, which induced from the pentafluorophenyl ester,32 increased

with each deposited bilayer, as can be seen in Figure 4a. In contrast to the LBL assembly of polyelectrolytes as well as small round nanoparticles,16 which show a linear increase of absorbance with each deposited layer, the absorbance increase of the (PAH/ MWCNTs-COOC6F5)n multilayer was not linear with respect to the number of bilayers (Inset of Figure 4a). Some recent studies also reported on an exponential growth regime during a multilayer deposition.33-35 In all of these reports, surface analysis revealed a rough and very irregular surface structure that leads to the idea that the evolution of a nonhomogenous flat surface during a LBL assembly results in a nonlinear deposition. Accordingly, the nonlinear film growth in our experiments may be attributed to the fact that the substrate is only partially coated with MWCNTsCOOC6F5 in the first deposition cycle, as MWCNTs are large and stiff nano-objects, and their deposition will not necessarily result in homogenously flat layers. In comparison, the LBL assembly of active ester functionalized gold nanoparticles resulted in a linear increase of the absorbance with respect to the number of bilayers, because the small size of the gold nanoparticles prevents the development of a rough surface.16 Looking at the changes in the absorbance in detail revealed that the layers between MWCNTs-COOC6F5 and PAH are indeed

Figure 4. (a) UV-vis absorbance spectra of [(PAH/PSS)2(PAH/MWCNTs-COOC6F5)n] (n ) 1-21) onto a quartz slide. Inset: evolution of the absorbance at λmax ) 550 nm vs the number of deposited bilayers. (b) Evolution of the absorbance at 210 nm as a function of the number of layers.

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Figure 5. (a) Surface resistance as a function of the number of deposited bilayers. (b) Photograph of [(PAH/PSS)2(PAH/MWCNTsCOOC6F5)15] deposited on both side of a quartz slide.

covalently cross-linked. Figure 4b shows the change in UV absorbance measured at 210 nm during the sequential deposition of MWCNTs-COOC6F5 and PAH. It is noteworthy that there was a consistent fluctuation in the absorbance. The absorption increased upon the deposition of the MWCNTs-COOC6F5 on the PAH-coated surface, suggesting that a considerable amount of active ester groups remained after the reaction with amine groups on the surface, which is required for the next deposition of PAH. When the next PAH layer was deposited, the UV absorbance decreased, indicating that some of the remaining active ester groups were consumed to form new amide bonds. To verify whether or not all active ester groups were consumed during the multilayer assembly, XPS measurements have been conducted and the obtained XPS spectra are shown in Figure 2c. The peak corresponding to N 1s (398-400 eV) likely came from amide bonds between the layers, and also from the unreacted amine groups of PAH. Deconvolution of the C 1s peak of the XPS spectrum showed various carbons with different binding energies such as graphite (284.5 eV), C-O (286.21-287.53 eV), >CdO (286.45-287.92 eV), -COO (288.39-289.54 eV), O-COO (289-291.6 eV), C-NHx (286-288.5 eV), and -CONH(287.7 eV). According to the XPS results of the multilayered film, [(PAH/PSS)2(PAH/MWCNTs-COOC6F5)20], a small F 1s peak was also detected, indicating that a trace of unreacted pentafluorophenyl ester groups still remained within the film. These might act as potential reactive sites (Figure 2c and Table 1). Figure 5b shows an image of the multilayered film, [(PAH/ PSS)2(PAH/MWCNTs-COOC6F5)15]. The film stability of covalently cross-linked multilayered film [(PAH/PSS)2(PAH/ MWCNTs-COOC6F5)20] was excellent. A free-standing film could be exfoliated from the quartz substrate by dissolving the sacrificial layers in a concentrated aqueous NaOH solution.36 Even though the multilayered film [(PAH/PSS)2(PAH/MWCNTsCOOC6F5)20] was dipped into a highly concentrated aqueous NaOH solution, the shape of the film did not change, as confirmed by TEM (Figure 6). In order to check if the MWCNTs are homogenously distributed within the multilayered film, surface resistance measurements have been conducted as a function of deposited bilayers (Figure 5a). The surface resistance dramatically decreased with the deposition of each PAH/MWCNTs-COOC6F5 bilayer and a plateau value was reached after nine bilayers [(PAH/ (32) Il’ina, A. V.; Davidovich, Y. A.; Rogozhin, S. V. Russ. Chem. Bull. 1988, 37, 2539–2541. (33) Palumbo, M.; Pearson, C.; Petty, M. C. Thin Solid Films 2005, 483, 114–121. (34) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458–4465.

Figure 6. TEM image of [(PAH/PSS)2(PAH/MWCNTs-COOC6F5)20] as a free-standing film.

PSS)2(PAH/MWCNTs-COOC6F5)7]. This suggests that the connectivity of the MWCNT network on the substrate reached the percolation threshold. The surface resistance of the multilayered film with 20 bilayers decreased to around 10 kΩ with reasonable transparency (70% at 500 nm). Even though, a slightly better surface resistivity of 6 kΩ has recently been reported for polyelectrolyte multilayers containing single-walled carbon nanotube,37 which in general show a better conductivity than multiwalled carbon nanotubes. However, taking the fair price of multiwalled carbon nanotubes and the fact of an extremely stable thin film due to the covalent bonding between the layers into account, our presented approach may provide the possibility for a broad application of transparent conductive thin films.

Conclusions In this article, we described the synthesis of pentafluorophenyl ester modified multiwalled carbon nanotubes (MWCNTsCOOC6F5) and their use for the preparation of a multilayered thin film. LBL assembly of MWCNTs-COOC6F5 and poly(allylamine hydrochloride) (PAH) was performed by taking advantage of the high reactivity of MWCNTs-COOC6F5 toward amines, which resulted in the formation of layers that were connected through amide bonds. The active ester method described herein opens up new possibilities for the use of CNTs in the fabrication of nanoscale electronic components. Further, the possibility to introduce bioactive compounds or biological complexes onto MWCNTs may lead easily to bionanoelectronic applications, such as the development of a biological probe. Experiments in this direction are in progress and the results will be published in due course. Acknowledgment. Financial support by the Center for Advanced Materials Processing of 21st Century Frontier R&D Program funded by the Ministry of Commerce Industry and Energy (MOCIE), Republic of Korea is gratefully acknowledged. This work was also supported by an international research training group (IRTG 1404) funded by the German Research Foundation (DFG) and the Korea Science and Engineering Foundation (KOSEF). J.Y.C. thanks the Global (35) Charlot, A.; Gabriel, S.; Detrembleur, C.; Je´roˆme, R.; Je´roˆme, C. Chem. Commun. 2007, 4656–4658. (36) Dubas, S. T.; Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2001, 123, 5368–5369.

Transparent ConductiVe Multilayered Films

Research Laboratory (GRL) program for financial support. H.J.P. acknowledges financial support from the German academic exchange service (DAAD). The authors thank Nadine Metz for providing NH2-PEG550. (37) Yu, X.; Rajamani, R.; Stelson, K. A.; Cui, T. J. Nanosci. Nanotechnol. 2006, 6, 1939–1944.

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Supporting Information Available: ATR/FT-IR spectra, UV-vis spectra, and thermogravimetric analysis results of active pentafluorophenyl ester modified MWCNTs and oxidized MWCNTs. This material is available free of charge via the Internet at http://pubs.acs.org. LA801341T