Langmuir Monolayers of Co Nanoparticles and Their Patterning by

J. Phys. Chem. B 2005, 109, 13119-13123

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Langmuir Monolayers of Co Nanoparticles and Their Patterning by Microcontact Printing Jong-Il Park,† Woo-Ram Lee,‡ Sung-Soo Bae,† Youn Joong Kim,§ Kyung-Hwa Yoo,‡ Jinwoo Cheon,*,‡ and Sehun Kim*,† Department of Chemistry, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Korea, Departments of Chemistry and Physics, Yonsei UniVersity, Seoul 120-749, Korea, and DiVision of Nano-Material and EnVironmental Science, Korea Basic Science Institute, Daejeon 305-333, Korea ReceiVed: December 13, 2004; In Final Form: April 18, 2005

In this paper, we describe an easy and reliable method for the production of patterned monolayers of Co nanoparticles. A two-dimensional monolayer of Co nanoparticles is fabricated by spreading a nanoparticle solution over an air-water interface and then transferring it to a hydrophobic substrate by using the LangmuirBlodgett (LB) method. Transmission electron microscopy (TEM) was used to show that, with increasing surface pressure, the Co nanoparticles become well-organized into a Langmuir monolayer with a hexagonal close-packed structure. By controlling the pH of the subphase, it was found that a monolayer of Co nanoparticles with long-range order could be obtained. Further, by transferring the Langmuir monolayer onto a poly(dimethoxysilane) (PDMS) mold, the selective micropatterning of the Co nanoparticles could be achieved on a patterned electronic circuit. The electronic transport properties of the Co nanoparticles showed the ohmic I-V curve.

Introduction Convenient and effective organization of nanomaterials (molecules, polymers, and nanoparticles) into one-, two-, and three-dimensional (1-D, 2-D, and 3-D) structures is the key to the realization of nanodevices.1-8 The most common method of constructing 2- and 3-D nanoparticle structures involves synthesizing the nanoparticles in solution by chemical methods and then attaching them to various substrates using suitable interactions such as van der Waals forces and electrostatic or covalent bonds.9-11 In the case of Au nanoparticles, 1-D chains and wires have been produced along nanofibrils derived from the self-assembly of DNA and synthesized peptides.9,10,12 Threedimensinal crystal structures have been formed by a selfassembly process with a ligand exchange reaction.13 During the self-assembly process, 2-D monolayers are produced as a result of interparticle interactions. To more effectively produce 2-D monolayers of nanoparticles on solid substrates, many researchers have tried using various techniques, such as layer by layer (LbL) deposition, the Langmuir-Blodgett method (LB), and the spin-coating method.14-16 The LB technique is the most promising method for producing well-organized 2-D monolayers of surfactants, polymers, and nanoparticles, because it provides fine control of the thickness and homogeneity of the monolayer and of multilayers. For example, Heath et al., Markovich et al., and Xi et al. have reported the fabrication of LB films of Ag, Fe3O4, and Fe2O3 nanoparticles in the presence of various capping molecules such as fatty acids and alkanethiols.16-18 Furthermore, the formation of 2-D patterned monolayers of Au, Fe2O3, and Pt@Fe2O3 nanoparticles by using a combination of LB with microcontact printing (µ-CP) has been reported.19-22 * To whom correspondence should be addressed. (S.K.) Phone: (+82) 42-869-2831. Fax: (+82) 42-869-2810. E-mail: [email protected]. (J.C.) Phone: (+82) 2-2123-5631. Fax: (+82) 2-364-7050. E-mail: [email protected]. † Korea Advanced Institute of Science and Technology. ‡ Yonsei University. § Korea Basic Science Institute.

However, highly ordered LB films and 2-D patterns of Co nanoparticles have not yet been reported because of difficulties in the synthesis of monodisperse Co nanoparticles. In this paper, we describe a method for the systematic and comprehensive fabrication of 2-D monolayers of Co nanoparticles via the LB technique and of 2-D patterned monolayers on various substrates, such as silicon wafers, using the microcontact printing method. Experimental Section Materials. Octadecyltrichlorosilane (OD-TCS) and sodium bis(2-ethylhexyl) sulfosuccinate (NaAOT) were purchased from Aldrich Co. Tetradecanoic acid and all solvents were purchased from Junsei Co. Dicobalt octacarbonyl was purchased from Strem Inc. Deionized (DI) water (18 MΩ‚cm) was obtained using a Millipore four-bowl purification system. PDMS elastomer (Sylgard 184) was obtained from Dow Corning. Preparation of Co Nanoparticles. Co nanoparticles were prepared using the thermolysis of Co2(CO)8 in a refluxing toluene solution containing tetradecanoic acid and NaAOT as stabilizer.23 The Co nanoparticles solution was filtered through a 0.45 µm nylon membrane filter to remove aggregated nanoparticles and residual organic impurities. Substrate Preparation. The Si(111) surface was cleaned by dipping it in piranha solution (H2SO4:H2O2 ) 7:3) at 90 °C for 1 h followed by washing with deionized water. After cleaning, the sample was sonicated in basic hydroxide solution (H2O: H2O2:NH3 ) 5:1:1) for 30 min, then sonicated in acidic peroxide solution (H2O:H2O2:HCl ) 6:1:1) for 30 min followed by washing with deionized water, and kept in an oven at 120 °C for 10 min. The slide glasses, oxidized Si(111) wafers, and mica substrates were also cleaned by sonication in 2-propanol. The OD-terminated surface was produced by immersion in a toluene solution (20 mL) containing OD-TCS (0.1 mL) at room temperature (RT) for 10 min. After the reaction, each

10.1021/jp0443308 CCC: $30.25 © 2005 American Chemical Society Published on Web 06/16/2005

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Figure 2. TEM images of LB films of Co nanoparticles at pH ) 11, 12, 13, and 14, respectively.

Figure 1. π-A isotherm graph of monodisperse Co nanoparticles (∼10 nm) and TEM images of monolayers of Co nanoparticles on carboncoated Cu grids fabricated with the LB method at surface pressures of 5, 15, 30, and 45 mN/m (A-D, respectively).

sample was washed with toluene and then used in the fabrication of an LB film or in µ-CP. LB Films. The NIMA 611 Langmuir trough was cleaned with chloroform and hexane and then filled with deionized water. The water surface was cleaned by pumping out the dust and organic molecules on the water surface using a circulating aspirator. A 2.2 × 10-6 M solution of Co nanoparticles in toluene (10-50 µL) was spread over the water surface, and, after waiting 10 min for the toluene to evaporate, a barrier compression was applied at a constant rate of 10-50 mm/min. At surface pressures in the range of 10-45 mN/m, Langmuir films formed on the various substrates, namely, TEM grids, slide glasses, and silicon wafers, as well as on mica and gold substrates. Preparation of Micropatterned PDMS Stamps. PDMS stamps with a line pattern (2 µm width, 1 µm spacing, and 500 nm depth) were replicated from a master silicon grating (TGZ03, 1 µm width, 2 µm spacing, and 500 nm depth) originally fabricated for atomic force microscopy (AFM) tip characterization. Preparation of Patterned LB Films. The well-organized LB films of Co nanoparticles were transferred onto the PDMS stamps using the LB technique. The Co nanoparticle-covered PDMS stamps were placed onto the OD-terminated Si surfaces or silicon dioxide substrates containing E-beam lithographically patterned gold electrodes and, after pressing by hand for 5 s, were carefully removed from the substrate. A cross-bar pattern was also produced by printing twice, with a 90° rotation of the substrate between the two stampings. Instruments. The TEM images were obtained with an EM 912 omega (Zeiss, KBSI) instrument operating at 120 kV. The deposited and patterned Langmuir films were analyzed with a FE-SEM (XL30FEG, Philips) and an optical microscope (Axio3.0, Zeiss).

Figure 3. (A) TEM image and (C) SEM image of a 2-D monolayer of Co nanoparticles fabricated with the LB technique under optimum conditions. B and D are enlarged views of A and C, respectively.

Results and Discussion After cleaning with chloroform and hexane, the LB trough was filled with DI water, which was cleaned by pumping out the dust and organic molecules on the water surface using a circulating aspirator. When 30 µL of a 2.2 × 10-6 M Co nanoparticle solution was spread over the water surface, the hydrophobic Co nanoparticles floated on the water surface, after evaporation of toluene for approximately 10 min. The barrier compression of the nanoparticles at a constant speed of 10 mm/ min causes an increase in the surface pressure and results in the close packing of the Co nanoparticles. Figure 1 shows the surface pressure versus area (π-A) isotherm for the Co nanoparticles on the air-water interface. The Co nanoparticles on the water surface are present in three different phases (gas, liquid, and solid); Interval A-B is the gas phase, B-C is the liquid phase, and C-D is the solid phase. The TEM images in Figure 1A-D show LB films on amorphous carbon-coated Cu grids attached to slide glasses at various surface pressures. At low pressures (∼5 mN/m), the

Langmuir Monolayers of Co Nanoparticles

Figure 4. Schematic views of (A) the formation of an LB film of Co nanoparticles and (B) the microcontact printing of Co nanoparticles.

Co nanoparticles are randomly distributed and form only small 2-D aggregates during the evaporation of toluene (Figure 1A); the number density of nanoparticles (counted from this TEM image) is about 1.8 × 1011 particles/cm2, which is well-matched with the surface area per particle (∼550 nm2). On compression of the nanoparticles, a two-dimensionally organized monolayer

J. Phys. Chem. B, Vol. 109, No. 27, 2005 13121 of Co nanoparticles gradually forms, and the number density of nanoparticles increases from 3.4 × 1011 to 6.2 × 1011 particles/cm2 in Figure 1B,C, respectively. The area per nanoparticle in Figure 1C is 160 nm2, which is near the close-packing area of Co nanoparticles. In the highly compressed state (∼45 mN/m), the nanoparticles become densely packed with a density of 6.4 × 1011 particles/cm2. On further increase of the pressure to beyond the highly compressed state, folded layers are observed to form. The highly compressed monolayer is slightly contaminated by free fatty acid, which appears as the fuzzy phase shown in Figure 1D. To improve the quality of the Langmuir films, it was found that further control of their conditioning was needed, such as through the adjustment of the basicity of the subphase. If the pH of the subphase is basic, the free fatty acid dissolves into the subphase and the numbers of large gaps and voids between the particles (due to areas occupied by free fatty acid) are diminished. Therefore, the monolayers of Co nanoparticles have a more closely packed structure in a strongly basic solution (1 mM NaOH, pH ) 11) than is achieved in DI water. The addition of further sodium hydroxide produces more dissolution of free fatty acid, but higher NaOH concentrations (e.g. 1 M NaOH, pH ) 14) are not suitable for forming a monolayer of Co nanoparticles because of contamination by NaOH and the ionization of the Co nanoparticles by the base. By carefully leaving a small amount of fatty acid in the Co nanoparticles solution and optimizing the deposition conditions (particle concentration, barrier rate, dipping rate, and varying the substrate), we were able to obtain an almost close-packed 2-D monolayer of Co nanoparticles (Figure 2). At optimum conditions (particle concentration ) 30 µL of 2.2 × 10-6 M solution, solvent ) toluene, pH of subphase ) 11, barrier rate ) 10 mm/min, dipping rate ) 10 mm/min), the Co nanoparticles were found to be well-organized with hex-

Figure 5. Patterns of microcontact printed LB films of nanoparticles: (A) optical microscope image, (B and C) SEM images of stripe patterns of Co nanoparticles, (D) optical microscope images, and (E and F) SEM images, which are cross-line patterns.

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Figure 6. (A) Optical microscope image of the gold electrode and (B) FE-SEM image of LB films of Co nanoparticles microcontact-printed across the gold electrode lines. C and D are enlarged views obtained with FE-SEM. E is the I-V curve of the LB films of Co nanoparticles microcontactprinted across the gold electrode lines.

agonal close-packing at the air-water interface and could easily be transferred with long-range ordering to a carbon-coated Cu grid attached to a slide glass (Figure 3A,B). This result was also obtained for other hydrophobic substrates, such as for the gold surface shown in Figure 3C. In this SEM image, the Co nanoparticles can be seen to be well-organized with a hexagonal close-packing structure and are widely distributed with a range greater than 10 µm. Some cracks in the LB films occur during the dipping processes, because the interaction between the particles is weaker than the dipping force. However, most of the LB films of Co nanoparticles were successfully transferred to the hydrophobic substrates (gold, carbon-coated Cu grids, H-passivated silicon wafers, octadecylterminated silicon wafers), in contrast to the nonuniform monolayers formed on hydrophilic substrates (slide glasses, mica, OH-terminated silicon wafers), because of the attractive interaction between the Co nanoparticles and the hydrophobic substrate. A well-organized LB film of Co nanoparticles can be used as the ink for a PDMS stamp. Figure 4 shows a schematic view of the fabrication of a patterned monolayer of Co nanoparticles. The PDMS stamps with a stripe pattern (2 µm width, 1 µm spacing, and 500 nm depth) were replicated from a master silicon grating (TGZ03) used in atomic force microscopy (AFM) tip characterization. By vertical or horizontal dipping at a surface pressure of 30 mN/m, the PDMS stamp could be covered with

a monolayer of Co nanoparticles, which was then imprinted on an octadecyl-terminated Si wafer for ∼5 s. Patterned monolayers of Co nanoparticles are not easy to see with the naked eye, but are easily observed with an optical microscope. Figure 5A shows a patterned monolayer of Co nanoparticles on an octadecyl-terminated silicon wafer. A 2 µm wide stripe pattern is visible with a black line in lowmagnification SEM images, which is composed of closely packed Co nanoparticles in enlarged view (Figure 5B,C). The cross-bar pattern shown in Figure 5E,F is produced by printing on the stripe pattern of Co nanoparticles with another striped line stamp coated with Fe3O4 nanoparticle. The striped lines of Fe3O4 nanoparticles are across the horizontal lines of Co nanoparticles. The stripe pattern can also be stamped onto a silicon dioxide substrate containing electron beam (E-beam) lithographically patterned gold electrodes (Figure 6A). This substrate is also terminated by the octadecyl functional group before µ-CP. Figure 6B shows a SEM image of monolayer stripes of Co nanoparticles printed across three sets of up-down gold electrode lines. Each set of up-down gold electrodes is made up of two parallel zigzag lines. The gap between the up electrode and the down electrode is 200 nm, and the electrode line width is 100 nm. As shown in Figure 6C, the gold electrodes have a zigzag pattern, and the Co nanoparticles were successfully printed onto this substrate across the gold electrodes. The electrical measurements were carried out by passing an electric

Langmuir Monolayers of Co Nanoparticles current through the monolayer of nanoparticles between the two electrodes. The patterned samples are electrically insulating because of the long-chain organic capping groups and the surface oxide layer.24 After annealing the substrate for 1 h under reducing conditions (300 °C, 5% H2), ohmic transport was observed in this pattern at room temperature instead of the Coulomb blockade, as shown in Figure 6E.24 Multichannel tunneling through the hexagonally packed nanoparticles is a major contribution to the electron transport, contributing more to the ohmic conductivity than single particle transport.25,26 Room-temperature resistance is about 120 kΩ, which is similar to the value for annealed Co nanoparticles obtained by Black et al.6 Low-temperature transport measurements were not carried out in this study, but the resulting transport is expected to be similar to Black’s results. If a suitable PDMS pattern and gold electrode substrate with a nanoscale pattern are prepared, the LB-µCP method could be used in the future fabrication of nanodevices, such as singleelectron transistors with a high-density array. Conclusion In summary, the forced assembly of Co nanoparticles on an air-water interface can be used to produce a wide range of ordered Co nanoparticle films that under optimum conditions can be easily transferred to hydrophobic substrates. The hexagonal close-packing structures of the LB films were controlled by variation of the particle concentration, the pH of the subphase, the barrier rate, and the dipping rate. Further, patterning of the Co nanoparticles was achieved by stamping a Langmuir film coated onto a PDMS stamp; these patterns were characterized by optical microscopy and FE-SEM. Furthermore, we suggest that future nanodevices can be manufactured by selectively imprinting Langmuir monolayers of nanoparticles onto patterned gold electrodes. Acknowledgment. We would like to thank KBSI for TEM analyses. This research was supported by grants from KOSEF through the Center for Nanotubes and Nanostructured Composites, the Brain Korea 21 Project, KRF (Grant R02-2004000-10096-0), NCRC (Grant R15-2004-024-02002-0), the National R&D Project for Nano Science and Technology, the National R&D for Cancer Control (0320250-2), Korea Health 21R&D(0405-MN01-0604-0007),andAOARD(FA520904P0406). Supporting Information Available: Two figures, showing (S1) X-ray photoelectron spectra and X-ray absorption spectra

J. Phys. Chem. B, Vol. 109, No. 27, 2005 13123 and (S2) TEM and FE-SEM images (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schmid, G. Nanoparticles: From Theory to Application; WileyVCH: Weinheim, Germany, 2004; p 1. (2) Pole, C. P.; Owens, F. J. Introduction to Nanotechnology; WileyInterscience: New Jersey, 2003; p 1. (3) Klabunde, K. J. Nanoscale Materials in Chemistry; Wiley-Interscience: New York, 2001; p 1. (4) Pileni, M. P. Metal Nanoparticles: Synthesis, Characterization, and Applications; Dekker: New York, 2001; p 1. (5) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (6) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. H. Science 2000, 290, 1131. (7) Petit, C.; Taleb, A.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 1805. (8) Boal, A. K.; Ilhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (9) Maeda, Y.; Tabata, H.; Kawai, T. Appl. Phys. Lett. 2001, 79, 1181. (10) Fu, X. Y.; Wang, Y.; Huang, L. X.; Sha, Y. L.; Gui, L. L.; Lai, L. H.; Tang, Y. Q. AdV. Mater. 2003, 15, 902. (11) Bae, S. S.; Lim, D. K.; Park, J. I.; Lee, W. R.; Cheon, J.; Kim, S. J. Phys. Chem. B 2004, 108, 2575. (12) Loweth, C. J.; Caldwell, W. B.; Peng, X. G.; Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808. (13) Baraton, M.-I. Synthesis, Functionalization and Surface Treatment of Nanoparticles; American Scientific Publishers: Stevenson Ranch, CA, 2003; p 1. (14) Basu, J. K.; Sanyal, M. K. Phys. Rep. 2002, 363, 1. (15) Hong, Y. K.; Kim, H.; Lee, G.; Kim, W.; Park, J. I.; Cheon, J.; Koo, J. Y. Appl. Phys. Lett. 2002, 80, 844. (16) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (17) Fried, T.; Shemer, G.; Markovich, G. AdV. Mater. 2001, 13, 1158. (18) Huo, H.; Li, W.; Cui, H.; Xi, S.; Wang, J.; Zhao, B.; Shen, Y.; Lu, Z. Chem. Mater. 2000, 13, 790. (19) Santhanam, V.; Liu, J.; Agarwal, R.; Andres, R. P. Langmuir 2003, 19, 7881. (20) Santhanam, V.; Andres, R. P. Nano Lett. 2004, 4, 41. (21) Guo, Q.; Teng, X.; Rahman, S.; Yang, H. J. Am. Chem. Soc. 2003, 125, 630. (22) Guo, Q.; Teng, X.; Yang, H. AdV. Mater. 2004, 16, 1337. (23) Park, J. I.; Kang, N. J.; Jun, Y. W.; Oh, S. J.; Ri, H. C.; Cheon, J. ChemPhysChem 2002, 3, 543. (24) As-prepared Co nanoparticles are oxidized during LB filming procedure in basic solution and ambient condition but easily reduced by annealing at 300 °C under H2 gas. (See Supporting Information.) (25) Petit, C.; Cren, T.; Roditchev, D.; Sacks, W.; Klein, J.; Pileni, M. P. AdV. Mater. 1999, 11, 1198. (26) We have compared the SEM images before and after reduction at 300 °C under H2. After the reducing process, the interparticle distance was shortened and the oxidized CoO layer and stabilizer of Co nanoparticles were removed. The shortened interparticle distance and reduction of the oxidized state are attributed to the conductance of Co nanoparticle films (Figure S2 in Supporting Information).