Direct Formation of SiO2 Nanohole Arrays via Iron Nanoparticle

Oct 10, 2008 - Iron NPs prepared by hydroxylamine-mediated synthesis method resulted in not only nanoholes but also nanotrenches because the diameter ...
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Articles Direct Formation of SiO2 Nanohole Arrays via Iron Nanoparticle-Induced Carbothermal Reduction Hye Ryung Byon, Bonghoon Chung, Taihyun Chang, and Hee Cheul Choi* Department of Chemistry, Pohang UniVersity of Science and Technology (POSTECH), San 31, Hyoja-Dong, Nam-Gu, Pohang, South Korea 790-784 ReceiVed July 7, 2008. ReVised Manuscript ReceiVed September 1, 2008

Hexagonally patterned SiO2 nanohole arrays having sub-10 nm of width were directly formed via carbothermal reduction of SiO2 with carbon-dissolved iron nanoparticles (NPs) (C(on iron NPs, s) + SiO2(s) T SiO(g) + CO(g)). Iron NPs prepared by hydroxylamine-mediated synthesis method resulted in not only nanoholes but also nanotrenches because the diameter of these iron NPs is suitable for the growth of single-walled carbon nanotubes (SWNTs) that further react with underneath SiO2 to produce nanotrenches. Higher yields of nanoholes were obtained by using larger-sized iron NPs (diameter: 3-8 nm) prepared from ferritins and by reducing the amount of active carbon precursor sources during the reaction, by which the growth of SWNTs were substantially suppressed. SiO2 nanohole arrays were then obtained from hexagonally self-arrayed iron NPs, which were fabricated using polystyrene-block-poly(2vinylpyridine) (PS-b-P2VP) micelles. In addition to the reactivity of carbothermal reduction, interparticular distance of the patterned iron NPs turned out to be a key factor to successfully form nanohole arrays. The interparticular distance was controlled by changing the composition of PS-b-P2VP micelles.

Introduction During the past two decades, continuous efforts to develop new chemical synthetic methodologies have facilitated remarkable progresses in productions of quantum-scale functional nanomaterials1-5 that are particularly advantageous for the next generation of electronics such as complementary metal-oxide semiconductor (CMOS) integrated circuits,6 memory,7 nanosensor,8 and optoelectronic devices,9 as well as energy-conversion systems.10-13 For example, a solution chemistry has generally succeeded in the syntheses * Corresponding author. E-mail: [email protected].

(1) (a) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545–610. (b) Norris, D. J.; Efros, A. L.; Erwin, S. C. Science 2008, 319, 1776–1779. (2) (a) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445. (b) Dai, H. Surf. Sci. 2002, 500, 218–241. (c) Goldberger, J.; Fan, R.; Yang, P. Acc. Chem. Res. 2006, 39, 239–248. (3) Lieber, C. M. MRS Bull. 2003, 28, 486–491. (4) (a) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947– 1949. (b) Wang, Z. L.; Pan, Z. Int. J. Nanosci. 2002, 1, 41–51. (5) (a) Dai, Z. R.; Gole, J. L.; Stout, J. D.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 1274–1279. (b) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. Solid State Commun. 2001, 118, 351–354. (6) (a) Chen, Z.; Appenzeller, J.; Lin, Y.-M.; Sippel-Oakley, J.; Rinzler, A. G.; Tang, J.; Wind, S. J.; Solomon, P. M.; Avouris, P. Science 2006, 311, 1735. (b) Ahn, J.-H.; Kim, H.-S.; Lee, K. J.; Jeon, S.; Kang, S. J.; Sun, Y.; Nuzzo, R. G.; Rogers, J. A. Science 2006, 314, 1754– 1757. (c) Kim, D.-H.; Ahn, J.-H.; Choi, W. M.; Kim, H.-S.; Kim, T.-H.; Song, J.; Huang, Y. Y.; Liu, Z.; Lu, C.; Rogers, J. A. Science 2008, 320, 507–511. (7) (a) Guo, L.; Leobandung, E.; Chou, S. Y. Science 1997, 275, 649– 651. (b) Kroutvar, M.; Ducommun, Y.; Heiss, D.; Bichler, M.; Schuh, D.; Abstreiter, G.; Finley, J. J. Nature 2004, 432, 81–84. (c) Fuhrer, M. S.; Kim, B. M.; Durkop, T.; Brintlinger, T. Nano Lett. 2002, 2, 755–759. (d) Lu, W.; Lieber, C. M. Nat. Mater. 2007, 6, 841–850.

of metal and semiconductor 0D nanocrystals1 while a chemical vapor deposition (CVD), vapor-liquid-solid (VLS), and vapor-solid (VS) processes have produced (8) (a) Kong, J.; Franklin, N.; Chou, C.; Pan, S.; Cho, K. J.; Dai, H. Science 2000, 287, 622–625. (b) Chen, R. J.; Bangsaruntip, S.; Drouvalakis, K. A.; Kam, N. W. S.; Shim, M.; Li, Y.; Kim, W.; Utz, P. J.; Dai, H. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4984–4989. (c) Byon, H. R.; Choi, H. C. J. Am. Chem. Soc. 2006, 128, 2188–2189. (d) Patolsky, F.; Zheng, G.; Lieber, C. M. Anal. Chem. 2006, 78, 4260–4269. (9) (a) Klimov, V. I.; Mikhailovsky, A. A.; Xu, S.; Malko, A.; Hollingsworth, J. A.; Leatherdale, C. A.; Eisler, H.-J.; Bawendi, M. G. Science 2000, 290, 314–317. (b) Ohtsu, M.; Kobayashi, K.; Kawazoe, T.; Sangu, S.; Yatsui, T. IEEE J. Sel. Top. Quant. Electron. 2002, 8, 839– 862. (c) Xia, Y. N.; Halas, N. J. MRS Bull. 2005, 30, 338–348. (d) Li, Y.; Qian, F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18–27. (e) Pauzauskie, P.; Yang, P. Mater. Today 2006, 9, 36–45. (10) (a) Greenham, N. C.; Peng, X. G.; Alivisatos, A. P. Phys. ReV. B 1996, 54, 17628–17637. (b) Nozik, A. J. Physica E 2002, 14, 115–120. (c) Kymakis, E.; Amaratunga, G. A. J. Appl. Phys. Lett. 2002, 80, 112– 114. (d) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455–459. (e) Tian, B.; Zheng, X.; Kempa, T. J.; Fang, Y.; Yu, N.; Yu, G.; Huang, J.; Lieber, C. M. Nature 2007, 449, 885–889. (11) (a) Li, W. Z.; Liang, C. H.; Zhou, W. J.; Qiu, J. S.; Zhou, Z. H.; Sun, G. Q.; Xin, Q. J. Phys. Chem. B 2003, 107, 6292–6299. (b) Wang, C.; Waje, M.; Wang, X.; Tang, J. M.; Haddon, R. C.; Yan, Y. Nano Lett. 2004, 4, 345–348. (c) Chan, C. K.; Peng, H.; Liu, G.; McIlwrath, K.; Zhang, X. F.; Huggins, R. A.; Cui, Y. Nat. Nanotechnol. 2008, 3, 31–35. (d) Chen, C. K.; Zhang, X. F.; Cui, Y. Nano Lett. 2008, 8, 307–309. (12) (a) Wang, Z. L.; Song, J. H. Science 2006, 312, 242–246. (b) He, R.; Yang, P. Nat. Nanotechnol. 2006, 1, 42–46. (c) Wang, X.; Song, J.; Liu, J.; Wang, Z. L. Science 2007, 316, 102–105. (13) (a) Heremans, J. P.; Thrush, C. M.; Morelli, D. T.; Wu, M.-C. Phys. ReV. Lett. 2002, 88, 216801–4. (b) Hochbaum, A. I.; Chen, R.; Delgado, R. D.; Liang, W.; Garnett, E. C.; Najarian, M.; Majumdar, A.; Yang, P. Nature 2008, 451, 163–167. (c) Boukai, A. I.; Bunimovich, Y.; Tahir-Kheli, J.; Yu, J.-K.; Goddard III, W. A.; Heath, J. R. Nature 2008, 451, 168–171.

10.1021/cm801850p CCC: $40.75  2008 American Chemical Society Published on Web 10/10/2008

Nanohole Arrays Via Fe NP-Induced Carbothermal Reduction

various 1D nanotubes,2 wires,2a,3 belts,4 and ribbons.5 More recently, a modified VS process also has been applied to organic precursor molecules, resulting in unprecedented organic helical nanobelts and C60 single crystalline nanodisks.14 Although individual nanomaterials are surely of great interests for the investigation of their fundamental characteristics and demonstration of prototypical device systems, more meaningful systems would require systematically patterned array structures. Several examples of patterned 0D and 1D nanomaterials have been obtained via various physicochemical surface passivation techniques including traditional Langmuir-Blodgett technique15 and surfaceprogrammed assembly.16 Among the various patterned array nanostructures, nanohole array attracts many scientists since they provide platforms for the fundamental studies about integrated quantum memory device17 and surface plasmon resonance sensor.18 Nanoholes are also applicable as a nanohost for single molecule spectroscopy,19 artificial channel for trafficking ionic and single biomolecular movements,20 and zeptoliter beakers as a nanometer scale reactor.21 So far, colloidal lithographic etching,22 nanoimprint lithography,23 anodic aluminum oxide template and its (14) (a) Yoon, S. M.; Hwang, I. C.; Shin, N.; Ahn, D.; Lee, S. J.; Lee, J. Y.; Choi, H. C. Langmuir 2007, 23, 11875–11882. (b) Shin, H. S.; Yoon, S. M.; Tang, Q.; Chon, B.; Joo, T.; Choi, H. C. Angew. Chem., Int. Ed. 2008, 47, 693–696. (15) (a) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255–1259. (b) Huang, J.; Kim, F.; Tao, A.; Conner, S.; Yang, P. Nat. Mater. 2005, 4, 896–900. (c) Li, X.; Zhang, L.; Wang, X.; Shimoyama, I.; Sun, X.; Seo, W.-S.; Dai, H. J. Am. Chem. Soc. 2007, 129, 4890–4891. (16) (a) Mitchell, G. P.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1999, 121, 8122–8123. (b) Rao, S. G.; Huang, L.; Setyawan, W.; Hong, S. Nature 2003, 425, 36–37. (c) Myung, S.; Lee, M.; Kim, G. T.; Ha, J. S.; Hong, S. AdV. Mater. 2005, 17, 2361–2364. (17) (a) Kouklin, N.; Bandyopadhyay, S.; Tereshin, S.; Varfolomeev, A.; Zaresky, D. Appl. Phys. Lett. 2000, 76, 460–462. (b) Black, C. T.; Guarini, K. W.; Milkove, K. R.; Baker, S. M.; Russel, T. P.; Tuominen, M. T. Appl. Phys. Lett. 2001, 79, 409–411. (c) Black, C. T.; Ruiz, R.; Breyta, G.; Cheng, J. Y.; Colburn, M. E.; Guarini, K. W.; Kim, H. C.; Zhang, Y. IBM J. Res. DeV. 2007, 51, 605–633. (18) (a) Gordon, R.; Brolo, A. G.; McKinnon, A.; Rajora, A.; Leathem, B.; Kavanagh, K. L. Phys. ReV. Lett. 2004, 92, 0374011–4. (b) Rindzevicius, T.; Alaverdyan, Y.; Dahlin, A.; Ho¨o¨k, F.; Sutherland, D. S.; Ka¨ll, M. Nano Lett. 2005, 5, 2335–2339. (c) Tetz, K. A.; Pang, L.; Fainman, Y. Opt. Lett. 2006, 31, 1528–1530. (d) Genet, C.; Ebbesen, T. W. Nature 2007, 445, 39–46. (19) Kumbhakar, M.; Nath, S.; Mukherjee, T.; Mittal, J. P.; Pal, H. J. Photochem. Photobiol., C 2004, 5, 113–137. (20) Dekker, C. Nat. Nanotechnol. 2007, 2, 209–215. (21) (a) Barton, J. E.; Odom, T. W. Nano Lett. 2004, 4, 1525–1528. (b) Cui, Y.; Bjo¨rk, M. T.; Liddle, J. A.; So¨nnichsen, C.; Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093–1098. (c) Milliron, D. J.; Caldwell, M. A.; Philip Wong, H.-S. Nano Lett. 2007, 7, 3504–3507. (d) Li, J.; Kamata, K.; Watanabe, S.; Iyoda, T. AdV. Mater. 2007, 19, 1267–1271. (e) Kim, S.-J.; Maeng, W. J.; Lee, S. K.; Park, D. H.; Bang, S. H.; Kim, H.; Sohn, B.-H. J. Vac. Sci. Technol., B 2008, 26, 189–194. (22) (a) Choi, D.-G.; Yu, H. K.; Jang, S. G.; Yang, S.-M. J. Am. Chem. Soc. 2004, 126, 7019–7025. (b) Jiang, P.; McFarland, M. J. J. Am. Chem.Soc. 2005, 127, 3710–3711. (c) Manzke, A.; Pfahler, C.; Dubbers, O.; Plettl, A.; Ziemann, P.; Crespy, D.; Schreiber, E.; Ziener, U.; Landfester, K. AdV. Mater. 2007, 19, 1337–1341. (23) (a) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J.-P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. DeV. 2001, 45, 697–719. (b) Oshima, H.; Kikuchi, H.; Nakao, H.; Itoh, K.-I.; Kamimura, T.; Morikawa, T.; Matsumoto, K.; Umada, T.; Tamura, H.; Nishio, K.; Masuda, H. Appl. Phys. Lett. 2007, 91, 022508–3.

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transference,24 and block copolymer-based optical lithographic patterning processes17c,21c-e have been reported to realize nanohole arrays. However, these methods still suffer from tedious or complicated stepwise processes. Hence, it is highly demanded to develop a novel method allowing “direct” formations of nanohole arrays having less than 20 nm diameters. Recently, we have developed a novel surface chemistry (called carbothermal reduction) occurring between singlewalled carbon nanotubes (SWNTs) and SiO2 surface, by which SiO2 surfaces are thermochemically etched by SWNTs, resulting in SiO2 nanotrenches having sub-10 nm width with SWNT-resembled shapes and trajectories.25,26 This carbothermal reduction of SiO2 has inspired us to further apply for the direct formation of nanoholes. Throughout the in depth investigation of the mechanism of carbothermal reduction, we have figured out that iron catalyst nanoparticle (NP) plays a critical role not only in the growth of SWNTs but also more importantly in the initiation of carbothermal reduction. This implies that amorphous carbons or graphitic layers wrapping iron NPs27 can thermally reduce SiO2 substrate, and the resultant will be SiO2 nanoholes (C(on iron NPs, s) + SiO2(s) T SiO(g) + CO(g)), whereas embedded iron NPs are simply removed from the SiO2 substrate by levitation.26 Herein, we report successful formations of nanoholes having sub-10 nm of diameters via carbothermal reduction of SiO2 by iron NPs, as well as nanohole arrays formed when iron NPs arrayed by using diblock copolymer pattern are used. Experimental Section Formation of Iron Nanoparticles (NPs). (1) Hydroxylaminemediated formation: A SiO2/Si substrate (thermally grown ca. 500 nm thickness of SiO2 on highly B doped Si, Silicon Valley Microelectronics) was immersed in a 10 mL of DI water, in which 10 µL of 5 mM FeCl3 · 6H2O(aq) and, after 30 s, 100 µL of 40 mM NH2OH · HCl (aq) were successively added. The total immersion time was 3 min. The SiO2/Si substrate was thoroughly rinsed with DI water, dried by a stream of N2 gas, and then calcined in air at 800 °C for 5 min.28 (2) Ferritin-mediated formation: A SiO2/Si substrate was immersed in a 2 µg/mL ferritin aqueous solution (Type VII: from Human Heart, Sigma) for 5 min then washed and dried. The sample was calcined in air at 800 °C for 5 min. Iron NP Arrays. 0.5 wt % (Micelle I) or 0.3 wt % (Micelle II) of polystyrene-block-poly(2-vinylpyridine), PS-b-P2VP, (MnPS ) 56 kg/mol, MnP2VP ) 21 kg/mol, Mw/Mn ) 1.06, Polymer Source, Inc.) was dissolved in toluene at 70 °C and stirred for 4-5 h. In (24) (a) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468. (b) Asoh, H.; Matsuo, M.; Yoshihama, M.; Ono, S. Appl. Phys. Lett. 2003, 83, 4408–4410. (c) Kang, S.; Su, P. C.; Park, Y. I.; Saito, Y.; Prinz, F. B. J. Electrochem. Soc. 2006, 153, A554-A559. (25) Byon, H. R.; Choi, H. C. Nat. Nanotechnol. 2007, 2, 162–166. (26) Byon, H. R.; Choi, H. C. Nano Lett. 2008, 8, 178–182. (27) (a) Li, Y.; Kim, W.; Zhang, Y.; Rolandi, M.; Wang, D.; Dai, H. J. Phys. Chem. B 2001, 105, 11424–11431. (b) Hofmann, S.; Sharma, R.; Ducati, C.; Du, G.; Mattevi, C.; Cepek, C.; Cantoro, M.; Pisana, S.; Parvez, A.; Cervantes-Sodi, F.; Ferrari, A. C.; Dunin-Borkowski, R.; Lizzit, S.; Petaccia, L.; Goldoni, A.; Robertson, J. Nano Lett. 2007, 7, 602–608. (28) Choi, H. C.; Kundaria, S.; Wang, D. W.; Javey, A.; Wang, Q.; Rolandi, M.; Dai, H. Nano Lett. 2003, 3, 157–161.

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Figure 1. Formation of nanotrenches and/or nanoholes via carbothermal reduction of SiO2 by iron NPs. AFM images of (a) iron NPs formed by hydroxylaminemediated method on a SiO2/Si substrate; (b) after O2-assisted CVD treatment of (a) under the conventional gas condition of O2/H2/CH4/C2H4/Ar ) 1.5/ 500/1000/20/100 sccm at 900 °C for 10min; (c) ferritin-derived iron NPs on a SiO2/Si substrate; (d) after O2-assisted CVD treatment under the same condition of (b); (e) after O2-assisted CVD treatment without ethylene gas (900 °C for 10 min, O2/H2/CH4/C2H4/Ar ) 1.5/500/1000/0/100 sccm). Insets in (c) and (e) are high-magnification images, and the scale bars indicate 100 nm.

Results and Discussion

Figure 2. Schematic illustration for the formation of nanohole arrays on a SiO2/Si substrate. (a) Overview of two steps: (1) formation of the iron NP arrays by using polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymers and O2 plasma treatment, (2) formation of the nanohole arrays via carbothermal reduction in an O2-assisted CVD system. (b) Detail view of step (1) in (a) depicting spontaneous formation of PS-b-P2VP micelle arrays containing iron ions in the core.29b

toluene, PS-b-P2VP spontaneously formed spherical micelles where the solvophobic P2VP became a core while the solvophilic PS formed corona (Figure 2b). After cooling down to room temperature, 0.3 molar ratio of FeCl2 · 4H2O vs 2VP units for Micelle I (or 0.2 molar ratio for Micelle II) was dissolved to the micelle solution and vigorously stirred at least 3 days for the sufficient percolation of Fe2+ salts into core P2VP moieties. For the formation of Fe ion-contained micellar monolayer film on the SiO2/Si substrate, the solution was spin-coated at 4000 rpm for 50 s. Finally, O2 plasma (SPI plasma-prep II) was treated at 50 mA and 100 mTorr for 5 min to remove organic moieties and the samples were calcined in air at 800 °C for 5 min. Formation of SiO2 Nanohole Arrays. The iron NP array samples were transferred in the O2-assisted CVD system. Ar (350 sccm) and O2 (3 sccm) gases were introduced as a carrier gas during heating up and O2, H2, CH4, and Ar gases were turned on at 900 °C for designed reaction time. After the carbothermal reduction reaction was finished, all of the gases were turned off, except Ar gas (350 sccm), and the samples were then cooled to room temperature.

For the demonstration of SiO2 nanohole formation via carbothermal reduction, iron NPs were prepared on a SiO2/ Si substrate by calcining hydroxylamine-mediated iron NPs at 800 °C for 5 min in air28 (Figure 1a). The resulting iron NPs showed quite a narrow diameter distribution with an average diameter of 1.5 nm. When the substrate was transferred into an O2-assisted CVD system where O2, H2, CH4, and C2H4 gases were introduced at 1.5, 500, 1000, and 20 sccm (standard cubic centimeters per minute), respectively, at 900 °C for 10 min, nanoholes were formed but at very low population (Figure 1b). Instead, nanotrenches were mainly obtained. The high population of nanotrench is essentially due to the size of iron NPs because the diameters of these iron NPs are very suitable for the growth of SWNTs, by which nanotrenches are eventually formed. To examine the relationship between iron NP size and the population of nanohole, carbothermal reduction was performed using ferritin-derived iron NPs that provide much wider size distribution (4-15 nm) with larger average diameter size (7.9 nm). The ferritin-mediated iron NPs were prepared on a SiO2/Si substrate by calcining predeposited ferritin proteins at 800 °C for 5 min in air (Figure 1c). As shown in Figure 1d, the population of nanohole was substantially increased although nanotrenches and unreacted SWNTs were still found. These results clearly imply that SWNT growth is effectively suppressed by using large iron NPs27a in the O2assisted CVD environment, which leads to more efficient creation of nanoholes. To further increase nanohole population, the amount of carbon gas source (C2H4) was decreased with an anticipation of severe inhibition of SWNT growth. C2H4 was selected over CH4 because C2H4 tends to be pyrolized actively and hence contributes to the growth of SWNTs more. Indeed, nanoholes were selectively formed without nanotrenches or unreacted SWNTs when the car-

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Figure 3. Randomly positioned nanoholes originated from the iron NP arrays. (a-c) Iron NP arrays on a SiO2/Si substrate (using Micelle I); (d-f) 16.5% density of nanoholes compared to the original NP population after carbothermal reduction under the gas conditions of O2/H2/CH4/C2H4/Ar ) 1.5/500/1000/ 0/100 sccm at 900 °C for 15 min; (g-i) 38.8% density of nanoholes compared to the orginal NP population after carbothermal reduction under the gas conditions of O2/H2/CH4/C2H4/Ar ) 1.5/1000/1000/0/100 sccm at 900 °C for 15 min. (b), (e), (h) and (c), (f), (i) are high-magnification and 3D AFM images of (a), (d), (g), respectively. Inset in (b) shows a Fourier transformed pattern.

bothermal reduction was performed without C2H4 (O2/H2/ CH4/C2H4/Ar ) 1.5/500/1000/0/100 sccm) at 900 °C for 10 min (Figure 1d and the Supporting Information, Figure S1). After confirming the optimized reaction condition for the dominant formation of nanoholes, we attempted to construct SiO2 nanohole arrays from regularly patterned iron NPs using diblock copolymers. Diblock copolymer micelles commonly offer hexagonally ordered frameworks with controlled amounts of iron ions.29 In the case of polystyrene-block-poly(2vinylpyridine) (PS-b-P2VP) diblock copolymer in toluene, iron ions are settled in the P2VP core whereas PS corona shells are stretched toward the solvent. To form patterned iron NP arrays, iron-ion-containing PS-b-P2VP micelle solutions prepared at two different compositions were spincasted on SiO2/Si substrates (Figure 2). First, 0.5 wt % PS-b-P2VP micellar solution dissolved in a 0.3 molar ratio of FeCl2 to 2VP units (Micelle I) was spin-casted on a SiO2/Si substrate. The iron NPs obtained (29) (a) Sohn, B.-H.; Choi, J.-M.; Yoo, S. I.; Yun, S.-H.; Zin, W.-C.; Jung, J. C.; Kanehara, M.; Hirata, T.; Teranishi, T. J. Am. Chem. Soc. 2003, 125, 6368–6369. (b) Lu, J.; Yi, S. S.; Kopley, T.; Qian, C.; Liu, J.; Gulari, E. J. Phys. Chem. B 2006, 110, 6655–6660. (c) Bhaviripudi, S.; Reina, A.; Qi, J.; Kong, J.; Belcher, A. M. Nanotechnology 2006, 17, 5080–5086. (d) Chung, B.; Choi, M.; Ree, M.; Jung, J. C.; Zin, W. C.; Chang, T. Macromolecules 2006, 39, 684–689.

after the O2 plasma treatment displayed a hexagonal array as confirmed by atomic force microscope (AFM) as well as Fourier transformed pattern (Figure 3b and the inset). The average values of diameter, distance between iron NPs, and number of iron NPs in 1 × 1 µm2 are 3.5 ( 0.6 nm, 45.6 ( 4.3 nm, and ca. 550, respectively (Figure 3a-c). When carbothermal reduction was conducted with this arrayed iron NP sample under the previously optimized reaction condition (O2/H2/CH4/C2H4/Ar ) 1.5/500/ 1000/0/100 sccm at 900 °C for 15 min), nanoholes were clearly attained without any observation of nanotrench (Figure 3d-f). Nanoholes were also investigated with cross-sectional images of high-resolution transmission electron microscope (HRTEM), which show a nanohole width of ca. 22.4 nm determined at a full-width at halfmaximum (fwhm) and depth of ca. 5.2 nm (see the Supporting Information, Figure S2). In addition, AFM profiles show 40-50 nm of top width in nanoholes (tip radius < 10 nm) (see the Supporting Information, Figure S2). Although nanoholes were successfully created, the acquired nanoholes were placed on random positions and the density of hole was down to 16.5% compared with the original iron NP population. Such irregular formations and small numbers of nanoholes indicate that iron NPs

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are moving on the surface during the reaction and some of them are even removed without thermochemical reduction (see the Supporting Information, Figure S3). The movement of iron NPs is mainly attributed to the levitation of iron NPs caused by carbothermal reduction occurring between iron NPs and underneath SiO2 under the continuous gas flow. To suppress the movement (or migration) of iron NPs, two reaction environment changes were attempted. First, H2 concentration was increased as a similar H2 effect on the population control of levitated iron NPs had been observed from the self-assembly of iron NPs on the sidewalls of SWNTs.26 However, when the amount of H2 gas was doubled to 1000 sccm, the array pattern was destroyed although the density of nanohole was somewhat increased up to 38.8% (Figure 3g-i). Second, a new iron NP array sample having longer interparticular distance was used because the labile movement of levitated iron NP would be further accelerated if a density of iron NPs is high enough to engender attractive forces between NPs.26 Hence, a solution of 0.3 wt % PSb-P2VP micelle including a 0.2 molar ratio of FeCl2 to 2VP units (Micelle II) was spin-casted on a substrate, and hexagonal arrays of iron NPs were well-formed as shown in Figure 4a and the inset. The average values of diameter, distance between iron NPs, and number of iron NPs in 1 × 1 µm2 are 3.2 ( 0.6 nm, 59.0 ( 5.6 nm, and ca. 270, respectively. Compared to the case when Micelle I was used, the interparticular distance was increased by ca. 30%, whereas the diameter remains unchanged. The London-van der Waals force between two iron NPs can be estimated using eq 1 by assuming both iron NPs have an identical size.30

[

( )]

A d2 d2 d2 + ln 1 - 2 + 2 (1) 2 2 6 2r 2(r - d ) r where A is the Hamaker constant, d is the diameter of a particle, and r is the center-to-center distance between the particles. When the Hamaker constant for iron NP (Fe2O3) in vacuum is 23.2 × 10-20 J,31 the van der Waals forces applied to the iron NPs shown in Micelle I (d ) 3.5 nm, r ) 45.6 nm) and Micelle II (d ) 3.2 nm, r ) 59.0 nm) are 4.23 × 10-24 J and 2.26 × 10-24 J, respectively. Therefore, ca. halved interparticle interaction force in Micelle II would be expected to restrain the mobility of iron NPs during the carbothermal reduction reaction. Indeed, the movement of iron NP turned out to be considerably restrained when the iron NP array sample prepared by Micelle II was reacted for the carbothermal reduction at the optimized condition (Figure 4b). However, no nanohole was created either, seemingly because of low reactivity. This is not a surprising result because the suppressed movement of iron NP at this specific situation circuitously indicates that the efficiency of carbothermal reduction is also quite low. Now, to increase the reactivity while the mobility of iron NP is still effectively suppressed, the optimized reaction condition was slightly tuned: (1) the amount of O2 gas was doubled, and (2) the reaction time Φ)-

(30) (a) Hamaker, H. C. Physica IV 1937, 4, 1058–1072. (b) Tadmor, R. J. Phys.: Condens. Matter 2001, 13, L195-L202. (31) Height, M. J.; Howard, J. B.; Tester, J. W.; Vander Sande, J. B. J. Phys. Chem. B 2005, 109, 12337-12346.

Figure 4. Nanohole arrays originated from the low population of iron NP arrays. AFM images of (a) iron NP arrays on the SiO2/Si substrate (using Micelle II). Inset is a Fourier transformed pattern. (b) After the O2-assisted CVD treatment in optimized condition (O2/H2/CH4/C2H4/Ar ) 1.5/500/ 1000/0/100 sccm at 900 °C for 15 min). (c) Partially formed nanoholes after 15 min reaction under the modified gas condiction of O2/H2/CH4/ C2H4/Ar ) 3/500/1000/0/100 sccm at 900 °C. (d) Almost-perfect nanoholes arrays obtained from prolonged reaction for 60 min. Insets of (c) and (d) show high-magnification images and scale bars are 50 nm. The red and green circles indicate the positions of iron NPs and nanoholes, respectively. (e) 3D image of inset in (d) and an AFM width profile of single nanohole.

was increased. Figure 4c shows an AFM image taken after 15 min of the reaction at 900 °C under O2/H2/CH4/C2H4/Ar ) 3/500/1000/0/100 sccm. Small numbers of nanoholes were produced, but most of the iron NPs were still kept at their original sites. Figure 4c inset shows two nanoholes (green circle) and five iron NPs (red circles) on hexagonal apexes. After a prolonged reaction for 60 min, almost perfect nanohole arrays were finally formed without a notable position movement (average hole distance is 59.8 ( 9.5 nm) and the hole density is greater than 80% compared to the iron NP arrays (Figure 4d). AFM profile showed that the top width of nanohole was ca. 16 nm (Figure 4e), by which the diameter of nanohole at the fwhm would be ca. 8 nm. Conclusion In summary, we successfully demonstrated the formation of SiO2 nanohole arrays via carbothermal reduction of SiO2 substrates by patterned iron NP arrays. The high density iron NPs having hexagonal orders with short interparticular distance were promptly displaced in the O2-assisted CVD

Nanohole Arrays Via Fe NP-Induced Carbothermal Reduction

system, resulting in the formation of randomly positioned nanoholes. On the contrary, SiO2 nanohole array was obtained from lower density iron NP array samples having increased interparticular distance at prolonged carbothermal reduction condition. Thus, the key for the successful formation of nanohole array is to keep the original patterned array of iron NPs throughout the reaction, which is accomplished by controlling the interparticular distance of iron NPs prepared by using a template of PS-b-P2VP micelles. We are currently studying to fabricate nanoholes having high aspect ratio to the depth direction for numerous applications including zeptoliter beaker for the nanometer size of molecular reaction.21

Chem. Mater., Vol. 20, No. 21, 2008 6605

Acknowledgment. This work was supported by the Nano/ Bio Science & Technology Program of MOST (2008-00979), Korean Research Foundation (MOEHRD, KRF-2005-005J13103, and KRF-2007-313-C00386), and KOSEF (2007-81158). T.C. acknowledges the support from KOSEF (R0A-2007000-20125-0). We thank Prof. Gyu-Chul Yi at POSTECH for use of O2 plasma. Supporting Information Available: AFM images of Figure S1-S3 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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