Functionalized Multi-Walled Carbon Nanotubes Filled Ultraviolet

Apr 17, 2009 - These prefabricated Ag/MMWNT nanofillers were dispersed homogeneously in an acrylic resin before cured under UV radiation. The curing ...
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Functionalized Multi-Walled Carbon Nanotubes Filled Ultraviolet Curable Resin Nanocomposites and Their Applications for Nanoimprint Lithography Putian Wang, Jinbao Guo, Huihui Wang, Yan Zhang, and Jie Wei* College of Materials Science and Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, P. R. China ReceiVed: January 23, 2009; ReVised Manuscript ReceiVed: March 13, 2009

A conductive UV curable resin was prepared by mixing nanosized silver particles growing on modified multiwalled carbon nanotubes (MMWNT). The MWNT was treated by a concentrated H2SO4/HNO3 mixture and then ultrasonicated with AgNO3 in ethylene glycol (EG) solution for nanosized Ag particles to be deposited on. To improve the reaction process and control particle size, additives such as polyvinyl pyrrolidone (PVP) and sodium lauryl sulfate (SDS) were added into the system. The microstructure and surface morphology of Ag/MMWNT composite was investigated by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analysis, and the optimal reaction time was discussed. These prefabricated Ag/MMWNT nanofillers were dispersed homogeneously in an acrylic resin before cured under UV radiation. The curing process was tracked by Fourier transform infrared spectroscopy and the electrical resistivity was measured by the Four-probe method. The nanoimprint lithography was characterized by scanning electron microscopy and atomic force microscopy, and the morphologies of products associated with applications were displayed. 1. Introduction Since the first use of the printed circuit board (PCB) in the electronic industry, it has been extensively studied in the recent 20 years. Currently, circuits become tiny and complicated due to the higher requirements in performance coming from computers and many other electronic products. Traditionally, lithography was involved with complex electroplating and etching processes in manufacturing the circuits. One promising alternative is inkjet printing technology, by which the conductive lines can be drawn onto the substrate in one step. Ink used in this process contains metallic nanoparticles such as gold, palladium, silver, nickel, and copper. Unfortunately, these particles need a high temperature to form a conductive network as their melting temperature is typically at 100-300 °C, which limits the use of the plastic and some flexible materials as a substrate.1-3 Nanoimprint lithography (NIL), which is recognized as one of the choices for next generation of nanolithography, has been developed in an effort to fabricate features smaller than 10 nm.4-6 The use of the UV curable conductive resin in “Step and Flash Imprint Lithography” allows finishing of the procedures at room temperature. During the past few years, increasing attention has been paid to UV curable resin in NIL fields.7-9 If resins imprinted have a degree of electrical conductivity, the NIL processes will be largely predigested. As most of the resins are insulating, some excellent conductors were considered to fill into the system to increase the electrical conductivity for NIL application. To obtain conductive resins in NIL, carbon nanotubes (CNT) is selected as the template. It is well-known that CNT has extrahigh mechanical properties and superior electrical properties: an electric current transfer capacity 1000 times greater than copper wire.10 It has been reported that CNT enhanced both the mechanical and electrical properties of matrix polymer more * To whom correspondence [email protected].

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Figure 1. Schemes of modification and deposition of nanosized Ag particles on MWNT.

Figure 2. Schematic diagram of the deposition of Ag/MMWNT composite.

effectively than conventional carbon fibers or graphite because of its large interfacial area and geometric aspect ratio.7 Hence, CNT has been applied to transistors, unipolars, catalysts, adsorbents, photovoltaic devices, field emission materials, medicine, H2 storage, batteries, and biosensors, and so forth.11,12 The methods of mixing CNT with polymer matrices have been published in ref 13. For example, the plastic transistors and logic gates with CNT doping were produced by Rogers et al.,14-17 through incorporating CNT as a conductive filler with resin to fabricate electrical devices by NIL. As a contrast to CNT filler, Chou et al.18-20 synthesized nanosized silver particles and studied the effect of silver colloids on polymeric conductive

10.1021/jp9007027 CCC: $40.75  2009 American Chemical Society Published on Web 04/17/2009

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Figure 3. SEM images of Ag/MMWNT composite morphologies at different reaction times. (a) 1 h, (b) 2 h, (c) 3 h, (d) 4 h, (e) 5 h, and (f) 10 h; TEM images of Ag/MMWNT composite at different reaction times (g) 1 h, (h) 2 h.

adhesives. Considering the research above, both CNT and metallic particles as fillers could cause the insulating resins to become electric in some extent depending on the content of the conductive fillers. Ideally, the concentration of fillers should be low enough not to increase the resin viscosity to negatively affect NIL processing. In this article, we applied modified multiwalled carbon nanotubes (MMWNT) with Ag nanoparticles grown on them as the main conductor to fabricate a conductive UV curable resin used in NIL area. MWNT can be considered as layers of CNT wrapped together and has larger interfacial area and electrical conductivity than CNT. Nanosized Ag particles as a good metallic conductor were synthesized by reducing AgNO3 under ultrasonication in ethylene glycol (EG) solution were homogeneously dispersed on the surface of MMWNT. Modification generated defects points on MWNT walls, providing points for Ag nanoparticles to assemble and grow. The morphology of particles was controlled by reaction time and additives. When Ag/MMWNT was dispersed in UV curable resin, the conductive percolation threshold would be diminished significantly compared with systems with only MMWNT or Ag

nanoparticles used. This process provided the conductive composite with better electrical properties with less MWNT, an inevitable requirement for its NIL application in the electric industry. 2. Experimental Section 2.1. Modification of MWNT. The multiwalled carbon nanotubes used in this work (Shenzhen Nanotechnologies Co. Ltd.) were synthesized by the chemical vapor deposition method (CVD). MWNT was about 80-100 nm in diameter, 5-10 µm in length, and greater than 95% purity. A concentrated H2SO4/ HNO3 mixture (3:1) was used to purify the catalysts and amorphous carbon and to generate surface functional groups on the wall of MWNT including hydroxyl groups, lactone groups, and carboxylic acid groups. The process took 2-3 h with ultrasonication. After filtering, the MWNT was collected by washing with deionized water more at least three times and then air drying. 2.2. Growth of Ag Nanoparticles on Modified MWNT (MMWNT). Modified multiwalled CNT (10 mg) was dispersed in 15 mL ethylene glycol (EG) and then ultrasonicated 15 min

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Figure 4. XPS survey scan of the Ag/MMWNT composite. The inserted is the Ag3d spectrum of Ag nanoparticles.

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Figure 6. FTIR spectra of acrylate Ag/MMWNT nanocomposites with 5 wt % Ag/MMWNT at different UV radiation times.

Figure 5. XRD pattern of Ag/MMWNT composite.

to completely separate the nanotube bundles. With the weight ratio of MMWNT/Ag ) 1:4, so 63 mg AgNO3 was first added into 15 mL EG solution, followed by polyvinyl pyrrolidone (PVP) as a stabilizer and sodium lauryl sulfate (SDS) as a surfactant into the solution with a weight ratio of PVP/SDS/Ag ) 1:1:1 simultaneously. EG is both the solution and the reducing agent to AgNO3. Solutions should be mixed thoroughly during ultrasonication until Ag nanoparticles were completely deposited on MMWNT. The production was filtered using a Millipore membrane and then washed with acetone and deionized water three times, respectively. Figure 1 illustrates the schematic diagram of Ag/MMWNT fabrication process. 2.3. UV Curable Resin Incorporated with Ag/MMWNT. Vijet100 (Surface Specialties Co. Ltd. USA) was a polymer precursor containing acrylic functional groups used as the UV curable resin. Irgacure184 (Ciba Specialty Chemicals Inc. Switzerland) was used as photoinitiator. The Ag/MMWNT composite was dispersed in 15 mL acetone containing resin and photoinitiator. Ultrasonication was applied for 30 min to obtain a homogeneous suspension at room temperature. A series of suspensions were prepared with different ratios of composite to the resin. After that, acetone was volatilized, resulting in the conductive UV curable resin with Ag/MMWNT well dispersed. 2.4. Measurements. Ag/MMWNT morphological and surface chemistry was characterized by scanning electron microscopy (Hitachi S-4700 SEM, Japan), transmission electron microscopy (HRTEM JEOL TEM-3010, Japan), and X-ray

Figure 7. Conversion of the double bonds vs UV radiation time (acrylate with 5 wt % Ag/MMWNT).

photoelectron spectroscopy (Thermo VG ESCALAB XPS-250, UK). The crystal lattice of Ag/MMWNT was determined by X-ray diffraction (Rigaku D/Max2500 VB2+/pc XRD, Japan). The cured process of the resin with Ag/MMWNT was tracked by fourier transform infrared spectroscopy (Thermo Nicolet FTIR NICOLET 5700, USA) during UV radiation. The UV curable resin incorporating conductive composite was dropped into grids (3 × 2 mm2) on a glass substrate and then UV-cured (main wavelength is 365 nm) at a given radiation time. Its electrical resistivity (F) was measured by the Four-probe Method (Baishen technologies Co. Ltd. SX1934, China). Finally, atomic force microscopy (Digital Instruments Multimode NanoScope IIIa AFM, USA) was used to show the morphology of NIL treated film surface. 3. Results and Discussion 3.1. Ag/MMWNT Synthesis. The original MWNT was first modified to generate acidic groups on the surface including hydroxyl groups, lactone groups, and carboxylic acid groups. Then the modified MWNT (MMWNT) was ultrasonicated in the EG solution containing AgNO3, PVP, and SDS to deposit nanosized Ag particles, as shown in Figure 2. When the concentration of Ag ion is high, it may cause problems because

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Figure 8. SEM rift image describing Ag/MMWNT composite dispersion in cured resin matrix. Inserted is the local region.

of the fact that Ag particles that were just synthesized were very active and inclined to adhere everywhere on the surfaces. To inhibit the side reactions, we were required to mix raw materials together so that MMWNT is the first choice for active Ag particles. Besides that, the complex formed by functional groups and Ag particles were much more stable than that of the other sites. Modified points more easily attracted Ag particles. However, the equality of these particles is difficult to control. In this work, symmetrical and moderate aggregation is expected. Therefore, additives should be used. SDS was a widely used surfactant and is able to reduce the hydrophobic property of MMWNT and Ag particles. The adsorptions of PVP both on Ag nanoparticles and the MMWNT surface combined made them stable and prevented large agglomeration.21 These additives were removed by abstersion and filtration. After that, the solution was clear and achromatic, as we know the Ag nanoparticle solution is straw yellow, so this phenomenon indicating that the Ag nanoparticles had completely reacted. 3.2. Influence of Reaction Time on the Morphology of Ag/ MMWNT. Reaction time plays an important role in determining the morphology of Ag/MMWNT composite. Figure 3 shows SEM and TEM images of the Ag/MMWNT composite with different reaction times. When the reaction time was 1 h, Ag nanoparticles were tiny, flat, and not very clear as shown in SEM. When we prolonged the time to 2 h, visible particles appeared like gears. When the time was extented to 3-5 h, it could be seen that the particles became bigger and denser. Finally, after 10 h of reaction, Ag particles and additives on the surface of MMWNT were totally agglomerated and hard to distinguish from one another. Obviously, too long of a reaction time is harmful to the system. On the basis of the results above, the optimal reaction time is 2-3 h. Calculated from SEM and TEM images ((b) and (h)) in Figure 3, the Ag particles’ diameter is approximately 30 nm. 3.3. XPS Analysis of Ag/MMWNT Composite. The surface of Ag/MMWNT composite was scanned by XPS analysis. Figure 4 shows the spectra of Ag/MMWNT composite fabricated by AgNO3 and MMWNT in EG solution at room temperature. The peaks with binding energies of 284.8, 368.8,

Figure 9. Electrical resistivity of different nanofillers.

and 374.8 eV corresponded to C1s, Ag3d5/2, and Ag3d3/2 components respectively, the typical values of carbon and silver. These results indicated that the formation of Ag nanoparticles on the walls of MMWNT was very pure, so there is no specific chemical binding between silver and MMWNT.22,23 3.4. XRD Analysis of Ag/MMWNT Composite. Figure 5 shows the XRD diffraction pattern of Ag/MMWNT composite fabricated by AgNO3 and MMWNT in EG solution at room temperature. It could be used to evaluate the crystallinity of Ag particles growth on MMWNT surface. It indicated the purity of the composite as there was no other peak shown in the XRD pattern. The diffraction peaks at 2θ of 38.12°, 44.31°, 64.45°, and 77.41° correspond to Ag (111), (200), (220), (311) reflections respectively, implying that Ag nanosized particles formed a cubic-face-centered structure. The size of these well crystallized Ag particles could be calculated by Scherrer’s formula: L ) kλ/B cos θ (Where L is the average crystal size, k is Scherrer factor: 0.89, and B is the full width at halfmaximum of the peak), and the calculated size of Ag particles is 28.4 nm, which is quite close to the size (30 nm) estimated from SEM and TEM.

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Figure 10. Images of Ag/MMWNT-resin nanocomposite NIL patterns. (a) SEM image and (b) AFM image of the square grating pattern with Ag/MMWNT nanofillers at loading rate of 5 wt %, (c) SEM images of rectangle grating pattern with line width transits from 4 to 1 µm, and (d) multiplex patterns with Ag/MMWNT nanofillers at loading rate of 5 wt %.

3.5. Blending and Curing Process of Ag/MMWNT Incorporating with UV Curable Resin Nanocomposites. The UV curing process was a double bond cross-linking reaction initiated by free radicals decomposing from photoinitiator under UV radiation, the same as the polymerization of acrylates. The reaction between free radicals and CdC double bonds transformed the CdC into C-C single bond and cross-linked together simultaneously, accompanying the occurrence of new free radicals to drive the reaction to continue. The FTIR spectra of Ag/MMWNT incorporating with UV curable resin composite during the curing process were shown in Figure 6. The strong absorption peak at 1633 cm-1 and 810 cm-1 corresponding to the CdC double bonds disappeared mostly after 5 min UV radiation, indicating that the CdC double bonds in the composite had reacted completely. The curing process was monitored by the CdC double bond conversion depending on UV radiation time, as shown in Figure 7. As the FTIR absorption peaks at 1633 cm-1 and 810 cm-1 ascribed to the CdC double bonds, the peaks intensity at given curing time (It) and peaks intensity before curing (I0) could be used to determine the CdC double bonds’ conversion efficiency. From Figure 7, the conversion of the CdC double bonds increased sharply at the first 30 s, indicating a fast curing reaction even though the system contained 5 wt % Ag/MMWNT composite. After 5 min of UV radiation, the conversion of CdC double bonds reached a relative plateau value of about 75% and went up slowly. The conversion of CdC double bonds could have reached a higher value with the prolonged UV radiation time but could by no means increase up to 100% because the viscosity increased dramatically during fast curing reaction and because the portion of CdC double bonds were isolated in the cured system, leaving unreacted bonds with strongly restricted mobility. Dispersion of Ag/MMWNT incorporation with UV curable resin nanocomposites in polymer matrix prepared by solution blending was very homogeneous after the ultrasonication. The morphology of the nanocomposites is shown in Figure 8. Generally speaking, well dispersed MMWNT has a higher interfacial area and geometric aspect ratio than those of MMWNT agglomerates, which increases the chances of contact between nanocomposites and decreases the percolation threshold.

3.6. Electrical Resistivity of Ag/MMWNT Incorporating with Resin Nanocomposites. The electrical resistivity of Ag/ MMWNT-resin nanocomposites is mainly ascribed to three contributions: self-resistance of Ag/MMWNT nanofillers, contact resistance, and tunneling resistance between nanofillers in the conductive network.24 To make the Ag/MMWNT-resin nanocomposite conductive, a continuous network of Ag/ MMWNT should be formed. The electrical resistivity will drop sharply when the percolation threshold decreases.25 A composite of acrylic resin with Ag/MMWNT, although it has less vol % at the same wt %, showed lower electrical resistivity compared to composite with only MMWNT. It is seen in Figure 9 that a conductive composite was obtained with only 1 wt % of Ag/ MMWNT, whereas materials made of only Ag nanoparticles or MMWNT were still insulators or semiconductors at similar composition. At 5 wt % nanofillers loading, the Ag/ MMWNT-resin electrical resistivity was of 42 Ω · cm, which was 2 orders magnitude lower than the MMWNT-resin, whereas at the same rate of loading the MMWNT-resin electrical resistivity was 2.7 × 103 Ω · cm and Ag-resin is insulating. Obviously, the Ag/MMWNT-resin has a lower percolation threshold and better conductivity than the other two at the same conditions. This phenomenon could be explained by the introduction of Ag nanoparticles, which changed the contact resistance and tunneling resistance between two particles in a conductive network. Ag nanoparticles deposited on the walls of MMWNT play a role like hooks or gears; once they touch each other they will occlude together, which will greatly increase the chances of their contact. Besides that, the perfect electrical performance of MWNT has a direction along the tube but on the vertical aspect it is helpless. Silver nanoparticles doped on the walls of MMWNT extend the tunneling effect on the vertical aspect obviously. These silver nanoparticles help to form a conductivity network in three dimensions, which is another reason why system conductivity gets improved. When there was only MMWNT in the resin, either the chances of contact or the tunnel effect between them was much poorer than those with MMWNT with Ag nanoparticles incorporated. Only Ag particles in resin would also not work because silver balls did not have such a high aspect ratio as MWNT and it was difficult for them to

Multi-Walled Carbon Nanotubes make contact with eachother. Therefore, it required at least 55 wt % to lead to a similar electrical resistivity as Ag/ MWNT-resin. 3.7. NIL Patterning Process of Ag/MMWNT-Resin Nanocomposites. The grating pattern was applied with a polydimethylsiloxane (PDMS) stamp, which has specific designs. The Ag/MMWNT-resin nanocomposites were dropped onto a polyethylene terephthalate (PET) wafer. The nanopatterns were imprinted into the nanocomposites by the PDMS stamp in contact on the top with an appropriate pressure. The nanocomposites after NIL patterning process were then exposed to UV illumination through the transparent PDMS stamp for at least 120 s to complete the solidification of the film. The stamp was released from the substrate after curing and patterns were left. The imprint image in Figure 10 shows an overall view of the Ag/MMWNT nanofillers at a loading rate of 5 wt %. The resolution of this grating pattern was (about 1 µm here) mainly dependent on the precision of the pattern stamp. 4. Conclusions In summary, UV curable resin incorporated modified multiwalled carbon nanotubes with nanosized silver particles growing on them were prepared by ultrasonication in EG solution containing additives with a weight ratio of PVP/SDS/Ag ) 1:1:1. The optimal reaction time is 2-3 h based on our experimental conditions. The component and morphology of Ag/MMWNT composite was characterized by XPS, XRD, SEM, and TEM. The results showed that the expected growth structure and the average size of Ag nanoparticles was approximately 30 nm. The dispersion of Ag/ MMWNT in acrylic resin was also investigated by SEM and showed homogeneous mixing, which was cured quickly under UV radiation monitored by FTIR. The minimal electrical resistivity of the product with 5 wt % Ag/MMWNT could drop to 42 Ω · cm, 2 orders of magnitude lower than that of resin with MMWNT only. The appearance of conductive grating patterns made by NIL solution was characterized through SEM and AFM, implying this nanocomposites capacious view of application. Acknowledgment. Financial support from National Natural Science foundation (Grant No. 50673007) is gratefully acknowledged.

J. Phys. Chem. C, Vol. 113, No. 19, 2009 8123 References and Notes (1) Menard, E.; Meitl, M. A.; Sun, Y. G.; Park, J. U.; Shir, D. J.; Nam, Y. S.; Jeon, S.; Rogers, J. A. Chem. ReV. 2007, 107, 1117. (2) Lee, H. H.; Chou, K. S.; Huang, K. C. Nanotechnology 2005, 16, 2436. (3) Lee, H. H.; Chou, K. S.; Shih, Z. W. Int. J. Adhes. Adhes. 2005, 25, 437. (4) Wu, W.; Tong, W. M.; Bartman, J.; Chen, Y. F.; Walmsley, R.; Yu, Z. N.; Xia, Q. F.; Park, I.; Picciotto, C.; Gao, J.; Wang, S. Y.; Morecroft, D.; Yang, J.; Berggren, K. K.; Williams, R. S. Nano Lett. 2008, 8, 3865. (5) Moran, I. W.; Briseno, A. L.; Loser, S.; Carter, K. R. Chem. Mater. 2008, 20, 4595. (6) Bruinink, C. M.; Burresi, M.; Boer, M. J.; Segerink, F. B.; Jansen, H. V.; Berenschot, E.; Reinhoudt, D. N.; Huskens, J.; Kuipers, L. Nano Lett. 2008, 8, 2872. (7) Choi, J. H.; Jung, S. U.; Choi, D. G.; Jeong, J. H.; Lee, E. S. Microelectron. Eng. 2007, 5, 7. (8) Tjong, S. C.; Liang, G. D.; Bao, S. P. Polym. Eng. Sci. 2008, 10, 177. (9) Lee, E. S.; Jeong, J. H.; Sim, Y. S.; Kim, K. D.; Choi, D. G.; Choi, J. H. Curr. Appl. Phys. 2006, 6, 92. (10) Wang, S. R.; Liang, Z. Y.; Liu, T.; Wang, B.; Zhang, C. Nanotechnology 2006, 17, 1551. (11) Kang, S. J.; Kocabas, C.; Ozel, T.; Shim, M.; Pimparkar, N.; Alam, M. A.; Rotain, S. V.; Rogers, J. A. Nature Nanotechnology 2007, 2, 230. (12) Miyata, Y.; Yanagi, K.; Maniwa, Y.; Kataura, H. J. Phys. Chem. C 2008, 112, 3591. (13) Jimenez, G. A.; Jana, S. C. Carbon 2007, 45, 2079. (14) Cao, Q.; Xia, M. G.; Shim, M.; Rogers, J. A. AdV. Funct. Mater. 2006, 16, 2355. (15) Zhou, Y. X.; Gaur, A.; Hur, S. H.; Kocabas, C.; Meitl, M. A.; Shim, M.; Rogers, J. A. Nano Lett. 2004, 4, 2031. (16) 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. (17) Cao, Q.; Xia, M. G.; Kocabas, C.; Shim, M.; Rogers, J. A. Appl. Phys. Lett. 2007, 90, 023516. (18) Chou, K. S.; Chen, C. C. Micromeso. Mater. 2007, 98, 208. (19) Chou, K. S.; Huang, K. C.; Lee, H. H. Nanotechnology 2005, 16, 779. (20) Chou, K. S.; Lu, Y. C. Thin Solid Films 2007, 515, 7217. (21) Chen, M.; Chen, C. M.; Yu, H. W.; Lu, S. C.; Koo, H. S. IEEE INEC. 2008, 2, 1699. (22) Dai, K.; Shi, L. Y.; Fang, J. H.; Zhang, Y. Z. Mater. Sci. Eng., A 2007, 465, 283. (23) Jiang, H. J.; Zhu, L. B.; Moon, K. S.; Wong, C. P. Carbon 2007, 45, 655. (24) Wu, H. P.; Wu, X. J.; Ge, M. Y.; Zhang, G. Q.; Wang, Y. W.; Jiang, J. Z. Compos. Sci. Technol. 2007, 67, 1116. (25) Xu, Y. J.; Higgins, B.; Brittain, W. J. Polymer 2005, 46, 799.

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