Fabrication of Micro− Nano Hybrid Patterns on a Solid Surface

A hybrid pattern with micropatterned nanoarray of Au−Pt core−shell particles in the background of Au nanoarray was fabricated via an approach comb...
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Fabrication of Micro-Nano Hybrid Patterns on a Solid Surface Peng Liu and Jiandong Ding* Key Laboratory of Molecular Engineering of Polymers of Ministry of Education, Department of Macromolecular Science, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, China Received June 15, 2009. Revised Manuscript Received August 13, 2009 A hybrid pattern with micropatterned nanoarray of Au-Pt core-shell particles in the background of Au nanoarray was fabricated via an approach combining photolithography, block copolymer micelle nanolithography, and controlled seed growth. Two pertinent patterns were also obtained: one is a hexagonal nanopattern of Au-Pt bimetallic nanodots with controlled thickness of Pt shells, and the other is an interlaced pattern of Pt microsheets and Au nanoarrays. We thus present a platform to generate regular micro-nano patterns of one or more than one metal on a solid surface.

Introduction Surface patterning with regular arrays in micro- or nanoscale has received increasing attention in the recent decade and potentially applied in many fields.1-16 In general, fabrication of micropatterns on a solid surface depends on conventional photolithography. Advanced ray lithography17 and proximalprobe lithography such as dip-pen lithography18 have the capacity to generate nanopatterns. Soft lithography,19 nanoimprint *Corresponding author. E-mail: [email protected].

(1) Geissler, M.; Xia, Y. N. Adv. Mater. 2004, 16, 1249–1269. (2) Briseno, A. L.; Mannsfeld, S. C. B.; Ling, M. M.; Liu, S. H.; Tseng, R. J.; Reese, C.; Roberts, M. E.; Yang, Y.; Wudl, F.; Bao, Z. N. Nature 2006, 444, 913– 917. (3) Whitesides, G. M.; Ostuni, E.; Takayama, S.; Jiang, X. Y.; Ingber, D. E. Annu. Rev. Biomed. Eng. 2001, 3, 335–373. (4) Lee, K. B.; Kim, E. Y.; Mirkin, C. A.; Wolinsky, S. M. Nano Lett. 2004, 4, 1869–1872. (5) Hodgson, L.; Chan, E. W. L.; Hahn, K. M.; Yousaf, M. N. J. Am. Chem. Soc. 2007, 129, 9264–9265. (6) Arnold, M.; Cavalcanti-Adam, E. A.; Glass, R.; Blummel, J.; Eck, W.; Kantlehner, M.; Kessler, H.; Spatz, J. P. ChemPhysChem 2004, 5, 383–388. (7) Lahann, J.; Balcells, M.; Rodon, T.; Lee, J.; Choi, I. S.; Jensen, K. F.; Langer, R. Langmuir 2002, 18, 3632–3638. (8) Hoover, D. K.; Chan, E. W. L.; Yousaf, M. N. J. Am. Chem. Soc. 2008, 130, 3280–3281. (9) Zhang, Y.; Matsumoto, E. A.; Peter, A.; Lin, P. C.; Kamien, R. D.; Yang, S. Nano Lett. 2008, 8, 1192–1196. (10) Bita, I.; Yang, J. K. W.; Jung, Y. S.; Ross, C. A.; Thomas, E. L.; Berggren, K. K. Science 2008, 321, 939–943. (11) Childs, W. R.; Nuzzo, R. G. J. Am. Chem. Soc. 2002, 124, 13583–13596. (12) Bardea, A.; Naaman, R. Langmuir 2009, 25, 5451–5454. (13) Gao, J.; Liu, Y. L.; Xu, H. P.; Wang, Z. Q.; Zhang, X. Langmuir 2009, 25, 4365–4369. (14) Graeter, S. V.; Huang, J. H.; Perschmann, N.; Lopez-Garcia, M.; Kessler, H.; Ding, J. D.; Spatz, J. P. Nano Lett. 2007, 7, 1413–1418. (15) Salber, J.; Grater, S.; Harwardt, M.; Hofmann, M.; Klee, D.; Dujic, J.; Huang, J. H.; Ding, J. D.; Kippenberger, S.; Bernd, A.; Groll, J.; Spatz, J. P.; Moller, M. Small 2007, 3, 1023–1031. (16) Sun, J. G.; Graeter, S. V.; Yu, L.; Duan, S. F.; Spatz, J. P.; Ding, J. D. Biomacromolecules 2008, 9, 2569–2572. (17) Ito, T.; Okazaki, S. Nature 2000, 406, 1027–1031. (18) Piner, R. D.; Zhu, J.; Xu, F.; Hong, S. H.; Mirkin, C. A. Science 1999, 283, 661–663. (19) Xia, Y. N.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153–184. (20) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Science 1996, 272, 85–87. (21) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735–738. (22) Sohn, B. H.; Yoo, S. I.; Seo, B. W.; Yun, S. H.; Park, S. M. J. Am. Chem. Soc. 2001, 123, 12734–12735. (23) Park, S.; Kim, B.; Wang, J. Y.; Russell, T. P. Adv. Mater. 2008, 20, 681–685. (24) Zhang, L.; Zou, B.; Dong, D.; Huo, F. W.; Zhang, X.; Chi, L. F.; Jiang, L. Chem. Commun. 2001, 1906–1907. (25) Zhang, G.; Wang, D. Y.; Mohwald, H. Nano Lett. 2005, 5, 143–146. (26) Gates, B. D.; Xu, Q. B.; Love, J. C.; Wolfe, D. B.; Whitesides, G. M. Ann. Rev. Mater. Res. 2004, 34, 339–372.

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lithography,20 molecular self-assembly,21-24 colloidal crystal template,25 and other “unconventional” approaches26 have also been developed to fabricate nanostructures. Concerning preparation of a high-throughout pattern of metal nanodots less than 20 nm, block copolymer micelle nanolithography has presented a powerful route.27-30 Patterns of alloy nanoparticles have also been prepared.31,32 Combinations of block copolymer micelle nanolithography with electron-beam lithography,33 photolithography,34,35 and soft lithography35 have been used to fabricate micropatterns of gold33,34 or iron oxide35 nanoparticles. To date, we have not found any publication about micro-nano hybrid patterns with both continuous and discontinuous microregions containing metals. Besides block copolymer micelle nanolithography, another approach we should mention in this paper is hydroxylamine seeding. In 1998, Natan’s group found that colloidal Au nanoparticles could be well grown due to reduction of gold acid (HAuCl4) by NH2OH, which was catalyzed by Au nanoparticles.36 In 2008, Spatz’s group developed this approach in order to achieve Au or platinum (Pt) nanoarrays with a series of defined particle sizes due to the “controlled” seed growth;37 otherwise, the Au or Pt particles could still be enlarged, but the spatial order of the initial pattern could not be kept. One of their controlling approaches is the modification of a glass surface by alkyltrimethoxysilane before seed growth but after block copolymer micelle nanolithography.

(27) Spatz, J. P.; Mossmer, S.; Hartmann, C.; Moller, M.; Herzog, T.; Krieger, M.; Boyen, H. G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407–415. (28) Kim, D. H.; Sun, Z. C.; Russell, T. P.; Knoll, W.; Gutmann, J. S. Adv. Funct. Mater. 2005, 15, 1160–1164. (29) 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. (30) Huang, J. H.; Grater, S. V.; Corbellinl, F.; Rinck, S.; Bock, E.; Kemkemer, R.; Kessler, H.; Ding, J. D.; Spatz, J. P. Nano Lett. 2009, 9, 1111–1116. (31) Li, X.; Goring, P.; Pippel, E.; Steinhart, M.; Kim, D. H.; Knoll, W. Macromol. Rapid Commun. 2005, 26, 1173–1178. (32) Ethirajan, A.; Wiedwald, U.; Boyen, H. G.; Kern, B.; Han, L. Y.; Klimmer, A.; Weigl, F.; Kastle, G.; Ziemann, P.; Fauth, K.; Cai, J.; Behm, R. J.; Romanyuk, A.; Oelhafen, P.; Walther, P.; Biskupek, J.; Kaiser, U. Adv. Mater. 2007, 19, 406– 410. (33) Glass, R.; Arnold, M.; Blummel, J.; Kuller, A.; Moller, M.; Spatz, J. P. Adv. Funct. Mater. 2003, 13, 569–575. (34) Gorzolnik, B.; Mela, P.; Moeller, M. Nanotechnology 2006, 17, 5027–5032. (35) Yun, S. H.; Sohn, B. H.; Jung, J. C.; Zin, W. C.; Ree, M.; Park, J. W. Nanotechnology 2006, 17, 450–454. (36) Brown, K. R.; Natan, M. J. Langmuir 1998, 14, 726–728. (37) Lohmueller, T.; Bock, E.; Spatz, J. P. Adv. Mater. 2008, 20, 2297–2302.

Published on Web 09/01/2009

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Figure 1. Schematic presentation of fabrication of three patterns composed of two metals on glass. (a) Preparation of initial Au nanoarray. The monolayer of micelles loaded with the gold precursor is dip-coated onto glass surfaces. After oxygen plasma treatment, gold nanodots are reduced and deposited in a hexagonal array. (b) Fabrication of pattern 1 (nanopattern of Au-Pt core-shell particles). Self-assembly monolayer (SAM) of octadecyltrimethoxysilane (OTMS) and the controlled seed growth are used to fabricate an array of Au-Pt core-shell nanoparticles. (c) Fabrication of pattern 2 (micro-nano hybrid patterns of Au-Pt bimetallic nanoarray and Au nanoarray) via combination of photolithography and controlled seed growth based upon an initial Au nanoarray. (d) Fabrication of pattern 3 (interlaced patterns of Pt microsheets and Au nanoarrays) via combination of photolithography and uncontrolled seed growth based upon an initial Au nanoarray.

Different from the nanoarrays of enlarged particles of either Au or Pt37 and suspensions or nonregular arrays of core-shell metallic nanoparticles,38-45 we herein fabricate regular nanopatterns of Au-Pt core-shell particles (pattern 1), as seen in Figure 1. A more significant novelty in the present paper comes from the first report of micro-nano hybrid patterns with both continuous and discontinuous microregions composed of metals. The present approach owns an advantage of fabrication of patterns with more than one metal, with representative bimetallic patterns shown in patterns 2 and 3 (Figure 1). Pattern 2 is composed of a nanoarray of Au and another array of bimetallic nanoparticles with Au as core and Pt as shell, and the two nanoarrays spatially distribute complementarily, forming a micropattern in a larger scale. The prototype of the micropattern is introduced via photolithography, and the Au nanoarray is generated via block copolymer micelle nanolithography. The (38) Schmid, G.; West, H.; Mehles, H.; Lehnert, A. Inorg. Chem. 1997, 36, 891– 895. (39) Scott, R. W. J.; Wilson, O. M.; Oh, S. K.; Kenik, E. A.; Crooks, R. M. J. Am. Chem. Soc. 2004, 126, 15583–15591. (40) Henglein, A. J. Phys. Chem. B 2000, 104, 2201–2203. (41) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718–724. (42) Lyon, J. L.; Fleming, D. A.; Stone, M. B.; Schiffer, P.; Williams, M. E. Nano Lett. 2004, 4, 719–723. (43) Rodriguez-Gonzalez, B.; Burrows, A.; Watanabe, M.; Kiely, C. J.; Liz-Marzan, L. M. J. Mater. Chem. 2005, 15, 1755–1759. (44) Mallik, K.; Mandal, M.; Pradhan, N.; Pal, T. Nano Lett. 2001, 1, 319–322. (45) Cao, L. Y.; Tong, L. M.; Diao, P.; Zhu, T.; Liu, Z. F. Chem. Mater. 2004, 16, 3239–3245.

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nanoarray of core-shell particles is prepared via the seed-growth approach with Au nanoparticles as nucleus and then grown in an aqueous solution of H2PtCl6 and NH2OH. Pattern 3 (interlaced patterns of Pt microsheets and Au nanoarrays) could be simply regarded as an extreme case of pattern 2 if the controlled seed growth proceeds for an infinitely long time. To significantly save time, we use an “uncontrolled” seed growth in preparation of pattern 3. The uncontrolled growth could be carried out in high-concentrated seeding solutions, with or without octadecyltrimethoxysilane (OTMS) modification. All of three patterns schematically presented in Figure 1 are new. Especially pattern 2 is very difficult to obtain. Fabrication of pattern 1 could act as a pretechnique of generation of pattern 2, and pattern 3 is an extension of pattern 2.

Experimental Section Materials Used in Experiment. Block copolymers of polystyrene and poly(2-vinylpyridine), PS-b-P2VP, were from Polymer Source Inc., Canada. The molecular weights (MWs) of PS and P2VP blocks were 52 400 and 28 100, and the polydispersity index of the polymer defined as weight-average MW over number-average MW (Mw/Mn) was 1.07. OTMS were obtained from Aldrich. Chloroauric acid tetrahydrate (HAuCl4 3 4H2O), hydroxylamine hydrochloride (NH2OH 3 HCl), hexachloroplatinic acid hexahydrate (H2PtCl6 3 6H2O), and other reagents were obtained from Shanghai Chemical Reagent Corp. All chemicals were of analytical grade. All aqueous solutions were made using ultrapure water from a Milli-Q system (Millipore). Microscopic cover glass was used after extensive and fresh cleaning. DOI: 10.1021/la9021504

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Fabrication of Au Nanoarray on Substrate. A solution of PS-b-P2VP (5 mg/mL) was made by dissolving the polymer in 3 mL of anhydrous toluene. After stirring for 5 h, the metal precursor (HAuCl4 3 4H2O) with a given molar ratio over pyridine unit in PS-b-P2VP was added and stirred for at least 24 h until the colloid became transparent. Substrates were fixed on a substrate holder, dip-coated with micelles loaded with gold precursors using a self-built pulling machine. The substrates were treated by oxygen plasma for 60 min at 100 W to reduce metal precursors to gold nanoparticles and meanwhile to remove polymers. Finally, a Au nanoarray was positioned on the substrate. Self-Assembly of OTMS on Glass Decorated with Au Nanoarray. OTMS solution (volume 0.5%) was prepared in anhydrous toluene. A Au nanoarray on glass was treated with O2 plasma for 10 min. Then the sample was placed immediately into OTMS solution at 60 °C for at least 24 h to form the self-assembly monolayer. The samples were rinsed with toluene, acetone, and ethanol, dried with nitrogen gas, and baked at 120 °C for 1 h.

Preparation of Nanopattern of Au-Pt Core-Shell Particles (Pattern 1). A Au nanoarray on glass decorated with OTMS was immersed into an low-concentrated aqueous solution containing 0.01 wt % H2PtCl6 and 2 mM NH2OH 3 HCl at 60 °C Gold nanoparticles as seeds were used to deposit Pt as shell (the controlled seed growth). The controlled seed growth based upon the initial Au nanoarray resulted in an array of Au-Pt core-shell nanoparticles.

Preparation of Micro-Nano Hybrid Patterns of Au-Pt Bimetallic Nanoarray and Au Nanoarray (Pattern 2). A photoresist was spin-coated onto glass decorated with Au nanoarray and SAM of OTMS at 4000 rpm, resulting in the films of about 1.5 μm in thickness. Microstructures of the photoresist were generated by standard photolithography. Substrates with Au nanoarray and photoresist micropatterns were immersed into a solution containing 0.01 wt % H2PtCl6 and 2 mM NH2OH 3 HCl. Thus, the controlled seed growth proceeded just in the unshielded region. Afterward, samples were washed with acetone ultrasonic bath for 10 min for the lift-off of photoresist and with Milli-Q water ultrasonic bath for another 10 min and then dried at 100 °C. After removal of photoresist, a hybrid pattern of Au-Pt bimetallic nanoarray and Au nanoarray in different micropatterns was produced.

Preparation of Interlaced Patterns of Pt Microsheet and Au Nanoarray (Pattern 3). Microstructures of photoresist on glass with Au nanoarray and OTMS monolayer were fabricated by standard photolithography. The Au nanoarrays in the microregions unshielded by photoresists undergo uncontrolled seed growth in conditions such as a high-concentrated aqueous solution of 0.5 wt % H2PtCl6 and 0.2 M NH2OH, and eventually covered by Pt (the uncontrolled seed growth). Samples were washed with acetone ultrasonic bath for 10 min for the lift-off of photoresist and Milli-Q water ultrasonic bath for another 10 min and dried at 100 °C, which resulted in interlaced patterns of Pt microsheet and Au nanoarray. Characterization of Resultant Patterns. UV-vis spectroscopic measurements were done using a UV-vis spectrophotometer (TU-1901, Beijing Puxi Inc., China) at room temperature in the range of 200-900 nm. X-ray photoelectron spectroscopy (XPS) was carried out on a RBD upgraded PHI-5000C ESCA system (Perkin-Elmer) using monochromatic Mg KR X-rays at 1253.6 eV operated at 250 W, and spectrum calibration was performed by taking the C 1s electron peak (BE = 284.6 eV) as internal reference. The data analysis was made by using the RBD AugerScan 3.21 software provided by RBD Enterprises. Atomic force microscopy (AFM) (Dimension 3100, Digital Instruments) was used to image the samples under ambient conditions in tapping mode with a silicon cantilever (40 N/m). Scanning electron microscopy (SEM) was carried out with a Hitachi S-4800 microscope at an acceleration voltage of 2 kV. Transmission electron microscopy (TEM) photographs were taken by 494 DOI: 10.1021/la9021504

Figure 2. AFM images of (a) an initial Au nanoarray on glass and (b) a nanopattern of Au-Pt core-shell particles. The bimetallic nanopattern was prepared after immersing the Au-patterned substrate in a solution containing 0.01% H2PtCl6 and 2 mM NH2OH at 60 °C for 16 h. The insets in (a) and (b) are the central part of the corresponding autocorrelation images. (c) and (d) show height profiles along the lines in (a) and (b), respectively. The sizes of Au-Pt core-shell nanoparticles resulting from different growth periods are summarized in (e). (f ) UV-vis spectra of glass substrates decorated with nanopattern of Au-Pt core-shell particles. The data at “0 h” in (e) and (f ) refer to the initial Au nanoarray modified with OTMS. using a JEOL JEM-2100F electron microscope at 100 kV. An inverted optical microscope (Zeiss Axiovert 200) equipped with an integrated digital camera was also used to observe some substrate surfaces.

Results and Discussion Preparation of Initial Au Nanoarrays on Glass. When PS-b-P2VP was dissolved in toluene, spherical reverse micelles were formed with the lipophobic P2VP blocks forming cores and the lipophilic PS blocks forming coronas. Next, a metal precursor (HAuCl4) was added into the suspension. The gold precursor migrated into the cores of the micelles due to their lipophobic character. PS-b-P2VP micelles loaded with HAuCl4 were deposited on substrate by dip-coating. Oxygen plasma was used to remove the block copolymers and meanwhile reduce the gold precursors into gold nanoparticles. This process enabled the preparation of ordered array of gold nanoparticles on substrate (Au nanoarray), as published previously.27,30 A typical image is shown in Figure 2a. Preparation of Nanopattern of Au-Pt Core-Shell Particles (Pattern 1). Core-shell nanoparticles with different components and thicknesses exhibit various catalytic activity as well as optical and electrical properties.39-43 In this experiment, a Au nanoarray on substrate was immersed into H2PtCl6 and Langmuir 2010, 26(1), 492–497

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Figure 3. Representative SEM images of an initial Au nanoarray on glass (a) and nanopatterns of Au-Pt core-shell particles prepared in solution contained 0.01% H2PtCl6 and 2 mM NH2OH at 60 °C after growing for 8 h (b) and 16 h (c).

NH2OH solution to clay Au particles by Pt. The nanopattern of Au-Pt core-shell particles was visualized by AFM, as exampled in Figure 2b. Although not perfectly ordered, the nanoparticles exhibited a high degree of short-range hexagonal order as evident from the corresponding autocorrelation function (see inset in Figure 2b), implying the preservation of a pseudohexagonal order of the original micelles and Au nanodots. The order arrays of nanoparticles were thus not disturbed by the controlled seed growth. The height in AFM measurements gave the sizes of nanoparticles, indicating 6 nm for the initial Au nanoparticles and about 15 nm for Au-Pt core-shell nanodots after 16 h growth, as seen in Figure 2c,d. The size of Au-Pt nanoparticles is almost linearly related to the growth time (Figure 2e). The growth rate is about 0.6 nm/h under our experimental conditions unless the growth lasts very long. The size enlargement indicates the successful deposition of Pt shell on the surface of Au core in a well-controlled manner. Figure 2f shows representative UV-vis spectra of Au nanoarray and nanopattern of Au-Pt core-shell particles. The spectral peak of Pt nanoparticles was reported at about 265 nm.46,47 Redshifting in our experiments (about 288 nm after 16 h growth) revealed an effect of Au cores on Pt shells. The absorbance peak at about 520 nm caused by the excitation of surface plasmon resonance of gold nanoparticles37 was not detected by us, which might be due to the low amount of gold on glass. While AFM better gives the particle height in the z direction, SEM and TEM better measure the x-y dimension. Figure 3 shows representative SEM images of the initial gold nanoarray and the nanopatterns of Au-Pt core-shell particles after 8 and 16 h growth. The enlargement of nanoparticles during growth is again confirmed. Before our TEM observations, a metallic nanopattern was treated first by hydrofluoric acid to free nanoparticles from glass substrate, and thus the order and the so-called pattern were lost. The free nanoparticles were then transferred onto a carbon film supported by a copper mesh. A typical TEM image of Au nanoparticles extracted from the initial Au nanoarray is shown in Figure 4a. After 8 h growth, the resulting Au-Pt nanoparticles were significantly enlarged (Figure 4b). Because both core and shell are metallic and the Pt shell is quite thick in our experiments, it is hard to visualize Au cores. The lattice plane with interval of 2.0 A˚ (Figure 4c) corresponds to gold (200), and the lattice plane with interval of 2.2 A˚ comes from Pt (111) (Figure 4d). The elemental information was further afforded by XPS, as shown in Figure 5. The XPS spectrum of the initial Au nanoarray (46) Yee, C.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 4314–4316. (47) Huang, J. C.; Liu, Z. L.; Liu, X. M.; He, C. B.; Chow, S. Y.; Pan, J. S. Langmuir 2005, 21, 699–704.

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Figure 4. TEM images of (a) initial Au nanoparticles and (b) a Au-Pt core-shell nanoparticle resulting from 8 h growth. (c) and (d) display magnified regions marked as “1” in (a) and “2” in (b), respectively, showing crystalline lattice planes of metals. Those nanoparticles were removed from glass and transferred into carbon films before TEM observations.

exhibit two obvious peaks of Au (4f ) at 83.4 and 87.2 eV. Two peaks at 72.6 and 76.0 eV occur after seed growth, which just correspond to Pt (4f ). These Au characteristic peaks disappeared after a long seed growth because the thickness of Pt shell exceeds the detectable depth of XPS. Combination of AFM (Figure 2b,d, e), SEM (Figure 3), TEM (Figure 4), XPS (Figure 5), and UV-vis absorption (Figure 2f ) convinces the formation of array of Au(core)-Pt(shell) nanoparticles. Preparation of Micro-Nano Hybrid Pattern of Au-Pt Bimetallic Nanoarray and Au Nanoarray (Pattern 2). Here we take advantage of conventional photolithography to fabricate DOI: 10.1021/la9021504

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Figure 5. XPS spectra of the initial Au nanoarray and nanopatterns of Au-Pt core-shell particles on glass substrate after growing with 0.01% H2PtCl6 and 2 mM NH2OH solution at 60 °C for 4 and 12 h.

Figure 7. (a) XPS curves of the initial Au nanoarray and micronano hybrid pattern (pattern 2) after a controlled growth for 16 h. Just local curves with the binding energy range characteristic of Au and Pt are displayed. (b) Calculated Pt content as a function of growth time. Here 100% refers to all of element signals (not limited to Pt and Au). The line is just for guidance of the eyes.

Figure 6. SEM images of glass surfaces decorated with initial Au nanoarray (a) and micro-nano hybrid patterns of Au-Pt nanoarray and Au nanoarray (pattern 2) (b-g). The inset in (a) displays a magnified region showing the Au nanodots. The hybrid patterns in (b)-(g) were obtained by growing in a solution with 0.01% H2PtCl6 and 2 mM NH2OH at 60 °C after 8 (b), 16 (d), and 48 h (f ). (c), (e), and (g) show magnified regions schematically marked in (b), (d), and (f ), respectively. Image (h) displays a three-dimensional schematic graph of pattern 2. 496 DOI: 10.1021/la9021504

microstructure of photoresist on Au nanoarray. Some Au nanoparticles were then covered by the photoresist microregion. Only those bare Au nanodots in the interval region among photoresist micropatterns served as seeds and then were claded by Pt, which resulted in formation of nanoarray of Au-Pt core-shell particles. After removal of photoresist, hybrid pattern of two nanoarrays with a micropatterned spatial arrangement were obtained, as shown in Figure 6. Nothing was observed in the initial Au nanoarray in low magnification (Figure 6a). The SEM images of micro-nano hybrid pattern in low magnification show micropatterns with gradually enhanced contrast upon longer growth time, as presented in Figure 6b,d,f. The corresponding highmagnification images (Figure 6c,e,g) clearly display two nanoarrays: one is the initial Au nanoarray, and the other is the nanoarray of Au-Pt core-shell particles with enlarged sizes but still a basically uniform size distribution at early growth stage. The SEM images observed under different magnifications confirmed the micro-nano patterns but did not offer any element information. The energy dispersive X-ray (EDX) analysis could not afford the desired evidence due to very weak signals (data not shown). So, we employed a more sensitive detect approach, XPS, which has a detectable depth of several nanometers. The XPS spectrum of initial Au nanoarray on silanized substrate in Figure 7a exhibits two peaks of Au (4f ) at 83.4 and 87.2 eV. After immersed into the growth solution (0.01% H2PtCl6/2 mM NH2OH solution) for 16 h, the two characteristic peaks of Au relatively decreased, and meanwhile two peaks at 72.6 and 76.0 eV corresponding to Pt (4f ) emerged. The signal of Pt demonstrated the formation of Pt shell in the microregion of At-Pt core-shell nanoparticle array, and the signal of Au came from the remaining region of Au nanoarray. Here we note that unlike XPS measurements for pattern 1 (Figure 5), the Au signal in pattern 2 did not Langmuir 2010, 26(1), 492–497

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grown microregion could be roughly thought as a Pt microsheet for a very long controlled seed growth. To save time to a large extent, we used an uncontrolled seed growth in a highconcentrated growth solution to fabricate interlaced pattern of quasi-Pt microsheet and Au nanoarray. The resultant micronano interlaced patterns characterized by both SEM and optical microscopy are shown in Figure 8. Images in low magnification show quasi-Pt microsheet in triangular shapes (bright microscale islands) (Figure 8a,b). The high-magnification SEM image of the border region (Figure 8c) confirms the coexistence of quasi-Pt microsheet and Au-nanodot array. The microsheet of Pt is so dense that even optical microscopy made the pattern of those microsheets visualized well (Figure 8d), while nothing was seen on the glass with the initial Au nanopattern (Figure 8e).

Conclusions

Figure 8. SEM images (a, b, and c) and optical micrographs (d, e) of patterned surfaces. Images of (a), (b), (c), and (d) are graphs of interlaced pattern of Pt microsheet and Au nanoarray (pattern 3) after an initial Au nanoarray proceeded an uncontrolled growth in 0.5% H2PtCl6 and 0.2 M NH2OH solution at 60 °C for 12 h. The SEM image at low magnification in (a) visualizes micropattern of Pt microsheet. The image at immediate magnification in (b) shows a corner of a Pt microsheet as schematically presented by the left rectangle marked in (a). The SEM in high magnification in (c) shows the border as schematically presented by the right rectangle in (a), especially displaying the remaining Au nanoarray beside Pt microsheet. The grown micropattern can even be observed clearly in an optical microscope (d), while the blank optical graph of the initial Au nanoarray (e) is as control.

disappear even after a long seed growth (Figure 7a). In the controlled growth process, the Pt content gradually increased, as shown in Figure 7b. Since the content here is related to volume instead of size, the growth trend is reasonably more significant than linear. The positions of the characteristic peaks of Au and Pt are consistent with the previous reports.27,37 Our XPS experiments provided strong evidence of metal elements in the formation of micropatterned nanoaray of Au-Pt core-shell particles and the background Au nanoarray. Preparation of Interlaced Pattern of Pt Microsheet and Au Nanoarray (Pattern 3). In the process of preparation of micro-nano hybrid pattern of Au-Pt bimetallic nanoarray and Au nanoarray (pattern 2), we observed that Au-Pt core-shell nanoparticles in designed micropattern tended to coalesce with longer growth time (Figure 6g). Therefore, as an extension, the

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In summary, we successfully fabricated a micro-nano pattern composed of micropatterned nanoarray of Au-Pt core-shell particles and regular Au nanoarray via the first combination of photolithography, block copolymer micelle nanolithography, and controlled seed growth (pattern 2). Two other related bimetallic patterns, namely, a nanopattern of Au-Pt core-shell particles (pattern 1) and an interlaced pattern of Pt microsheet and Au nanoarray (pattern 3), were meanwhile generated for the first time. It was also easy to prepare a micro-nano pattern composed of just one metal such as Au (data not shown). As is known, photolithography is flexible to produce patterns with various shapes and microscale sizes; the sizes and intervals of initial nanoarrays could be adjusted via block copolymer composition and loading amount of metallic precursor;6,27,30,37 block copolymer micelle nanolithography have been applied to fabricate nanoarrays of metals including Au, Ag, Pt, Pd, etc.27 More than two metals could also be deposited layer-by-layer via multiple seed-growth steps,40 and even bimetallic nanoparticles could be of multishell.43 So the combinatorial approach suggested in the present paper is limited neither to fabricating Au-Pt bimetallic nanoarray nor to the demonstrated patterns. The methodology is ready to be extended to generate more hybrid micro-nano structures designed toward potential applications such as studies of catalysis, surface-enhanced Raman spectrum and other nanoparticle-related optics, and even cell-biomaterial interactions on the molecular and supermolecular levels. Acknowledgment. The authors are grateful for the financial support from Chinese Ministry of Science and Technology (973 program No. 2009CB930000), NSF of China (grants 50533010 and 20774020), Science and Technology Developing Foundation of Shanghai (grant 07JC14005), and Shanghai Education Committee (project B112). Help by Dr. Renchao CHE in TEM observations is also appreciated.

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