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Fabrication of Metallized Nanoporous Films from the Self-Assembly of a Block Copolymer and Homopolymer Mixture Xue Li,*,† Shuying Zhao,† Shuxiang Zhang,† Dong Ha Kim,‡,§ and Wolfgang Knoll§ School of Chemistry and Chemical Engineering, UniVersity of Jinan, 106 Jiwei Road, Jinan 250022, People’s Republic of China, DiVision of Nano Sciences and Department of Chemistry, Ewha Womans UniVersity, 11-1 Daehyun-Dong, Seodaemun-Gu, Seoul 120-750, Korea, and Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany ReceiVed January 30, 2007. In Final Form: April 24, 2007 Inorganic compound HAuCl4, which can form a complex with pyridine, is introduced into a poly(styrene-block2-vinylpyridine) (PS-b-P2VP) block copolymer/poly(methyl methacrylate) (PMMA) homopolymer mixture. The orientation of the cylindrical microdomains formed by the P2VP block, PMMA, and HAuCl4 normal to the substrate surface can be generated via cooperative self-assembly of the mixture. Selective removal of the homopolymer can lead to porous nanostructures containing metal components in P2VP domains, which have a novel photoluminescence property.
Introduction Films of block copolymers have been recognized as promising candidates for the preparation of various functional nanostructures, which may be used in surface patterning, lithography, and templating for the fabrication of organic-inorganic electronic, optical, and magnetic devices.1-7 To this end, well-aligned microdomains, especially those in an orientation normal to the substrate, are desirable.8,9 However, without further constraint, the perpendicular alignment of the domains is difficult to obtain because of the preferential interactions of one of the blocks with the substrate and air interfaces.10-12 In recent years, numerous research effort has been devoted to improving the alignment of cylindrical domains of block copolymers. A block copolymer film with oriented microdomains can be obtained by several approaches such as the “neutral surface” of the solid substrate,13 an external electric field,14 solvent-induced orientation,15,16 solvent prewetting,17 directional crystallization,18 the introduction of geometric substrate anisotropy,19 and a polymeric supramolecular * Corresponding author. E-mail:
[email protected]. † University of Jinan. ‡ Ewha Womans University. § Max Planck Institute for Polymer Research. (1) Hamley, I. W. The Physics of Block Copolymers; Oxford University Press: New York, 1998. (2) Mansky, P.; Chaikin, P.; Thomas, E. L. J. Mater. Sci. 1995, 30, 19871992. (3) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401-1404. (4) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126-2129. (5) Asakawa, K.; Hiraoka, T.; Hieda, H.; Sakurai, M.; Kamata, Y.; Naito, K. J. Photopolym. Sci. Technol. 2002, 15, 465-470. (6) Cheng, J. Y.; Ross, C. A.; Thomas, E. L.; Smith, H. I.; Lammertink, R. G. H.; Vancso, G. J. IEEE Trans. Magn. 2002, 38, 2541-2543. (7) Bal, M.; Ursache, A.; Tuominen, M. T.; Goldbach, J. T.; Russell, T. P. Appl. Phys. Lett. 2002, 81, 3479-3481. (8) Sidorenko, A.; Tokarev, I.; Minko, S.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 12211-12216. (9) (a) Fustin, C.-A.; Lohmeijer, B. G. G.; Duwez, A.-S.; Jonas, A. M.; Schubert, U. S.; Gohy, J.-F. AdV. Mater. 2005, 17, 1162-1165. (b) Li, C.-P.; Wei, K.-H.; Huang, J. Y. Angew. Chem., Int. Ed. 2006, 45, 1449-1453. (10) Sundrani, D.; Darling, S. B.; Sibener, S. J. Nano Lett. 2004, 4, 273-276. (11) Hahm, J.; Sibener, S. J. J. Chem. Phys. 2001, 114, 4730-4740. (12) Harrison, C.; Adamson, D. A.; Cheng, Z.; Sebastian, J. M.; Sethuraman, S.; Huse, D. A.; Register, R. A.; Chaikin, P. M. Science 2000, 290, 1558-1560. (13) Huang, E.; Russell, T. P.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Hawker, C. J.; Mays, J. Macromolecules 1998, 31, 7641-7650. (14) Morkved, T. L.; Lu, M.; Urbas, A. M.; Ehrichs, E. E.; Jaeger, H. M.; Mansky, P.; Russell, T. P. Science 1996, 273, 931-933.
assembly with a low molar mass additive.8,20,21 After the removal of one of the blocks in block copolymer films with cylindrical microdomains perpendicular to the substrate via UV exposure or plasma etching methods,3,22 ordered nanoporous films can be fabricated. It has been demonstrated that the addition of a homopolymer or low-molecular-weight additives to a block copolymer is another simple route to producing porous nanostructures with multiple length scales.8,23 The addition of the low-molecular-weight compounds may change the properties of one of the blocks or the interactions at interfaces, resulting in the improvement of the orientation of microdomains. Well-ordered nanostructured thin polymer films have been fabricated from the assembly of poly(styrene-block-4-vinylpyridine) (PS-b-P4VP) and 2-(4′-hydroxybenzeneazo) benzoic acid (HABA). The alignment of the cylindrical nanodomains formed by P4VP-HABA associates is insensitive to the composition of the confining surface. After the removal of HABA, a nanomembrane is obtained.8 Recent computer simulations have predicted that the synergistic interactions between self-organizing particles and a selfassembling matrix material can influence the phase behavior and ordering of the block copolymers.24 In experiments, Kramer’s group reported a nanoparticle-induced phase transition in a block copolymer thin film as well as the formation of different structures of gold nanoparticles.25 Russell’s group showed that the cooperative, coupled self-assembly of mixtures of diblock (15) (a) Fukunaga, K.; Elbs, H.; Magerle, R.; Krausch, G. Macromolecules 2000, 33, 947-953. (b) Elbs, H.; Drummer, C.; Abetz, V.; Krausch, G. Macromolecules 2002, 35, 5570-5577. (16) (a) Kim, G.; Libera, M. Macromolecules 1998, 31, 2569-2577. (b) Kim, G.; Libera, M. Macromolecules 1998, 31, 2670-2672. (17) Hahm, J.; Sibener, S. J. Langmuir 2000, 16, 4766-4769. (18) Park, C.; Rosa, C. D.; Thomas, E. L. Macromolecules 2001, 34, 26022606. (19) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. AdV. Mater. 2006, 18, 2505-2521. (20) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa, R.; tenBirke, G.; Ikkala, O. Science 1998, 280, 557-560. (21) Ma¨ki-Ontto, R.; de Moel, K.; de Odorico, W.; Ruokolainen, J.; Stamm, M.; ten Brinke, G.; Ikkala, O. AdV. Mater. 2001, 13, 117-121. (22) Turn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12, 787-791. (23) Jeong, U.; Kim, H.-C.; Rodriguez, R. L.; Tsai, I. Y.; Stafford, C. M.; Kim, J. K.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2002, 14, 274-276. (24) (a) Lee, J. Y.; Shou, Z.; Balazs, A. C. Phys. ReV. Lett. 2003, 91, 136103. (b) Lee, J. Y.; Shou, Z.; Balazs, A. C. Macromolecules 2003, 36, 7730-7739. (c) Schultz, A. J.; Hall, C. K.; Genzer, J. Macromolecules 2005, 38, 3007-3016.
10.1021/la7002589 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/26/2007
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copolymers and surface-active nanoparticles could orient the copolymer domains normal to the surface in thin films, even when one of the blocks was strongly attracted to the substrate.26a Very recently, the same group also showed that the complexation of alkali halide or metal salts with the poly(ethylene oxide) (PEO) block of a PS-b-PEO copolymer could greatly affect the ordering process of the copolymer films during solvent annealing, and the microdomains of the block copolymer were found to orient normal to the surface of the film with markedly enhanced lateral order.26b Wei’s group observed a morphological transformation of PSb-P4VP from a hexagonally packed cylindrical structure to a lamellar structure upon sequestering CdS nanoparticles in the P4VP block. The binding of CdS nanoparticles into the P4VP domains is caused by the formation of hydrogen bonds between them.27a It is also found that the dispersion of TiO2 nanoparticles can be controlled in one of the two blocks of lamellar polystyreneblock-poly(methyl methacrylate) (PS-b-PMMA) by using hydrophobic or hydrophilic surfactants. The location of TiO2 nanoparticles affects the block copolymer’s luminescence at different wavelengths.27b It is expected that nanoporous thin films of block copolymers may exhibit novel electronic and optical properties if a metal or semiconductor component is introduced.9 Nanoporous thin films containing Ru have been obtained using metallosupramolecular block copolymers.9a Here, we report a simple method to fabricate functional nanoporous thin films containing Au using mixtures of asymmetric PS-b-P2VP block copolymers and PMMA homopolymers. Usually, the cylindrical microdomains of P2VP parallel to the substrate surface are formed by the preferential interaction of P2VP with the substrate surface. Chloroauric acid, which can form a complex with pyridine units in this block copolymer, is introduced to adjust the interaction between P2VP and the substrate surface. The alignment of the cylindrical nanodomains formed by P2VP-HAuCl4/PMMA has been produced after the introduction of HAuCl4. Selective removal of the homopolymer can generate porous nanostructures containing Au in P2VP blocks, which have a novel optical property. The scheme to generate a nanoporous film containing tiny gold spots is illustrated in Figure 1. Experimental Section Materials. Polystyrene-block-poly(2-vinyl pyridine) (PS-b-P2VP) with a polydispersity index of 1.07 was purchased from Polymer Source, Inc. The number-average molecular weights of the PS and P2VP blocks were 54 900 and 18 600 g/mol, respectively. PMMA of Mw ) 23 600 was obtained from Polymer Source, Inc. Tetrachloroauric(III) acid (HAuCl4‚xH2O, Mw ) 333.79) was purchased from Fluka and used as received. Analytical-grade tetrahydrofuran (THF) was purchased from Laborbedarf GmbH. Substrates. Silicon (Si) and SiO2 wafers with a native oxide layer (ca. 2.5 cm × 2.5 cm) were cleaned in piranha solution (70/30 v/v concd H2SO4/30% H2O2) at 80 °C for 30 min, thoroughly rinsed with Mili-Q water, and then blown dry with nitrogen gas. Caution! Piranha solution reacts Violently with organic compounds and should not be stored in a closed container. The gold substrates were prepared by first evaporating a 2 nm chrome layer and then a 50 nm gold layer on silicon substrates. Film Preparation. Given amounts of PS-b-P2VP, PMMA (5 wt % PMMA, relative to the P2VP block), and HAuCl4‚xH2O were (25) Kim, B. J.; Chiu, J. J.; Yi, G.-R.; Pine, D. J.; Kramer, E. J. AdV. Mater. 2005, 17, 2618-2622. (26) (a) Lin, Y.; Bo¨ker, A.; He, J.; Sill, K.; Xiang, H.; Abetz, C.; Li, X.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55-59. (b) Kim, S. H.; Misner, M. J.; Yang, L.; Gang, O.; Ocko, B. M.; Russell, T. P. Macromolecules 2006, 39, 8473-8479. (27) (a) Yeh, S.-W.; Wei, K.-H.; Sun, Y.-S.; Jeng, U.-S.; Liang, K. S. Macromolecules 2005, 38, 6559-6565. (b) Weng, C.-C.; Wei, K.-H. Chem. Mater. 2003, 15, 2936-2941.
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Figure 1. Schematic illustration of the process used to fabricate the nanoporous PS-b-P2VP film containing Au. (A) Block copolymer, homopolymer, HAuCl4, and the solvent form one phase in solution. (B) During spin coating, the solvent evaporates, and microphase separation takes place. (C) The film is first irradiated with UV light, followed by treatment with acetic acid to remove PMMA. Finally, a nanoporous film containg Au forms. dissolved in THF and stirred for at least for 24 h to make a homogeneous solution. The molar ratio of HAuCl4/2VP was 0.5. Polymer films were produced simply by spin coating from the 1.0 wt % THF solution onto silicon (with native oxide on the surface), SiO2, and gold substrates at 2000 rpm. The film thickness measured by a surface profiler (Tencor P-10, KLA Tencor) was about 60 nm. Gold salts in the mixture films were reduced by UV exposure or were dipped into 1 wt % NaBH4 solutions for 10 s.28,29 To remove the PMMA, the film was exposed to deep UV irradiation and then washed with acetic acid.22 To obtain the AFM image of the bottom surface of the mixture film containing gold after treatment with UV irradiation and acetic acid, the film was floated onto the surface of a 2 wt % HF water solution, transferred to a water bath to remove HF, and then transferred to a Si substrate with the bottom of the film up. Characterization. SFM height and phase contrast images were obtained using a Digital Instruments Dimension 3100 scanning force microscope in tapping mode with an Olympus cantilever with a spring constant ranging from 33.2 to 65.7 N/m and a resonance frequency of 277.3-346.3 Hz (as specified by the manufacturer). X-ray photoelectron spectroscopy (XPS) was performed on a PerkinElmer-Physical Electronics 5100 with Mg KR excitation (400 W). Spectra were obtained at a takeoff angle of 15°. Photoluminescence (PL) spectra were measured using a Spex Fluorolog II (212) instrument at an excitation wavelength of 350 nm.
Results and Discussions It is well known that P2VP cylindrical microdomains orient parallel to the substrate surface in thin block copolymer films because of the preferential interaction of P2VP with the substrate and low surface energy of PS with the air interface.26a It has been reported that a mixture of a PS-b-PMMA with a small amount (28) Kim, J.-U.; Cha, S.-H.; Shin, K.; Jho, J. Y.; Lee, J.-C. AdV. Mater. 2004, 16, 459-464. (29) Sohn, B. H.; Seo, B. H. Chem. Mater. 2001, 13, 1752-1757.
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Figure 2. AFM height (A, C) and phase images (B, D) of films of PS-b-P2VP/PMMA mixtures: (A, B) as spin cast and (C, D) after UV irradiation and treatment with acetic acid.
of PMMA homopolymer can change the phase behavior of the system and enhance the orientation of the PMMA cylindrical microdomains normal to the surface.30,31 However, the enhancement effect of the orientation by the addition of a homopolymer is relatively weak for the PS-b-P2VP system because of the strong interaction between VP units and the OH groups at the SiOx substrate surface. Figure 2 shows the AFM height and phase contrast images of the PS-b-P2VP/PMMA blend films on silicon substrates before (Figure 2A,B) and after (Figure 2C,D) the removal of PMMA. Brighter dots and darker areas in the phase image of Figure 2B correspond to the cylindrical microdomains normal to the substrate and the PS matrix, respectively. After UV irradiation and acetic acid treatment (to cross-link PS-bP2VP and remove PMMA),22,32 a mixed morphology of perpendicular and parallel orientations was observed (Figure 2C,D). The results indicate that the addition of a small amount of PMMA produces only part of the cylindrical microphaseseparated domains of P2VP and PMMA normal to the substrate. Similar behaviors were observed for PS-b-P2VP/PMMA blend films on SiO2 and gold substrates. It is well known that at low homopolymer concentration or when the molecular weight of the homopolymer is below or comparable to that of one (30) Orso, K. A.; Green, P. F. Macromolecules 1999, 32, 1087-1092. (31) Jeong, U.; Ryu, D. Y.; Kho, D. H.; Kim, J. K.; Goldbach, J. T.; Kim, D. H.; Russell, T. P. AdV. Mater. 2004, 16, 533-536. (32) Doycheva, M.; Stamenov, R.; Tsvetanov, Ch. B.; Adler, H. P. J.; Kuckling, D. Macromol. Mater. Eng. 2001, 286, 151-155.
solubilizing block, the homopolymer is solubilized in the copolymer microdomains.1 For PS, PMMA, and PVP, the PMMA-PVP interaction parameter (χPMMA-PVP ) 0.007) is much lower than the values for PS-PVP (χPS-PVP ) 0.1) and PSPMMA (χPS-PMMA ) 0.02).33 The molecular weight of PMMA is larger than that of the P2VP block. In terms of the interaction parameters and the molecular weight of PMMA, it is reasonable to deduce that the added PMMA is located in P2VP microdomains and surrounded by P2VP. With the addition of a gold (Au) precursor, HAuCl4, to the PS-b-P2VP/PMMA mixtures, it is found that the perpendicular orientations of the microphase domains are markedly enhanced. Figure 3A-D shows the AFM height and phase contrast images of the PS-b-P2VP/PMMA/HAuCl4 films before and after removal of PMMA. The height variation and phase contrast are relatively small for the as-prepared film. However, arrays of pores with an average diameter of ∼38 nm on the film surface were observed (Figure 3C) after the removal of PMMA, illustrating that nanoscopic cylindrical domains are perpendicular to the film surface. It has been reported that deep UV irradiation of the mixture cross-links PS and P2VP, degrades PMMA, and reduces HAuCl4.22,28,32 Though it is not clear if UV irradiation can promote the microphase separation between PMMA and Au-containing P2VP domains, nanoporous block copolymer films containing a Au component in the P2VP blocks (Figure 3C), which are (33) Walheim, S.; Ramstein, M.; Steiner, U. Langmuir 1999, 15, 4828-4836.
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Figure 3. AFM height (A, C) and phase images (B, D) of films of PS-b-P2VP/PMMA/HAuCl4 mixtures: (A, B) as spin cast and (C, D) after UV irradiation and treatment with acetic acid. AFM height (E) and phase (F) images of the back side of the film of PS-b-P2VP/ PMMA/HAuCl4 mixtures after UV irradiation and treatment with acetic acid.
different from the porous films generated from neat PS-bPMMA,22,23 are obtained. To investigate further if the cylindrical microdomains are normal to the substrate surface, the bottom surface of the PSb-P2VP/PMMA/HAuCl4 mixture film on the SiO2 substrate after the removal of PMMA with UV irradition and acetic acid treatment was examined with AFM. The height and phase images shown in Figure 3E,F imply that the cylindrical microdomains containing P2VP, PMMA, and Au span the entire film thickness. The average center-to-center distance of the pores is measured
to be about 50 nm. As we know, the PVP block has a preferential interaction with the polar substrate, and the PS block prefers an air interface because of its lower surface energy. Thus, lamellae parallel to the substrate with an asymmetric wetting configuration have to be induced during the high-temperature annealing process.29 However, after thermal annealing in vacuum at 170 °C for 2 days, UV irradiation, and acetic acid treatment, a porous film is also observed (Figure 4), indicating that the orientation of the cylindrical domains normal to the substrate is induced by the introduction of HAuCl4. X-ray photoelectron spectroscopy
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Figure 5. High-resolution XPS Au 4f spectrum of the film of the PS-b-P2VP/PMMA/HAuCl4 mixture after the reduction of gold salts.
Figure 4. AFM height (A) and phase (B) images of the films of PS-b-P2VP/PMMA/HAuCl4 mixtures after thermal annealing at 170 °C for 2 days, UV irradiation, and acetic acid treatment.
(XPS) measurements were carried out on the mixture films after the reduction of Au salts. The characteristic Au0 peaks at ∼84.0 eV (Au 4f7/2) and ∼87.7 eV (Au 4f5/2) are clearly observed (Figure 5).34 From the results, one can conclude that the Au salts become metallic Au after reduction. As reported previously, the incorporation of HAuCl4 results in local protonation of the pyridine units followed by electrostatic interaction of the quaternary ammonium species with AuCl4anions (i.e., complexation between HAuCl4 and the pyridine units).35,36 The complexation of HAuCl4 to the pyridine units leads to the change in interaction between P2VP and the substrate interface and finally to a change in the structure of the film. The same results obtained from films on Au, Si, and SiO2 substrates illustrate that the alignment of the cylindrical nanodomains in the mixture film is insensitive to the surface energy of the substrates because of P2VP-HAuCl4 complexation, which is consistent with the results of the PS-b-P2VP/HABA system.8 To understand the optical properties of the nanoporous films containing Au, photoluminescence (PL) measurements were carried out. For comparison, PL spectra of pure Au nanoparticles fabricated from PS-b-P2VP/HAuCl4 micelles combined with (34) Jaramillo, T. F.; Baeck, S.-H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148-7149. (35) Sidorov, S. N.; Bronstein, L. M.; Kabachii, Y. A.; Valetsky, P. M.; Soo, P. L.; Maysinger, D.; Eisenberg, A. Langmuir 2004, 20, 3543-3550. (36) Spatz, J. P.; Mo¨sser, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P. Langmuir 2000, 16, 407-415.
Figure 6. (a, b) PL spectra of pure Au nanoparticles on Si substrates. (c, d) PL spectra of a porous PS-b-P2VP film containing Au on Si substrates. The excitation wavelengths are 260 nm (a, c) and 350 nm (b, d).
oxygen plasma etching36 are also shown in Figure 6. One can see that pure Au nanoparticles exhibit a broad band with the main peak centered at 330 nm at an excitation wavelength of 260 nm and another broad band with the main peak at 440 nm at an excitation wavelength of 350 nm. In comparison, a strong PL band consisting of two emission peaks (330 and 367 nm) is observed for the nanoporous film containing Au at an excitation wavelength of 260 nm. The peak at 330 nm is in agreement with that obtained for pure Au nanoparticles, and the peak at 367 nm is a new one and is observed for the first time from this type of Au nanostructure with extraordinary morphology. It has been reported that small Au nanoparticles (∼5 nm) show PL at 440 nm with an excitation wavelength at 230 nm, which is assigned to the radiative recombination of Fermi-level electrons and sp- or d-band holes. When the size of bare Au nanoparticles is larger than ∼5 nm, PL has not been observed.37 Surface-passivated Au nanoclusters (
