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In situ GISAXS Investigation of Gold Sputtering onto a Polymer Template E. Metwalli,† S. Couet,‡ K. Schlage,‡ R. Ro¨hlsberger,‡ V. Ko¨rstgens,† M. Ruderer,† W. Wang,† G. Kaune,† S. V. Roth,‡ and P. Mu¨ller-Buschbaum*,† TU Mu¨nchen, Physik Department LS E13, James-Franck-Strasse 1, 85747 Garching, Germany, and HASYLAB at DESY, Notke Strasse 85, 22603 Hamburg, Germany. ReceiVed December 11, 2007. In Final Form: January 21, 2008 Microphase-separation structures in mixed diblock-triblock copolymer thin films are used for the incorporation of gold atoms inside the polymer matrix via sputtering of gold. Polystyrene (PS) spheres are arranged in a liquidlike type with a well defined nearest neighbor distance inside a polyisoprene matrix acting as a template for directing the gold atoms. Sputtering conditions are selected with a very low sputtering rate to avoid clustering in the atmosphere so that gold reaches the polymer surface in its atomic state. Due to the mobility of the gold atoms and the selective interaction with the PS parts of the microphase separation structure, gold is accumulated inside the polymer film in the PS spheres, as probed in situ with grazing incidence small-angle X-ray scattering (GISAXS). Nominally 4.3 Å of gold is deposited, which by diffusion is spread out vertically over a thickness of 280 nm. UV-vis spectroscopy reveals a small blue shift for the gold sputtered polymer film. Atomic force microscopy proves the absence of gold clusters on the film surface. For low sputtering rate, GISAXS proves good sensitivity for gold migration inside the polymer film and opens new possibilities for studying polymer-metal interaction.
1. Introduction Recent advances in the patterning of polymers have enabled the fabrication of integrated micro- and nanosystems with high degree of complexity and functionality.1-3 For example, block copolymers have attracted immense interest for nanotechnology applications4 because of easy processability and low-cost fabrications. The chemically distinct and immiscible polymer blocks in block copolymers microphase-separate and selfassemble into ordered patterns on the scale of nanometers. This soft nanostructured polymer film can further be used as a template for patterning of hard inorganic materials such as metal nanoparticles.5-11 Metal nanoclusters in a matrix of insulating polymer have unique physical properties and have been proposed for optical, electrical, and magnetic applications.7,12-15 * Corresponding author. E-mail:
[email protected]. Phone: +49 89 289 12451. Fax: +49 89 28912473. † TU Mu ¨ nchen. ‡ HASYLAB at DESY. (1) Xia, Y. N.; Kim, E.; Zhao, X. M.; Rogers, J. A.; Prentiss, M.; Whitesides, G. M. Science 1996, 273, 347. (2) Quake, S. R.; Scherer, A. Science 2000, 290, 1536. (3) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. AdV. Mater. 1997, 9, 61. (4) Hamley, I. W. Ang. Chem., Int. Ed. 2003, 42, 1692. (5) Krishnan, R. S.; Mackay, M. E.; Duxbury, P. M.; Pastor, A.; Hawker, C. J.; Van Horn, B.; Asokan, S.; Wong, M. S. Nano Lett. 2007, 7, 484. (6) Tjandra, W.; Yao, J.; Ravi, P.; Tam, K. C.; Alamsjah, A. Chem. Mater. 2005, 17, 4865. (7) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126. (8) Lu, J. Q.; Yi, S. S. Langmuir 2006, 22, 3951. (9) Minelli, C.; Hinderling, C.; Heinzelmann, H.; Pugin, R.; Liley, M. Langmuir 2005, 21, 7080. (10) Horiuchi, S.; Fujita, T.; Hayakawa, T.; Nakao, Y. Langmuir 2003, 19, 2963. (11) Adachi, M.; Okumura, A.; Sivaniah, E.; Hashimoto, T. Macromolecules 2006, 39, 7352. (12) Black, C. T.; Murray, C. B.; Sandstrom, R. L.; Sun, S. H. Science 2000, 290, 1131. (13) Sanchez, C.; Julian, B.; Belleville, P.; Popall, M. J. Mater. Chem. 2005, 15, 3559. (14) Sanchez, C.; Lebeau, B. MRS Bull. 2001, 26, 377.
Previous studies16-19 have shown that noble metal particles can preferentially decorate a particular domain in a diblock copolymer film. X-ray standing wave fluorescence20 was used to prove that gold nanoparticles tend to diffuse toward the center of the poly(2-vinylpyridine) (P2VP) domains of polystyreneblock-poly(2-vinylpyridine), P(S-b-2VP), diblock copolymers. The contact or van der Waals interaction was used to account for adsorption of metal nanoparticles on one block of the copolymer. For example, the contact interaction of gold with P2VP in P(S-b-2VP) diblock copolymer via bonding with nitrogen atoms21 explained the preferential adsorption to the P2VP domains. Gold decoration of polystyrene on polystyrene-blockpolymethylmethacrylate P(S-b-MMA) diblock copolymer thin films is explained by the stabilization effect of the polar (PMMA) block via van der Waals interactions.22,23 In general, the specific nature of the selective gold-polymer interaction that causes the self-assembly is still far from being completely understood. Patterning of metal nanoparticles within polymer films has been achieved using four main routes. The first method is vaporphase codeposition of polymers/nanoparticles in high vacuum followed by thermal annealing.24-27 Annealing of the polymer (15) Roth, S. V.; Walter, H.; Burghammer, M.; Riekel, C.; Lengeler, B.; Schroer, C.; Kuhlmann, M.; Walther, T.; Sehrbrock, A.; Domnick, R.; Mu¨ller-Buschbaum, P. Appl. Phys. Lett. 2006, 88, 021910. (16) Balazs, A. C. Curr. Opin. Colloid Interface Sci. 1999, 4, 443. (17) Balazs, A. C.; Emrick, T.; Russell, T. P. Science 2006, 314, 1107. (18) Thompson, R. B.; Ginzburg, V. V.; Matsen, M. W.; Balazs, A. C. Science 2001, 292, 2469. (19) Morkved, T. L.; Wiltzius, P.; Jaeger, H. M.; Grier, D. G.; Witten, T. A. Appl. Phys. Lett. 1994, 64, 422. (20) Lin, B. H.; Morkved, T. L.; Meron, M.; Huang, Z. Q.; Viccaro, P. J.; Jaeger, H. M.; Williams, S. M.; Schlossman, M. L. J. Appl. Phys. 1999, 85, 3180. (21) Tsai, W. H.; Boerio, F. J.; Clarson, S. J.; Parsonage, E. E.; Tirrell, M. Macromolecules 1991, 24, 2538. (22) Hasegawa, H.; Hashimoto, T. Polymer 1992, 33, 475. (23) Kunz, M. S.; Shull, K. R.; Kellock, A. J. J. Appl. Phys. 1992, 72, 4458. (24) Takele, H.; Schurmann, U.; Greve, H.; Paretkar, D.; Zaporojtchenko, V.; Faupel, F. Eur. Phys. J. Appl. Phys. 2006, 33, 83. (25) Biswas, A.; Marton, Z.; Kanzow, J.; Kruse, J.; Zaporojtchenko, V.; Faupel, F.; Strunskus, T. Nano Lett. 2003, 3, 69. (26) Biswas, A.; Aktas, O. C.; Kanzow, J.; Saeed, U.; Strunskus, T.; Zaporojtchenko, V.; Faupel, F. Mater. Lett. 2004, 58, 1530. (27) Kay, E. Z. Phys. D 1986, 3, 251.
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film above the glass transition temperature (Tg) of the polymer allows structural relaxation of the polymer matrix and was proven to be responsible for the dispersion of the metal nanoparticles within the polymer film. The second method is based on the deposition from a mixture of block copolymer and organic-coated nanoparticles in solution onto a solid surface followed by the annealing step.28-34 The third method employs the dewetting of polymer films made from low concentrations of mixed solutions of polymer and polymer-grafted nanoparticles to create metal nanostructures.35-38 The fourth method uses the self-organization characteristic of evaporated nanoparticles on a self-assembled polymer film to create nanopatterning by selective adsorption.39 The different methods used for the metal patterning along with the various mechanisms (adsorption, surface diffusion, nucleation, and agglomeration) involved in the dispersion process40 of the nanoparticles within the polymer film make it difficult to understand the kinetics of nanoparticle formation in polymer matrix. For example, in the coevaporation method, both metal and organic particles impinge on the solid surface and codeposition occurs in conditions that are far from thermodynamic equilibrium.24 In wet chemical synthesis processes, problems due to residual solvents may influence the aggregation behavior of nanoparticles. The physical vapor deposition (PVD) method, however, offers a better method with the advantage of good conformity on a variety of complex topographies. The evaporated or sputtered nanoparticles decorate one block on the surface of the phase-separated polymer film by selective wetting. In the present study, we use the sputtering technique to investigate the deposition behavior of gold onto commercially important types of copolymers, the thermoplastic elastomers. A mixture of polystyrene-block-polyisoprene-block-polystyrene P(S-b-I-b-S) triblock and polystyrene-block-polyisoprene P(S-b-I) diblock copolymers is known for its use in pressure sensitive adhesive (PSA) applications.41 Pure copolymers used as base materials for PSA applications can show some self-tack but are typically not sufficient tacky on solid surfaces for commercial applications. In order to obtain PSA properties, the entanglements of the rubbery phase must be diluted to lower the elastic modulus of the physically crosslinked gel. In addition, the possibility of the material to dissipate energy and resist crack propagation at the interface needs to be increased. Both modifications of the properties are achieved by (28) Lauter-Pasyuk, V.; Lauter, H. J.; Ausserre, D.; Gallot, Y.; Cabuil, V.; Hamdoun, B.; Kornilov, E. I. Physica B 1998, 248, 243. (29) Lin, Y.; Boker, A.; He, J. B.; Sill, K.; Xiang, H. Q.; Abetz, C.; Li, X. F.; Wang, J.; Emrick, T.; Long, S.; Wang, Q.; Balazs, A.; Russell, T. P. Nature 2005, 434, 55. (30) Hamdoun, B.; Ausserre, D.; Joly, S.; Gallot, Y.; Cabuil, V.; Clinard, C. J. Phys. II 1996, 6, 493. (31) Lauter-Pasyuk, V.; Lauter, H. J.; Gordeev, G. P.; Mu¨ller-Buschbaum, P.; Toperverg, B. P.; Jernenkov, M.; Petry, W. Langmuir 2003, 19, 7783. (32) Fro¨msdorf, A.; Kornowski, A.; Putter, S.; Stillrich, H.; Lee, L. T. Small 2007, 3, 880. (33) Hashimoto, T.; Harada, M.; Sakamoto, N. Macromolecules 1999, 32, 6867. (34) Jain, A.; Hall, L. M.; Garcia, C. B. W.; Gruner, S. M.; Wiesner, U. Macromolecules 2005, 38, 10095. (35) Abul Kashem, M. M. A.; Perlich, J.; Schulz, L.; Roth, S. V.; Petry, W.; Mu¨ller-Buschbaum, P. Macromolecules 2007, 40, 5075. (36) Barnes, K. A.; Karim, A.; Douglas, J. F.; Nakatani, A. I.; Gruell, H.; Amis, E. J. Macromolecules 2000, 33, 4177. (37) Barnes, K. A.; Douglas, J. F.; Liu, D. W.; Karim, A. AdV. Colloid Interface Sci. 2001, 94, 83. (38) Krishnan, R. S.; Mackay, M. E.; Duxbury, P. M.; Hawker, C. J.; Asokan, S.; Wong, M. S.; Goyette, R.; Thiyagarajan, P. J. Phys.: Condens. Matter 2007, 19, 356003. (39) Lopes, W. A. Phys. ReV. E 2002, 65, 031606. (40) Faupel, F.; Zaporojtchenko, V.; Strunskus, T.; Erichsen, J.; Dolgner, K.; Thran, A.; Kiene, M. In Metallization of Polymers 2, 1st ed.; Sacher, E., Ed.; Kluwer Academic/Plenum: New York, 2002; Vol. 2, p 73. (41) Roos, A.; Creton, C. Macromolecules 2005, 38, 7807.
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Figure 1. Schematic side view of the sputtering chamber.
blending with a low molecular weight but high-Tg (glass transition temperature) tackifying resin, which is typically miscible with the rubbery phase but immiscible with the glassy domains. The major component of the nanoparticles in the mixture is the soft rubbery polyisoprene and the minor component is the glassy polystyrene. Here, we use a portable, remotely controlled DC magnetron sputter deposition system mounted on the beamline of grazing incidence small-angle X-ray scattering (GISAXS) to investigate the in situ formation and growth of gold nanoparticles in the polymer film.
2. Description of the Portable Sputtering System The sputtering chamber is shown schematically in Figure 1. An ultra high vacuum (UHV) sputtering system, including 2 gas inlets, 3 DC magnetron sputter sources, and a load/lock system, has been designed to fit in many beam lines42 including a typical small-angle X-ray scattering beamline for in situ and real time experiments. The sputtering chamber has two beryllium windows, making small-angle X-ray scattering measurements feasible. Sputter chamber, gas and water inlets, and sputter head feedthrough were all made of stainless steel by HASYLAB DESY. The gas flow controllers, pumps, valves, and piping were assembled for vacuum control. The main chamber is pumped down by a two-stage turbo molecular pump that brings the chamber into a base pressure of 5 × 10-8 mbar. A load/lock is separated from the main chamber by a gate valve and allows the exchange of samples in a short time without breaking the main chamber vacuum. The samples were mounted in a sample holder designed to be translated into the system horizontally (perpendicular to the X-ray beam) through a central flange on the chamber. A vertical manipulator was used to align the sample in the desired position. The portable sputtering system, attached to a mechanical support, was mounted on a two-circle goniometer for precise sample alignments. The displacement along z direction (normal to the (42) Diederich, T.; Couet, S.; Roehlsberger, R. Phys. ReV. B 2007, 76, 054401.
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plane of the sample surface) was achieved by translating the entire system (chamber, mechanical support, and goniometer) using a translational table. After rough alignments of the entire chamber, the sample surface was able to be precisely aligned either parallel or perpendicular to the incoming X-ray beam. Prior to each deposition experiment, the target was cleaned by sputtering for a short time with its shutter closed to prevent any deposition during this cleaning step. The deposition was performed at an argon pressure of 4 × 10-3 mbar, and the deposition rate was set to 4.3 Å/min. All parts of the system including the sputter guns, shutters, and gas inlets were controlled by a computer that allowed for precise and reproducible control of the sputtering process and also provided a complete remote control outside the inaccessible X-ray hutch.43 3. Experimental 3.1. Substrate Cleaning and Polymer Coating. A silicon wafer 100 (n-type, Silchem) was cut into 2.5 × 2.5 cm2 pieces. The silicon substrates were cleaned as follows: sonication in dichloromethane at 35 °C for 15 min, water rinsing for 5 min, and then soaking in the cleaning bath at 80 °C for 15 min. The cleaning solution was composed of 100 mL of 96% H2SO4, 35 mL of 35% H2O2, and 65 mL deionized water. The cleaned substrates were further rinsed in deionized water for 10 min and finally spin-dried. A mixture of a triblock copolymer polystyrene-block-polyisoprene-block-polystyrene, denoted P(S-b-I-b-S), with a molecular weight Mw ) 155 kg/ mol and a diblock copolymer polystyrene-block-polyisoprene, denoted P(S-b-I), with a molecular weight Mw ) 90 kg/mol was dissolved in toluene and used for the coating step. The total content of styrene and isoprene in the copolymer mixture was 16% and 84%, respectively. As-cleaned silicon substrates were coated with a thin polymer film using the spin coating method (2500 rpm, 30 s). The polymer films were heat annealed in a vacuum oven at 120 °C for 2 h. 3.2. Real-Space Surface Characterizations. Film thickness was measured with a variable-angle single-wavelength (532 nm) imaging ellipsometer (i-elli2000, Nanofilm Technologie). Topographical and phase images of the polymer film were obtained using a commercial AFM AutoProbe CP Research Instrument. Microfabricated V-shaped silicon cantilevers of theoretical spring constant k ) 3.2 N/m and resonance frequency of approximately 80 kHz with a silicon conical tip of typical radius of 10 nm were used. Areas of 1 µm2 were scanned under constant applied force conditions (noncontact mode), and all AFM images were collected in air. Reflectance UV-vis absorption spectra were recorded on a Perkin-Elmer Lambda 35 spectrophotometer equipped with a reflection holder for nontransparent samples. 3.3. Grazing Incidence Small-Angle X-ray Scattering (GISAXS) Measurements. The GISAXS measurements were carried out at beamline BW4 of DORIS III storage ring44 at HASYLAB (DESY, Hamburg). In GISAXS, the incoming X-ray beam impinges onto the sample surface at a small incidence angle Ri, and the scattered signal is collected using a 2D detector. The selected wavelength was λ ) 0.138 nm. The pathway of the X-ray beam was evacuated, and the beam was focused to the size of 30 × 60 µm2 using an assembly of beryllium lenses. The X-ray beam divergence in and out of the plane of reflection was set by high-quality entrance cross-slits. Twodimensional detector (MARCCD, 2048 × 2048 pixels) was placed at a distance of LSD ) 1.88 m from the center of rotation of the sputtering chamber (where the sample was located). For the experiments presented here, a rodlike beam stop made of tantalum with a diameter of 1.5 mm mounted close to the detector was used to protect the detector from the intense reflected beam in the plane of incidence, and a pointlike movable beam stop in front of the (43) Couet, S.; Diederich, T.; Roehlsberger, R. DeVelopment of a computer controlled UHV deposition chamber; Hasylab Annual Report, 2006; p 323. (44) Roth, S. V.; Dohrmann, R.; Dommach, M.; Kuhlmann, M.; Kroger, I.; Gehrke, R.; Walter, H.; Schroer, C.; Lengeler, B.; Mu¨ller-Buschbaum, P. ReV. Sci. Instrum. 2006, 77, 085106.
Langmuir, Vol. 24, No. 8, 2008 4267 detector was used to block the specular reflection from the sample. The sample inside the sputtering chamber was placed horizontally (xy plane) and at an incident angle of Ri ) 0.44° to the X-ray beam. The incidence angle is well above the critical angle of both the polymer film and the substrate (Rc(SIS/IS) ) 0.133°, Rc(Si) ) 0.2°); therefore, the Yoneda45 peaks of both materials and specular peak are well separated on the 2D detector. At this angle of incidence both surface and bulk nanostructures of the polymer film are accessible. Structural information is obtained with horizontal (qy) and vertical (qz) cuts of the 2D intensity distribution with q ) 1.7 × 10-4 Å-1. The qy cut is parallel to the sample surface at qz position of maximum intensity, and the qz cut is perpendicular to the surface at qy position of the maximum interference. The horizontal cut qy provides the lateral structure information, including particle shape, size distributions, and spatial distributions.46,47 The horizontal cut qy is normally named out-of-plane GISAXS cut in reference to a cut normal to the plane of incidence beam. All cuts were integrated over a limited q-range to improve statistics. In the applied stop-sputtering mode, the gold nanoparticles were sputter deposited on top of the polymer film at a rate of 4.3 Å/min for 6 s and the scattered intensity was accumulated over 30 s of counting time. Ten repetitions of 6 s sputtering followed by 30 s GISAXS measurements were perfomed on the same polymer film; hence, 10 2D GISAXS images were collected to follow the in situ growth of gold nanoparticles in the polymer film. After these 10 repetitions, the film was kept in the chamber to relax for 10 h before a final GISAXS measurement was taken. 3.4. Modeling. The resulting intensity profiles qy were fitted using the program IsGISAXS.48 The program is intended for the simulation and analysis of islands supported on a substrate, buried inside a substrate, or encapsulated in a layer on a substrate. The island shape, the interference function between islands, and the experimental parameters are given as input. The program uses the distorted-wave Born approximation49,50 for computation of the intensity. The interference function (structure factor, S(q)) is the FT of the island position autocorrelation function. A 1D paracrystal model, which is a regular one-dimensional lattice with special cumulative disorder inducing a loss of long range order, was used. The structure factor was fitted with two parameter functions; the average distance, D, and a disorder parameter, ω (width of the distance distributions). In the present study, the decoupling approximation (DA) was used. DA supposes that the kind of the scatterers and their positions are not correlated in such a way that the partial pair correlation functions depend only on the relative positions of the scatterers (homogeneous system) and not on the class kind. In the simulation, the spherical shaped particles were used with a radius R and the form factor is calculated using:48 Psph(q, R) ) 4πR3
sin qR - qR sin qR exp(iqzR) (qR)3
A Lorenz type of distribution probability for the parameter R was selected, with the relative width σR/R of the distribution of the radii. Various different island shapes, such as cylinders, boxes, hemispheres, and ellipsoids, with different aspect ratios were tested. The best agreement with the experimental pattern was obtained using a spherical shape. The program IsGISAXS allowed us to determine the interparticle distance, D, lateral size, and size distribution.51 (45) Yoneda, Y. Phys. ReV. 1963, 131, 2010. (46) Salditt, T.; Metzger, T. H.; Peisl, J.; Reinker, B.; Moske, M.; Samwer, K. Europhys. Lett. 1995, 32, 331. (47) Mu¨ller-Buschbaum, P. Anal. Bioanal. Chem. 2003, 376, 3. (48) Lazzari, R. J. Appl. Crystallogr. 2002, 35, 406. (49) Rauscher, M.; Paniago, R.; Metzger, H.; Kovats, Z.; Domke, J.; Peisl, J.; Pfannes, H. D.; Schulze, J.; Eisele, I. J. Appl. Phys. 1999, 86, 6763. (50) Busch, P.; Posselt, D.; Smilgies, D. M.; Rauscher, M.; Papadakis, C. M. Macromolecules 2007, 40, 630. (51) Revenant, C.; Leroy, F.; Lazzari, R.; Renaud, G.; Henry, C. R. Phys. ReV. B 2004, 69, 035411.
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Figure 2. Composite images showing 2D scattering patterns of (a) phase separated thin blend copolymer film, (b-k) 10 repetitions of 6 s sputtering onto the polymer film at a rate of 4.3 Å/min, and (l) after 10 h relaxation of the 4.3 Å gold covered polymer film. The intensity is shown on a logarithmic scale. The gray rectangle in the middle of the images indicates the rod beam stop, and the horizontal black line with pointlike end is the specular beam stop. Two white arrows indicate the two scattering intensity maxima along the qy direction.
4. Results 4.1. Morphology of the Polymer Film. The formation of PS-rich spherical domains in P(S-b-I-b-S) triblock,52-55 P(S-bI) diblock copolymers,56 and their blends57 has been previously reported, and the arrangements of these domains in the microphase-separated polymer samples were identified as bodycentered cubic (bcc). In this study, the GISAXS 2D image (Figure 2a) of a similar thin blend polymer film shows two intensity maxima (marked with arrows), separated by the rod beamstop visible along qy direction, that originates from the interference by PS domains. This interference effect arises because the PS domains are separated by a preferential nearest neighbors (centerto-center) distance, D. The detection of only one interdomain interference and the absence of additional secondary bcc peaks (at q2/q1 ) x2 and q3/q1 ) x3) indicate that there are no regular (52) O’Connor, A. E.; Macosko, C. W. J. Appl. Polym. Sci. 2002, 86, 3355. (53) Lee, S. H.; Char, K.; Kim, G. Macromolecules 2000, 33, 7072. (54) Prasman, E.; Thomas, E. L. J. Polym. Sci., Part B: Poly. Phys. 1998, 36, 1625. (55) Choi, S.; Lee, K. M.; Han, C. D.; Sota, N.; Hashimoto, T. Macromolecules 2003, 36, 793. (56) Khandpur, A. K.; Forster, S.; Bates, F. S.; Hamley, I. W.; Ryan, A. J.; Bras, W.; Almdal, K.; Mortensen, K. Macromolecules 1995, 28, 8796. (57) Daoulas, K. C.; Theodorou, D. N.; Roos, A.; Creton, C. Macromolecules 2004, 37, 5093.
spatial arrangements of the PS-rich domains and a more liquidlike ordered structure best describes the morphology of the polymer film. The absence of long-range ordering is due to the annealing condition, which is not expected to lead to a fully equilibrated structure. The long-range highly ordered structures of polymer films can be experimentally55 obtained by annealing for long periods (up to 1 month) at a given temperature in vacuum to reach the thermodynamically favorable equilibrium state. Thus with respect to application as well as to a reasonable time demanding sample preparation, the installed liquidlike structure of PS spheres in a PI matrix is favorable. The main characteristic interparticle distance and the radius of the PS-domains are well predicted by the simulation (Figure 3), and so is the fit of the experimental qy scattering profile of the polymer film (Figure 4). A form factor maximum causing a peak, characterizing the spherical shape of the PS-rich domains, is not detected in the qy profile58 of the polymer film (Figure 4). Imperfect shape of the PS-rich domains and the polydispersity of their sizes are shown in the AFM micrographs (Figure 5). Our fitting, based on spherical-shaped particles, indicated that the average particle size is 55 Å, it varies between 20 and 120 Å, (58) Yokoyama, H.; Dutriez, C.; Li, L.; Nemoto, T.; Sugiyama, K.; Sasaki, S.; Masunaga, H.; Takata, M.; Okuda, H. J. Chem. Phys. 2007, 127, 014904.
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Figure 3. 2D GISAXS intensity pattern (in qyqz) of the copolymer film measured (left) before sputter deposition of gold together with (right) simulated 2D intensity using IsGISAXS. The intensity is plotted on a logarithmic scale. The gray rectangle in the middle of the images indicates the rod beamstop.
Figure 4. GISAXS parallel cuts (qy) to the sample surface. Both the intensity and the qy are represented on logarithmic scales. The solid lines are the best fits using IsGISAXS. Bottom to top: the phase-separated polymer film, ten repetitions of 6 s gold sputtering, and the relaxed gold decorated film.
and the mean interparticle distance is 420 Å. Therefore, a polymer film (thickness ) 280 nm) with a morphology that is composed of 3D liquidlike packing of PS domains in PI matrix was successfully prepared. The model polymer film will be used as a template to follow the growth of nanoparticles in a polymer film consisting of two chemically different regions (PS and PI). 4.2. Growth of Gold Nanoparticles in the Polymer Film. The deposition of gold on the polymer film was performed for 6 s periods, and the in situ GISAXS measurements were collected for 30 s immediately after the deposition step. This sequence was repeated 10 times, and the 2D intensity profiles are shown in Figure 2b-k. As the amount of deposited gold increases, the prominent peak at qy ) 0.015 Å-1 (corresponding to the mean interparticle distance of 420 Å) on both sides of the beam stop gradually grows in intensity along the qz axis (Figure 2b-k). Two soaring increases of the scattering on both sides of the beam stop (for qy < 0.006 Å-1) are the parasitic scattering because of the reflection by the beam stop. The increase of the parasitic scattering around the beam stop intensifies with the successive accumulation of gold on the polymer film. This increase is attributed to the increase of the overall scattered X-ray intensity
from the sample surface, due to the high scattering cross section of gold atoms. The fitting of the out-of-plane cuts (qy) of the 2D scattering intensity profile (at qz position of 0.025 Å-1) was performed for qy > 0.006 Å-1 (Figure 4) and indicates no strong influence of the accumulated gold particles on the overall lateral structures of the polymer film. In addition, no additional characteristic scattering peak is observed for the gold particles in the qy direction. The surface profile of the polymer film after gold deposition (4.3 Å), as obtained by AFM (Figure 5), shows no strong indication of surface gold decoration. These results indicate that gold nanoparticles are indeed inside the polymer film and not at the film surface. The absence of gold layer decorating the PS domains of the polymer film at the air interface is attributed to possible depletion of the film59 during the deposition process, which is in agreement with X-ray standing wave fluorescence spectroscopy study.7,20 On the other hand, the vertical cuts (qz) obtained at the prominent peak (at qy position 0.015 Å-1) show a gradual increase in the intensity (marked by dashed arrow in Figure 6) with increasing of the amount of gold. The polymer film model was simulated using the higher refractive index of gold instead of the PS domains. The results of this simulation compared with the experimental one are shown in Figure 7 and indicate that the gold atoms diffuse inward and aggregate inside the PS domains. The polymer film covered with a nominal gold layer thickness of 4.3 Å gold was kept to relax for 10 h and then the GISAXS measurement was performed to investigate a possible structural reorganization of the gold nanoparticles inside the film. The GISAXS study indicates negligible changes in the film structure after 10 h of relaxation time of the gold doped polymer film and reveals that the gold nanoparticles aggregate and form a stable morphology within the film. Due to limited beam time, the effect of longer relaxation time was not tested. It is well-known that the aggregation of gold nanoparticles changes color from red to violet or blue. The UV-vis reflection spectrum (Figure 8) of the polymer film versus that of the gold covered polymer film shows a blue shift (reflection minima from 540 to 505 nm), which indicates the presence of gold nanoparticles. The resonance wavelength of the surface plasmon in metallic particles is highly dependent on the environment around (59) Thran, A.; Strunskus, T.; Zaporojtchenko, V.; Faupel, F. Appl. Phys. Lett. 2002, 81, 244.
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Figure 5. AFM topography (a,c) and phase (b,d) images of the copolymer film before sputtering (a,b) and after gold sputtering of nominally 4.3 Å (c,d). The visible structure originates from the microphase separation of the copolymer (spherical rigid PS domains in soft PI matrix). The structure heights increase with the increase in the brightness of structures in the image and the z scale is 30 Å.
systematic decrease in the wavelength at maximum absorption from 850 to 600 nm with decreasing size of the gold nanoparticles from 1200 Å to 200 Å. This may support the UV-vis blue shift effect due to presence of 55 Å gold nanoparticles in the polymer film, as concluded from the GISAXS results.
5. Discussion
Figure 6. GISAXS cuts perpendicular (qz) to the surface at qy position of maximum scattering interference (0.015 Å-1). Both the intensity and qz are represented in logarithmic scales. Bottom to top: the phase-separated polymer film, ten repetitions of 6 s gold sputtering, and the relaxed gold decorated film. The dashed arrow indicates the increase of intensity with the progressive increase of the amount of sputtered gold.
the particles as well as the particle size.60 Due to the absorption of the polymer film in the visible region, further analyses on the optical reflection data is not possible. A recent work61 shows the (60) Okamoto, T. In Near-Field optics and surface plasmon polaritons; Kawata, S., Ed.; Springer: New York, 2001; Vol. 81, p 97. (61) Okamoto, T.; Yamaguchi, I. J. Phys. Chem. B 2003, 107, 10321.
Previous investigations19,20,39,62,63 have employed the annealing of the copolymer with internally attached nanoparticles to obtain nanostructured metal films. Such an annealing process has been avoided in the present investigation. Instead, we used a copolymer film in which the majority of the polymer is soft at room temperature, and only a minor portion of it is glassy. The metallic decoration of the copolymer ultrathin film was studied using a block copolymer template, and its morphology, as given, was investigated to focus on the aggregation behavior of the metal sputter deposited onto the template. In the sputtering chamber used in this study, we assume that gold reaches the polymer surface in its atomic state because the mean free path of the sputtered gold atoms in an Ar atmosphere of 4 × 10-3 mbar is much larger than the target-substrate distance. The following mechanisms39,40 were suggested for the atom upon arrival at the surface: bounce off the surface, diffuse laterally on the surface, diffuse within the film,64 or evaporate off the (62) Zehner, R. W.; Lopes, W. A.; Morkved, T. L.; Jaeger, H.; Sita, L. R. Langmuir 1998, 14, 241. (63) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (64) Tuteja, A.; Mackay, M. E.; Narayanan, S.; Asokan, S.; Wong, M. S. Nano Lett. 2007, 7, 1276.
Gold Sputtering onto a Polymer Template
Figure 7. 2D GISAXS intensity pattern for 4.3 Å gold sputtered polymer film (a) experimental and (b) simulated with parameters obtained using the qy fit and the X-ray index of refraction for gold. The intensity is represented on a logarithmic scale. The gray rectangle in the middle of the images indicates the rod beam stop.
Figure 8. Reflectance UV-vis spectra of copolymer film (dashed line) and gold covered (4.3 Å) copolymer film (solid line).
surface. The rate of each of the above processes along with the rate at which these atoms segregate into nanoparticles determine the final nanostructure of the deposed metal film. In our study, increasing the amount of gold caused a progressive agglomeration of gold particles within the buried PS domains in the polymer film, indicating that gold atoms diffuse in the PI regions as well as in the PS regions of the polymer film. In PS
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domains, the gold-gold particle interaction seems to overwhelm the gold-polymer interactions,15,65 and the gold atoms aggregate and ignore the template. A possible reason for such high interaction between gold particles in PS is the high surface tension of gold (1.3 J/m2) compared with the low interaction energy (0.1-0.3 J/m2) between gold and PS.39 The gold atoms in the PS domains reach equilibrium by minimizing their surface area by forming large spherical nanoparticles. Because of the very small magnitudes of surface energies of both PS and PI compared with surface energy of gold, adsorption of metal particles at the boundary between PS and PI regions is excluded. PI having the lower surface energy, as compared to PS, covers as a very thin layer the whole polymer film surface. Thus, gold atoms land on the polymer surface, diffuse through the soft PI region in both regions (PS and PI), and tend to agglomerate within the PS. Our results show consistency with a recent TEM investigation39 of samples made by silver evaporation onto both PS and PMMA homopolymer films. The volume of silver particles on the polar PMMA homopolymer film is less than that on the PS homopolymer film. In the latter study, this smaller volume of silver particles on PMMA was attributed to both silver diffusion within the film and silver evaporation off the PMMA surface. Our results indicate that the evaporation off the polar PI block regions of our polymer film is less likely to occur. The gold atoms diffuse mainly within both the PS and PI polymeric domains, and in the PS domains, the particles coalesce to form nanoparticles. It has been reported by Zaporojtchenko et al.66 that during early stage of nobel metal deposition on polymer surfaces, such as PS, the growth process is dominated by surface diffusion. Also a recent investigation64 showed that the diffusion of nanoparticles is as much as 200 times faster than that predicted by continuum Stokes-Einstein relation. Additional important questions are why gold particles do not aggregate in the PI matrix as they diffuse in both PS and PI regions and whether the gold atoms/particles diffuse at the same speed in both blocks. In the PI regions, the polymer-metal interaction is relatively higher than that in the PS because of the presence of diene groups with delocalized π-bonded electrons on PI chains that may produce a “capping” effect on the metal. Therefore, the metal atoms are stabilized in the PI matrix, and the formation of metal aggregates is greatly suppressed. Metal atoms in PI regions can be seen as isolated metal atom dispersion in the polymer matrix. For concentrated metal particles, it has been shown previously67 that the dynamics (diffusion) of particle dispersions in polymeric matrices are reduced dramatically even in situations where the polymer-metal interactions are very weak. This may explain the absence of surface gold decoration on the PS domains, since the total gold deposited in this study after 10 repetitions of sputtering step is only 4.3 Å. We have not reached the high gold concentration, where the diffusion is reduced and a surface gold decorated PS layer is formed. Within the context of this discussion, it is expected that metal diffusion in the PS is faster than in PI regions as a result of higher metal-PI interactions. This does not contradict the previously observed high density of metal clusters (large metal particle volume) on the PS39 compared with polar polymers (e.g., PMMA and PI), because the metal-metal interactions will dominate on (65) Roth, S. V.; Burghammer, M.; Riekel, C.; Mu¨ller-Buschbaum, P.; Diethert, A.; Panagiotou, P.; Walter, H. Appl. Phys. Lett. 2003, 82, 1935. (66) Zaporojtchenko, V.; Behnke, K.; Thran, A.; Strunskus, T.; Faupel, F. Appl. Surf. Sci. 1999, 145, 355. (67) Cole, D. H.; Shull, K. R.; Rehn, L. E.; Baldo, P. Phys. ReV. Lett. 1997, 78, 5006.
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the PS surface and segregation of metal atoms on the PS surface seems to occur at faster rate than the diffusion step.
6. Summary The formation and growth of gold nanoparticles in dry phaseseparated block copolymer film composed of spherical glassy PS domains in a rubbery PI matrix is investigated using in situ GISAXS. The in situ GISAXS investigation is critical for understanding how the arrangements of metal nanoparticles correlate with the structure of copolymer domains within the film and opens new possibilities for the investigation of metalpolymer interactions. We showed that gold migrates to the central regions of the polymer film and decorates the spherical PS domains. The gold-gold interaction in PS regions dominates the gold-polymer interaction. In PI matrix, the gold-gold
Metwalli et al.
interaction is suppressed by a “capping” effect of the delocalized π-bonds on the PI chains that stabilize the dispersed gold atoms. Metal aggregation occurs at a rate faster than the diffusion of gold atoms in the polymer. Our study introduces the concept that, without thermal annealing of the gold attached polymer film, the gold assembly is not limited to the flat two-dimensional but is also included in three-dimensional structures. Acknowledgment. We thank A. Timmann for his help during the alignment of BW4 beamline at HASYLAB. We thank B. Russ, TU Munich, for her support during the surface characterization experiments. Financial support by DFG (Grant MU1487/6) is gratefully acknowledged. LA7038587
