Three-Dimensional Nanoscale Alignment of Metal Nanoparticles

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Three-Dimensional Nanoscale Alignment of Metal Nanoparticles Using Block Copolymer Films as Nanoreactors Shin Horiuchi* and Takashi Fujita Research Center of Macromolecular Technology (Macrotech), National Institute of Advanced Industrial Science and Technology (AIST), Tokyo Waterfront, 2-41-6, Aomi, Kohtoh-ku, 135-0064 Tokyo, Japan

Teruaki Hayakawa Research Center of Macromolecular Technology (Macrotech), National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, 305-8565 Ibaraki, Japan

Yukimichi Nakao Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan Received August 27, 2002. In Final Form: December 27, 2002 We present a simple dry process to create nanoscale arrangements of metal nanoparticles within block copolymer films. This process involves only one step such that the vapor of palladium(II) acetylacetonato (Pd(acac)2) is exposed to a polymer film in a nitrogen atmosphere at 180 °C for periods up to 2 h. The Pd(acac)2 vapor penetrates into the film and then is decomposed and reduced to a metallic state to form nanoparticles therein. When block copolymer films having nanoscale microdomain structures are used, the metal complex is selectively reduced in one of the phases to give nanoscale arrangements of the metal particles that reflect the microdomain structure of the used block copolymer. In symmetric diblock copolymers of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA), Pd(acac)2 is selectively reduced in the PS lamellae to yield Pd nanoparticles with the average diameter ranging from 3 to 4 nm with a narrow size distribution, depending on the molecular weight. As a result, the Pd nanoparticles are arranged in a periodic lamellar manner within the film. In an asymmetric diblock copolymer having an alcohol group in the side chain of one of the blocks, poly(styrene)-block-poly(hydroxylated polyisoprene) (PS-b-PIOH), where the PIOH block forms spherical domains in the PS matrix, the Pd particles are selectively assembled in the PIOH domains, not in the PS phase. This approach would be of general applicability to any block copolymers, and the particles should be assembled in the phase having relatively stronger reducing power. The vapor of the metal complex can penetrate into the film deeply, and thus the arrangements of the particles are in a three-dimensional array within a film. Co(acac)2 is also evaluated and gives Co nanoparticles within the PIOH domains in the PS-b-PIOH film as does as Pd(acac)2. Electron spectroscopic imaging and electron energy loss spectroscopy on an energy-filtering transmission electron microscope were performed for nanoscale chemical analysis to investigate the reaction products of Co(acac)2 within the polymer films in detail.

Introduction Micropatterning technology is of great importance for manufacturing electronic, optical, and mechanical devices in a wide range of applications. However, the increasing demands for next generation devices with smaller, faster, and denser systems require the development of nanopatterning technology. Although efforts have been made to decrease the patterning size by the standard semiconductor lithography technology, fabricating nanostructured materials by “bottom-up” methods using self-assembling systems particularly has been recognized as an elegant and powerful approach to establish novel nanolithography technologies. The use of block copolymer systems having well-organized multiphase structures with nanoscale spatial regulation has been recognized as one of such approaches to create nanopatterned materials.1 Most of the studies on block copolymer systems, to date, have been * To whom correspondence should be addressed. Tel: +81-33599-8309. Fax: +81-298-3599-8166. E-mail: [email protected].

concentrated on purely organic polymers in terms of the phase behavior and its dynamics, being well-understood. Now research is at a stage of utilizing and designing those systems to create nanostructured materials to bring them close to their potential applications. One approach combines self-assembling block copolymer systems with the electric, magnetic, or photonic properties of inorganic components. In particular, mixtures of nanoparticles and block copolymers can yield highly ordered composites for next generation catalysts, selective membranes, and photonic band gap materials.2 The structure and properties of metal nanoparticles, whose properties are characteristic of their sizes and shapes, have been extensively studied over the past decades.3 Research regarding the syntheses and the stabilization of various kinds of metal particles has been reported, and also efforts (1) (a) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401-1404. (b) Spatz, J. P.; Eibeck, P.; Mo¨ssmer, S.; Mo¨ller, M.; Herzog, T.; Ziemann, P. Adv. Mater. 1998, 10, 849-852.

10.1021/la020745d CCC: $25.00 © 2003 American Chemical Society Published on Web 02/28/2003

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Horiuchi et al. Scheme 1. The Synthesis of PS-b-PIOH

have been devoted to the incorporation of metal particles in an organized fashion within suitable matrixes. For this purpose, the self-assembly of nanodomain structures of block copolymers has been frequently utilized as templates or scaffolds. Many proposals to realize the nanoscale arrangements of metal nanoparticles within polymeric matrixes have been reported. The synthetic approach to serve organometallic block copolymers having metal complexes attached selectively to one block and to thermally decompose the metal complexes in solvent-cast films yields metal particles in the nanodomains of the block copolymers.4 To avoid such complex synthesis procedures, various ways using amphiphilic block copolymers for the selective loading of metal complexes in one of the domains in colloidal suspensions have been reported.5 Subsequent thermal treatment reduces metal ions in the nanodomain space, and then metal particles are arranged in nanoscale along the nanodomain patterns after the evaporation of the solvent. Recently, Lopes et al. reported that thermally evaporated metals were deposited onto a block copolymer thin film and guided to the preferred domains of the block copolymer due to the different wetting properties, and then the metal particles aggregated selectively into the domains.6 We have reported in a short communication a simple and easy dry process to offer a nanoscale arrangement of metal nanoparticles within block copolymer films under mild conditions.7 The evaporation of Pd(II) acetylacetonato (Pd(acac)2) onto free-standing diblock copolymer films in a nitrogen atmosphere at 180 °C for a certain length of time causes a reduction of Pd(acac)2 to produce Pd nanoparticles inside of the polymer films. As a result of this treatment, Pd nanoparticles selectively grow and are self-assembled in the phase which has the stronger reduction power against Pd(acac)2, aligning themselves (2) (a) Bronstein, L.; Antonietti, M.; Valetsky, P. In Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, 1998; Chapter 7. (b) Templin, M.; Franck, A.; Chesne, A. D.; Leist, H.; Zang, Y.; Ulrich, R.; Schadler, V.; Wiesner, U. Science 1997, 278, 17951798. (c) Zao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stuky, G. D. Science 1998, 279, 548-552. (d) Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.; Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. Adv. Mater. 2001, 13, 412-425. (e) Thompson, R. B.; Ginzburg, V. V.; Matsen, M. W.; Balazs, A. C. Science 2001, 292, 2469-2472. (3) Nanoparticles and Nanostructured Films; Fendler, J. H., Ed.; Wiley-VCH: Weinheim, 1998. (4) (a) Yue, J.; Sankaran, V.; Cohen, R. E.; Schrock, R. R. J. Am. Chem. Soc. 1993, 115, 4409-4410. (b) Sankaran, V.; Yue, J.; Cohen, R. E.; Schrock, R. R.; Silbey, R. J. Chem. Mater. 1993, 5, 1133-1142. (c) Chan, Y. N. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 24-27. (d) Chan, Y. N. C.; Craig, G. S. W.; Schrock, R. R.; Cohen, R. E. Chem. Mater. 1992, 4, 885-894. (5) (a) Saito, R.; Okamura, S.; Ishizu, K. Polymer 1993, 34, 11891188. (b) Tsutsumi, K.; Funaki, Y.; Hirokawa, Y.; Hashimoto, T. Langmuir 1999, 15, 5200-5203. (c) Spatz, J. P.; Mossmer, S.; Hartmann, C.; Moller, M.; Herzog, T.; Krieger, M.; Boyen, H. G.; Ziemann, P. Langmuir 2000, 16, 407-415. (6) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735-738. (7) Horiuchi, S.; Sarwar, M. I.; Nakao, Y. Adv. Mater. 2000, 12, 15071511.

along the nanodomain structures of the block copolymers. In contrast to the method by Lopes et al., the vapor of the complex diffuses deeply inside of the film in our case, and the chemical reaction is involved together with the physical diffusion process. This indicates that the block copolymer films work as nanoreactors to create the three-dimensional nanoscale arrangements of nanoparticles with tunable periodic lengths. In our previous paper, a symmetrical poly(styrene)-block-poly(methyl methacrylate) (PS-bPMMA) diblock copolymer was used, and Pd particles with the average diameter of 3.5 nm aligned themselves along the lamellar nanodomains of the PS block. From that study, we recognized that polymers have a self-catalytic effect to reduce the metal complex. Within our experiments, all of the evaluated polymers had the capability to reduce the metal complex, but the ability of reducing the complex seems to be varied among the polymers. We expect that this method is of general applicability to any type of block copolymer and also to other metal complexes that have different metal ions. This paper describes our continuing work to investigate the structures within the polymer films created by our method and also to introduce our method to other types of block copolymers and to metal complexes. For investigating the state of the dispersion and the nature of the produced particles, we carried out the local analysis in a nanometer scale by energy-filtering transmission electron microscopy (EFTEM). Experimental Section Materials. Pd(acac)2 and Co(acac)2, where acac denotes CH3COCHCOCH3, were purchased from Johnson Matthey Materials Technology and were recrystallized from acetone before use. Atactic polystyrene (PS) and poly(methyl methacrylate) (PMMA) homopolymers were purchased from Aldrich Chemical Co., and their number-average molecular weights (Mn’s) are 200 000 and 15 000, respectively. Reprecipitation was done twice from a methylene chloride solution to methanol for purification. Three symmetric diblock copolymers of PS-b-PMMA with different molecular weights were used as received from Polymer Source Inc. An asymmetric diblock copolymer having a hydroxyl group in one of the blocks was synthesized as illustrated in Scheme 1.8 A poly(styrene-block-isoprene) (PS-b-PI) was prepared by classical anionic polymerization using sec-BuLi as an initiator in tetrahydrofuran (THF) at -78 °C. Gel permeation chromatography gave an Mn of 171 000 and a polydispersity (Mw/Mn) of 1.08 for this block copolymer. The composition of 1,2-addition and 3,4-addition in the PI block was determined at 40/60 by 1H NMR spectroscopy in CDCl3. The molecular weight of the PI block was calculated also from the 1H NMR spectrum. The modification of the PI blocks to yield a poly(styrene-blockhydroxylated isoprene) (PS-b-PIOH) was carried out by the following procedure. In a 100 mL flask equipped with a stopcock was placed dried PS-b-PI (130K/39K) (1.0 g, 3.38 mmol of vinyl and methyl vinyl groups in the PI block). Distilled THF (30 mL) was transferred to the flask via cannula, and the solution was (8) Mao, G.; Wang, J.; Clingman, S. R.; Ober, C. K.; Chen, J. T.; Thomas, E. L. Macromolecules 1997, 30, 2556-2567.

Nanoscale Alignment of Metal Nanoparticles

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Table 1. Characterization of the Block Copolymers Used in This Study number average molecular weight (Mn) × 10-4 code

polymer (A-b-B)

A block

B block

polymer 1 polymer 2 polymer 3 polymer 4

PS-b-PMMA PS-b-PMMA PS-b-PMMA PS-b-PIOH

2.5 4.7 7.3 12.5

2.6 4.0 7.0 4.0

microdomain interdomain morphology spacing (nm) lamellae lamellae lamellae spherical

21 41 49 52

cooled to -15 °C. Then 9-borabicyclo[3.3.1] nonane (9-BBN, 0.5 M THF solution, 8.1 mL, 4.05 mmol) was transferred into the solution at this temperature, and the solution was stirred at room temperature for 24 h. The solution was cooled to -25 °C, and anhydrous methanol (1 mL) was injected into the solution to remove unreacted 9-BBN. The solution was stirred at this temperature for 30 min, and then NaOH aq (6 N, 0.8 mL, 4.8 mmol) was added to the solution. After 30 min, a 30% aqueous solution of hydrogen peroxide (1.5 mL) was slowly added to the solution. After stirring for 2 h, the solution was slowly heated to 50 °C and stirred for 2 h. Upon cooling to room temperature, the solution clearly phase-separated with an aqueous layer on the bottom. The solution was cooled with a cooling bath to freeze the aqueous phase. The organic layer was poured into NaOH aq (0.25 M, 500 mL). The product was then filtered and washed with water. The product was dissolved in THF and reprecipitated into NaOH aq (0.25 M, 500 mL), filtered, and washed with water three more times. The polymer was finally washed with a large amount of water before drying in vacuo. The yield was 97%. IR (NaCl): ν (cm-1) ) 3330-3355 (OH), 3050, 3025, 2930, 2855 (C-H, st), 1060 (C-O, st). 1H NMR (270 MHz, DMSO): δ (ppm) ) 0.77-2.30 (br, CH3, CH2, CH), 3.35 (br, -CH2-OH, 2H), 4.36 (br, -CH2-OH, 1H), 6.32-7.23 (br, benzene, 5H). The films of those polymers were prepared from a 1 wt % solution of toluene for PS-b-PMMA and of pyridine for PS-bPIOH. The solutions were evaporated in an ambient condition overnight and then dried on a hot plate at 50 °C for 1 day and finally in a vacuum oven at 150 °C for 2 days. The thickness of the films was about 100 µm. The characteristics of these films are summarized in Table 1. The periodicities of the block copolymers formed in the films were measured by a small-angle X-ray scattering (SAXS) apparatus (RIGAKU Ultrax4153A172B) consisting of a 45 kW rotating-anode X-ray generator. Incorporation of Metal Particles into the Block Copolymer Films. The process of our method to incorporate the metal particles into the polymer films was described in our previous paper.7 The bottom of a glass vessel with 10 mg of a metal complex is heated at 180 °C in vacuo to sublime the metal complex, and then the metal complex is immediately solidified on the upper side of the glass wall within a few minutes. In the next step, a polymer film is loaded into the glass vessel, and the glass vessel is put into the oil bath at 180 °C for a defined length of time after nitrogen replacement. The progress of the absorption and the reduction of the metal complex was monitored by UVvis spectroscopy on a V-550 JASCO spectrometer. EFTEM. A LEO922 energy-filtering transmission electron microscope, in which an Omega-type energy filter is integrated, was used at an accelerating voltage of 200 kV to investigate the state of the dispersion of the metal particles in the polymer films. In EFTEM, the transmitted electrons are selected not only according to their scattered angle but also according to their energy. The energy filter disperses the inelastically scattered electrons generated by electron-specimen interactions according to their energy. This enables us to perform electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS). Detailed descriptions of this equipment appear elsewhere,9 and the applications of this technique to the local elemental analysis of polymer materials are also listed in the references.10 ESI was employed to obtain zero-loss images and elemental maps. The zero-loss images are equivalent to conventional TEM images, (9) (a) Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer-Verlag: Berlin, 1995. (b) Brydson, R. Electron Energy Loss Spectroscopy; BIOS: Oxford, 2001.

but their image quality is superior to that of conventional images, having sharpness with enhanced contrast by removing the inelastically scattered electrons that cause blurring in conventional bright field images.11 All normal TEM images shown in this paper are zero-loss images that are formed by inserting the energy-selecting slit below the filter with the 20 eV width centered at the zero-loss peak. Elemental maps were created in accordance with the “three-window power law” method.12 This method involves several steps: First, an image in the vicinity of the element absorption edge is acquired as an image containing element-specific information, and then two images are acquired below the absorption edge that define the background intensities. Following this, a background image below the absorption edge is computed pixel by pixel by extrapolation to the energy loss value above the absorption edge. Finally, this background image is subtracted from the element-specific image. For the creation of a background image, the background curve is modeled as the following equation:

I ) AE-r

(1)

where I is the signal intensity, E is the energy loss, and A and r are the adjustable parameters. A and r are calculated pixel by pixel from the two images below the edge. Image recording and processing were performed using a slow-scan CCD camera, Proscan HSC2 (Proscan Co. Ltd.), and an image processing system, analySIS (Soft Imaging System, Co. Ltd.), on a PC connected to the microscope. EELS was employed for detailed chemical analysis in selected nanoscale areas. Two detection methods for EELS spectra were used: “parallel EELS” and “image EELS”. In parallel EELS, the specimen area to be analyzed is determined by inserting an aperture at the entrance of the filter. The spectrum is imaged on the slow-scan CCD camera, and the image analysis system measures the intensity and converts it into a spectrum.13 In image EELS, a series of images is acquired sequentially with a predetermined energy increment, and those images are stacked in the image analysis system. The image analysis system measures the mean gray values inside selected arbitrarily shaped areas in each image of the stack and plots the values against the energy losses of the corresponding images to display the EELS spectrum of the areas.14 For ESI and EELS analysis, the observation was performed cryogenically at 100 K. The critical dose of polymers, Dc, which is defined as that at which some spectral feature or diffracted intensity falls below a threshold, typically 1/e of the unirradiated signal, has been reported to be 0.01-10 C/cm2.15 It was reported that PS is resistant to electron beams up to 1.0 C/cm2 at 127 K.16 For creation of elemental maps and collection of image-EELS spectra, a large number of images have to be acquired in the same specimen area. Thus, it is not possible to limit the dose below the Dc. Cryogenic operation does not reduce the number of broken bonds but prevents atoms from leaving the irradiated area by reducing their diffusion rate. In our case, cooling specimens is also effective to reduce the evaporation of the dissolved metal complexes from the irradiated areas. The number-average diameter of the particles was calculated from the TEM images using image processing software, Ultimage Pro 2.6 (Graftek France). More than 200 particles were counted, (10) (a) Horiuchi, S.; Yase, K.; Kitano, T.; Higashida, N.; Ougizawa, T. Polym. J. 1997, 29, 380-383. (b) Horiuchi, S.; Hanada, T.; Yase, K.; Ougizawa, T. Macromolecules 1999, 32, 1312-1314. (c) Horiuchi, S.; Ishii, Y. Polym. J. 2000, 32, 339-347. (11) Reimer, L. In Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer-Verlag: Berlin, 1995; p 347. (12) Reimer, L. In Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer-Verlag: Berlin, 1995; p 383. (13) (a) Egerton, R. F.; Leapman, R. D. In Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer-Verlag: Berlin, 1995; p 271. (b) Brydson, R. Electron Energy Loss Spectroscopy; BIOS: Oxford, 2001; p 46. (14) Kortje, K. H. J. Microsc. 1994, 174, 149-159. (15) Reimer, L. Transmission Electron Microscopy, 3rd ed.; SpringerVerlag: Berlin, 1997; Chapter 11. (16) Varlot, K.; Martin, J. M.; Quet, C. J. Microsc. 1998, 191, 187194.

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Figure 1. (a) Morphologies of the block copolymer films used in this study shown by three different modes in EFTEM. (a), (b), and (c) are PS-b-PMMA (polymer 3), and (d), (e), and (f) are PS-b-PIOH. (a,d) Zero-loss filtering images; (b,e) π-π* resonance images; (c,f) oxygen maps. and individual diameters were assigned to those circles with equivalent area. Specimens for EFTEM analysis were prepared by cutting the films parallel to the surface of the films on an ultramicrotome, RMC MT-XL (Boeckeler Inst., Inc.), with a diamond knife after embedding a film in a light-cured resin, D-800 (JEOL DATUM Co. Ltd.). Sections with the thickness of about 50 nm were collected on a 600 mesh copper grid for observation. Electron diffraction studies of metal particles were also performed on EFTEM using an Imaging Plate system, DITABIS (Micron Co. Ltd.). Like the imaging studies, the energy-selecting slit with a width of 20 eV was inserted to remove the inelastic scattering background. The camera constant was obtained from the diffraction pattern of gold powder. Radii of the Debye rings were measured from the photographs and then compared to the calculated values using the camera constant and the unit cell data in the Powder Diffraction Files (PDF).17

Results and Discussion Self-Assembly of Pd Nanoparticles in Block Copolymer Films. First, the underlying morphologies of the used block copolymers using three different modes available in ESI are shown in Figure 1. The zero-loss images of PS-b-PMMA and PS-b-PIOH (images a and d of Figure 1, respectively) show their lamellar and spherical morphologies, respectively, with better contrast than conventional TEM images even though specimens were unstained. π-π* resonance images (Figure 1b,e) and oxygen elemental maps (Figure 1c,f) can obviously distinguish the PS phase from either the PMMA or the PIOH phase. The π-π* peak at around 6 eV (inset of Figure 1e) in an EELS spectrum can be used for imaging of the phase having conjugated carbon-carbon double bonds, which is referred to as chemical imaging.18 Thus, the ES images formed by the inelastically scattered electrons with an energy loss of 6 ( 2 eV (∆E ) 6 ( 2 eV) identify the PS phases in the block copolymers. On the other hand, oxygen (17) Selected Power Diffraction Data for Metals and Alloys Data Book, 1st ed.; Weissmann, S., Ed.; JCPDS-International Centre for Diffraction Data: Newtown Square, PA, 1978; Vol. 1. (18) Siagchaew, K.; Libera, M. Philos. Mag. A 2000, 80, 1001-1016.

maps shown in images c and f of Figure 1 reveal the PMMA and the PIOH phases, respectively. For creating those elemental maps, ES images at ∆E ) 495 ( 10, 525 ( 10, and 545 ( 10 eV were used. The micrographs of the PSb-PMMA block copolymer show the area just below the free surface together with the embedding resin. The lamellae near the free surface are oriented parallel to the surface, and the chemical and elemental maps indicate that the PS lamellae are located at the air/copolymer interface. This surface-induced orientation of symmetric diblock copolymers has been extensively studied, and it is known that the component having lower surface tension is preferentially located at the air/copolymer interface. In the case of PS-b-PMMA, it has been reported that the PS phase is present at the surface.19 As a result of the exposure of the metal complex vapor to a polymer film in a nitrogen atmosphere, a transparent PS film turns opaque and black upon exposure to the Pd(acac)2 vapor while the Pd(acac)2 fixed on the inside glass wall keeps its initial yellow color. TEM photographs of the cross sections of the PS films exposed for 10, 30, and 60 min and the PS-b-PMMA films of differing molecular weights after exposure for 30 min are shown in Figure 2. Single nanosized particles are dispersed in all the cases, and the electron diffraction pattern (inset of Figure 2c) gives Debye rings that can be assigned to {111}, (d ) 2.25 Å); {200}, (1.94 Å); {220}, (1.38 Å); and {311}, (1.17 Å) planes of face-centered-cubic Pd. This confirms that the polymer films reduce the Pd(II) ions to the metallic state. The reduced Pd does not exist as isolated atoms, but rather forms particles through the nucleated growth process. Histograms indicating the particle size distributions estimated from the corresponding TEM images are also presented in Figure 2. The particles are randomly distributed in the PS films, while they are selectively located within the lamellae of the PS-b-PMMA diblock (19) (a) Kunz, M.; Shull, K. Polymer 1993, 34, 2427-2430. (b) Russell, T. P.; Coulon, G.; Deline, V. R.; Miller, D. C. Macromolecules 1989, 22, 4600-4606. (c) Mayes, A. M.; Russell, T. P.; Basseau, P.; Baker, S. M.; Smith, G. S. Macromolecules 1994, 27, 749-755.


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Figure 2. TEM micrographs of the cross sections of the PS and the PS-b-PMMA films exposed to the Pd(acac)2 vapor presenting the dispersion of Pd nanoparticles together with histograms describing the statistical distribution of the Pd particle size. (a), (b), and (c) are the PS films exposed to the Pd(acac)2 vapor for 10, 30, and 60 min, respectively. (d), (e), and (f) are PS-b-PMMA films of differing molecular weights (samples 1, 2, and 3, respectively) exposed to the Pd(acac)2 vapor for 30 min. The inset of Figure 2c is an electron diffraction pattern obtained from this specimen.

copolymers. The metal particles are located in the lamella at the free surface of the film; thus, the Pd particles are assembled in the PS lamellae in the PS-b-PMMA diblock copolymer films. Also, the alternative lamellar microdomain structure is visualized with sufficiently good contrast where the PS lamellae appear dark although no conventional heavy metal staining was employed. This suggests that Pd(acac)2 is condensed in the PS phase to give a “staining effect”. In the PS films, the number of particles increases with the exposure time, while the particle size is almost constant at around 4 nm, being independent of the exposure times. On the other hand, in the PS-b-PMMA block copolymer films, the particle size depends on the molecular weight, that is, the width of the lamellae. The block copolymer film having the lowest molecular weight obviously yields smaller particles than those in the PS films. Thermal decomposition of Pd(acac)2 in a bulk form is reported to occur above 185 °C.20 Therefore, the polymer films have catalytic activity to reduce the Pd(II) ions to the metallic state. The reduction of Pd(acac)2 to produce metal nanoparticles in various organic solvents has been reported by Esumi et al.21 Among the solvents they evaluated, methyl isobutyl ketone at its boiling point of 115.9 °C gave stable Pd particles with the smallest size ranging between 8 and 10 nm. Their work also indicated that organic compounds promote the thermal decomposition of Pd(acac)2 at temperatures lower than the decomposition temperature of the bulk state. The detailed mechanism of the catalytic effect of the polymer films on the reduction of the metal complex in our method is unknown, but the solid state polymer films probably work in the same manner as the solvents in terms of the reduction of Pd(acac)2. Gross et al. reported thermoanalytical studies on the thermal decomposition of palladium acetate induced by a photothermal laser direct-write process.22 They determined the primary reaction products

as acetic acid and carbon dioxide. It is presumed that Pd(acac)2 also decomposes in a similar manner. Parts a and b of Figure 3 show the changes in the UVvis absorption spectra of the PS and the PMMA homopolymer films, respectively, upon the exposure of Pd(acac)2 vapor. The absorption band with a maximum at 330 nm corresponding to Pd(acac)2 appears at the beginning of the exposure in both the PS and the PMMA films. In the PS film, the intensity of the peak increases together with its tailing toward longer wavelengths. Finally, the peak disappears and is replaced with monotonically broad spectra throughout the wavelength range displayed. On the other hand, the intensity of the peak at 330 nm increases with exposure time, but the tailing of the peak in the PS film does not occur for exposure times up to 60 min in the PMMA film. After 60 min, absorbance in the longer wavelength region jumps up, and finally, the monotonically broad spectrum is obtained after 120 min. The absorbance at 330 and 400 nm is plotted against exposure time in parts c and d of Figure 3, respectively. Those show a steep increase in the absorbance at 330 nm within 60 min followed by a slight decrease both in the PS and in the PMMA. The increase in the absorbance at 400 nm, on the other hand, is steady with the exposure time in the PS, while the PMMA keeps the lower values within 60 min and afterward it jumps rapidly to the same level as the PS, suggesting that PMMA has an induction period in terms of the absorption of the metal complex and the formation of the nanoparticles. The difference in the behavior of the absorption spectra agrees with the visual observation of those films as shown in Figure 4. The PS film becomes darker gradually from the beginning of the exposure, while the PMMA remains its transparency at 60 min and turns black later on. This relative difference in terms of the reduction power against Pd(acac)2 between PS and PMMA causes the selective growth of the Pd nanoparticles in the PS-b-PMMA block copolymer films. The PS phase reduces the Pd(II) ions immediately after

(20) Nakao, Y. J. Colloid Interface Sci. 1995, 171, 386-391. (21) Esumi, K.; Tano, T.; Meguro, K. Langmuir 1989, 5, 268-270.

(22) Gross, M. E.; Appebaum, A.; Gallagher, P. K. J. Appl. Phys. 1987, 61, 1628-1632.

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Figure 3. UV-vis absorption spectra of (a) PS and (b) PMMA homopolymer films exposed to the Pd(acac)2 vapor for different times and the absorbance at (c) 330 nm and (d) 400 nm plotted against the exposure times.

Figure 4. Appearances of PS and PMMA homopolymer films after exposure to Pd(acac)2 vapor for (a) 60 min and (b) 120 min.

Figure 5. Zero-loss filtering TEM micrographs of the cross sections of PS-b-PMMA (sample 2) exposed to the Pd(acac)2 vapor for 2 h showing the assembly of the Pd nanoparticles in a lamellar manner. (b) is a magnified image of (a).

the absorption, while the PMMA keeps its ionic state for approximately 1 h. Further exposure of the Pd(acac)2 vapor to the block copolymer films up to 2 h gives the well-aligned Pd nanoparticles in a periodical lamellar manner as shown in Figure 5. The gray level contrast between the PS and the PMMA lamellae observed after the exposure for 30 min is eliminated. This means that most of the absorbed

metal complex is converted to the metal particles. The Pd particles seem to be connected to each other in the PS lamellae, while few particles exist in the PMMA lamellae. Those observations indicate that the vapor of Pd(acac)2 penetrates into the inside of the film and is consumed predominantly in the PS phase to construct a threedimensional alignment of the Pd metal nanoparticles. The formation of Pd nanoparticles reaches a depth of 100 µm by exposure for 2 h using a PS homopolymer thick film. As presented in Figure 2, the absorbed Pd(acac)2 is not homogeneously dissolved in the block copolymers but rather is condensed in the PS phase. However, the UVvis absorption spectra shown in Figure 3 indicate that PMMA absorbs the Pd(acac)2 equally as compared to PS. It is suggested that the formation of the Pd particles in the PS lamellae provides a thermodynamic driving force for the diffusion of the metal complex from the PMMA to the PS lamellae. The fact that the PS-b-PMMA film having the lowest Mn (sample 1) in our samples produces much smaller particles (3.0 nm) than the other two samples (3.8 and 4.0 nm, respectively) suggests that the space of the phase influences the growth of the particles. It is reasonable to assume that the nucleation rates of Pd in the PS phase are comparable in all the cases including both the PS homopolymer and the block copolymers. It is generally known that the relative rates of nucleation and growth manage the particle size. Hence, one can speculate that a narrow lamellar spacing suppresses the nuclear growth to yield the smaller particles. To produce larger particles at the same nucleation rate, more Pd atoms have to be provided to the nuclei for the growth of particles. As mentioned above, the PS lamellae dissolve Pd(acac)2 much


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Figure 7. (a) A zero-loss filtering TEM micrograph of the PSb-PIOH film exposed to the Co(acac)2 vapor for 45 min. (b) A selected-area electron diffraction pattern of (a).

Figure 6. Zero-loss filtering TEM micrographs of the cross sections of PS-b-PIOH films exposed to the Pd(acac)2 vapor for (a) 10, (b) 15, and (c) 60 min and (d) a selected-area electron diffraction pattern obtained from (b).

more than the PMMA phase. Therefore, it is speculated that the nuclei in the narrowest lamellae may lie under a circumstance where enough Pd atoms are not provided for the growth of the particles. As reported in our previous papers, we have evaluated many types of polymers in terms of the reduction of Pd(acac)2 including various methacrylate polymers, epoxy resins, polyamides, and so on.7,23 PMMA was the only polymer which had less catalytic behavior in terms of the reduction of Pd(acac)2. That is, PMMA shows the induction period between the absorption and the reduction of Pd(acac)2 as presented in Figure 3b. Other polymers showed a behavior similar to that of PS as shown in Figure 3a. We speculate that the carbonyl group in the PMMA side chain contributes to the stabilization of the Pd(II) ions. The other methacrylate polymers, on the other hand, have larger molecules attached to the carbonyl group. Hence, the interaction between Pd(II) ions and the carbonyl group is weaker due to the steric hindrance to promote the reduction of the Pd(II) ions. We expect that this self-assembly of the Pd particles can be obtained in any type of block copolymer having a relative difference in the reducing power between the components. Thus, we prepared an asymmetric block copolymer having alcoholic moieties in one of the blocks dispersed as spherical domains in the PS matrix, PS-bPIOH. It has been known that alcohols act as reducing agents of metal salts in the following manner.24

Pd(II) + R-CH2OH f Pd + R-CHO + 2H+ (2) Therefore, it is predicted that the Pd particles will assemble in the PIOH domains, not in the PS matrix, in the PS-b-PIOH film. Figure 6 shows the TEM photographs of PS-b-PIOH films exposed to the Pd(acac)2 vapor for 5, 15, and 60 min. Those photographs clearly show that the Pd particles are selectively incorporated into the PIOH domains. As compared to the PS-b-PMMA films, the particles are produced significantly faster in the PS-bPIOH film. The number of particles in the PIOH domains increases from 5 min (Figure 6a) to 15 min (Figure 6b), (23) Nakao, Y. Chem. Lett. 2000, 766-767. (24) Hashimoto, T.; Harada, M.; Sakamoto, N. Macromolecules 1999, 32, 6867-6870.

while the numbers after 15 and 60 min (Figure 6c) seem to be nearly the same. The PIOH domains have a stronger reducing power than the PS matrix as predicted. Even after exposure for a longer period up to 60 min, a few Pd particles exist in the PS matrix. A diffraction pattern from the specimen exposed for 15 min is shown in Figure 6d. This gives a clearer pattern than that from the PS film (Figure 2c) because a larger number of particles is generated in an equivalent area, and the Debye ring patterns agree with the {111}, {200}, {220}, and {311} planes of face-centered-cubic Pd. It is difficult to estimate the average particle size quantitatively because the particles are hardly separated from each other. The size is nearly the same as that obtained in the PS-b-PMMA. The Pd particles are selectively located in the PS domains in the case of PS-b-PMMA, while they are located in the PIOH domains, not in the PS, in the case of PSb-PIOH. Thus, we can conclude that Pd nanoparticles selectively grow in the phase having the stronger reducing power in block copolymer films. The order of the reducing power against the Pd(II) ion is PIOH > PS > PMMA. When the PIOH domains are embedded in the PS matrix, the PIOH domains provide a restricted reaction environment that is regarded as a “nanoreactor”. Reduction of Co(acac)2 in Polymer Films. Magnetic properties for some metal or oxide nanoparticles can strongly depend on the particle size and shape.2a,25 The arrangement of such magnetic small particles at a nanoscale can lead to ultrahigh-density storage media.26 Thus, we evaluated the reduction of Co(acac)2 in polymer films by the same procedure as was done for the reduction of Pd(acac)2. Figure 7a shows a TEM micrograph of a PS-b-PIOH film exposed to Co(acac)2 vapor for 45 min. It shows that quite small particles are located in the PIOH domains in the PS-b-PIOH film. Figure 7b shows the electron diffraction pattern obtained from this specimen, giving Debye rings assigned to {002} (d ) 2.04 Å) and {21 h 0} (1.25 Å) planes of the hexagonal close-packed Co crystal structure. Figure 8 depicts the spectral changes in UV-vis absorption with increasing the exposure time. The spectrum of 1 wt % of Co(acac)2 dissolved in a PS film, which was prepared by dissolving both Co(acac)2 and PS in pyridine and subsequently evaporating the solvent, is also shown. This spectrum reveals that Co(acac)2 existing in a polymer film as isolated molecules has a sharp absorption band at around 330 nm. The PS film with dissolved Co(acac)2 has (25) (a) Stamm, C.; Marty, F.; Vaterlaus, A.; Weich, V.; Egger, S.; Maier, U.; Ramsperger, U.; Fuhrmann, H.; Pescia, D. Science 1998, 282, 449-451. (b) Stahl, B.; Gajbhiye, N. S.; Wilde, G.; Kramer, D.; Ellrich, J.; Ghafari, M.; Hahn, H.; Gleiter, H.; Weissmu¨ler, J.; Wu¨rschum, R.; Schlossmacher, P. Adv. Mater. 2002, 14, 24-27. (c) Verelst, M.; Ely, T. O.; Amiens, C.; Snoeck, E.; Lecante, P.; Alain, M.; Respaud, M.; Broto, J. M.; Chaudret, B. Chem. Mater. 1999, 11, 2702-2708. (26) 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.

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Figure 8. UV-vis absorption spectra of the PS-b-PIOH film exposed to the Co(acac)2 vapor for different times. The spectrum of the PS film with 1 wt % of dissolved Co(acac)2 is included.

Figure 9. (a) An ES image at ∆E ) 250 ( 10 eV of the PSb-PIOH film exposed to Co(acac)2 vapor for 45 min. (b) An EELS spectrum showing the cobalt L2,3-edge obtained from a selected region in (a) with a diameter of 500 nm. (c) A cobalt elemental map created by the three-window power law method.

a pale pink color, while the PS-b-PIOH film exposed to Co(acac)2 vapor has an opaque red-brown color. The spectral changes shown in Figure 8 result in the disappearance of the band at 330 nm and the appearance of a broad band at around 400 nm through the exposure of Co(acac)2 vapor. This new absorption band is broad and has a tail toward longer wavelengths. This is characteristic of surface plasmon absorption27 and suggests that the Co atoms do not exist as isolated atoms, but rather as nanoparticles. As compared to the Pd particles, these Co particles are quite small. Hence, the zero-loss image (Figure 7a) does not show the state of the dispersion of the Co particles clearly. On the other hand, an ES image at ∆E ) 250 ( 10 eV offers negative contrast to show the location of the metal particles clearly (Figure 9a). Such an image is called a “structure-sensitive image” formed at energy loss levels just below the carbon K-edge at 285 eV, which allows an image to be formed with a minimum (27) (a) Galletto, P.; Brevet, P. F.; Girault, H. H.; Antoine, R.; Broyer, M. Chem. Commun. 1999, 581-582. (b) Link, S.; El-Sayed, M. J. Phys. Chem. B 1999, 103, 4212-4217.

Horiuchi et al.

contribution from carbon and a strong contribution from noncarbon atoms in a carbon-rich specimen.28 Cobalt displays two sharp and intense peaks at around 780 eV in an EELS spectrum from an L2,3 ionization edge. These are known as white lines29 and are shown in Figure 9b, taken by means of parallel EELS from a selected area with a diameter of 500 nm in a specimen of Figure 7a. Those two peaks result from the excitation of an electron from the 2d state to an energetically narrow 3d band. This is characteristic of transition-metal and rare-earth elements. The appearance of the two peaks is due to the spin-orbit splitting of the ionized 2p core level. An L3 peak is followed at higher energy loss by an L2 peak. This feature makes detection and quantification of the elements extremely easy. Pd, on the contrary, exhibits a broad and weak ionization edge at around 330 eV,30 where the strong carbon K-edge at 285 eV overlaps and makes the characterization difficult. Thus, the nanoscale chemical analysis of cobalt atoms in the polymer films was performed by means of ESI and EELS. Figure 9c is a cobalt elemental map created by the three-window power law method of the PS-b-PIOH film exposed to the vapor of Co(acac)2 for 45 min. For the imaging of the cobalt distribution, the two ES images at ∆E ) 725 ( 10 and 750 ( 10 eV were used to create the background image, and then the background image was subtracted from the cobalt core loss image at ∆E ) 780 ( 10 eV. This qualitatively shows that the cobalt atoms are located within the isolated PIOH domains. Moreover, image EELS offers additional evidence supporting this fact. For collection of the spectra containing the cobalt ionization edge by image EELS, a sequence of ES images with the energy window of 10 eV were acquired by an energy increment of 2 eV in the energy range from 740 to 820 eV. As shown in Figure 10, EELS spectra of the arbitrary areas decided on the image can be obtained. The spectrum extracted from the PIOH domains displays the intense L2,3-edge of cobalt as the result of the average values of all the pixels within the areas indicated in blue, while the spectrum from the areas in the PS matrix (indicated in red) gives no signals for the existence of the element. The image EELS analysis offers evidence that the absorbed Co(acac)2 vapor migrates into the PIOH and is consumed dominantly there. The UV-vis absorption spectroscopy and the electron diffraction study reveal that the produced particles are metallic cobalt particles. Therefore, the Co(acac)2 can be reduced selectively in the PIOH domains in the PS-b-PIOH block copolymer film in the same manner as the case of Pd(acac)2. On the other hand, the sizes of the particles produced in the PS film are in the range from 20 to 30 nm by the exposure of Co(acac)2 vapor for 45 min. This size is more than 10 times larger than that yielded in the PS-b-PIOH film as presented in Figure 11a. The oxygen and the cobalt maps (images b and c of Figure 11, respectively) reveal that there are two kinds of particles, one of which is oxygen rich and the other of which is cobalt rich. The energy loss values used for creating those maps are 495 ( 10, 525 ( 10, and 545 ( 10 eV for oxygen and 725 ( 10, 750 ( 10, and 780 ( 10 eV for cobalt. In Figure 11d, the oxygen and the cobalt maps are superimposed on its structuresensitive image at ∆E ) 250 ( 10 eV, where the oxygen map is indicated in red and the cobalt map is in green. (28) (a) Reimer, L. In Energy-Filtering Transmission Electron Microscopy; Reimer, L., Ed.; Springer-Verlag: Berlin, 1995; p 273. (b) Horiuchi, S.; Matchariyakul, N.; Yase, K.; Kitano, T.; Choi, H. K.; Lee, Y. M. Polymer 1996, 37, 3065-3078. (29) Wang, Z. L.; Yin, J. S.; Jiang, Y. D. Micron 2000, 31, 571-580. (30) Reimer, L.; Zepke, U.; Moesch, J.; Hillert, S. S.; Messemer, M. R.; Probst, W.; Weimer, E. EELS Spectroscopy; Carl Zeiss: Oberkochen, Germany, 1992.


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Figure 10. Image EELS spectra of the PS-b-PIOH film exposed to the Co(acac)2 vapor for 45 min, displaying the cobalt L2,3-edge extracted from the selected areas indicated in the ES image at ∆E ) 250 ( 10 eV. The spectrum shown in blue is obtained as the average values of the pixels within the PIOH domains indicated in blue in the image, while the spectrum in red is from the pixels within the areas from the PS matrix indicated in red.

Figure 11. Elemental maps of the PS homopolymer film exposed to the Co(acac)2 vapor for 45 min: (a) a zero-loss image, (b) an oxygen map, and (c) a cobalt map. (d) The oxygen and the cobalt maps are superimposed in green and red, respectively, on the ES image at ∆E ) 250 ( 10 eV.

The area containing both oxygen and cobalt appears yellow due to the composition of the red and the green pixels. It obviously indicates that the most of particles are oxygen rich with no cobalt atoms, whereas the rest of the particles contain cobalt atoms together with oxygen. Image EELS enables us to obtain EELS spectra from individual particles to give information on the relative composition of oxygen and cobalt. For collection of the spectra containing both the oxygen and the cobalt ionization edges by image EELS, a sequence of ES images with

an energy window of 10 eV were acquired by an energy increment of 5 eV in the energy range from 500 to 800 eV. The acquisition time for each image was 5 s at the electron dose rate of 0.34 C/cm2. Figure 12 presents the spectra extracted from the corresponding particles indicated in the image together with the spectra from the PS matrix. The spectra can be classified into three patterns based on the intensities and shapes of the element-specified peaks. That is, the spectra from the particles indicated in blue circles in the image contain both the oxygen K-edge and

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Figure 12. Image EELS spectra of the PS homopolymer exposed to the Co(acac)2 vapor for 45 min, displaying the oxygen K-edge and the cobalt L2,3-edge obtained from the regions indicated in the image.

the cobalt L2,3-edge; the spectra from the particles indicated in red have only the oxygen K-edge with the strongest intensity; and those from the PS matrix indicated in yellow show only the oxygen K-edge with relatively weaker intensities. The results suggest that the thermal decomposition of Co(acac)2 to produce the metallic cobalt particles in the PS homopolymer film is not complete, yielding the oxygen-rich particles as byproducts. The spectra also suggest that some reaction products are dissolved in the PS matrix. Although the detailed structure of the cobaltcontaining particles is unknown, cobalt atoms may not be reduced and form some compounds with oxygen. Finally, we carried out a quantitative study of EELS in terms of oxygen and cobalt for following three samples: (a) the PS-b-PIOH film exposed to the Co(acac)2 vapor for 45 min, (b) the PS film exposed to the Co(acac)2 vapor for 45 min, and (c) the PS film with 1 wt % of dissolved Co(acac)2. EELS spectra were collected by parallel EELS over the whole energy range from 0 to 800 eV from the areas with a 100 nm diameter. The spectra were then deconvoluted using the Fourier-Log method to remove the contribution of multiple inelastic scattering.31 Figure 13 shows four EELS spectra of the above three samples and a plain PS homopolymer film ranging from 500 to 800 eV after deconvolution to show the oxygen K-edge and the cobalt L2,3-edge. To minimize the evaporation of the Co(acac)2 from the specimens and the contamination of the irradiated area, the acquisitions were carried out with electron doses below 0.1 C/cm2. To estimate the relative atomic concentration ratios of cobalt to oxygen, the intensities coming from solely the ionization events were determined as shown in Figure 13. The continuously decreasing background was fitted using the power law function of the form I ) AE-r using the intensity values prior to the edge of interest to determine the parameters of A and r, and then the background curve was extrapolated under (31) Brydson, R. Electron Energy Loss Spectroscopy; BIOS: Oxford, 2001; p 59.

Figure 13. EELS spectra displaying the oxygen K-edge and the cobalt L2,3-edge of (a) the PS-b-PIOH and (b) the PS films exposed to the Co(acac)2 vapor for 45 min and (c) a PS film with 1 wt % of dissolved Co(acac)2. Fitted power law backgrounds are indicated by dotted lines, and the shaded areas above each edge are used to quantify the elemental composition.

the edge as indicated by dotted lines. Next, the values of the background were subtracted from the original values. Then, the shaded region was integrated with the fixed energy width to be used for the quantification of elemental compositions. Ratios of the integrated values with the energy window of 70 eV under the cobalt L2,3-edge to the oxygen K-edge (Co/O ratios) were calculated for the spectra shown in Figure 13 and are indicated beside the corresponding spectra. Although those values are not the true ratios of the number of atoms, we can discuss compara-


tively the elemental concentrations among our samples. Obviously, the concentration of cobalt relative to oxygen is significantly higher in the PS-b-PIOH film than in the other samples despite the polymer itself having a great amount of oxygen. This suggests that most of the reaction products as the result of the reduction of Co(acac)2 go out of the film as gaseous products in the PS-b-PIOH film. The spectrum from the plain PS film presents no ionization edges in the displayed energy range, indicating that the oxygen K-edges are contributed by Co(acac)2 in samples b and c. The Co/O ratios obtained from those two samples are significantly small, suggesting that thermal decomposition of Co(acac)2 in the PS film is incomplete and most of the reaction products remain in the film. Conclusions We have developed a simple dry process in which Pd and Co metal nanoparticles are selectively doped into the domains of block copolymer films to provide nanoscale arrangements of nanoparticles. In this method, the vapor of the metal complex can penetrate into the films deeply, which makes possible the spatially ordered arrangements of the nanoparticles in three dimensions. Pd(acac)2 can be reduced and forms metal nanoparticles with narrow size distributions in all the evaluated polymers. On the other hand, Co(acac)2 can be reduced by the PIOH due to a strong reducing power in the PS-b-PIOH block copolymer film to produce the Co nanoparticles, whereas the PS homopolymer cannot reduce the Co(II) ions and yields some large particles with diameters of tens of nanometers containing cobalt and oxygen. Those results suggest that the tendency

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of the reduction and formation of metal particles depends on the metal species of the acetylacetonato complexes. We qualitatively investigated the mechanism of the construction of the alignment of the metal nanoparticles within block copolymer films by UV-vis spectroscopy and EFTEM. We successfully analyzed by EFTEM the nanoscale local structures and the products in the films exposed to Co(acac)2 vapor. Quantitative studies in terms of the diffusion and the reduction of the metal complexes, however, are necessary for further detailed investigations on the mechanism of this process. To clarify those two effects on the formation of the metal nanoparticles, the incorporated amount and reduced amount of the metal complex should be estimated separately. X-ray photoelectron spectroscopy would be a useful technique for estimating the absorbed metal complex and the reduced metals. But signals corresponding to the metal species with enough intensity were not obtained because sufficient amounts of products could not be incorporated on the surfaces of the films. In this paper, we have not yet proposed satisfying information on the chemical, physical, and morphological changes of the polymers during the process. We will continuously investigate the mechanism by introducing other techniques. Also, we will evaluate chemical, magnetic, or optical properties of the materials prepared by this method. Acknowledgment. Financial support by New Energy and Industrial Technology Development (NEDO) for the Nanostructured Polymer Project is greatly acknowledged. LA020745D

Three-Dimensional Nanoscale Alignment of Metal Nanoparticles

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