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Spatially Correlated Metallic Nanostructures on Self-Assembled Diblock Copolymer Templates Amir W. Fahmi*,† and Manfred Stamm‡ Department of Physics, University of Nottingham, Nottingham NG7 2RD, United Kingdom, and Institut fu¨ r Polymerforschung Dresden e.V. (IPF), Hohe Strasse 6, 01069 Dresden, Germany Received August 10, 2004. In Final Form: November 5, 2004 Polymeric complexes based on diblock copolymers (polystyrene-block-4-vinylpyridine) hydrogen bonded with pentadecylphenol self-assemble under oscillatory shear flow into a highly ordered lamellar structure (Ikkala et al. Science 2002). Microtomed films of the lamellar structure form an array of “nanosheets” following immersion in methanol. We have exploited this nanosheet array as an extremely effective template to direct the spatial organisation of metallic (Pd) nanoclusters. The electroless deposition metal on the nanotemplates leads to morphologically complex nanostructured metallic films which were observed using atomic force microscopy and scanning electron microscopy.
Introduction Multiscale ordering of materials is a fundamental prerequisite for the application of molecular systems1,2 in device technology.3,4 Typically, the highest degree of order is associated with self-assembled structures corresponding to equilibrium states of the system. These equilibrium structures can be found in colloidal crystals,5 nanocrystal superlattices,6 metal wire formation on stepped crystal surfaces,7 block copolymer8 systems,9 and many others. The formation of spatially ordered arrangements of metallic nanocrystals10-12 is of particular importance for many (bio)chemical, optical, magnetic, and electronic applications. One approach to controlling the organization of nanoclusters is to exploit the pattern-forming properties of diblock copolymers.13 Block copolymers14 form various highly ordered morphologies with nanometer scale correlation lengths arising from microphase separation. The lamellar structure is the most widely studied of block copolymer morphologies. In thin films, in addition to composition and molecular * Corresponding author. E-mail: Amir.Fahmi@ Nottingham.ac.UK. † University of Nottingham. ‡ Institut fu ¨ r Polymerforschung Dresden e.V. (IPF). (1) Anelli, P. L.; Stoddart, J. F. J. Am. Chem. Soc. 1991, 113, 5131. (2) Otsuki, J. J. Am. Chem. Soc. 1997, 119, 7895. (3) Alivisatos, A. P. Adv. Mater. 1998, 10, 1297. (4) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G. M. Science 1999, 284, 948. (5) Aastuen, D. J. W.; Clark, N. A.; Cotter, L. K.; Ackerson, B. J. Phys. Rev. Lett. 1986, 57, 1733. (6) Coller, C. P.; Sakally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978. (7) Viernow, J.; Petrovykh, D. Y.; Men, F. K.; Lin, J. L.; Himpsel, F. J. Appl. Phys. Lett. 1999, 74, 2125. (8) Sundrani, D.; Darling, S. B.; Sibener, S. J. Nano Lett. 2004, 4, 273. (9) Bates, F. S.; Fredrickson, G. H. Annu. Rev. Phys. Today 1999, 52, 32. (10) Yu, K.; Wu, X.; Brinker, C. J.; Ripmeester, J. Langmuir 2003, 19, 7282. (11) Fahmi, A. W.; Braun, H. G.; Stamm, M. Adv. Mater. 2003, 15, 1201. (12) Templin, M.; Franck, A.; Chesne, A.; Leist, H.; Zhang, Y.; Ulrich, R.; Schaedler, V.; Wiesner, U. Science 1997, 278, 1795. (13) Rajaram, A. P.; Raashina, H.; Schulberg, M. T.; Sengupta, A.; Sun, J.; Watkins, J. J. Science 2004, 303, 507. (14) Hamley, I. W., Ed., The Physics of Block Copolymers; Oxford University Press: Oxford, New York, Tokyo, 1998.
weight, the domain structure is also dependent on the surface energies of the blocks and on geometrical constraints introduced by confinement in a thin film. Therefore, the lamellar structure has a strong tendency to orientate parallel to the film surface, leading to unstructured film surfaces. The reason for this is assigned to the strong interaction of one of the blocks with the substrate. Several attempts have been performed to overcome these shortcomings, e.g., by applying electric fields15 or by choosing nonselective walls16 or prepatterned surfaces.17 As all these approaches require additional effort, a spontaneous formation of lateral structures would be desirable. It is now well established that long-range order of alternating layers can be induced by the application of large amplitude oscillatory shear (LAOS).18-20 Early work on orientation in lamellar block copolymers was undertaken by Keller and co-workers21 using extrusion techniques which are relevant to processing. However, shear fields are a more convenient method of producing highly orientated bulk structures; thus they have been employed in this research. To prepare a lamellar structure in thin film and obviate the strong tendency of the lamellar structure to orientate parallel to the film surface, we sliced ultrathin films from the bulk sample after using LAOS. Then the ultrathin films with 50 nm thickness and >250 µm2 were supported on a Si-wafer substrate. As a result, highly orientated thin films were formed at the desired location on the substrate surface. In this work, a technique will be described that employs diblock copolymers as a template for direct self-assembly of metallic nanoparticles. With this technique we can produce metal patterns that are an order of magnitude smaller than those that can be achieved by conventional (15) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J. Adv. Mater. 2000, 12, 787. (16) Kellogg, G. J.; Walton, D. G.; Mayes, A. M.; Lambooy, P.; Russell, T. P.; Gallagher, P. D.; Satija, S. K. Phys. Rev. Lett. 1996, 76, 2503. (17) Rockford, L.; Liu, Y.; Mansky, P.; Russell, T. P.; Yoon, M.; Mochrie, S. G. J. Phys. Rev. Lett. 1999, 82, 2602. (18) Polis, D. I.; Winey, K. I. Macromolecules 1996, 29, 8180. (19) Laurer, J. H.; Pinheiro, B. S.; Polis, D. L.; Winey, K. I. Macromolecules 1999, 32, 4999. (20) Zhang, Y.; Wiesner, U. J. Chem. Phys. 1995, 103, 4784. (21) Folkes, M. J.; Keller, A. J. Polym. Sci., Part B: Polym. Phys. 1976, 14, 833.
10.1021/la0479962 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/24/2004
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optical lithography.22 Furthermore, this method provides a route to polymer-based nanostructure formation that avoids the standard staining procedures by harmful heavy metal compounds such as OsO4 or RuO4. There are two primary methods by which a polymernanoparticle hybrid can be created. The first is the preparation of metallic nanoparticles by coordinating and stabilizing the particle by small molecules, which have side groups chemically the same in one of the side block chains of the microphase-separated domains.23,24 The second method, as noted above, involves incorporation of the nanoparticles into one of the diblock copolymer microphases. In situ preparation of nanoparticles is achieved by reduction of metal ions in the microdomain space, which are selectively incorporated into one of the microphases of the block copolymer.25-27 Here, we focus on nanohybrids prepared by the latter method. The driving force for self-assembly is a selective interaction between the metal ions and one of the polymeric components. During recent years Ikkala and Ten Brinke have introduced a new concept to prepare functional polymeric materials based on self-assembly of comb-shaped supramolecules.28-30 They are obtained by attaching “short” chain molecules via physical interactions to one of the blocks of a diblock copolymer to construct hierarchically structured materials.31,32 In this work, a method will be described that utilize functional polymeric materials as a template for direct self-assembly of metallic nanoparticles. The limits of resolution are dictated by the size of the metallic particles and the width of the copolymer interface. We used a polymeric complex based on diblock copolymers, namely, polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) hydrogen-bonded stiochiometrically with amphiphilic 3-npentadecylphenol (PDP).24-28 The bulk polymeric complex self-assembled to form macroscopically orientated structures under LAOS, with well-defined size and periodicity. Microtomed slices of the polymeric complex were supported on a Si wafer. Arrays of “nanosheets” following immersions in methanol were formed. An electroless method was used to metallize the nanosheets. Experimental Section Materials and Methods. Polystyrene-block-4-vinylpyridine (PS-b-P4VP) was purchased from Polymer Source, Inc., with a polydispersity Mw/Mm ) 1.07 and molecular weights of 92000 and 29000 for PS and P4VP, respectively. The amphiphile pentadecylphenol (PDP) was purchased from Aldrich (purity 9095%). PDP was distilled under vacuum conditions (10-3 bar at 185 °C) and twice recrystallized with cyclohexane to obtain a high-purity sample. The polymeric complex was prepared by dissolving both components (PS-b-P4VP and PDP) in analytical grade chloroform (22) Schmid, H.; Biebuyck, H.; Michel, B.; Martin, O. J. F. Appl. Phys. Lett. 1998, 72, 2379. (23) Fogg, D. E.; Radzilowski, L. H.; Blanski, R.; Schrock, R. R.; Thomas, E. L. Macromolecules 1997, 30, 417. (24) Tsutsumi, K.; Funaki, Y.; Hirokawa, Y.; Hashimoto, T. Langnuir 1999, 15, 5200. (25) Nakao; Y. J. J. Colloid Interface Sci. 1995, 171, 386. (26) Zehner, R. W.; Lopes, W. A.; Morkoved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 241. (27) Lopes, W. A.; Jaeger, H. Nature 2001, 414, 735. (28) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa, R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557. (29) Ikkala, O.; Ten Brinke, G. Science 2002, 295, 2407. (30) Ruokolainen, J.; Saariaho, M.; Ikkala, O.; ten Brinke, G.; Thomas, E. L.; Torkkeli, M.; Serimaa, R. Macromolecules 1999, 32, 1152. (31) Ma¨kinen, R.; Ruokolainen, J.; Ikkala, O.; de Moel, K.; ten Brinke, G.; De Odorico, W.; Stamm, M. Macromolecules 2000, 33, 3441. (32) Ruotsalainen, T.; Torkkeli, M.; Serimaa, Ma¨kela¨, T.; Maki-Ontto, R.; Ruokolainen, J.; ten Brinke, G.; Ikkala, O. Macromolecules 2003, 36, 9437.
Langmuir, Vol. 21, No. 3, 2005 1063 at 60 °C. The solvent was then slowly removed, and the sample was vacuum-dried at 65 °C for 24 h. The samples were prepared by placing the polymeric complex in a 7.9 mm diameter, 1 mm deep mold inserted between two PTFE plates. The polymer was then heated to 125 °C under vacuum for 1 h to ensure a homogeneous melt form between the PTFE plates. The temperature was then reduced to 115 °C, and the sample was pressed to form a tablet at 5 kN. The sample was rapidly cooled to 30 °C and then removed from the press. To transform the macroscopically unorientated (isotropic) microphase separation in the block copolymer sample to macroscopically aligned microdomains, we used an oscillatory shear flow technique.33-35 A strong shear field applied to macroscopically disordered bulk structures leads to the rearrangement of the individual domains in a way that ultimately results in macroscopic ordering.36 By use of this technique,37,38 orientated domains of lamellar structure can be formed in sizes of up to several millimeters, in contrast to electric39,40 field techniques which are used in thin films. Oscillatory shear flow was applied (at 0.5 Hz, 50% strain amplitude and at T ) 115 °C for 12 h) using an ARES Rheometric instrument with a plate-plate geometry (1 mm gap size) and a transducer having an operating range of 0.2-200 g cm. The probe and orientation conditions were carried out as a time sweep experiment; i.e., the strain amplitude, frequency, and temperature were held constant. Following large amplitude oscillation shear flow, the sample was annealed in the rheometer for 1 h at 115 °C to allow the rheological response to reach steady state. Then the sample was quenched rapidly to room temperature done to “freeze in” the equilibrium structure of the polymeric complex. The sample was then embedded in epoxy resin. Ultrathin specimens were obtained using a Reichert Ultracut E ultramicrotome and a diamond knife at -50 °C. The ∼50 nm sections were picked up on a polished silicon wafer. Subsequently, the films on the Si wafer were immersed in methanol for 20 min to extract the PDP. Following the sample preparation detailed above, a Digital Instruments D3100/Nanoscope III atomic force microscope (AFM) system was used to acquire tapping mode AFM images with a silicon cantilever (f0 ) 250-400 kHz) and a scan speed of 6 µm s-1. To provide more detailed structural information, small-angle X-ray scattering (SAXS) measurements were acquired on the bulk sample using a SAXS instrument consisting of a Rigaku Rotaflex 18 kW rotating anode X-ray source with an osmic multilayer optic monochromator operating at a wavelength of λ ) 1.54 Å (Cu KR). Three pinholes collimate the beam diameter from 1.5 mm to less than 1 mm. a Siemens multiwire type area detector was used. The sample-detector distance was 1.35 m.
Results and Discussion It is clear from a consideration of the complementary SAXS and AFM data (Figures 1 and 2, respectively) that the block copolymer complex PS-b-P4VP(PDP) has formed highly orientated lamellar structures. The SAXS measurement in Figure 1a demonstrates macroscopically orientation of structure-within-structure morphology characterized by two different length scales. The diblock copolymers have formed the large scale of the lamellar structure, where the comb copolymer-like su(33) Winey, K. I.; Patel, S. S.; Larson, R. G.; Watanabe, H. Macromolecules 1993, 26, 4373. (34) Kannan, R. M.; Kornfield, J. A. Macromolecules 1994, 27, 1177. (35) Gupta, V. K.; Krishnamoori, R.; Kornfield, J. A.; Smith, S. D. Macromolecules 1996, 29, 1359. (36) Zhong-Ren, C.; Kornfield, J. A.; Smith, S. D.; Grothaus, J. T.; Sattkowski, M. M. Science 1997, 277, 1248. (37) Pinheiro, B. S.; Winey, K. I. Macromolecules 1998, 31, 4447. (38) Castelletto, V.; Hamley, I. W.; Holmqvist, P.; Rekatas, C.; Booth, C.; Grossmann, J. G. Colloid Polym. Sci. 2001, 279, 621. (39) Huang, E.; Rockford, L.; Russell, T. P.; Hawker, C. J.; Mays, J. Nature 1998, 395, 757. (40) 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.
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Figure 1. (a) 2D SAXS pattern in tangential direction demonstrates two perpendicular structures with different dimensions. (b) Profile intensity (I) as a function in scattering vector (q), obtained by integration in a sector of a twodimensional SAXS pattern qnor,qrad.
pramolecules form the smaller scale. This is orientated perpendicular to the large lamellar morphology. Figure 1b shows scattering intensity versus scattering vector obtained by radial integration of the two-dimen-
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sional (2D) SAXS patterns. We observe a series of peaks in the position ratio 1:2:3:4:5: ... This sequence of peaks is a reflection of a lamellar structure. The alternating blocks of PS and P4VP(PDP) are stacked with a long period of 60 nm (large structure), whereas in the P4VP(PDP) matrix containing layers, alternating orientated layers of polar and nonpolar material with a long period of 4 nm are present (small structures). Figure 2 shows an AFM height image of the bulk sample after microtoming the surface of the polymeric complex P(S-b-4VP)(PDP). The image shows highly orientated lamellar structure with a period of 60 nm. The results of SAXS and AFM (Figure 2) agree well for the large structure. Additionally, the extra resolution of SAXS provides evidence that the small structures were present (Figure 1a) perpendicular to the large structure. Microtomed ultrathin films were supported on a Si wafer to produce the nanosheets. The advantage of this method which produces a film 50 nm thick is that it avoids the parallel orientation of the lamellar structure to the film surface through the decreasing of the interaction with the substrate surface, a problem that occurs, for example, in spin coating. It also has a greater control over the lamellar structure direction on the surface and the occupied area on the substrate surface. Transformation of the film morphology from a lamellar structure to an array of nanosheets takes place during methanol treatment. Figure 3 shows the topography of the nanosheets following immersion in methanol for 20 min. The sheets arrange parallel to each other with width about 30 nm located at spacing between 60 and 70 nm, and an average height of 14 nm. It appears that the PSb-P4VP(PDP) morphology is changed significantly upon methanol treatment into PS-b-P4VP domains orientated in sheets structures. This change arises because every repeated PS sheet in the PS92k-b-P4VP29k nanosheet array is directly adsorbed upon treatment with organic solvent onto the solid substrate. Consequently, P4VP chains are situated on the surface of PS blocks. As a result, the exposed surfaces of P4VP domains are extended on the substrate surface (PS domains 92 kg/ mol) as indicated by the scheme outlined in Figure 4. A direct swelling of the P4VP blocks (which are set above the PS blocks) produces P4VP (29 kg/mol) domains confined in thin shells on the glassy PS matrix. An
Figure 2. (a) Height image of AFM measurement of the polymeric complex PS-b-P4VP(PDP) surface in bulk after microtoming some nanosheets from the bulk sample. (b) Cross section of the height image.
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Figure 3. AFM phase Image of PS-b-P4VP nanosheets on Si wafer substrate after extracting the PDP amphiphile.
Figure 4. Illustration of the decoration steps of nanoclusters above the nanosheets on the Si wafer.
important point is that before the decoration of nanodomains with the metal nanoclusters (see discussion below), scanning electron microscopy (SEM) was not capable of imaging the morphology of the PS-b-P4VP nanotemplates due to the negligible difference in the electron density between PS and P4VP blocks. The primary focus of our work, however, is the in situ synthesis of palladium nanoclusters in the microdomain space of the diblock copolymer derived system described above. The method we have chosen involves reduction of palladium metal ions Pd(II) on P4VP blocks of the PSb-P4VP diblock copolymer. After reduction, the Pd atoms aggregate into nanoclusters which are selectively incorporated in microdomains comprised of P4VP block chains. The ratio between metal ions and monomers at the surface of the nanotemplate plays a fundamental role due to the large surface energy of the metal exceeding that of the copolymer by orders magnitude. Consequently, metalmetal bonds will overwhelm metal-polymer bonds, but for a very small metal concentration the metal-polymer bonds will form first until a certain concentration of the
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Figure 5. SEM micrograph shows Pd nanopartticles metal onto the block copolymer PS-b-P4VP film after the metallization process.
ions on the polymeric surface is reached, then the metallic nanoparticles start bonding together to form metallic aggregates. The metallization process involved using palladium acetate Pd(OAc)2, dimethylamine borane (DMB), 3-sodium citrate, and lactose (Aldrich). An aqueous solution of Pd(OAc)2 was prepared with 5 × 10-3 N in acidic medium (HCl, pH ) 3). Pd(OAc)2 solution was added dropwise to the polymer films on Si wafers for 2 min. The wafer was then rinsed several times with water. Finally, Pd2+ ions were reduced by treating them with mixed solution based on 0.4 M aqueous solution of DMB, sodium citrate 0.8, and 0.8 M lactose. Sodium citrate and lactose were controlling the size formation of Pd nanoclusters into the nanostructured material. The formation of the nanoclusters in the polymer phase was performed in two stages (see Figure 4): (i) Coordination of metal ions in nanotemplates: Transition metals exhibit a high ability to coordinate with P4VP blocks due to the nitrogen lone pair of the pyridine ring. In this work homogeneous aqueous solutions of palladium acetate were prepared with pH ) 2-3. The nanotemplates were then coated with the Pd-acetate solutions to allow Pd cations associate with P4VP chains. (ii) Reduction of the metal ions: A reduction of Pd(II) ions in one of the nanodomains was achieved to obtain a film specimen in which the metallic nanoparticles are selectively included in one of the nanodomains. The size distribution of the nanoparticles within the nanodomains is dependent on two factors: first, the reduction solution in which dimethylboron (DMB) is active was mixed with sodium citrate to control the size of the Pd nanoparticles in a range of 2-4 nm; second, the diffusion of the metal atoms into the polymer medium, by the irreversible interaction of the forming metal cluster in the P4VP chains. Following the decoration of the polymer templates with nanoparticles, scanning electron micrographs and atomic force microscope (AFM) images were taken. SEM was performed with a ZEISS DSM 982 GEMINI at low electron beam energy (1 keV) in order to avoid surface charging. Under these conditions, no additional coating of sample was necessary. The aggregation of Pd nanoclusters within the P4VP domains dramatically
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Figure 6. AFM phase image scanning along the line demonstrates the diffusion of Pd nanoclusters into the P4VP phase. The clusters appear brighter in the polymer phase and in the 3D-FFT of the image.
increases the SEM contrast compared to the nanoparticlefree templates (see Figure 5). AFM images show definite aggregation of the Pd nanoclusters on the polymer nanosheets. As shown in the AFM phase image in Figure 6, individual nanoclusters are arranged in almost spherical aggregations that are connected together by lines of clusters from one particle agglomeration to the next particle agglomeration. A clearer depiction shows the Pd particles in phase segregated nanodomains ranging in size from 120 to 140 nm in width, with heights of approximately 30 nm. Additionally, there are threedimensional fast Fourier transforms of the larger lamellar image with scale bar of 3 µm. Conclusions We have described an electroless route to achieve nanoscale alignment of metal nanoclusters (with a narrow size distribution) within ultrathin block copolymer films. In situ Pd(II) ions were first reduced to a metallic state and then into well-organized nanosheet morphology. The architecture of this system conformed in bulk to an expected lamellar structure of PS and P4VP(PDP). Under oscillatory shear flow in the nonlinear regime the mi-
crophase domains were macroscopically orientated. The conversion of the ultrathin film lamellar structure to nanosheet morphology took place after immersing the films in methanol. The preferential incorporation of the Pd nanocluster into the P4VP domains has swelling effects (height, width) on the nanosheets. This facile methodology provides control over surface morphology and deposition rate by careful modulation of parameters such as metal ion concentration, immersion time, and type of reduction agent. This novel simple concept is capable of achieving the difficult task of connection between the organic nanophase (polymer materials) and the inorganic phase (transition elements) to produce conducting metallic nanosheet arrays with different thickness by repeating the deposition process. These present thin film arrays are believed to be the first and are promising candidates as functional material needed for nanoelectronics devices and many other technological applications. Acknowledgment. We acknowledge the assistance of Dr. H. G. Braun in producing the SEM image and Dr. P. Moriarty for his advice. LA0479962
